ENA Dictionary of BiologyOxford University PressOxford University Press

A Dictionary of Biology

© Oxford University Press 2000, 2003
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A

A band

The region of a striated muscle fibre that contains both thick (myosin) and thin (actin) filaments. It is visible as a dark band with a lighter central zone (see H zone) in the middle of a sarcomere.

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abdomen

The posterior region of the body trunk of animals. In vertebrates it contains the stomach and intestines and the organs of excretion and reproduction. It is particularly well defined in mammals, being separated from the thorax by the diaphragm. In many arthropods, such as insects and spiders, it may be segmented.

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abductor (levator)

A type of muscle whose function is to move a limb away from the body. Abductors work antagonistically with adductors.

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abiogenesis

The origin of living from nonliving matter, as by biopoiesis. See also spontaneous generation.

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abiotic factor

Any of the nonliving factors that make up the abiotic environment in which living organisms occur. They include edaphic factors and all the aspects of climate, geology, and atmosphere that may affect the biotic environment. Compare biotic factor.

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abomasum

The fourth and final chamber of the stomach of ruminants. It fills from the omasum and empties into the small intestine. The abomasum is referred to as the `true stomach' as it is in this chamber that protein digestion occurs, in acidic conditions. See Ruminantia.

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ABO system

One of the most important human blood group systems. The system is based on the presence or absence of antigens A and B on the surface of red blood cells and antibodies against these in blood serum. A person whose blood contains either or both these antibodies cannot receive a transfusion of blood containing the corresponding antigens as this would cause the red cells to clump (see agglutination). The table illustrates the basis of the system: people of blood group O are described as `universal donors' as they can give blood to those of any of the other groups. See also immune response.

Group

Antigens on red cell surface

Antibodies in serum

Blood group of people donor can receive blood from

Blood group of people donor can give blood to

A

A

anti-B

A, O

A, AB

B

B

anti-A

B, O

B, AB

AB

A and B

none

A, B, AB, O

AB

O

neither A nor B

anti-A and anti-B

O

A, B, AB, O

The ABO blood group system

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abscisic acid (ABA)

A naturally occurring plant growth substance that appears to be involved primarily in seed maturation and in regulating closure of leaf pores (stomata). In seeds, it promotes the synthesis of storage protein and prevents premature germination. In leaves, abscisic acid is produced in large amounts when the plant lacks sufficient water, promoting closure of stomata and hence reducing further water losses. It was formerly believed to play a role in abscission, hence the name.

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abscission

The separation of a leaf, fruit, or other part from the body of a plant. The process is controlled by growth substances, notably auxins; it involves the formation of an abscission zone , at the base of the part, within which a layer of cells (abscission layer ) breaks down.

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absolute refractory period

See refractory period.

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absorbed dose

See dose.

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absorption

The movement of fluid or a dissolved substance across a cell membrane. In many animals, for example, soluble food material is absorbed into cells lining the alimentary canal and thence into the blood. In plants, water and mineral salts are absorbed from the soil by the roots. See osmosis.

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absorption spectrum

See spectrum.

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abyssal zone

The lower depths of the ocean (below approximately 2000 metres), where there is effectively no light penetration. Abyssal organisms are adapted for living under high pressures in cold dark conditions. See also aphotic zone.

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Acarina

An order of small arthropods belonging to the class Arachnida and comprising the mites and ticks. There are over 30000 described species, with perhaps 20 times this number still unknown, distributed worldwide in a wide variety of terrestrial and aquatic habitats. Many are free-living in soil or on vegetation, feeding on organic matter or preying on other small arthropods, while a significant number are parasites of plants and animals, including domesticated animals and humans. The adult body is generally globular or ovoid, with four pairs of legs. Unlike spiders, there is no `waist', the abdomen being fused to the more anterior prosoma. At the front of the body the capitulum bears the mouthparts, variously adapted for cutting, crushing, or piercing. The eggs hatch into a three-legged larva, which subsequently moults to a nymph resembling the adult. Ticks (up to 3 cm long) are ectoparasites of vertebrates, feeding on blood drawn through the skin of their host. They transmit a wide range of diseases, including certain forms of encephalitis and Lyme disease. Mites are much smaller (up to 4 mm long) and are parasitic or free-living. They tend to feed on feathers, hair, skin secretions, or skin debris, causing, for example, scabies in humans and mange in domesticated animals. The house-dust mite (Dermatophagoides) can provoke allergies or dermatitis. Spider mites are damaging parasites of plants and may infest some arable and greenhouse crops.

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acceleration

A form of heterochrony in which, during the course of evolution, the rate of development of an organism is speeded up and new stages are added to the end of the ancestral developmental sequence without prolonging the total development time. The morphological outcome is an example of peramorphosis, and the developmental sequence (ontogeny) conforms to the theory of recapitulation.

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acceptor

1 (in chemistry) A compound, molecule, ion, etc., to which electrons are donated in the formation of a coordinate bond.

2 (in biochemistry) A receptor that binds a hormone without any apparent biological response.
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accessory bud

A bud that is situated at the side of or above an axillary bud (see axil).

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accessory chromosome (B chromosome)

Any nonessential chromosome that is found in addition to the regular karyotype of a species. Accessory chromosomes are heterochromatic (see chromatin). The frequency of accessory chromosomes in a species varies among populations but their presence does not appear to alter the appearance (phenotype) of the organisms.

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accessory pigment

A photosynthetic pigment that traps light energy and channels it to chlorophyll a, the primary pigment, which initiates the reactions of photosynthesis. Accessory pigments include the carotenoids, phycobiliproteins, and chlorophylls b, c, and d.

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acclimatization

1 The progressive adaptation of an organism to any change in the environment that subjects it to physiological stress.

2 The overall sum of processes by which an organism attempts to compensate for conditions that would substantially reduce the amount of oxygen delivered to its cells.
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accommodation

1 (in animal physiology) Focusing: the process by which the focal length of the lens of the eye is changed so that clear images of objects at a range of distances are displayed on the retina. In man and some other mammals accommodation is achieved by reflex adjustments in the shape of the lens brought about by relaxation and contraction of muscles within the ciliary body.

2 (in botany) The ability of a plant to adapt itself to gradually changing environmental conditions.
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3 (in animal behaviour) Adjustments made by an animal's nervous or sensory systems in response to continuously changing environmental conditions.
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acellular

Describing tissues or organisms that are not made up of separate cells but often have more than one nucleus (see syncytium). Examples of acellular structures are muscle fibres. Compare unicellular.

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acentric

Describing an aberrant chromosome fragment that lacks a centromere. Such fragments are normally lost because they are unable to orientate properly during cell division.

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acetabulum (cotyloid cavity)

The semicircular cavity in the pelvic girdle that houses the ball-shaped head of the femur.

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acetate (ethanoate)

A salt or ester of acetic acid.

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acetic acid (ethanoic acid)

A carboxylic acid, CH_3COOH, that, is used as a carbon source by certain green algae. Combined with coenzyme A (see acetyl coenzyme A), it plays a crucial role in the energy metabolism of all organisms.

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acetone

See ketone; ketone body.

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acetylation

The introduction of an acetyl group (CH_3CO–) into a compound. The acetylation of coenzyme A to acetyl coenzyme A is an important stage in the Krebs cycle: the acetyl group is derived from pyruvate after the removal of a molecule of carbon dioxide and two hydrogen atoms.

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acetylcholine (ACh)

One of the main neurotransmitterss of the vertebrate nervous system. It is released at certain (cholinergic ) nerve endings and may be excitatory or inhibitory; it initiates muscular contraction at neuromuscular junctions. Once acetylcholine has been released it has only a transitory effect because it is rapidly broken down by the enzyme cholinesterase.

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acetylcholinesterase

See cholinesterase.

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acetyl coenzyme A (acetyl CoA)

A compound formed in the mitochondria when an acetyl group (CH_3CO–), derived from the breakdown of fats, proteins, or carbohydrates (via glycolysis), combines with the thiol group (–SH) of coenzyme A. Acetyl CoA feeds into the energy generating the Krebs cycle and also plays a role in the synthesis and oxidation of fatty acids.

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achene

A dry indehiscent fruit formed from a single carpel and containing a single seed. An example is the feathery achene of clematis. Variants of the achene include the caryopsis, cypsela, nut, and samara. See also etaerio.

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acid

A type of compound that contains hydrogen and dissociates in water to produce positive hydrogen ions. The reaction, for an acid HX, is commonly written:
HX H+ + X–

In fact, the hydrogen ion (the proton) is hydrated, and the complete reaction is:
HX + H_2O H_3O+ + X–

The ion H_3O+ is the hydroxonium ion . The strength of an acid depends on the extent to which it dissociates: strong acids (e.g. sulphuric acid and hydrochloric acid) are almost completely dissociated in water; weak acids (e.g. carbonic acid) are only partially dissociated. See also buffer; pH scale. Compare base.

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acid–base balance

The regulation of the concentrations of acids and bases in blood and other body fluids so that the pH remains within a physiologically acceptable range. This is achieved by the presence of natural buffer systems, such as the haemoglobin, hydrogencarbonate ions, and carbonic acid in mammalian blood. By acting in conjunction, these effectively mop up excess acids and bases and therefore prevent any large shifts in blood pH. The acid–base balance is also influenced by the selective removal of certain ions by the kidneys and the rate of removal of carbon dioxide from the lungs.

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acid growth theory

A theory, originally proposed in 1970 by R. Cleland, A. Hager, and co-workers, that describes how auxins stimulate cell expansion in certain plant tissues, notably in coleoptiles of cereals and other grasses, such as oat (Avena). It asserts that the auxins induce acidification of the immediate cell-wall environment, thereby activating enzymes that loosen load-bearing bonds within the cell wall. This permits expansion of the walls by the cell's internal turgor pressure, and thus enlargement of the cell. It is thought that the auxin binds with auxin-binding proteins in the plasma membrane and stimulates proton pumps (also in the cell's plasma membrane) to excrete protons (H+) into the cell wall from the cytoplasm. The consequent lowered cell-wall pH activates the wall enzymes.

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acidic stains

See staining.

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acidosis

A condition in which the body fluids become more acidic, i.e. the pH is less than 7.4, and the capacity of the body to buffer hydrogen ions is diminished. A decrease in the elimination of carbon dioxide from the body gives rise to respiratory acidosis , while a deficiency of hydrogencarbonate results in metabolic acidosis .

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acid protease

A protein-digesting enzyme (see protease) that exhibits maximum activity and stability in acid conditions (pH 2.0–5.0) and is inactivated at pH values above 6.0. Acid proteases have a low isoelectric point and are low in basic amino acids. Two types are widely used in the food and beverage industries: those from Aspergillus, which resemble pepsin; and those from Mucor, which resemble rennin.

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acid rain

Precipitation having a pH value of less than about 5.0, which has adverse effects on the fauna and flora on which it falls. Rainwater typically has a pH value of 5.6, due to the presence of dissolved carbon dioxide (forming carbonic acid). Acid rain results from the emission into the atmosphere of various pollutant gases, in particular sulphur dioxide and various oxides of nitrogen, which originate from the burning of fossil fuels and from car exhaust fumes, respectively. These gases dissolve in atmospheric water to form sulphuric and nitric acids in rain, snow, or hail (wet deposition ). Alternatively, the pollutants are deposited as gases or minute particles (dry deposition ). Both types of acid deposition affect plant growth – by damaging the leaves and impairing photosynthesis and by increasing the acidity of the soil, which results in the leaching of essential nutrients. This acid pollution of the soil also leads to acidification of water draining from the soil into lakes and rivers, which become unable to support fish life. Lichens are particularly sensitive to changes in pH and can be used as indicators of acid pollution (see indicator species).

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acinus

(pl. acini) The smallest unit of a multilobular gland, such as the pancreas. Each acinus in the pancreas is made up of a hollow cluster of acinar cells , which produce the digestive enzymes secreted in pancreatic juice. Minute ducts from the pancreatic acini eventually drain into the pancreatic duct.

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acoelomate

Describing any bilaterally symmetrical animal of the subkingdom Eumetazoa that does not possess a coelom (see also body cavity). Examples of acoelomate animals are the platyhelminths.

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acquired characteristics

Features that are developed during the lifetime of an individual, e.g. the enlarged arm muscles of a tennis player. Such characteristics are not genetically controlled and cannot be passed on to the next generation. See also Lamarckism; neo-Lamarckism.

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acquired immune deficiency syndrome

See AIDS.

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acquired immunity

See immunity.

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acridine

A chemical (see formula) that is capable of causing frameshift mutations in the DNA sequence. Several derivatives of acridine (such as acridine orange) are used as dyes or biological stains.

Acridine

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acrocentric

See centromere.

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acromegaly

A chronic condition developing in adulthood due to overproduction of (or oversensitivity to) growth hormone, usually caused by a tumour in the pituitary gland. This leads to a gradual enlargement of the bones, causing characteristic coarsening of the facial features and large hands and feet.

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acrosome

See spermatozoon.

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ACTH (adrenocorticotrophic hormone, corticotrophin)

A hormone produced by the anterior pituitary gland that controls secretion of certain hormones (the corticosteroids) by the adrenal glands. Its secretion, controlled by corticotrophin-releasing hormone, occurs in short bursts every few hours, and is increased by stress. It can be administered by injection to treat such disorders as rheumatic diseases and asthma, but it only relieves symptoms and is not a cure.

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actin

A contractile protein found in muscle tissue, in which it occurs in the form of filaments (called thin filaments). Each thin filament consists of two chains of globular actin molecules, around which is twisted a strand of tropomyosin and interspersed troponin. Units of muscle fibre (see sarcomere) consist of actin and myosin filaments, which interact to bring about muscle contraction (see also sliding filament theory). Actin is also found in the microfilaments that form part of the cytoskeleton of all cells.

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Actinobacteria (Actinomycetes; Actinomycota)

A phylum of Gram-positive mostly anaerobic nonmotile bacteria. Many species are fungus-like, with filamentous cells producing reproductive spores on aerial branches similar to the spores of certain moulds. The phylum includes bacteria of the genera Actinomyces, some species of which cause disease in animals and man; and Streptomyces, which are a source of many important antibiotics (including streptomycin).

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actinomorphy

See radial symmetry.

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Actinomycetes

See Actinobacteria.

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action potential

The change in electrical potential that occurs across a cell membrane during the passage of a nerve impulse. As an impulse travels in a wavelike manner along the axon of a nerve, it causes a localized and transient switch in electric potential across the cell membrane from –60 mV (millivolts; the resting potential) to +45 mV. The change in electric potential is caused by an influx of sodium ions. Nervous stimulation of a muscle fibre has a similar effect.

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action spectrum

A graphical plot of the efficiency of electromagnetic radiation in producing a photochemical reaction against the wavelength of the radiation used. For example, the action spectrum for photosynthesis using light shows a peak in the region 670–700nm. This corresponds to a maximum absorption in the absorption spectrum of chlorophylls in this region.

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activated sludge process

A sewage and waste-water treatment. The sludge produced after primary treatment is pumped into aeration tanks, where it is continuously stirred and aerated, resulting in the formation of small aggregates of suspended colloidal organic matter called floc . Floc contains numerous slime-forming and nitrifying bacteria, as well as protozoans, which decompose organic substances in the sludge. Agitation or air injection maintains high levels of dissolved oxygen, which helps to reduce the biochemical oxygen demand. Roughly half the sewage in Britain is treated using this method.

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activation energy

Symbol Ea. The minimum energy required for a chemical reaction to take place. In a reaction, the reactant molecules come together and chemical bonds are stretched, broken, and formed in producing the products. During this process the energy of the system increases to a maximum, then decreases to the energy of the products. The activation energy is the difference between the maximum energy and the energy of the reactants; i.e. it is the energy barrier that has to be overcome for the reaction to proceed. The activation energy determines the way in which the rate of the reaction varies with temperature. It is usual to express activation energies in joules per mole of reactants.

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activator

1 A type of transcription factor that enhances the transcription of a gene by binding to a region of DNA called an enhancer. Compare repressor.

2 A substance that – by binding to an allosteric site on an enzyme – enables the active site of the enzyme to bind to the substrate.
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3 Any compound that potentiates the activity of a drug or other foreign substance in the body.
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active immunity

Immunity acquired due to the body's response to a foreign antigen.

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active site (active centre)

The site on the surface of an enzyme molecule that binds the substrate molecule. The properties of an active site are determined by the three-dimensional arrangement of the polypeptide chains of the enzyme and their constituent amino acids. These govern the nature of the interaction that takes place and hence the degree of substrate specificity and susceptibility to inhibition.

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active transport

The movement of substances through membranes in living cells, often against a concentration gradient: a process requiring metabolic energy. Organic molecules and inorganic ions are transported into and out of both cells and their organelles. The substance binds to a carrier molecule embedded in the membrane, which carries it through the membrane and releases it on the opposite side. Active transport serves chiefly to maintain the normal balance of ions in cells, especially the concentration gradients of sodium and potassium ions crucial to the activity of nerve and muscle cells. Compare facilitated diffusion.

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actomyosin

The complex formed from the interaction of the proteins actin and myosin during the process of muscle contraction. See also sliding filament theory.

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acyclovir (acycloguanosine)

A drug used to treat cold sores, shingles, genital blisters, or other lesions caused by herpesvirus infection. It is an analogue of the base guanine and acts by interfering with DNA replication of the virus.

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adaptation

1 (in evolution) Any change in the structure or functioning of an organism that makes it better suited to its environment. Natural selection of inheritable adaptations ultimately leads to the development of new species. Increasing adaptation of a species to a particular environment tends to diminish its ability to adapt to any sudden change in that environment.

2 (in physiology) The alteration in the degree of sensitivity (either an increase or a decrease) of a sense organ to suit conditions more extreme than normally encountered. An example is the adjustment of the eye to vision in very bright or very dim light.
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adaptive radiation (divergent evolution)

The evolution from one species of animals or plants of a number of different forms. As the original population increases in size it spreads out from its centre of origin to exploit new habitats and food sources. In time this results in a number of populations each adapted to its particular habitat: eventually these populations will differ from each other sufficiently to become new species. A good example of this process is the evolution of the Australian marsupials into species adapted as carnivores, herbivores, burrowers, fliers, etc. On a smaller scale, the adaptive radiation of the Galapagos finches provided Darwin with crucial evidence for his theory of evolution (see Darwin's finches).

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adductor (depressor)

A type of muscle whose function is to pull a limb inwards, towards the body of an animal. Compare abductor.

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adenine

A purine derivative. It is one of the major component bases of nucleotides and the nucleic acids DNA and RNA.

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adenohypophysis

See pituitary gland.

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adenosine

A nucleoside comprising one adenine molecule linked to a D-ribose sugar molecule. The phosphate-ester derivatives of adenosine, AMP, ADP, and ATP, are of fundamental biological importance as carriers of chemical energy.

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adenosine diphosphate (ADP)

See ATP.

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adenosine monophosphate (AMP)

See ATP.

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adenosine triphosphate

See ATP.

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adenovirus

One of a group of DNA-containing viruses found in rodents, fowl, cattle, monkeys, and man. In man they produce acute respiratory-tract infections with symptoms resembling those of the common cold. They are also implicated in the formation of tumours (see oncogenic).

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adenylate cyclase

The enzyme that catalyses the formation of cyclic AMP. It is bound to the inner surface of the plasma membrane. Many hormones and other chemical messengers exert their physiological effects by increased synthesis of cyclic AMP through the activation of adenylate cyclase. The hormone binds to a receptor on the outer surface of the plasma membrane, which then activates adenylate cyclase on the inner surface via G-protein or calmodulin.

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ADH

See antidiuretic hormone.

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adipocyte

See fat cell.

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adipose tissue

A body tissue comprising cells containing fat and oil. It is found chiefly below the skin (see subcutaneous tissue) and around major organs (such as the kidneys and heart), acting as an energy reserve and also providing insulation and protection.

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adjuvant

A nonantigenic substance (such as aluminium hydroxide) that, in combination with an antigen, enhances antibody production by inducing an inflammatory response, which leads to a local influx of antibody-forming cells. Adjuvants are used therapeutically in the preparation of vaccines, since they increase the production of antibodies against small quantities of antigen and lengthen the period of antibody production.

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adolescence

The period in human development that occurs during the teenage years, between the end of childhood and the start of adulthood, and is characterized by various physical and emotional changes associated with development of the reproductive system. It starts at puberty , when the reproductive organs begin to function, and is marked by the start of menstruation (see menstrual cycle) in females and the appearance of the secondary sexual characteristics in both sexes. In males the secondary sexual characteristics are controlled by the hormone testosterone and include deepening of the voice due to larynx enlargement, the appearance of facial and pubic hair, rapid growth of the skeleton and muscle, and an increase in sebaceous gland secretions. In females the secondary sexual characteristics are controlled by oestrogens and include growth of the breasts, broadening of the pelvis, redistribution of fat in the body, and appearance of pubic hair.

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ADP

See ATP.

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adrenal cortex

The outer layer of the adrenal gland, in which several steroid hormones, the corticosteroids, are produced.

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adrenal glands

A pair of endocrine glands situated immediately above the kidneys (hence they are also known as the suprarenal glands ). The inner portion of the adrenals, the medulla , contains chromaffin tissue and secretes the hormones adrenaline and noradrenaline; the outer cortex secretes small amounts of sex hormones (androgens and oestrogens) and various corticosteroids, which have a wide range of effects on the body. See also ACTH.

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adrenaline (epinephrine)

A hormone (see formula), produced by the medulla of the adrenal glands, that increases heart activity, improves the power and prolongs the action of muscles, and increases the rate and depth of breathing to prepare the body for `fright, flight, or fight' (see alarm response). At the same time it inhibits digestion and excretion. Similar effects are produced by stimulation of the sympathetic nervous system. Adrenaline can be administered by injection to relieve bronchial asthma and reduce blood loss during surgery by constricting blood vessels.

Adrenaline

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adrenal medulla

The inner part of the adrenal gland, in which adrenaline is produced.

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adrenergic

1 Describing a cell, especially a neurone, or a cell receptor that is stimulated by adrenaline, noradrenaline, or related substances. See also beta adrenergic receptor.

2 Describing a nerve fibre or neurone that releases adrenaline or noradrenaline when stimulated. Compare cholinergic.
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adrenocorticotrophic hormone

See ACTH.

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Adrian, Edgar Douglas, Baron

(1889–1977)
British neurophysiologist, who became a professor at Cambridge in 1937, where he remained until his retirement. He is best known for his work on nerve impulses, establishing that messages are conveyed by changes in the frequency of the impulses. He shared the 1932 Nobel Prize for physiology or medicine with Sir Charles Sherrington for this work.

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adsorption

The formation of a layer of solid, liquid, or gas on the surface of a solid or, less frequently, of a liquid. There are two types depending on the nature of the forces involved. In chemisorption a single layer of molecules, atoms, or ions is attached to the adsorbent surface by chemical bonds. In physisorption adsorbed molecules are held by the weaker physical forces. The property is utilized in adsorption chromatography.

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adventitious

Describing organs or other structures that arise in unusual positions. For example, ivy has adventitious roots growing from its stems.

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aeciospore (aecidiospore)

An asexual spore formed in the rust fungi (see rusts) from the fusion of cells. The nuclei from the two cells do not fuse and the spore is binucleate. Aeciospores develop in a sorus known as an aecidium .

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aeolian soil

A type of soil that is transported from one place to another by the wind.

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aerobe

See aerobic respiration.

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aerobic respiration

A type of respiration in which foodstuffs (usually carbohydrates) are completely oxidized to carbon dioxide and water, with the release of chemical energy, in a process requiring atmospheric oxygen. The reaction can be summarized by the equation:
C_6H_12O_6 + 6O_2 6CO_2 + 6H_2O + energy

Most organisms have aerobic respiration (i.e. they are aerobes ); exceptions include certain bacteria and yeasts. Compare anaerobic respiration.

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aerotaxis

See taxis.

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aestivation

1 (in zoology) A state of inactivity occurring in some animals, notably lungfish, during prolonged periods of drought or heat. Feeding, respiration, movement, and other bodily activities are considerably slowed down. See also dormancy. Compare hibernation.

2 (in botany) The arrangement of the parts of a flower bud, especially of the sepals and petals.
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aetiology

The study of causes, especially the causes of medical conditions.

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afferent

Carrying (nerve impulses, blood, etc.) from the outer regions of a body or organ towards its centre. The term is usually applied to types of nerve fibres or blood vessels. Compare efferent.

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affinity chromatography

A type of chromatography that is dependent on the affinity between specific molecules. The matrix, which can be packed into a column, is made from a material that is able to specifically bind to the molecule under investigation. Antibody purification can be achieved by this method when an appropriate antigen is bound to the matrix.

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aflatoxin

Any of four related toxic compounds produced by the mould Aspergillus flavus. Aflatoxins bind to DNA and prevent replication and transcription. They can cause acute liver damage and cancers: humans may be poisoned by eating stored peanuts and cereals contaminated with the mould.

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afterbirth

The placenta, umbilical cord, and extraembryonic membranes, which are expelled from the womb after a mammalian fetus is born. In most nonhuman mammals the afterbirth, which contains nutrients and might otherwise attract predators, is eaten by the female.

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agamospermy

See apomixis.

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agar

An extract of certain species of red seaweeds that is used as a gelling agent in microbiological culture media, foodstuffs, medicines, and cosmetic creams and jellies. Nutrient agar consists of a broth made from beef extract or blood that is gelled with agar and used for the cultivation of bacteria, fungi, and some algae.

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ageing

See senescence.

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agglutination

The clumping together by antibodies of microscopic foreign particles, such as red blood cells or bacteria, so that they form a visible pellet-like precipitate. Agglutination is a specific reaction, i.e. a particular antigen will only clump in the presence of its specific antibody; it therefore provides a means of identifying unknown bacteria and determining blood group. When blood of incompatible blood groups (e.g. group A and group B – see ABO system) is mixed together agglutination of the red cells occurs (haemagglutination ). This is due to the reaction between antibodies in the plasma (agglutinins ) and agglutinogens (antigens) on the surface of the red cells.

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agglutinogen

Any of the antigens that are present on the outer surface of red blood cells (erythrocytes). There are more than 100 different agglutinogens and they form the basis for identifying the different blood groups. Antibodies in the plasma, known as agglutinins , react with the agglutinogens in blood of an incompatible blood group (see agglutination).

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aggressin

A toxic substance that is secreted by certain parasitic microorganisms and inhibits the natural defence mechanisms of a host organism.

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aggression

Behaviour aimed at intimidating or injuring another animal of the same or a competing species. Aggression between individuals of the same species often starts with a series of ritualized displays or contests that can end at any stage if one of the combatants withdraws, leaving the victor with access to a disputed resource (e.g. food, a mate, or territory) or with increased social dominance (see dominant). It is also often seen in courtship. Aggression or threat displays usually appear to exaggerate the performer's size or strength; for example, many fish erect their fins and mammals and birds may erect hairs or feathers. Special markings may be prominently exhibited, and intention movements may be made: dogs bare their teeth, for example. Some animals have evolved special structures for use in aggressive interactions (e.g. antlers in deer) but these are seldom used to cause actual injury; the opponent usually flees first or adopts appeasement postures. Fights `to the death' are comparatively rare. See agonistic behaviour; display behaviour; ritualization.

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agitator

A bladelike instrument used in fermenters and bioreactors to mix the medium continuously in order to maintain the rate of oxygen transfer and to help keep the cells in suspension.

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Agnatha

A subphylum or superclass of marine and freshwater vertebrates that lack jaws. They are fishlike animals with cartilaginous skeletons and well-developed sucking mouthparts with horny teeth. The only living agnathans are lampreys and hagfishes (class Cyclostomata), which are parasites or scavengers. Fossil agnathans, covered in an armour of bony plates, are the oldest known fossil vertebrates. They have been dated from the Silurian and Devonian periods, 440–345 million years ago. Compare Gnathostomata.

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agonist

A drug, hormone, or transmitter substance that forms a complex with a receptor site, thereby triggering an active response from a cell. Compare antagonist.

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agonistic behaviour

Any form of behaviour associated with aggression, including threat, attack, appeasement, or flight. It is often associated with defence of a territory; for example, a threat display by the defending individual is often met with an appeasement display from the intruder, thus avoiding harmful conflict.

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agranulocyte

Any white blood cell (see leucocyte) with a nongranular cytoplasm and a large spherical nucleus; lymphocytes and monocytes are examples. Agranulocytes are produced either in the lymphatic system or in the bone marrow and account for 30% of all leucocytes. Compare granulocyte.

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agriculture

The study and practice of cultivating land for the growing of crops and the rearing of livestock. The increasing demands for food production during the second half of the 20th century have seen many developments in agricultural technology and practices that have greatly increased crop and livestock production. However, these advances in modern intensive farming techniques have had their impact on the environment, particularly with increased use of fertilizers and pesticides. The now widespread practice of crop monoculture (in which one crop is grown densely over an extensive area) has required an increase in the use of pesticides, as monoculture provides an ideal opportunity for crop pests. Monoculture also requires vast areas of land, which has meant that natural habitats have been destroyed. Deforestation has resulted from the clearing of forests for crop production and cattle rearing. Advances in technology have included ploughing machines with hydraulic devices that can control the depth to which the soil is ploughed, and seed drills that automatically implant seeds in the soil so that ploughing is not necessary. Food supply in many less-developed countries relies on subsistence farming , in which the crops and livestock produced are used solely to feed the farmer and his family. In such countries a system known as slash and burn is common, in which the vegetation in an area is cut down and then burnt, thus returning the minerals to the soil. The area can then be used for crop cultivation until the soil fertility drops, at which point it is then abandoned for a number of years and another site is cultivated.
The selective breeding of crop plants and farm animals has had an enormous impact on productivity in agriculture. Modern varieties of crop plants have increased nutritional value and greater resistance to disease, while animals have been selectively bred to enhance their yields of milk, meat, and other products. Developments in genetic engineering have enabled the introduction to commercial cultivation of genetically modified crop plants, such as tomatoes and soya, which contain genes from other organisms to enhance crop growth, nutritional properties, or storage characteristics. Genetic modification can also confer resistance to herbicides, thereby allowing more effective weed control, as well as improved resistance to insects and other pests and to diseases. The application of similar technology to animal production is being researched. See also genetically modified organisms.

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Agrobacterium tumefaciens

A Gram-negative soil bacterium that infects a wide range of plants and causes tumorous growths galls, especially at the root/stem junction (crown gall). It is of interest because the bacterial cells contain a plasmid, the Ti plasmid (tumour-inducing plasmid), a segment of which is transferred to cells of the plant host. This T-DNA (transfer DNA) segment, which comprises the genes responsible for the tumorous growth, becomes integrated into the genome of infected plant cells. Possession of the Ti plasmid has made A. tumefaciens an important tool in genetic engineering for the introduction of foreign genes into plant tissue. The tumour-inducing genes are usually replaced with the gene of interest, and a marker gene (e.g. the antibiotic resistance gene) is added to enable selection of transformed cells. See genetically modified organisms.

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AI

See artificial insemination.

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AIDS (acquired immune deficiency syndrome)

A disease of humans characterized by defective cell-mediated immunity and increased susceptibility to infections. It is caused by the retrovirus HIV (human immunodeficiency virus). This infects and destroys helper T cells, which are essential for combating infections. HIV is transmitted in blood, semen, and vaginal fluid; the major routes of infection are unprotected vaginal and anal intercourse, intravenous drug abuse, and the administration of contaminated blood and blood products. A person infected with HIV is described as HIV-positive ; after the initial infection the virus can remain generally dormant for up to ten years before AIDS develops. Antiviral drugs, including zidovudine (AZT), can delay the development of full-blown AIDS, in some cases for many years.

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air bladder

See swim bladder.

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air pollution (atmospheric pollution)

The release into the atmosphere of substances that cause a variety of harmful effects to the natural environment. Most air pollutants are gases that are released into the troposphere, which extends about 8 km above the surface of the earth. The burning of fossil fuels, for example in power stations, is a major source of air pollution as this process produces such gases as sulphur dioxide and carbon dioxide. Released into the atmosphere, both these gases are thought to contribute to the greenhouse effect. Sulphur dioxide and nitrogen oxides, released in car exhaust fumes, are air pollutants that are responsible for the formation of acid rain; nitrogen oxides also contribute to the formation of photochemical smog. See also ozone layer; pollution.

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air sac

1 Any one of a series of thin-walled sacs in birds that are connected to the lungs and increase the efficiency of ventilation. Some of the air sacs penetrate the internal cavities of bones.

2 A structural extension to the trachea in insects, which increases the surface area available for the exchange of oxygen and carbon dioxide in respiration.
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akinete

A nonmotile reproductive cell of certain filamentous blue-green bacteria (Cyanobacteria). An akinete is an enlarged resting cell with a thick wall and large amounts of food reserves and DNA. After cell division has occurred within the akinete the cell wall ruptures, releasing a filament of cells.

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alanine

See amino acid.

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alarm response

An immediate response to any stimulus that potentially threatens the wellbeing of an organism. It involves the release of adrenaline and noradrenaline from the adrenal glands, triggered by increased sympathetic nervous activity. These hormones enhance the effects of the sympathetic nervous system (e.g. by increasing heart and breathing rates) and promote glycogen breakdown, which supplies large amounts of glucose for increased respiration and energy release. Compare resistance response.

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alarm signal

A warning signal given by an animal to other members of its population in response to perceived danger, usually the approach of a predator. This is a form of altruism, since the animal that has perceived the danger may waste valuable time in giving the signal (or attract the attention of the predator by doing this) and thus reduce its own chances of survival. For example, the alarm signal of a rabbit to a threatening situation involves thumping the ground and then flashing the white of its tail as it runs, which alerts the rabbits nearby.

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albinism

Hereditary lack of pigmentation (see melanin) in an organism. Albino animals and human beings have no colour in their skin, hair, or eyes (the irises appear pink from underlying blood vessels). The allele responsible is recessive to the allele for normal pigmentation.

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albumen

See albumin.

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albumin

One of a group of globular proteins that are soluble in water but form insoluble coagulates when heated. Albumins occur in egg white (the protein component of which is known as albumen ), blood, milk, and plants. Serum albumins, which constitute about 55% of blood plasma protein, help regulate the osmotic pressure and hence plasma volume. They also bind and transport fatty acids. -lactalbumin is one of the proteins in milk.

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albuminous cell

See companion cell.

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alburnum

See sapwood.

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alcohol

An organic compound that contains the –OH group bound to a carbon atom. In systematic chemical nomenclature alcohol names end in the suffix -ol. Examples are methanol, CH_3OH, and ethanol, C_2H_5OH. Alcohols that have two –OH groups in their molecules are diols (or dihydric alcohols ), those with three are triols (or trihydric alcohols ), etc.

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alcoholic fermentation

See fermentation.

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aldohexose

See monosaccharide.

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aldose

See monosaccharide.

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aldosterone

A hormone produced by the adrenal glands (see corticosteroid) that controls excretion of sodium by the kidneys and thereby maintains the balance of salt and water in the body fluids. See also angiotensin.

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aleurone layer

The outer layer of living cells of the endosperm of wheat and other grain species. It is a single layer of cells that synthesizes the enzyme -amylase, which is secreted during germination into the starch-filled endosperm and breaks down the starch into maltose and glucose. Studies on the barley grain have shown that gibberellins control the synthesis of the enzyme by switching on the genes for the synthesis of the specific RNA that codes for the -amylase protein.

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algae

(sing. alga) A group of unrelated simple organisms that contain chlorophyll (and can therefore carry out photosynthesis) and live in aquatic habitats and in moist situations on land. The algal body may be unicellular or multicellular (filamentous, ribbon-like, or platelike). Formerly regarded as plants, algae are now classified as members of the kingdom Protoctista; they are assigned to separate phyla based primarily on the composition of the cell wall, the nature of the stored food reserves, and the other photosynthetic pigments present. See Bacillariophyta; Chlorophyta; Chrysomonada; Phaeophyta; Rhodophyta.
The organisms formerly known as blue-green algae are now classified as bacteria (see Cyanobacteria).

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algal bloom

The rapid increase in populations of algae and other phytoplankton, in particular the blue-green bacteria, that occurs in inland water systems, such as lakes. The density of the organisms may be such that it may prevent light from passing to lower depths in the water system. Algal blooms are caused by an increase in levels of nitrate, a mineral ion essential for algal and bacterial growth. The source of increased nitrate may be from agricultural fertilizers, which are leached into water systems from the land, or sewage effluent. Algal blooms contribute to the eutrophication of water systems. See also eutrophic. Compare red tide.

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algin (alginic acid)

A complex polysaccharide occurring in the cell walls of the brown algae (Phaeophyta). Algin strongly absorbs water to form a viscous gel. It is produced commercially from a variety of species of Laminaria and from Macrocystis pyrifera in the form of alginates , which are used mainly as stabilizers and texturing agents in the food industry.

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alien (exotic

A species of organism that is not native to a locality, having been moved there from its natural range by humans or other agents. Some alien species, such as rats, are introduced mainly by accident in cargoes or transport vessels, while others are transferred intentionally, often for their ornamental or economic value. An alien that establishes a self-sustaining wild population is described as naturalized , whereas one that depends on continual introduction is termed a casual .

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alimentary canal (digestive tract; gut)

A tubular organ in animals that is divided into a series of zones specialized for the ingestion, digestion, and absorption of food and for the elimination of indigestible material (see illustration). In most animals the canal has two openings, the mouth (for the intake of food) and the anus (for the elimination of waste). Simple animals, such as cnidarians (e.g. Hydra and jellyfish) and flatworms, have only one opening to their alimentary canal, which must serve both functions.

The human alimentary canal

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alkali

A base that dissolves in water to give hydroxide ions (OH–).

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alkaline phosphatase

An enzyme that catalyses the hydrolysis of phosphoric acid esters under conditions of alkaline pH. In humans the blood levels of alkaline phosphatase are measured as part of the assessment of liver function; in the blood the enzyme also breaks down phosphates required for mineralization of bone. There are two distinct forms of the enzyme (see isozyme), liver alkaline phosphatase and bone alkaline phosphatase.

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alkaloid

One of a group of nitrogenous organic compounds derived from plants and having diverse pharmacological properties. Alkaloids include morphine, cocaine, atropine, quinine, and caffeine, most of which are used in medicine as analgesics or anaesthetics. Some alkaloids are poisonous, e.g. strychnine and coniine, and colchicine inhibits cell division.

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alkalosis

A condition in which the body fluids become more alkaline, i.e. the pH is more than 7.4.

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alkaptonuria (alcaptonuria)

An inherited metabolic disorder that results from a deficiency of the enzyme homogentisic acid oxidase, which is required for the complete breakdown of the amino acids tyrosine and phenylalanine. The accumulation of the intermediate product, homogentisic acid, which imparts a dark colour to the urine, damages connective tissue, and causes joint disease.

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allantois

One of the membranes that develops in embryonic reptiles, birds, and mammals as a growth from the hindgut. It acts as a urinary bladder for the storage of waste excretory products in the egg (in reptiles and birds) and as a means of providing the embryo with oxygen (in reptiles, birds, and mammals) and food (in mammals; see placenta). See also extraembryonic membranes.

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allele (allelomorph)

One of the alternative forms of a gene. In a diploid cell there are usually two alleles of any one gene (one from each parent), which occupy the same relative position (locus) on homologous chromosomes. One allele is often dominant to the other (known as the recessive), i.e. it determines which aspects of a particular characteristic the organism will display. Within a population there may be numerous alleles of a gene; each has a unique nucleotide sequence.

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allele frequency (gene frequency)

The occurrence of an allele in a population in relation to all alleles of that gene at the same locus, expressed as a fraction.

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allelochemical

A substance (see semiochemical) produced by members of one species that influences the behaviour of members of another species. Allelochemicals can be divided into several categories. Kairomones benefit the receiving organism but cause disadvantage to the producer. For example, many plants (e.g. cabbages) release aromatic chemicals that attract insect predators, while parasites often exploit the pheromones released by their hosts to locate a suitable host; certain insect predators detect their prey in a similar way. Allomones benefit the producer but have no effect on the receiver. For example, many members of the beetle family Lycidae emit pungent chemicals that warn potential predators of their distasteful nature. Hence they are protected from predation, while the impact on the potential predator is neutral. The flowers of certain orchids emit allomones that mimic the sex pheromones of their bee or wasp pollinator. Males of the respective insect species attempt to copulate with the orchid flower, and pollinate it in the process, thus benefiting the orchid, while the cost to the deceived male insect is minimal. Synomones are beneficial to both producer and recipient. For example, pine trees damaged by beetles often emit terpenes that attract parasitoid insects that parasitize the pest beetles. Hence the parasitoid finds a suitable host, and the tree's pests are controlled.

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allelomorph

See allele.

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allelopathy

The secretion by plants of chemicals, such as phenolic and terpenoid compounds, that inhibit the growth or reproduction of other plant species, with which they are competing. For example, the aromatic oils released by certain shrubs of the Californian chaparral pass into the soil and inhibit the growth of herbaceous species nearby. Some plants produce chemicals that are toxic to grazing herbivorous animals.

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allergen

An antigen that provokes an abnormal immune response. Common allergens include pollen and dust (see allergy).

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allergy

A condition in which the body produces an abnormal immune response to certain antigens (called allergens ), which include dust, pollen, certain foods and drugs, or fur. In allergic individuals these substances, which in a normal person would be destroyed by antibodies, stimulate the release of histamine, leading to inflammation and other characteristic symptoms of the allergy (e.g. asthma or hay fever). This response is a type of hypersensitivity. See also anaphylaxis.

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allochthonous

Describing an organism that originates from a place other than that in which it is found. The organism is usually a transient member of a community. Compare autochthonous.

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allogamy

Cross-fertilization in plants. See fertilization.

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allogenic

1 Relating to or caused by a change in the environment or an individual organism brought about by some external factor. For example, the increased predation in a habitat caused by an immigrant predator would be described as allogenic. Compare autogenic.

2 (allogeneic) Describing the variation that exists in the genotypes of different individuals, usually when these belong to the same species.
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allograft

See graft.

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allometric growth

The regular and systematic pattern of growth such that the mass or size of any organ or part of a body can be expressed in relation to the total mass or size of the entire organism according to the allometric equation: Y = bx, where Y = mass of the organ, x = mass of the organism, = growth coefficient of the organ, and b = a constant.

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allomone

See allelochemical.

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allopatric

Describing or relating to groups of similar organisms that could interbreed but do not because they are geographically separated. Compare sympatric. See speciation.

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allopolyploid

A polyploid organism, usually a plant, that contains multiple sets of chromosomes derived from different species. Hybrids are usually sterile, because they do not have sets of homologous chromosomes and therefore pairing cannot take place. However, if doubling of the chromosome number occurs in a hybrid derived from two diploid (2n) species, the resulting tetraploid (4n) is a fertile plant. This type of tetraploid is known as an allotetraploid ; as it contains two sets of homologous chromosomes, pairing and crossing over are now possible. Allopolyploids are of great importance to plant breeders as advantages possessed by different species can be combined. The species of wheat, Triticum aestivum, used to make bread is an allohexaploid (6n), possessing 42 chromosomes, which is six times the original haploid number (n) of 7. See also amphidiploid. Compare autopolyploid.

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all-or-none response

A type of response that may be either complete and of full intensity or totally absent, depending on the strength of the stimulus; there is no partial response. For example, a nerve cell is either stimulated to transmit a complete nervous impulse or else it remains in its resting state; a stinging thread cell of a cnidarian is either completely discharged or it is not.

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allosteric enzyme

An enzyme that has two structurally distinct forms, one of which is active and the other inactive. In the active form, the quaternary structure (see protein) of the enzyme is such that a substrate can interact with the enzyme at the active site (see enzyme–substrate complex). The conformation of the substrate-binding site becomes altered in the inactive form and interaction with the substrate is not possible. Allosteric enzymes tend to catalyse the initial step in a pathway leading to the synthesis of molecules. The end product of this synthesis can act as a feedback inhibitor (see inhibition) and the enzyme is converted to the inactive form, thereby controlling the amount of product synthesized.

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allosteric site

A binding site on the surface of an enzyme other than the active site. In noncompetitive inhibition, binding of the inhibitor to an allosteric site inhibits the activity of the enzyme. In an allosteric enzyme, the binding of a regulatory molecule to the allosteric site changes the overall shape of the enzyme, either enabling the substrate to bind to the active site or preventing the binding of the substrate.

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allozyme

Any one of a number of different forms of the same enzyme that are coded by different alleles at the same locus.

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alpha helix

The most common form of secondary structure in proteins, in which the polypeptide chain is coiled into a helix. The helical structure is held in place by weak hydrogen bonds between the N–H and C=O groups in successive turns of the helix (see illustration). Compare beta sheet.

Alpha helix

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alpha-naphthol test

A biochemical test to detect the presence of carbohydrates in solution, also known as Molisch's test (after the Austrian chemist H. Molisch (1856–1937), who devised it). A small amount of alcoholic alpha-naphthol is mixed with the test solution and concentrated sulphuric acid is poured slowly down the side of the test tube. A positive reaction is indicated by the formation of a violet ring at the junction of the two liquids.

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alternation of generations

The occurrence within the life cycle of an organism of two or more distinct forms (generations), which differ from each other in appearance, habit, and method of reproduction. The phenomenon occurs in some protoctists, certain lower animals (e.g. cnidarians and parasitic flatworms), and in plants. The malaria parasite (Plasmodium), for example, has a complex life cycle involving the alternation of sexually and asexually reproducing generations. In plants the generation with sexual reproduction is called the gametophyte and the asexual generation is the sporophyte, either of which may dominate the life cycle, and there is also alternation of the haploid and diploid states. Thus in ferns the dominant plant is the diploid sporophyte; it produces spores that germinate into small haploid gametophytes bearing sex organs. In mosses the gametophyte is the dominant plant and the sporophyte is the spore-bearing capsule.

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alternative respiratory pathway

A pathway for cellular respiration occurring in many plants that permits electron transport and reduction of oxygen to water in the presence of cyanide and other substances that completely inhibit respiration in animals. It thus acts as an alternative to the electron transport pathways common to both plants and animals. Like the components of the normal electron transport chain, those of the alternative pathway reside in the mitochondrial inner membrane. The principal enzyme is the so-called alternative oxidase , which transfers electrons directly to oxygen and in so doing bypasses at least two sites for ATP formation by phosphorylation. Thus instead of being conserved as ATP, energy flowing through the alternative pathway (in the form of electrons) is converted mainly to heat. One hypothesis is that this provides a mechanism for `burning off' energy – produced by photosynthesis in the form of carbohydrate – that is temporarily more than the plant can cope with. Another interesting observation is that the alternative pathway may act as a short-term heat-generating system in certain tissues. For example, before pollination the spadix of the skunk cabbage (Sympocarpus foetidus) undergoes a temperature rise of about 10°C, which causes the emission of volatile chemicals that attract insect pollinators.

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altricial species

Any species of bird in which hatching occurs at an early stage of development. The offspring of altricial species tend to be born with no feathers and remain in the nest for a comparatively long period of time; they are described as nidicolous . Compare precocial species.

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altruism

Behaviour by an animal that decreases its chances of survival or reproduction while increasing those of another member of the same species. For example, a lapwing puts itself at risk by luring a predator away from the nest through feigning injury, but by so doing saves its offspring. Altruism in its biological sense does not imply any conscious benevolence on the part of the performer. Altruism can evolve through kin selection, if the recipients of altruistic acts tend on average to be more closely related to the altruist than the population as a whole. See also alarm signal; inclusive fitness.

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Alu family

A group of closely related DNA sequences arising from a single ancestral sequence (see gene family) and found dispersed and repeated many times within the genome of humans and other primates. Alu sequences are about 280 bp long and are cleaved by the restriction enzyme AluI (hence the name). The human genome contains roughly 500000 copies of Alu, making it the most abundant single sequence in the entire genome and constituting a large proportion of the class of moderately repetitive DNA known as SINEs. Alu sequences occur mainly in the nontranslated regions (introns) of genes and are regarded as pseudogenes; their function is unknown. The variations in structure and distribution of Alu sequences among primates can be used both to help track the evolutionary history of the genome and as genetic markers (see marker gene) for physical mapping.

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alveolus

1 The tiny air sac in the lung of mammals and reptiles at the end of each bronchiole. It is lined by a delicate moist membrane, has many blood capillaries, and is the site of exchange of respiratory gases (carbon dioxide and oxygen).

2 The socket in the jawbone in which a tooth is rooted by means of the periodontal membrane.
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Alzheimer's disease

A neurological disease characterized by progressive loss of intellectual ability. The disease, which is named after German physician Alois Alzheimer (1864–1915), is associated with general shrinkage of the brain tissue, with deposits of a glycoprotein, ß-amyloid protein, in the brain (see amyloid), and changes in the neurotransmitter systems within the brain that include a loss in the activity of cholinergic neurones. It is now thought that inheritance of certain genes may predispose to development of the disease in later life.

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amacrine cell

A type of nerve cell found in the retina of the eye. Amacrine cells receive sensory information from the rod and cone receptor cells in the retina and are able to integrate sensory information before sending this to the brain.

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amber

A yellow or reddish-brown fossil resin. The resin was exuded by certain trees and other plants and often contains preserved insects, flowers, or leaves that were trapped by its sticky surface before the resin hardened. Amber is used for jewellery and ornaments. It also has the property of acquiring an electrical charge when rubbed (the term electricity is derived from electron , the Greek name for amber). It occurs throughout the world in rock strata from the Cretaceous to the Pleistocene, but most commonly in Cretaceous and Tertiary rocks.

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ambient

Describing the conditions and factors that are present in the immediate environment. For example, the ambient temperature is the local temperature in a specific environment.

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ameloblast

Any of the epithelial cells that secrete enamel during tooth formation. They die before the tooth erupts, therefore damaged enamel in a tooth cannot be replaced. Compare odontoblast.

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amensalism

An association between two species that is detrimental to one of the species but has no effect on the other. A common example of amensalism is the release of chemical toxins by plants that can inhibit the growth of other plant species (see allelopathy).

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Ames test (Salmonella mutagenesis test)

A test to determine the effects of a chemical on the rate of mutation in bacterial cells, and hence its likely potential for causing cancer in other organisms, including humans. Devised by US biologist Bruce Ames (1928– ), it is widely used in screening chemicals occurring in the environment for possible carcinogenic activity (see xenobiotic). The chemical is applied to plates inoculated with a special mutant strain of bacteria, usually Salmonella typhimurium, that require the amino acid histidine for growth. Cells that mutate back to the wild type are detected by the occurrence of colonies able to synthesize their own histidine and therefore to grow on the medium.

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ametabolous

Describing insect development in which there is no metamorphosis and immature stages appear very similar to the adults, except that they lack genitalia. It occurs, for example, in silverfish. Compare hemimetabolous; holometabolous.

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amine

Any one of a group of organic compounds derived by replacing one or more of the hydrogen atoms in ammonia by organic groups. Primary amines have one hydrogen replaced, e.g. methylamine, CH_3NH_2. They contain the functional group –NH_2 (the amino group ). Secondary amines have two hydrogens replaced, e.g. methylethylamine, CH_3(C_2H_5)NH. Tertiary amines have all three hydrogens replaced, e.g. trimethylamine. Amines are produced by the decomposition of organic matter.

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amino acid

Any of a group of water-soluble organic compounds that possess both a carboxyl (–COOH) and an amino (–NH_2) group attached to the same carbon atom, called the -carbon atom. Amino acids can be represented by the general formula R–CH(NH_2)COOH. R may be hydrogen or an organic group, which may be nonpolar, basic, acidic, or polar; the nature of the R group determines the properties of any particular amino acid. Through the formation of peptide bonds, amino acids join together to form short chains (peptides) or much longer chains (polypeptides). Proteins are composed of various proportions of about 20 commonly occurring amino acids (see table). The sequence of these amino acids in the protein polypeptides determines the shape, properties, and hence biological role of the protein. Some amino acids that never occur in proteins are nevertheless important, e.g. ornithine and citrulline, which are intermediates in the urea cycle.
Plants and many microorganisms can synthesize amino acids from simple inorganic compounds, but animals rely on adequate supplies in their diet. The essential amino acids must be present in the diet whereas others can be manufactured from them.

The amino acids occurring in proteins

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aminopeptidase

Any enzyme that cleaves amino acids from the N-terminus of peptides or polypeptides. For example, membrane-bound aminopeptidases in the small intestine break down peptides and dipeptides into amino acids.

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amino sugar

Any sugar containing an amino group in place of a hydroxyl group. The hexosamines are amino derivatives of hexose sugars and include glucosamine (based on glucose) and galactosamine (based on galactose). The former is a constituent of chitin and the latter occurs in cartilage.

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ammonia

A colourless gas, NH_3, with a strong pungent odour. Ammonia is produced by the deamination of excess amino acids in the liver. Industrially it is made from its constituent elements by the Haber process for use in the manufacture of nitric acid, ammonium nitrate, ammonium phosphate, and urea (the last three as fertilizers), explosives, dyestuffs, and resins. The participation of ammonia in the nitrogen cycle is a most important natural process. By means of nitrogenase enzymes, nitrogen-fixing bacteria are able to achieve similar reactions to those of the Haber process, but under normal conditions of temperature and pressure. The reactions release ammonium ions, which are converted by nitrifying bacteria into nitrite and nitrate ions.

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ammonite

An extinct aquatic mollusc of the class Cephalopoda. Ammonites were abundant in the Mesozoic era (225–65 million years ago) and are commonly found as fossils in rock strata of that time, being used as index fossils for the Jurassic period. They were characterized by a coiled shell divided into many chambers, which acted as a buoyancy aid. The external suture lines on these shells increased in complexity with the advance of the group.

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ammonotelic

Describing animals that excrete nitrogenous waste in the form of ammonia. Most aquatic animals are ammonotelic. Compare ureotelic; uricotelic.

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amniocentesis

The taking of a sample of amniotic fluid from a pregnant woman to determine the condition of an unborn baby. A hollow needle is inserted through the woman's abdomen and wall of the uterus and the fluid drawn off. Chemical and microscopical examination of cells shed from the embryo's skin into the fluid are used to detect spina bifida, Down's syndrome, or other serious biochemical or chromosomal abnormalities.

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amnion

A membrane that encloses the embryo of reptiles, birds, and mammals within the amniotic cavity. This cavity is filled with amniotic fluid, in which the embryo is protected from desiccation and from external pressure. See also extraembryonic membranes.

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amniote

A vertebrate whose embryos are totally enclosed in a fluid-filled sac – the amnion. The evolution of the amnion provided the necessary fluid environment for the developing embryo and therefore allowed animals to breed away from water. Amniotes comprise the reptiles, birds, and mammals. Compare anamniote.

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amniotic egg

The type of egg produced by reptiles, birds, and prototherian (egg-laying) mammals (amniotes), in which the embryo develops inside an amnion. The shell of the egg is either calcium-based or leathery.

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Amoeba

A genus of protoctists (see also protozoa) of the phylum Rhizopoda, members of which have temporary body projections called pseudopodia. These are used for locomotion and feeding and result in a constantly changing body shape (see amoeboid movement). Most species are free-living in soil, mud, or water, where they feed on smaller protoctists and other single-celled organisms, but a few are parasitic. The best known species is the much studied A. proteus.

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amoebocyte

An animal cell whose location is not fixed and is therefore able to wander through the body tissues. Amoebocytes are named after their resemblance, especially in their movement, to Amoeba (see amoeboid movement) and they feed on foreign particles (including invading bacteria). They occur, for example, in sponges and mammalian blood (e.g. some leucocytes).

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amoeboid movement

The mechanism of movement demonstrated by Amoeba and other cells that are capable of changing their shape (e.g. phagocytes). The cytoplasm of Amoeba consists of a central fluid plasmasol surrounded by a more viscous plasmagel. The plasmagel is converted to plasmasol, which slides towards the front of the cell, forming a pseudopodium and propelling the cell forward. On reaching the tip of the pseudopodium, this plasmasol is reconverted into plasmagel; at the same time the plasmagel at the rear of the cell is converted into plasmasol and streams forward, thus maintaining continuous movement.
Amoeboid movement is brought about by reversible changes in the actin filaments of the cell's cytoskeleton. Cross-linking of these filaments by other proteins creates a three-dimensional network with gel-like properties in the plasmagel region. Disassembly of this network causes reversion to the sol state of plasmasol.

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amount of substance

Symbol n. A measure of the number of entities present in a substance. The specified entity may be an atom, molecule, ion, electron, photon, etc., or any specified group of such entities. The amount of substance of an element, for example, is proportional to the number of atoms present. The SI unit of amount of substance is the mole.

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AMP

See ATP; cyclic AMP.

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amphetamine

A drug, 1-phenyl-2-aminopropane (or a derivative of this compound), that stimulates the central nervous system by causing the release of the transmitters noradrenaline and dopamine from nerve endings. It inhibits sleep, suppresses the appetite, and has variable effects on mood; prolonged use can lead to addiction.

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Amphibia

The class of vertebrate chordates (see Chordata) that contains the frogs, toads, newts, and salamanders. The amphibians evolved in the Devonian period (about 370 million years ago) as the first vertebrates to occupy the land, and many of their characteristics are adaptations to terrestrial life. All adult amphibians have a passage linking the roof of the mouth with the nostrils so they may breathe air and keep the mouth closed. The moist scaleless skin is used to supplement the lungs in gas exchange. They have no diaphragm, and therefore the muscles of the mouth and pharynx provide the pumping action for breathing. Fertilization is usually external and the eggs are soft and prone to desiccation, therefore reproduction commonly occurs in water. Amphibian larvae are aquatic, having gills for respiration; they undergo metamorphosis to the adult form.

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amphibolic pathway

A biochemical pathway that serves both anabolic and catabolic processes. An important example of an amphibolic pathway is the Krebs cycle, which involves both the catabolism of carbohydrates and fatty acids and the synthesis of anabolic precursors for amino-acid synthesis (e.g. -ketogluturate and oxaloacetate).

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amphidiploid

Describing an organism, cell, or nucleus that contains diploid sets of chromosomes originating from two different species. Crosses between taxonomically unrelated organisms are usually infertile, principally because the chromosomes lack a partner with which to pair during meiosis. However, if there is doubling of the parental sets of chromosomes, pairing can take place within each set, and meiosis may proceed to produce fertile gametes. For example, the F_1 hybrid resulting from a cross between a cabbage (Brassica sp.) and a radish (Raphanus sp.) is sterile. Yet such crosses may produce a few seeds that are capable of germinating into fertile F_2 plants. These seeds form following the chance fusion of two unreduced gametes, each of which contain all parental chromosomes. Hence, instead of the 18 chromosomes of the F_1 hybrid (nine from each parent), the F_2 has 36 chromosomes in its somatic nuclei, and thus contains the full diploid sets of both its progenitor species. During meiosis the chromosomes undergo pairing just like a normal diploid plant, and the result is a generally true-breeding hybrid, named Raphanobrassica. See also allopolyploid.

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amphimixis

True sexual reproduction, involving the fusion of male and female gametes and the formation of a zygote. Compare apomixis.

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amphioxus

Another name for the lancelet: see Cephalochordata.

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amphistylic jaw suspension

A type of jaw suspension seen in certain sharks, in which the upper jaw is braced against the cranium and is also supported by the hyomandibular (see hyoid arch). See illustration. Compare autostylic jaw suspension; hyostylic jaw suspension.

Amphistylic jaw suspension

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amphoteric

Describing a compound that can act as both an acid and a base. Amino acids, which contain both acidic and basic groups in their molecules, can be described as amphoteric. Solvents, such as water, that can both donate and accept protons are usually described as amphiprotic (see solvent).

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ampulla

1 An enlargement at one end of each of the semicircular canals of the inner ear. Each ampulla contains a group of receptors – sensory hair cells – embedded in a gelatinous cap (cupula ), which detects movement in one particular dimension, corresponding to the plane of the canal. Movement of the head causes the cupula (and the hairs within it) to bend in a direction opposite to that of the head movement (see illustration); this stimulates nerve impulses in the receptors, which are interpreted by the brain as movement in a particular dimension.

Ampulla

2 Any small vesicle or saclike process.
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3 (ampulla of Lorenzini) See electroreceptor.
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amylase

Any of a group of closely related enzymes that degrade starch, glycogen, and other polysaccharides. Plants contain both - and ß-amylases; the name diastase is given to the component of malt containing ß-amylase, important in the brewing industry. Animals possess only -amylases, found in pancreatic juice (as pancreatic amylase ) and also (in humans and some other species) in saliva (as salivary amylase or ptyalin ). Amylases cleave the glycosidic bonds of the long polysaccharide chains, producing a mixture of glucose and maltose.

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amyloid

Tissue consisting of protein fibrils that may accumulate between cells in various animal tissues, especially in the disorder amyloidosis . Amyloid deposits are insoluble and can exert pressure on various vital organs. These deposits are generally detected by staining with the dye Congo red. A build-up of amyloid tissue in the brain is a feature of Alzheimer's disease, Creutzfeldt–Jakob disease, and bovine spongiform encephalopathy.

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amylopectin

A polysaccharide comprising highly branched chains of glucose molecules. It is one of the constituents (the other being amylose) of starch.

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amyloplast

An organelle in plants that stores starch. Amyloplasts are often found in nonphotosynthetic tissue, such as roots and storage tubers.

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amylose

A polysaccharide consisting of linear chains of between 100 and 1000 linked glucose molecules. Amylose is a constituent of starch. In water, amylose reacts with iodine to give a characteristic blue colour.

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anabolic steroid

Any steroid compound that promotes tissue growth, especially of muscles. Naturally occurring anabolic steroids include the male sex hormones (androgens). Synthetic forms of these are used medically to help weight gain after debilitating diseases; their use by athletes to build up body muscles can cause liver damage and is banned by most athletic authorities.

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anabolism

The metabolic synthesis of proteins, fats, and other constituents of living organisms from molecules or simple precursors. This process requires energy in the form of ATP. See metabolism. Compare catabolism.

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anadromous

Describing the migration of certain fish, such as salmon, that spend most of their lives in oceanic waters before travelling to breed in the upper reaches of rivers and streams. Compare katadromous.

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anaemia

A condition that arises when either there are too few erythrocytes (red blood cells), the erythrocytes do not contain sufficient amounts of haemoglobin, or the erythrocytes are abnormal in other respects. Anaemia often results from loss of blood or from a deficiency in the factors necessary to synthesize haemoglobin (e.g. iron) or erythrocytes (e.g. folic acid and vitamin B_12). Increased destruction of erythrocytes may be induced by certain drugs or severe infection, and an abnormal form of haemoglobin results in sickle-cell anaemia (see polymorphism).

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anaerobe

See anaerobic respiration.

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anaerobic respiration

A type of respiration in which foodstuffs (usually carbohydrates) are partially oxidized, with the release of chemical energy, in a process not involving atmospheric oxygen. Since the substrate is never completely oxidized the energy yield of this type of respiration is lower than that of aerobic respiration. It occurs in some yeasts and bacteria and in muscle tissue when oxygen is absent (see oxygen debt). Obligate anaerobes are organisms that cannot use free oxygen for respiration; facultative anaerobes are normally aerobic but can respire anaerobically during periods of oxygen shortage. Alcoholic fermentation is a type of anaerobic respiration in which one of the end products is ethanol.

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anaesthetic

Any compound that can render an animal unconscious of painful stimuli. Anaesthetics can be used either at a local level, when sensation is removed from a specific area of the body; or at a general level, when a state of complete unconsciousness is induced.

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analgesic

A substance that reduces pain without causing unconsciousness, either by reducing the pain threshold or by increasing pain tolerance. There are several categories of analgesic drugs, including morphine and its derivatives (see opiate), which produce analgesia by acting on the central nervous system; nonsteroidal anti-inflammatory drugs (e.g. aspirin); and local anaesthetics.

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analogous

Describing features of organisms that are superficially similar but have evolved in different ways. The wings of butterflies and birds are analogous organs.

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anamniote

A vertebrate that lacks an amnion and whose embryos and larvae must therefore develop in water. Anamniotes comprise the agnathans, fishes, and amphibians. Compare amniote.

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anaphase

One of several stages of cell division. In mitosis the chromatids of each chromosome move apart to opposite ends of the spindle. In the first anaphase of meiosis, the paired homologous chromosomes separate and move to opposite ends; in the second anaphase the chromatids move apart, as in mitosis.

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anaphylaxis

An abnormal immune response that occurs when an individual previously exposed to a particular antigen is re-exposed to the same antigen. Anaphylaxis may follow an insect bite or the injection of a drug (such as penicillin). It is caused by the release of histamine and similar substances and may produce a localized reaction or a more generalized and severe one, with difficulty in breathing, pallor, or drop in blood pressure, unconsciousness, and possibly heart failure and death. See also allergy.

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anaplerotic

Describing a metabolic pathway that replenishes an intermediate in one of the main metabolic pathways. For example, some Krebs cycle intermediates function not only in energy metabolism but are also used as building blocks for the biosynthesis of various compounds. Unless these intermediates are replaced, these crucial pathways will slow down or halt completely. In plants, mitochondrial oxaloacetate, which is a Krebs cycle intermediate used for the synthesis of amino acids, can be replenished by the conversion of phosphoenolpyruvate derived from glycolysis in the cytosol. The cytosolic oxaloacetate is then reduced to malate, which enters the mitochondria, where it is reoxidized to oxaloacetate.

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anatomy

The study of the structure of living organisms, especially of their internal parts by means of dissection and microscopical examination. Compare morphology.

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ancestral trait

See plesiomorphy.

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androecium

The male sex organs (stamens) of a flower. Compare gynoecium.

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androgen

One of a group of male sex hormones that stimulate development of the testes and of male secondary sexual characteristics (such as growth of facial and pubic hair in men). Testosterone is the most important. Androgens are produced principally by the testes when stimulated with luteinizing hormone but they are also secreted in smaller amounts by the adrenal glands and the ovaries. Injections of natural or synthetic androgens are used to treat hormonal disorders of the testes and breast cancer and to build up body tissue (see anabolic steroid).

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anemophily

Pollination of a flower in which the pollen is carried by the wind. Examples of anemophilous flowers are those of grasses and conifers. Compare entomophily; hydrophily.

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aneuploid

Describing a nucleus, cell, or organism in which one or more chromosomes have been added to or deleted from the complete set, so that the total number of chromosomes is not an exact multiple of the haploid number (n); for example, 2n + 1 (see trisomy) or 2n – 1 (monosomy ). Compare euploid.

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angiosperms

See Anthophyta.

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angiotensin

Any of three related peptide hormones, two of which raise blood pressure. Angiotensin I is derived, by the action of the enzyme renin, from a protein (-globulin) secreted by the liver into the bloodstream. As blood passes through the lungs, another enzyme (angiotensin-converting enzyme ; ACE ) splits angiotensin I, forming angiotensin II. This causes constriction of blood vessels and stimulates the release of antidiuretic hormone and aldosterone, which increase blood pressure. Angiotensin III, formed by removal of a single amino acid from angiotensin II, also stimulates aldosterone release by the adrenal gland.

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angstrom

Symbol Å. A unit of length equal to 10–10 metre. It was formerly used to measure wavelengths and intermolecular distances but has now been replaced by the nanometre. 1 Å = 0.1 nanometre. The unit is named after the Swedish pioneer of spectroscopy A. J. Ångstrom (1814–74).

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anhydrobiosis

See cryptobiosis.

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animal

Any member of the kingdom Animalia, which comprises multicellular organisms that develop from embryos formed by the fusion of haploid eggs and sperm. Unable to manufacture their own food, they feed on other organisms or organic matter (see heterotrophic nutrition). Animals are therefore typically mobile (to search for food) and have evolved specialized sense organs for detecting changes in the environment; a nervous system coordinates information received by the sense organs and enables rapid responses to environmental stimuli. Animal cells lack the cellulose cells walls of plant cells. For a classification of the animal kingdom, see animal kingdom.

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animal behaviour

The activities that constitute an animal's response to its external environment. Certain categories of behaviour are seen in all animals (e.g. feeding, reproduction) but these activities involve different movements in different species and develop in different ways. Some movements are highly characteristic of a species (see instinct), whereas others are more variable and depend on the interaction between innate tendencies and learning during the individual's lifetime. Physiologists study how changes in the body (e.g. hormone levels) affect behaviour, psychologists study the mechanisms of learning, and ethologists study the behaviour of the whole animal: how this develops during the individual's lifetime and how it evolved through natural selection (see ethology).

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animal kingdom, simplified classification of

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animal starch

See glycogen.

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anion

A negatively charged ion, such as the chloride ion (Cl–). Compare cation.

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anisogamy

Sexual reproduction involving the fusion of gametes that differ in size and sometimes also in form. See also oogamy. Compare isogamy.

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Annelida

A phylum of invertebrates comprising the segmented worms (e.g. the earthworm). Annelids have cylindrical soft bodies showing metameric segmentation, obvious externally as a series of rings separating the segments. Each segment is internally separated from the next by a membrane and bears stiff bristles (see chaeta). Between the gut and other body organs there is a fluid-filled cavity called the coelom, which acts as a hydrostatic skeleton. Movement is by alternate contraction of circular and longitudinal muscles in the body wall. The phylum contains three classes: Polychaeta, Oligochaeta, and Hirudinea.

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annual

A plant that completes its life cycle in one year, during which time it germinates, flowers, produces seeds, and dies. Examples are the sunflower and marigold. Compare biennial; ephemeral; perennial.

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annual rhythm

The occurrence of a process or a function in a living organism on a yearly basis. Events that display an annual rhythm can include life cycles, such as those of annual plants; mating behaviour; some kinds of movement, such as migration; or growth patterns, such as the growth rings of woody plant stems. See also biorhythm.

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annual ring

See growth ring.

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annulus

1 (in botany)
a A ragged ring of tissue that remains on the stalk of a mushroom or toadstool. Also called a velum , it is formed from the ruptured membrane that originally covered the lower surface of the cap.
b The region of the wall of a fern sporangium that is specialized for spore dispersal. It consists of cells that are thickened except on their outer walls. On drying out, the cells contract and the sporangium ruptures, releasing the spores. The annulus springs back into position when the residual water in the cells vaporizes and any remaining spores are dispersed.

2 (in zoology) Any of various ring-shaped structures in animals, such as any of the segments of an earthworm or other annelid.
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Anoplura

See Siphunculata.

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anoxic

Lacking or not involving or requiring oxygen. For example, a culture of anaerobic microorganisms is called an anoxic culture .

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anoxic reactor

A bioreactor in which the organisms being cultured are anaerobes or in which the reaction being exploited does not require oxygen.

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ANS

See autonomic nervous system.

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antagonism

1 The interaction of two substances (e.g. drugs, hormones, or enzymes) having opposing effects in a system in such a way that the action of one partially or completely inhibits the effects of the other. For example, one group of anti-cancer drugs acts by antagonizing the effects of certain enzymes controlling the activities of the cancer cells. See also antagonist.

2 An interaction between two muscles, known as antagonistic muscles , in which contraction of one prevents that of the other. For example, the biceps and triceps are an antagonistic pair. See voluntary muscle.
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3 An interaction between two organisms (e.g. moulds or bacteria) in which the growth of one is inhibited by the other. Compare synergism.
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antagonist

A drug that inhibits the effect of an agonist in such a way that the combined biological effect of the two substances becomes smaller than the sum of their individual effects. Competitive antagonists act by binding to agonist receptors, while noncompetitive antagonists do not bind to the same receptor sites as the agonist. A functional antagonist binds to other receptors that elicit an effect opposite to that of the agonist.

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antenna

A long whiplike jointed mobile paired appendage on the head of many arthropods, usually concerned with the senses of smell, touch, etc (see sensillum). In insects, millipedes, and centipedes they are the first pair of head appendages and are specialized and modified in many insects. In crustaceans they are the second pair of head appendages, the first pair (the antennules ) having the sensory function, while the antennae are modified for swimming and for attachment.

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antennule

See antenna.

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anterior

1 Designating the part of an animal that faces to the front, i.e. that leads when the animal is moving. In man and bipedal animals the anterior surface corresponds to the ventral surface.

2 Designating the side of a flower or axillary bud that faces away from the flower stalk or main stem, respectively. Compare posterior.
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anther

The upper two-lobed part of a plant stamen, usually yellow in colour. Each lobe contains two pollen sacs within which are numerous pollen grains, which are released when the anther ruptures.

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antheridium

(pl. antheridia) The male sex organ of algae, fungi, bryophytes, clubmosses, horsetails, and ferns. It produces the male gametes (antherozoids ). It may consist of a single cell or it may have a wall that is made up of one or several layers forming a sterile jacket around the developing gametes. Compare archegonium.

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antherozoid (spermatozoid)

The motile male gamete of algae, fungi, bryophytes, clubmosses, horsetails, ferns, and certain gymnosperms. Antherozoids usually develop in an antheridium but in certain gymnosperms, such as Ginkgo and Cycas, they develop from a cell in the pollen tube.

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Anthocerophyta

A phylum comprising about 100 species of simple nonvascular plants, the hornworts (or horned liverworts), found worldwide in temperate and tropical regions on tree trunks, riverbanks, and other damp locations. They resemble thallose liverworts (see Hepatophyta), but produce long horn-shaped green sporophytes, which split longitudinally to release the spores. Hornwort cells each contain a single chloroplast inside which, uniquely among plants, is a pyrenoid, associated with starch production. Some species have separate male and female plants, while others have both types of sexual organ on the same plant. The motile sperm swim through the surface water film to fertilize the female gametes, and the resultant embryo gives rise to young sporophytes. Young gametophytes arise directly from the germinating spores. Hornworts were formerly classified as a class (Anthocerotae) of the Bryophyta.

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anthocyanin

One of a group of flavonoid pigments. Anthocyanins occur in the cell vacuoles of various plant organs and are responsible for many of the blue, red, and purple colours in plants (particularly in flowers). Compare betacyanin.

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Anthophyta (Angiospermophyta)

A phylum comprising the flowering plants (angiosperms). The gametes are produced within flowers and the ovules (and the seeds into which they develop) are enclosed in a carpel (compare Coniferophyta). The angiosperms are the dominant plant forms of the present day. They show the most advanced structural organization in the plant kingdom, enabling them to inhabit a very diverse range of habitats. There are two classes within this group: the Monocotyledonae with one seed leaf (cotyledon) in the seed, and the Dicotyledonae with two seed leaves.

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Anthozoa

See Cnidaria; coral.

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antibiotics

Substances that destroy or inhibit the growth of microorganisms, particularly disease-producing bacteria and fungi. Antibiotics are obtained from microorganisms (especially moulds) or synthesized. Common antibiotics include the penicillins, streptomycin , and tetracyclines . They are used to treat various infections but tend to weaken the body's natural defence mechanisms and can cause allergies. Overuse of antibiotics can lead to the development of resistant strains of microorganisms.

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antibody

A protein (see immunoglobulin) produced by certain white blood cells (lymphocytes) in response to entry into the body of a foreign substance (antigen) in order to render it harmless. An antibody–antigen reaction is highly specific. Antibody production is one aspect of the immune response and is stimulated by such antigens as invading bacteria, foreign red blood cells (see ABO system), inhaled pollen grains or dust, and foreign tissue grafts. Specific monoclonal antibodies are now used in various types of immunoassay. See also immunity.

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anticlinal

(in botany) At right angles to the surface of an organ or part. In anticlinal cell division the plane of division is at right angles to the surface of the plant body. Compare periclinal.

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anticoagulant

A substance that prevents the formation of blood clots. Heparin is a natural anticoagulant, which is extracted to treat such conditions as thrombosis and embolism. Synthetic anticoagulants include warfarin.

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anticoding strand (noncoding strand; template strand)

The strand in duplex DNA that by convention contains the sequence of bases that is complementary to that of the messenger RNA (mRNA) transcribed from the DNA (except that the RNA has uracil substituting for thymine). It is the strand used as a template for mRNA assembly during transcription, and is complementary to the other strand of the DNA molecule, the coding strand.

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anticodon

A sequence of three nucleotides (trinucleotide) on a strand of transfer RNA that can form base pairs (see base pairing) with a specific trinucleotide sequence (see codon) on a strand of messenger RNA during translation. See also protein synthesis.

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antidiuretic hormone (ADH; vasopressin)

A hormone, secreted by the posterior pituitary gland, that stimulates absorption of water by the kidneys and thus controls the concentration of body fluids. ADH is produced by specialized nerve cells in the hypothalamus of the brain and is transported to the posterior pituitary in the bloodstream. Deficiency of ADH results in a disorder known as diabetes insipidus , in which large volumes of urine are excreted; it is treated by administration of natural or synthetic hormone.

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antifreeze molecule

Any substance produced by an organism in order to prevent freezing of its tissues or body fluids when subject to subzero environmental temperatures. Many animals living in cold climates adopt a strategy of preventing ice formation in their tissues when subject to freezing conditions. One way of achieving this is to accumulate solutes in their blood, thereby raising the osmotic concentration and so depressing the supercooling point. Salts and sugars contribute to this, but organisms also produce relatively inert molecules, notably glycerol and other polyhydric alcohols (polyols), such as sorbitol and ribitol, specifically for this purpose. For example, high concentrations of glycerol can enable the survival of certain cold-hardy invertebrates at temperatures as low as –60°C. Some families of teleost fish inhabiting polar regions manufacture antifreeze peptides or antifreeze glycopeptides , which are effective antifreeze agents at relatively low concentrations. They bind to the edges of ice crystal lattices and prevent the addition of further water molecules, causing a phenomenon termed 'thermal hysteresis', in which the freezing point is depressed well below the melting point – hence these peptides are also called thermal hysteresis proteins . Similar peptides occur in certain insects, spiders, and mites. See also cryoprotectant.

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antigen

Any substance that the body regards as foreign and that therefore elicits an immune response, particularly the formation of specific antibodies capable of binding to it. Antigens may be formed in, or introduced into, the body. They are usually proteins. Histocompatibility antigens are associated with the tissues and are involved in the rejection of tissue or organ grafts (see histocompatibility); an example is the group of antigens encoded by the HLA system. A graft will be rejected if the recipient's body regards such antigens on the donor's tissues as foreign. See also antibody. Compare hapten.

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antigenic variation

The ability of certain pathogenic microorganisms, particularly viruses, to alter the antigens on their outer surface. This prevents the pathogen from being easily recognized and destroyed by the immune system of the host.

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antigen-presenting cell

A cell, such as a macrophage, that `presents' foreign antigens to helper T cells as part of the immune response. A direct reaction between these lymphocytes and foreign antigens is not common. The antigen-presenting cell takes in the foreign protein, processes it, and then displays peptide fragments on its surface as a complex with MHC class II histocompatibility proteins.

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antihistamine

Any drug that inhibits the effects of histamine in the body and is therefore used to relieve and prevent the symptoms associated with allergic reactions, such as hay fever. Since one of the side-effects produced by antihistamines is sleepiness, some are used to prevent motion sickness and induce sleep.

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anti-oncogene

See tumour-suppressor gene.

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antioxidants

Substances that slow the rate of oxidation reactions. Various antioxidants are used to preserve foodstuffs and to prevent the deterioration of rubber, synthetic plastics, and many other materials. Some antioxidants inhibit the oxidation reaction by removing oxygen free radicals. Naturally occurring antioxidants with this ability include vitamin E and ß-carotene; they limit the cell and tissue damage caused by foreign substances, such as toxins and pollutants, in the body.

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antipodal cells

The three haploid cells in the mature embryo sac of flowering plants that are situated at the opposite end to the micropyle.

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antiporter

A membrane protein that effects the active transport of a solute across a biological membrane but only when it also transports a second solute (usually of a different type) in the opposite direction. Compare symporter; uniporter.

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antipyretic

A drug that reduces fever by lowering body temperature. Certain analgesic drugs, notably paracetamol, aspirin, and phenylbutazone, also have antipyretic properties.

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antisense DNA

DNA that prevents the expression of certain genes. It consists of a stretch of single-stranded DNA that is complementary to a messenger RNA (mRNA) molecule; by binding to the mRNA it prevents the synthesis of a specific protein. Antisense DNA and RNA have been produced by techniques of genetic engineering and are undergoing clinical trials for their ability to inhibit the action of particular genes, e.g. ones involved in cancer.

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antiseptic

Any substance that kills or inhibits the growth of disease-causing microorganisms but is essentially nontoxic to cells of the body. Common antiseptics include hydrogen peroxide, the detergent cetrimide, and ethanol. They are used to treat minor wounds. Compare disinfectant.

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antiserum

Serum containing antibodies, either raised against particular antigens, and hence of known specificity, or a broad mixture of antibodies. It is used to provide short-term passive immunity, e.g. against hepatitis A virus, and to treat an infection to which the patient has no immunity. Antisera may be obtained from large animals, such as horses, that have been inoculated with particular antigens, or from pooled donated human serum.

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antitoxin

An antibody produced in response to a bacterial toxin.

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antiviral

Describing a drug or other agent that kills or inhibits viruses and is used to combat viral infections. Several types of antiviral drug are now in use, such as acyclovir, effective against herpesviruses, and zidovudine (AZT), which is used to treat HIV infection. The body's own natural antiviral agents, interferons, can now be produced by genetic engineering and are sometimes used therapeutically. However, many antiviral agents are extremely toxic, and viruses evolve rapidly so that a drug's effectiveness can soon be lost.

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anucleate

Describing any cell that does not contain a nucleus. For example, mature erythrocytes are anucleate.

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anus

The terminal opening of the alimentary canal in most animals, through which indigestible material (faeces) is expelled.

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aorta

The major blood vessel in higher vertebrates through which oxygenated blood leaves the heart from the left ventricle. The aorta branches to form many smaller arteries, which in turn branch many times to supply oxygen and essential nutrients to all living cells in the body. See also dorsal aorta; ventral aorta.

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aortic arches (arterial arches)

Six pairs of arteries in vertebrate embryos, which connect the ventral aorta to the dorsal aorta by running between the gill slits. The arches are numbered I to VI from the anterior end. In adult tetrapods arches I and II are lost, III gives rise to the carotid arteries, IV (one side of which is lost in birds and mammals) becomes the systemic arch supplying the trunk and limbs, V is lost (see ductus arteriosus), and VI gives rise to the pulmonary arch supplying the lungs (see pulmonary artery). In adult fish four to six arches persist as branchial arteries supplying the gills.

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aortic body

See ventilation centre.

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aortic valve

See semilunar valve.

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apatite

A complex mineral form of the salt calcium phosphate, Ca_5(PO_4)3(OH,F,Cl), that is the main constituent of the enamel of teeth.

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aphotic zone (bathypelagic zone)

The region of a lake or sea where no light penetrates; it is situated beneath the euphotic zone. The aphotic zone contains no algae or phytoplankton, and its inhabitants are exclusively carnivorous animals or organisms that feed on sediment or detritus, all reliant on energy inputs from the euphotic zone. It extends downwards from a depth of about 1000 m, or less in turbid waters, and includes the abyssal zone.

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apical dominance

Inhibition of the growth of lateral buds in a plant by the presence of a growing apical bud. It is brought about by the action of auxins (produced by the apical bud) and abscisic acid.

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apical meristem

A region at the tip of each shoot and root of a plant in which cell divisions are continually occurring to produce new stem and root tissue, respectively (see meristem). The new tissues produced are known collectively as the primary tissues of the plant. See ground meristem; procambium; protoderm. Compare cambium.

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Apicomplexa (Sporozoa)

A phylum of parasitic protoctists (see also protozoa) whose members may have a number of different animal hosts. Their complex life cycle involves the alternation of asexual reproduction (multiple fission) and sexual reproduction and the production of resistant spores (see illustration). The phylum includes the agents causing malaria (Plasmodium) and toxoplasmosis (Toxoplasma).

Life cycle of Plasmodium vivax (malaria parasite)

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apocarpy

The condition in which the female reproductive organs (carpels) of a flower are not joined to each other. It occurs, for example, in the buttercup. Compare syncarpy.

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apocrine secretion

See secretion.

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apodeme

An inward projection from the exoskeleton of an arthropod, to which muscles that enable movement of the limbs are attached.

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apoenzyme

An inactive enzyme that must associate with a specific cofactor molecule or ion in order to function. Compare holoenzyme.

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apomixis (agamospermy)

A reproductive process in plants that superficially resembles normal sexual reproduction but in which there is no fusion of gametes. In apomictic flowering plants there is no fertilization by pollen, and the embryos develop simply by division of a diploid cell of the ovule. See also parthenocarpy; parthenogenesis.

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apomorphy (derived trait)

A novel evolutionary trait that is unique to a particular species and all its descendants and which can be used as a defining character for a species or group in phylogenetic terms. Hence, the possession of feathers is unique to birds and defines all members of the class Aves. An apomorphy that is restricted to a single species is termed an autapomorphy . It alone cannot provide any information about the phylogenetic relations of that species, although it can indicate the degree of divergence of a species from its nearest relatives. An example is speech, which is found solely in humans (Homo sapiens) and not in other primates. An apomorphy that is shared by two or more species or groups is termed a synapomorphy . Such traits define the strictly monophyletic groups, or clades, which are the basis of cladistic classification systems (see cladistics). Compare plesiomorphy.

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apoplast

An interconnected system in plants that consists of all the cell walls and the water that exists in them (the cell wall is composed of cellulose fibres, between which are spaces filled with water). The movement of water (and dissolved ions and solutes) through the cell walls is known as the apoplast pathway . This is the main route by which water taken up by a plant travels across the root cortex to the endodermis (see also Casparian strip). Compare symplast.

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apoptosis

The process of cell death, which occurs naturally as part of the normal development, maintenance, and renewal of tissues within an organism. During embryonic development it plays a vital role in determining the final size and form of tissues and organs. For example, the fingers are `sculpted' on the spadelike embryonic hand by apoptosis of the cells between them; the tubules of the embryonic kidney are hollowed out by a similar process. Cancer is associated with the suppression of apoptosis (see also growth factor), which also occurs when viruses infect cells – in order to inhibit the activity of killer cells. Apoptosis differs from cell necrosis, in which cell death may be stimulated by a toxic substance.

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aposematic coloration

See warning coloration.

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appeasement

Behaviour that inhibits aggression from another animal of the same species, frequently taking the form of a special posture or display emphasizing the weakness of the performer. Threatening structures (e.g. antlers) and markings are covered or turned away, and vulnerable parts of the body may be exposed. Appeasement is seen in courtship, in greeting ceremonies, and often (from the loser) after a fight.

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appendicular skeleton

The components, collectively, of the vertebrate skeleton that are attached to the main supporting, or axial, skeleton. The appendicular skeleton is made up of paired appendages (e.g. legs, wings, arms) together with the pelvic girdle and pectoral girdle.

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appendix (vermiform appendix)

An outgrowth of the caecum in the alimentary canal. In humans it is a vestigial organ containing lymphatic tissue and serves no function in normal digestive processes. Appendicitis is caused by inflammation of the appendix.

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apposition

The addition of layers of cellulose to the inner surface of a plant cell wall, at its junction with the plasmalemma. This type of growth results in thickening and strengthening the cell wall and usually occurs when elongation of the cell is complete. Compare intussusception.

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Apterygota

A subclass of small primitive wingless insects in which metamorphosis is slight or absent. It includes the orders Thysanura and Diplura (bristletails), Collembola (springtails), and Protura. Compare Pterygota. See Hexapoda.

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aqueous humour

The fluid that fills the space between the cornea and the lens of the vertebrate eye. In addition to supplying the cornea and lens with nutrients, the aqueous humour helps to maintain the shape of the eye. It is produced and renewed every four hours by the ciliary body.

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Arabidopsis

A genus of flowering plants of the family Cruciferae (Brassicaceae). The species A. thaliana (Thale cress) is widely used as a research tool in molecular genetics and developmental biology because it has a small and simple genome (five pairs of chromosomes), over half of which codes for protein, and it can be easily cultured, having a life cycle of only 6–8 weeks.

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arachidonic acid

A polyunsaturated fatty acid, CH_3(CH_2)3(CH_2CH:CH)4(CH_2)3COOH, that is essential for growth in mammals (see eicosanoid). It can be synthesized from linoleic acid. Arachidonic acid acts as a precursor to several biologically active compounds, including prostaglandins, and plays an important role in membrane production and fat metabolism. The release of arachidonic acid from membrane phospholipids is triggered by certain hormones. See essential fatty acids.

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Arachnida

A class of terrestrial arthropods of the phylum Chelicerata, comprising about 65000 species and including spiders, scorpions, harvestmen, ticks, and mites. An arachnid's body is divided into an anterior prosoma and a posterior opisthosoma. The prosoma bears chelicerae, pedipalps, and four pairs of walking legs. The opisthosoma may bear various sensory or silk-spinning appendages (see spinneret). Arachnids are generally carnivorous, feeding on the body fluids of their prey or secreting enzymes to digest prey externally. Spiders immobilize their prey with poison injected by the fanglike chelicerae, while scorpions grasp their prey in large clawed pedipalps and may poison it using the posterior stinging organ. Ticks and some mites are parasitic but most arachnids are free-living. They breathe either via tracheae (like insects) or by means of thin highly folded regions of the body wall called lung books. See also Acarina.

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arachnoid membrane

One of the three membranes (meninges) that surround the brain and spinal cord of vertebrates. It lies between the pia mater and the dura mater. The arachnoid membrane is very delicate and carries cerebrospinal fluid, which sustains and cushions the nervous tissue.

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arbovirus

Obsolete name for any RNA-containing virus that is transmitted from animals to man through the bite of mosquitoes and ticks (i.e. arthropods, hence arthropod-borne viruses). They cause various forms of encephalitis (inflammation of the brain) and serious fevers, such as dengue and yellow fever.

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Archaea

A subkingdom (or domain) of prokaryotic organisms containing the archaebacteria, including the methanogens, which produce methane; the thermoacidophilic bacteria, which live in extremely hot and acidic environments (such as hot springs; see thermophilic); and the halophilic bacteria, which can only function at high salt concentrations and are abundant in the world's oceans. Archaebacteria are distinguished from the Eubacteria in that their membrane lipids are ether-linked as opposed to being ester-linked, and they lack peptidoglycan in their cell walls. However, the archaebacteria are grouped together principally on the basis of similarities in the base sequence of their ribosomal RNA. For some authors, this molecular evidence shows the archaebacteria to be phylogenetically distinct from either the eubacteria or the eukaryotes, and they are regarded as constituting a separate domain of life. They are possibly descendants of the earliest forms of life.

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Archaean

The earliest eon of geological time, in which there is the first evidence of life on earth. It follows the Hadean eon of pregeological time and extends from the time of the earliest known rocks, roughly 3800 million years ago, to the beginning of the Proterozoic eon, about 2600 million years ago. Rock formations called stromatolites, dated at 3500 million years old or older, are among the oldest of all fossil remains. They are thought to have been produced by the activities of microbial mats of filamentous purple and green bacteria. These prokaryotes performed photosynthesis anaerobically, perhaps using hydrogen sulphide as an electron donor instead of water. Some of their descendants evolved the ability to use water as an electron donor, producing oxygen as a by-product, and eventually brought about the change in atmospheric conditions necessary for aerobic life.

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archaebacteria

See Archaea.

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archegonium

(pl. archegonia) The multicellular flask-shaped female sex organ of bryophytes, clubmosses, horsetails, ferns, and many gymnosperms. Such plants are described as archegoniate to distinguish them from algae, which do not possess archegonia. The dilated base, the venter , contains the oosphere (female gamete). The cells of the narrow neck liquefy to allow the male gametes to swim towards the oosphere. The archegonium is thus an adaptation to the terrestrial environment as it provides a means for the male gametes to reach the female gamete. Compare antheridium.

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archenteron (gastrocoel)

A cavity within an animal embryo at the gastrula stage of development. All or part of the archenteron eventually forms the cavity of the gut. It is connected to the outside by an opening (the blastopore ), which becomes either the mouth, the mouth and anus, or the anal opening of the animal.

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areolar connective tissue

A type of connective tissue consisting of a gel-like matrix incorporating strands of protein fibres (collagen and elastin) and such cells as fibroblasts, mast cells, macrophages, and fat cells. This tissue is found throughout the body under the skin and linking organs and other tissues.

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arginine

See amino acid.

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aril

An outgrowth that grows around and may completely enclose the testa (seed coat) of a seed. It develops from the placenta, funicle, or micropyle of an ovule. The aril surrounding the nutmeg seed forms the spice mace. See also caruncle.

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arousal

A level of physiological and behavioural responsiveness in an animal, which tends to vary between sleep and full alertness. It is controlled by a particular part of the brain (the reticular activating system ) and can be detected by changes in brain electrical activity, heart rate and muscle tone, responsiveness to new stimuli, and general activity.

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arrector pili

A small muscle in the dermis of the skin that is attached to the base of a hair follicle. Contraction of the arrector pili, in response to cold or fear, pulls the hair into a vertical position, thus trapping an insulating layer of air around the body and causing `goose flesh' in humans.

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arrhenotoky

The phenomenon occurring in the reproduction of certain animals in which fertilized eggs give rise to females and unfertilized eggs to males. It is found among certain insect groups, notably the wasps and bees (Hymenoptera), and in mites, as well as rotifers and nematodes. The males are haploid and transmit only the maternal genome – their production represents arrhenotokous parthenogenesis – whereas the females are diploid. In pseudo-arrhenotoky both males and females arise from fertilized eggs and are diploid, but males subsequently become effectively haploid by inactivation of the paternal genome, either in all cells or only in germ-line cells. This occurs in certain scale insects and mites. Compare thelytoky.

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arrhythmia

A disturbance in the normal rhythm of the heart, usually caused by reduced blood flow within the heart or a defect in the pacemaker (the sinoatrial node) of the heart.

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arteriole

A small muscular blood vessel that receives blood from the arteries and carries it to the capillaries.

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artery

A blood vessel that carries blood away from the heart towards the other body tissues. Most arteries carry oxygenated blood (the pulmonary artery is an exception). The large arteries branch to form smaller ones, which in turn branch into arterioles. All arteries have muscular walls, whose contraction aids in pumping blood around the body. The accumulation of fatty deposits in the walls of the arteries leads to atherosclerosis , which limits and may eventually block the flow of blood. Compare vein.

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arthropod

Any invertebrate animal that, characteristically, possesses an outer body layer – the cuticle – that functions as a rigid protective exoskeleton; growth is thus possible only by periodic moults (see ecdysis). There are over one million species of arthropods, inhabiting marine, freshwater, and terrestrial habitats worldwide. The arthropod body is composed of segments (see metameric segmentation) usually forming distinct specialized body regions, e.g. head, thorax, and abdomen. These segments may possess hardened jointed appendages, modified variously as mouthparts, limbs, wings, reproductive organs, or sense organs. The main body cavity, containing the internal organs, is a blood-filled haemocoel, within which lies the heart. Although in older classifications arthropods are placed in a single phylum, Arthropoda, the origins and relationships of the various groups of arthropods remain uncertain, and they are now usually assigned to three separate phyla according to the basic structure of their appendages: Crustacea (shrimps, barnacles, crabs, etc.); Uniramia (or Mandibulata), including the classes Hexapoda (insects), Chilopoda (centipedes), and Diplopoda (millipedes); and Chelicerata, including the Arachnida (spiders, scorpions, mites, and ticks).

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articulation

The attachment of two bones, usually by means of a joint. The thigh bone (femur), for instance, articulates with the pelvic girdle.

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artificial insemination (AI)

The deposition of semen, using a syringe, at the mouth of the uterus to make conception possible. It is used in the selective breeding of domestic animals and also in humans in some cases of impotence and infertility. It is timed to coincide with ovulation in the female.

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artificial selection

The modification of species by selective breeding. Animals or plants with desirable characteristics are interbred with the aim of altering the genotype and producing a new strain of the organism for a specific purpose. For example, sheep are bred by means of artificial selection in order to improve wool quality.

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Artiodactyla

An order of hooved mammals comprising the even-toed ungulates, in which the third and fourth digits are equally developed and bear the weight of the body. The order includes cattle and other ruminants (see Ruminantia), camels, hippopotamuses, and pigs. All except the latter are herbivorous, having an elongated gut and teeth with enamel ridges for grinding tough grasses. Compare Perissodactyla.

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ascending tracts

See spinal cord.

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ascocarp

The reproductive body of fungi of the phylum Ascomycota, which contains ascus cells. An ascocarp may be a closed sphere (cleistothecium ), a flask-shaped structure (perithecium ) with a small opening (ostiole ), or an open cup-shaped structure (apothecium ). The ascocarp consists of both ascus-bearing and vegetative hyphae.

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ascogenous hyphae

The hyphae, in fungi of the Ascomycota, that grow from the ascogonium after it has fused with the antheridium. The ascogenous hyphae are made up of binucleate cells containing one nucleus derived from the male antheridium and the other from the female ascogonium. This condition is represented as n + n, rather than 2n, as the cells are not true diploid cells. Asci (see ascus) develop from the ascogenous hyphae.

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ascogonium

The female gametangium of certain fungi of the Ascomycota, from which ascogenous hyphae develop.

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Ascomycota

A phylum of fungi, formerly classified as a class (Ascomycetes) or a subdivision (Ascomycotina). It includes the yeasts, some species of edible fungi, and Claviceps purpurea, which causes ergot in rye. Many are fungal partners of lichens. Sexual reproduction is by means of ascospores produced within an ascus. The asci are usually grouped together in an ascocarp.

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ascorbic acid

See vitamin C.

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ascospore

The spore of fungi belonging to the phylum Ascomycota. See ascus.

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ascus

(pl. asci) A specialized cell in fungi of the phylum Ascomycota, which contains two haploid nuclei that fuse during sexual reproduction and then undergo meiosis, giving rise to eight ascospores contained within the ascus.

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asepsis

The condition of an environment that is free of pathogens. This is achieved by sterilization techniques.

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asexual reproduction

Reproduction in which new individuals are produced from a single parent without the formation of gametes. It occurs chiefly in lower animals, microorganisms, and plants. In microorganisms and lower animals the chief methods are fission (e.g. in protoctists), fragmentation (e.g. in some aquatic annelid worms), and budding (e.g. in cnidarians and yeasts). The principal methods of asexual reproduction in plants are by vegetative propagation (e.g. bulbs, corms, tubers) and by the formation of spores. Spore formation occurs in mosses, ferns, and other plants showing alternation of generations, as a dormant stage between sporophyte and gametophyte, and in some algae and fungi, to produce replicas of the organism. Compare sexual reproduction.

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asparagine

See amino acid.

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aspartic acid

See amino acid.

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aspirin (acetylsalicylic acid)

A drug that reduces inflammation, combats fever, and alleviates pain. Aspirin works by inhibiting the formation of prostaglandins, which are major factors in the inflammation process. It also reduces the aggregation of blood platelets, hence its use in maintaining blood flow following heart and circulatory disorders.

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assimilation

The utilization by a living organism of absorbed food materials in the processes of growth, reproduction, or repair.

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association

An ecological unit in which two or more species occur in closer proximity to one another than would be expected on the basis of chance. Early plant ecologists recognized associations of fixed composition on the basis of the dominant species present (e.g. a coniferous forest association). Associations now tend to be detected by using more objective statistical sampling methods. See also consociation.

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association centre

The part of the brain that links a primary sensory area (the part of the cerebral cortex that receives primary sensory impulses) with other parts of the brain, such as memory and motor areas, and deals with the interpretation and meaning of the primary sensory imput. For example, the auditory association area interprets a `moo' sound as that coming from a cow.

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aster

A starlike arrangement of microtubules radiating from a centrosome. Asters become conspicuous in animal cells at the ends of the spindle when cell division starts.

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astigmatism

A lens defect in which when rays in one plane are in focus those in another plane are not. The eye can suffer from astigmatism, usually when the cornea is not spherical. It is corrected by using an anastigmatic lens, which has different radii of curvature in the vertical and horizontal planes.

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astrocyte

See glia.

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atherosclerosis

Obstruction of the arteries by localized deposits of fatty material (including cholesterol) on their inner walls. Atherosclerosis is associated with high blood levels of cholesterol, particularly in the form of low-density lipoprotein; it can result in heart failure if it affects the coronary arteries (see coronary vessels).

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atlas

The first cervical vertebra, a ringlike bone that joins the skull to the vertebral column in terrestrial vertebrates. In advanced vertebrates articulation between the skull and atlas permits nodding movements of the head. See also axis.

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ATP (adenosine triphosphate)

A nucleotide that is of fundamental importance as a carrier of chemical energy in all living organisms. It consists of adenine linked to D-ribose (i.e. adenosine); the D-ribose component bears three phosphate groups, linearly linked together by covalent bonds (see formula). These bonds can undergo hydrolysis to yield either a molecule of ADP (adenosine diphosphate ) and inorganic phosphate or a molecule of AMP (adenosine monophosphate ) and pyrophosphate (see ATPase). Both these reactions yield a large amount of energy (about 30.6 kJ mol–1) that is used to bring about such biological processes as muscle contraction, the active transport of ions and molecules across cell membranes, and the synthesis of biomolecules. The reactions bringing about these processes often involve the enzyme-catalysed transfer of the phosphate group to intermediate substrates. Most ATP-mediated reactions require Mg2+ ions as cofactors.
ATP is regenerated by the rephosphorylation of AMP and ADP using the chemical energy obtained from the oxidation of food. This takes place during glycolysis and the Krebs cycle but, most significantly, is also a result of the reduction–oxidation reactions of the mitochondrial electron transport chain, which ultimately reduces molecular oxygen to water (oxidative phosphorylation). ATP is also formed by the light reactions of photosynthesis.

ATP

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ATPase

Any of a group of enzymes that bring about the hydrolysis of ATP. This results in the cleavage of either one phosphate group, with the formation of ADP and inorganic phosphate (Pi), or of two phosphate groups, with the formation of AMP and pyrophosphate (PPi); the second reaction yields twice as much energy as the first. ATPase activity is associated with many energy-consuming processes; for example, in muscle contraction it is associated with myosin when activated by actin. One form of ATPase, ATP synthetase , can catalyse the synthesis of ATP, for example in the mitochondrial electron transport chain (see also chemiosmotic theory).

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atrioventricular node (AVN)

A specialized group of cardiac muscle fibres situated in the fibrous ring between the right atrium and ventricle of the heart. The AVN is the only pathway between the atria and the ventricles through which electrical impulses can pass. Thus, following the contraction of the atria, the AVN initiates a wave of contraction in the ventricles via the bundle of His.

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atrioventricular valve

See bicuspid valve; tricuspid valve.

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atrium

1 (or auricle) A chamber of the heart that receives blood from the veins and forces it by powerful muscular contraction into the ventricle(s). Fish have a single atrium but all other vertebrates have two.

2 Any of various cavities or chambers in animals, such as the chamber surrounding the gill slits of the lancelet and other invertebrate chordates.
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atrophy

The degeneration or withering of an organ or part of the body.

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atropine

A poisonous crystalline alkaloid, C_17H_23NO_3. It can be extracted from deadly nightshade and other solanaceous plants and is used in medicine to treat colic, to reduce secretions, and to dilate the pupil of the eye.

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attenuation

1 (in medicine) A process of reducing the disease-producing ability of a microorganism. It can be achieved by chemical treatment, heating, drying, irradiation, by growing the organism under adverse conditions, or by serial passage through another organism. Attenuated bacteria or viruses are used for some vaccines.

2 (in mycology) The conversion by yeasts of carbohydrates to alcohol, as in brewing and wine and spirit production.
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attenuator

See operon.

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atto-

Symbol a. A prefix used in the metric system to denote 10–18. For example, 10–18 second = 1 attosecond (as).

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audibility

The limits of audibility of the human ear are between about 20 hertz (a low rumble) and 20000 hertz (a shrill whistle). With increased age the upper limit falls quite considerably.

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audiometer

An instrument that generates a sound of known frequency and intensity in order to measure an individual's hearing ability.

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auditory

Of or relating to the ear. For example, the auditory meatus is the canal leading from the pinna to the tympanum (eardrum).

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auditory nerve

The nerve that transmits sensory information from the ear to the brain. See also organ of Corti.

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auricle

1 See atrium.

2 See pinna.
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Australian region

See faunal region.

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Australopithecus

A genus of fossil primates that lived 4–1.3 million years ago, coexisting for some of this time with early forms of humans (see Homo). They walked erect and had teeth resembling those of modern man, but the brain capacity was less than half that of a human. Various finds have been made, chiefly in East and South Africa (hence the name, which means `southern ape'). Australopithecus and related genera are known as australopithecines .

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autapomorphy

See apomorphy.

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autecology

The study of ecology at the level of the species. An autecological study aims to investigate the ecology of populations or individuals of a particular species, including habitat, distribution, life cycle, etc. This should enable a full description of the ecological niche of the organism to be made. Compare synecology.

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autochthonous

Describing an organism that is native to the place in which it is found. Compare allochthonous.

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autoclave

A strong steel vessel used for carrying out chemical reactions, sterilizations, etc., at high temperature and pressure.

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autoecious

Describing rust fungi (see rusts) that pass their life cycle in association with only one host plant. Compare heteroecious.

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autogamy

1 A type of reproduction that occurs in single isolated individuals of ciliate protozoans of the genus Paramecium. The nucleus divides into two genetically identical haploid nuclei, which then fuse to form a diploid zygote. The onset of autogamy is associated with changing environmental conditions and may be necessary to maintain cell vitality.

2 Self-fertilization in plants. See fertilization.
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autogenic

Relating to or caused by a change in the environment or an individual organism due to some endogenous factor, i.e. one that comes from within the environment or organism. Compare allogenic.

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autograft

See graft.

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autoimmunity

A disorder of the body's defence mechanism in which an immune response is elicited against its own tissues. Rheumatoid arthritis is basically an autoimmune disease.

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autolysis

The process of self-destruction of a cell, cell organelle, or tissue. It occurs by the action of enzymes within or released by lysosomes. See also lysis.

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autonomic nervous system (ANS)

The part of the vertebrate peripheral nervous system that supplies stimulation via motor nerves to the smooth and cardiac muscles (the involuntary muscles) and to the glands of the body. It is divided into the parasympathetic and the sympathetic nervous systems, which tend to work antagonistically on the same organs (see illustration). The activity of the ANS is controlled principally by the medulla oblongata and hypothalamus of the brain.

The autonomic nervous system (showing one side of each system of the ANS)

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autopolyploid

A polyploid organism in which the multiple sets of chromosomes are all derived from the same species. For example, doubling of the chromosome number during mitotic cell division, possibly induced by colchicine, gives rise to a tetraploid known as an autotetraploid . Compare allopolyploid.

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autoradiography

An experimental technique in which a radioactive specimen is placed in contact with (or close to) a photographic plate, so as to produce a record of the distribution of radioactivity in the specimen. The film is darkened by the ionizing radiation from radioactive parts of the sample. Autoradiography is used to study the distribution of particular substances in living tissues, cells, and cultures. A radioactive isotope of the substance is introduced into the organism or tissue, which is killed, sectioned, and examined after enough time has elapsed for the isotope to be incorporated into the substance. Another common application of autoradiography is the location of radioactively labelled DNA probes or antibodies employed in such techniques as Southern blotting and Western blotting.

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autosome

Any of the chromosomes in a cell other than the sex chromosomes.

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autostylic jaw suspension

A type of jaw suspension seen in the lungfish (see Dipnoi) and tetrapods, in which the upper jaw is connected directly to the cranium (see illustration). Compare amphistylic jaw suspension; hyostylic jaw suspension.

Autostylic jaw suspension

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autotomy

The shedding by an animal of part of its body followed by the regeneration of the lost part. Autotomy is achieved by the contraction of muscles at specialized regions in the body. It serves as a protective mechanism if the animal is damaged or attacked (e.g. tail loss in certain reptiles) and is common as a method of asexual reproduction in polychaete worms, in which both new head and tail regions may be regenerated.

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autotrophic nutrition

A type of nutrition in which organisms synthesize the organic materials they require from inorganic sources. Chief sources of carbon and nitrogen are carbon dioxide and nitrates, respectively. All green plants are autotrophic and use light as a source of energy for the synthesis, i.e. they are photoautotrophic (see photosynthesis). Some bacteria are also photoautotrophic; others are chemoautotrophic , using energy derived from chemical processes (see chemosynthesis). Compare heterotrophic nutrition.

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auxanometer

Any mechanical instrument or measuring device used to study the growth or movement of plant organs. One type of auxanometer consists of a recording device that translates any increase in stem height into movement of a needle across a scale.

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auxin

Any of a group of plant growth substances responsible for such processes as the promotion of growth by cell enlargement, the maintenance of apical dominance, and the initiation of root formation in cuttings. Auxins are also involved in the abscission of leaves, fruit, or other plant organs and in the development of flowers and fruits. Naturally occurring auxins, principally indoleacetic acid (IAA), are synthesized in actively growing regions of the plant, from where they are transported to other parts of the plant. Synthetic auxins include 2,4-D, which is used as a weedkiller, and indolebutyric acid and naphthaleneacetic acid , which are sold in preparations of `rooting hormones'.

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auxin-binding protein

A protein found in plant cells that binds auxins and thus may act as the initial receptor of the signal transduction pathway for these plant growth substances. Likely candidates have been found in various plant tissue cultures, including pith cells of tobacco and coleoptile tissue of maize. Such binding proteins are thought to act in a manner analogous to hormone receptor proteins in animal cells. According to one model, auxin stimulates the cell by binding to an auxin-binding protein (ABP) on the exterior surface of the plasma membrane. The ABP is held in place by a membrane-spanning docking protein. This activated complex of auxin, ABP, and docking protein then triggers the auxin-stimulated response in the cell, for example cell expansion (see acid growth theory).

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Avery, Oswald Theodore

(1877–1955)
Canadian bacteriologist who worked at the Rockefeller Institute Hospital in New York (1913–48), where he and his colleagues Maclyn McCarty and Colin Macleod identified DNA as the hereditary material in the cells of pneumococcus bacteria. It had previously been thought that protein was the hereditary material; Avery's work was an important step in leading Watson and Crick to the discovery of the true chemical basis of heredity.

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Aves

The birds: a class of bipedal vertebrate chordates (see Chordata) with feathers, wings, and a beak. They evolved from reptilian ancestors in the Jurassic period (190–136 million years ago) and modern birds still have scaly legs, like reptiles. Birds are warm-blooded (see homoiothermy). The skin is dry and loose and has no sweat glands, so cooling is effected by panting. Their efficient lungs and four-chambered heart (which completely separates oxygenated and deoxygenated blood) ensure a good supply of oxygen to the tissues. Birds can therefore sustain a high body temperature and level of activity necessary for flight. The breastbone bears a keel for the attachment of flight muscles. The skeleton is very light; many of the bones are tubular, having internal struts to provide strength and air sacs to reduce weight and provide extra oxygen in flight. Their feathers are vital for flight, streamlining the body, and insulation against heat loss.
Many birds show a high degree of social behaviour in forming large flocks and pair bonding for nesting, egg incubation, and rearing young. Fertilization is internal and the female lays hard-shelled eggs. See also Ratitae.

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avidin

A glycoprotein component of egg white that binds strongly to the vitamin biotin. Proteins and nucleic acids can be linked to biotin (biotinylated ), and the avidin–biotin reaction can then be used in a number of assay methods, such as antigen–antibody reactions or DNA hybridization. For example, enzymes conjugated with avidin can be used to bind to biotinylated antibodies.

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AVN

See atrioventricular node.

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awn

A stiff bristle that projects from the tip of a plant part or organ. The bracts in a grass inflorescence commonly bear awns (see spike).

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axenic culture

A culture medium in which only one type of microorganism is growing. Such cultures are widely used in microbiology to determine the basic growth requirements or degree of inhibition by antibiotics or other chemicals of a particular species.

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axial skeleton

The main longitudinal section of the vertebrate skeleton, including the skull, the vertebral column, and the rib cage. Compare appendicular skeleton.

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axil

The angle between a branch or leaf and the stem it grows from. Axillary (or lateral ) buds develop in the axil of a leaf. The presence of axillary buds distinguishes a leaf from a leaflet.

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axillary bud

See axil.

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axis

The second cervical vertebra, which articulates with the atlas (the first cervical vertebra, which articulates with the skull). The articulation between the axis and atlas in reptiles, birds, and mammals permits side-to-side movement of the head. The body of the axis is elongated to form a peg (the odontoid process ), which extends into the ring of the atlas and acts as a pivot on which the atlas (and skull) can turn.

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axon

The long threadlike part of a nerve cell (neurone). It carries the nerve impulse (in the form of an action potential) away from the cell body of a neurone towards either an effector organ or the brain. See also nerve fibre.

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axoneme

The core of an undulipodium (cilium or eukaryotic flagellum), consisting of two central microtubules surrounded by nine other pairs of microtubules. The outer microtubules are associated with the protein dynein, which is responsible for the movement of the organelle.

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B

Bacillariophyta

A phylum of algae comprising the diatoms. These marine or freshwater unicellular organisms have cell walls (frustules ) composed of pectin impregnated with silica and consisting of two halves, one overlapping the other. Diatoms are found in huge numbers in plankton and are important in the food chains of seas and rivers. Past deposition has resulted in diatomaceous earths (kieselguhr) and the oil reserves of these species have contributed to oil deposits.

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bacillus

Any rod-shaped bacterium. Generally, bacilli are large, Gram-positive, spore-bearing, and have a tendency to form chains and produce a capsule. Some are motile, bearing flagella. They are ubiquitous in soil and air and many are responsible for food spoilage. The group also includes Bacillus anthracis, which causes anthrax.

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backbone

See vertebral column.

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back cross

A mating between individuals of the parental generation (P) and the first generation of offspring (F_1) in order to identify hidden recessive alleles. If an organism displays a dominant characteristic, it may possess two dominant alleles (i.e. it is homozygous) or a dominant and a recessive allele for that characteristic (i.e. it is heterozygous). To find out which is the case, the organism is crossed with one displaying the recessive characteristic. If all the offspring show the dominant characteristic then the organism is homozygous, but if half show the recessive characteristic, then the organism is heterozygous. See also test cross.

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background radiation

Low-intensity ionizing radiation present on the surface of the earth and in the atmosphere as a result of cosmic radiation and the presence of radioisotopes in the earth's rocks, soil, and atmosphere. The radioisotopes are either natural or the result of nuclear fallout or waste gas from power stations.

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bacteria

A diverse group of ubiquitous microorganisms all of which consist of only a single cell that lacks a distinct nuclear membrane and has a cell wall of a unique composition (see illustration). Bacteria constitute the prokaryotic organisms of the living world. However, their classification is a controversial issue. It is now recognized, on the basis of differences in ribosomal RNA structure and nucleotide sequences (see molecular systematics), that prokaryotes form two evolutionarily distinct groups. Some authorities place them all in a single kingdom, variously named Bacteria or Prokaryotae, which is divided into two subkingdoms: Archaea (archaebacteria), which includes the descendants of ancient bacterial groups; and Eubacteria, which represents the vast majority of present-day bacteria. Others argue that these groups of prokaryotes are so distinct that they should each be raised to the status of domain: Archaea (the archaebacteria, containing a variable number of kingdoms) and Bacteria (containing a single kingdom, Eubacteria). Generally speaking, the term `bacteria' includes both archaebacteria and eubacteria.
Bacteria can be characterized in a number of ways, for example by their reaction with Gram's stain or on the basis of their metabolic requirements (e.g. whether or not they require oxygen: see aerobic respiration; anaerobic respiration) and shape. A bacterial cell may be spherical (see coccus), rodlike (see bacillus), spiral (see spirillum), comma-shaped (see vibrio), corkscrew-shaped (see spirochaete), or filamentous, resembling a fungal cell. The majority of bacteria range in size from 0.5 to 5 µm. Many are motile, bearing flagella, possess an outer slimy capsule, and produce resistant spores (see endospore). In general bacteria reproduce only asexually, by simple division of cells, but a few groups undergo a form of sexual reproduction (see conjugation). Bacteria are largely responsible for decay and decomposition of organic matter, producing a cycling of such chemicals as carbon (see carbon cycle), oxygen, nitrogen (see nitrogen cycle), and sulphur (see sulphur cycle). A few bacteria obtain their food by means of photosynthesis, including the Cyanobacteria; some are saprotrophs; and others are parasites, causing disease. The symptoms of bacterial infections are produced by toxins.

A generalized bacterial cell

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bacterial growth curve

A curve on a graph that shows the changes in size of a bacterial population over time in a culture. The bacteria are cultured in sterile nutrient medium and incubated at the optimum temperature for growth. Samples are removed at intervals and the number of viable bacteria is counted. A logarithmic growth curve is plotted, which shows various phases (see graph).
In the lag (or latent ) phase there is only a small increase in numbers as the bacteria imbibe water, and synthesize ribosomal RNA and subsequently enzymes, in adjusting to the new conditions. The length of this phase depends on which medium was used to culture the bacteria before the investigation and which phase the cells are already in. As the life span (generation time) of the cells decreases, they enter the log (or exponential ) phase , in which the cells reach a maximum rate of reproduction and the number of bacteria increases directly with time, giving a straight slope on a logarithmic scale (see exponential growth). For example, the fastest generation time for E. coli is 21 minutes. Growth rate can be estimated in this phase. With time, as the population grows, it enters the stationary phase , when the nutrients and electron acceptors are depleted and the pH drops as carbon dioxide and other waste poisons accumulate. As the cell's energy stores are depleted the rate of cell division decreases. The death (or final ) phase occurs when the rate at which the bacteria die exceeds the rate at which they are produced; the population declines as the levels of nutrients fall and toxin levels increase. See also population growth.

Bacterial growth curve

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bactericidal

Capable of killing bacteria. Common bactericides are some antibiotics, antiseptics, and disinfectants. Compare bacteriostatic.

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bacteriochlorophyll

A form of chlorophyll found in photosynthetic bacteria, notably the purple and green bacteria. There are several types, designated a to g. For example, bacteriochlorophyll a and bacteriochlorophyll b are structurally similar to the chlorophyll a and chlorophyll b found in plants; the purple bacteria contain either of these two types of bacteriochlorophyll, depending on the species. Bacteriochlorophyll is located in specialized membrane systems (chromatophores ); in purple bacteria these are in the form of sheets, tubes, or vesicles arising inside the cell from the cell membrane, whereas in green bacteria they are cylindrical structures (chlorosomes ) underlying the cell membrane.

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bacteriology

The study of bacteria, including their identification, form, function, reproduction, and classification. Much attention is focused on the role of bacteria as agents of disease in animals, including humans, and in plants, and on methods of controlling pathogenic bacteria in the food chain and elsewhere in the environment. However, bacteriologists also investigate the many benefits of bacteria, e.g. in the production of antibiotics, enzymes, and amino acids, and in sewage treatment.

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bacteriophage (phage)

A virus that is parasitic within a bacterium. Each phage is specific for only one type of bacterium. Most phages (virulent phages ) infect, quickly multiply within, and destroy (lyse) their host cells. However, some (temperate phages ) remain dormant in their hosts after initial infection: their nucleic acid becomes integrated into that of the host and multiplies with it, producing infected daughter cells (see lysogeny). Lysis may eventually be triggered by environmental factors Phages are used experimentally to identify bacteria, to control manufacturing processes (such as cheese production) that depend on bacteria, and, because they can alter the genetic make-up of bacterial cells, they are important tools in genetic engineering as cloning vectors. See lambda phage.

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bacteriorhodopsin

A membrane-bound protein manufactured by the halophilic (salt-resistant) archaebacterium Halobacterium salinarum that exploits light energy to make ATP without the involvement of chlorophyll-like pigments. When activated by light, it pumps protons out of the cell; this creates a concentration gradient, which enables ATP to be synthesized. Bacteriorhodopsin is composed of seven -helix segments, which span the membrane and are joined together by short amino-acid chains. It contains the prosthetic group retinal, which is also found in the pigment rhodopsin in the rod cells of vertebrates.

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bacteriostatic

Capable of inhibiting or slowing down the growth and reproduction of bacteria. Some antibiotics are bacteriostatic. Compare bactericidal.

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bacteroid

The form adopted by a nitrogen-fixing bacterium when active within the root nodule of a host plant. For example, Rhizobium bacteria change into enlarged irregularly shaped branching cells when they infect the root cells of their legume host. These bacteroids become surrounded by a peribacteroid membrane , derived from membranes of their host cells, and differentiate to produce the key enzymes and other components of nitrogen fixation, such as nitrogenase. As the infection process progresses, the bacteroids become housed in a root nodule, where they are totally dependent on their host for the energy required for nitrogen. In return they supply their host with an assimilable form of nitrogen, i.e. ammonia, which is incorporated into amino acids. See nitrogen fixation.

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Baer, Karl Ernst von

(1792–1876)
Estonian-born German biologist and embryologist. He studied medicine and comparative anatomy before becoming professor of zoology at Königsberg University in 1817. Ten years later he discovered the mammalian ovum, and traced its development from the Graafian follicle to the embryo. He demonstrated that similar organs in different animals developed from the same germ layers. He also noted the similarities between the young embryos of widely different species (the biogenetic law), which provided Charles Darwin with important evidence for his theory of evolution.

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baker's yeast

Strains of the yeast Saccharomyces cerevisiae that are used in bread-making to enable the dough to rise. Water is added to flour, which activates the amylase enzymes that hydrolyse the starch in flour to glucose. Baker's yeast is then added, which uses the glucose as a substrate for aerobic respiration. The carbon dioxide produced from yeast respiration causes bubbles to form in the dough; these become larger during heating in an oven, giving bread its typical texture.

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balance

1 (in animal physiology) Equilibrium in the posture of the body. In vertebrates balance is sensed and maintained by the vestibular apparatus of the inner ear.

2 (in nutrition) See diet.
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balanced polymorphism

See polymorphism.

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baleen

See whalebone.

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barb

1 (in zoology) Any one of the stiff filaments forming a row on each side of the longitudinal shaft of a feather (see illustration). Together the barbs form the expanded part (vane ) of the feather. See barbule.

Interlocking barbs of a contour feather

2 (in botany) A hooked hair.
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barbiturate

Any one of a group of drugs derived from barbituric acid, which have a depressant effect on the central nervous system. Barbiturates were originally used as sedatives and sleeping pills but their clinical use is now limited due to their toxic side-effects; prolonged use can lead to addiction. Specific barbiturates in clinical use include phenobarbitone, for treating epilepsy, and methohexitane sodium, used as an anaesthetic.

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barbule

Any of the minute filaments forming a row on each side of the barb of a feather. In a contour feather adjacent barbules interlock by means of hooks (barbicels ) and grooves, forming a firm vane. Down feathers have no barbicels.

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Barfoed's test

A biochemical test to detect monosaccharide (reducing) sugars in solution, devised by the Swedish physician C. T. Barfoed (1815–99). Barfoed's reagent , a mixture of ethanoic (acetic) acid and copper(II) acetate, is added to the test solution and boiled. If any reducing sugars are present a red precipitate of copper(II) oxide is formed. The reaction will be negative in the presence of disaccharide sugars as they are weaker reducing agents.

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bark

The protective layer of mostly dead cells that covers the outside of woody stems and roots. It includes the living and dead tissues external to the xylem, including the phloem and periderm. The term can be used more specifically to describe the periderm together with other tissues isolated by the activity of the cork cambium. In some species, such as birch, there is one persistent cork cambium but in the older stems of certain other species a second cork cambium becomes active beneath the periderm and further periderm layers are formed every few years. The result is a composite tissue called rhytidome , composed of cork, dead cortex, and dead phloem cells.

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baroreceptor

A receptor that responds to changes in pressure. The carotid sinus in the carotid artery contains baroreceptors that respond to changes in arterial pressure and are therefore involved in the regulation of blood pressure and heart beat.

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Barr body

A structure consisting of a condensed X chromosome (see sex chromosome) that is found in nondividing nuclei of female mammals. The presence of a Barr body is used to confirm the sex of athletes in sex determination tests. It is named after the Canadian anatomist M. L. Barr (1908–95), who identified it in 1949.

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basal body (kinetosome)

See undulipodium.

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basal ganglia

Small masses of nervous tissue within the brain that connect the cerebrum with other parts of the nervous system. They are involved with the subconscious regulation of voluntary movements. The largest of the basal ganglia is the corpus striatum .

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basal metabolic rate (BMR)

The rate of energy metabolism required to maintain an animal at rest. BMR is measured in terms of heat production per unit time and is usually expressed in kilojoules of heat released per square metre of body surface per hour (kJm–2h–1). It indicates the energy consumed in order to sustain such vital functions as heartbeat, breathing, nervous activity, active transport, and secretion. Different tissues have different metabolic rates (e.g. the BMR of brain tissue is much greater than that of bone tissue) and therefore the tissue composition of an animal determines its overall BMR. For any comparable group of animals (such as mammals) BMR is proportional to body weight according to the allometric equation (see allometric growth); small animals tend to have a higher metabolic rate per unit weight than large ones.

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base

A compound that reacts with an acid to give water (and a salt). A base that dissolves in water to produce hydroxide ions is called an alkali . For example, ammonia reacts as follows:
NH_3 + H_2O NH_4+ + –OH

Similar reactions occur with organic amines (see also nitrogenous base).

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basement membrane

A thin sheet of collagen in animals that underlies and supports the cells of an epithelium, separating this from the underlying tissue. The basement membrane is made up of glycosaminoglycan (mucopolysaccharide) and fibrous material. In some tissues, such as the kidney glomerulus and the lung alveoli, the basement membrane acts as a filter (see ultrafiltration).

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base pairing

The chemical linking of two complementary nitrogenous bases in DNA and in certain types of RNA molecules. Of the four such bases in DNA, adenine pairs with thymine and cytosine with guanine. In RNA, thymine is replaced by uracil. Base pairing is responsible for holding together the two strands of a DNA molecule to form a double helix and for faithful reproduction and reading of the genetic code. The links between bases take the form of hydrogen bonds.

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basic stains

See staining.

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Basidiomycota

A phylum of fungi, formerly classified as a class (Basidiomycetes) or a subdivision (Basidiomycotina). Sexual reproduction is by means of basidiospores produced externally on a club-shaped or cylindrical basidium. Basidia are often grouped together on fruiting structures, such as mushrooms, puffballs, and bracket fungi. Exceptions are the rusts and smuts, which do not produce obvious fruiting bodies.

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basidiospore

The characteristic spore of fungi belonging to the phylum Basidiomycota. See basidium.

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basidium

(pl. basidia) A specialized cell in fungi of the phylum Basidiomycota, in which nuclear fission and meiosis occur. This results in the formation of four basidiospores , which are attached to the basidium by means of sterigmata (short stalks).

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basilar membrane

See organ of Corti.

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basket cell

A type of cell found in the cerebral cortex. Basket cells form a layer next to the Purkyne cells, from which they receive stimuli.

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basophil

A type of white blood cell (leucocyte) that has a lobed nucleus surrounded by granular cytoplasm (see granulocyte). Basophils are produced continually by stem cells in the red bone marrow; like mast cells, they produce histamine and heparin as part of the body's defences at the site of an infection or injury (see inflammation).

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bast

An old name for phloem.

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batch culture

A technique used to grow microorganisms or cells. A limited supply of nutrients for growth is provided; when these are used up, or some other factor becomes limiting, the culture declines. Cells, or products that the organisms have made, can then be harvested from the culture.

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Batesian mimicry

See mimicry.

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Bateson, William

(1861–1926)
British geneticist, who worked at Cambridge University. In 1900 he translated and championed the rediscovered work of Mendel and went on to study inheritance in chickens, demonstrating Mendelian ratios in the inheritance of comb shapes. He also found that some traits are controlled by more than one gene. In experiments with sweet peas he showed that some traits are inherited together, but he did not accept T. H. Morgan's explanation of linkage to account for this. Bateson coined the term `genetics' (in 1905).

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bathypelagic zone

See aphotic zone.

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bats

See Chiroptera.

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B cell (B lymphocyte)

A lymphocyte that is derived from stem cells in the bone marrow but does not mature in the thymus (compare T cell); in birds it matures in the bursa of the cloaca (hence B cell). Each B cell has a unique set of receptor molecules on its surface, designed to recognize a specific antigen. When the appropriate antigen binds with the receptors, the B cell divides repeatedly to form a clone of cells, which mature into plasma cells; these produce large amounts of specific antibody (see immunoglobulin), which circulates in the blood and lymph and binds to the corresponding antigen. After a few days of antibody production the plasma cells die. However, some cells from the clone remain in the form of memory cells , which initiate a more rapid immune response on subsequent exposure to the same antigen. See also clonal selection theory.

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B-chromosome

See accessory chromosome.

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Beadle, George Wells

(1903–89)
US geneticist who, after holding several professorships, went to Stanford University, where he worked with Edward Tatum (1909–75). Using moulds, they deduced that the function of genes is to control the production of enzymes, which in turn control metabolic processes. They found that mutant genes result in abnormal (and non-operative) enzymes. For this `one gene–one enzyme' theory (see one gene–one polypeptide hypothesis), they were awarded the 1958 Nobel Prize in physiology or medicine.

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becquerel

Symbol Bq. The SI unit of activity (see radiation units). The unit is named after the discoverer of radioactivity, A. H. Becquerel (1852–1908).

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bees

See Hymenoptera.

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beetles

See Coleoptera.

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beet sugar

See sucrose.

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behaviour

The sum of the responses of an organism to internal or external stimuli. The behaviour of an animal can be either instinctive (see instinct) or learned. See animal behaviour.

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behavioural genetics

The branch of genetics concerned with determining the relative importance of the genetic constitution of animals as compared to environmental factors in influencing animal behaviour.

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Benedict's test

A biochemical test to detect reducing sugars in solution, devised by the US chemist S. R. Benedict (1884–1936). Benedict's reagent – a mixture of copper(II) sulphate and a filtered mixture of hydrated sodium citrate and hydrated sodium carbonate – is added to the test solution and boiled. A high concentration of reducing sugars induces the formation of a red precipitate; a lower concentration produces a yellow precipitate. Benedict's test is a more sensitive alternative to Fehling's test.

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benthos

Flora and fauna occurring on the bottom of a sea or lake. Benthic organisms may crawl, burrow, or remain attached to a substrate. Compare nekton; neuston; plankton.

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beriberi

A disease caused by a low intake of vitamin B_1 (thiamine; see vitamin B complex), resulting in damage to peripheral nerves and heart failure. Beriberi is most common in regions of the Far East where the diet is based on polished white rice, which lacks the thiamine-rich seed coat.

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berry

A fleshy fruit formed from either one carpel or from several fused together and containing many seeds. The fruit wall may have two or three layers but the inner layer is never hard and stony (as in some drupes). Examples of berries are grapes and tomatoes. A berry, such as a cucumber, that develops a hard outer rind is called a pepo . One that is segmented and has a leathery rind, such as a citrus fruit, is called a hesperidium . The rind contains oil glands and is lined by the white mesocarp, commonly called pith .

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beta adrenergic receptor (beta adrenoceptor)

Any of a class of adrenergic cell receptors that are stimulated by adrenaline, noradrenaline, or related substances and cause the relaxation of smooth muscle in blood vessels, bronchi, the uterus, bladder, and other organs. This leads to dilation of the arteries and airways, among other effects. One subgroup, called ß_1-adrenergic receptors, mediate an increase in the rate and force of the heartbeat. See also beta blocker.

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beta blocker (beta-adrenoceptor antagonist)

Any of a group of drugs that bind preferentially to beta adrenergic receptors and hence block their stimulation by the body's own neurotransmitters, adrenaline and noradrenaline. Beta blockers, such as propranolol, oxprenolol, and atenolol, are used to treat disorders of the cardiovascular system, including high blood pressure (hypertension), angina pectoris, and irregularities of heartbeat (arrhythmias). They are also effective in treating anxiety and glaucoma (as eye drops) and in preventing migraine. They tend to dampen the effects of exercise or stress on heart rate, heart output, and blood pressure, as well as improving the oxygenation of the heart muscles. The release of the enzyme renin from the kidneys is also reduced, leading to an overall fall in arterial blood pressure.

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beta cells (ß cells)

See islets of Langerhans.

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betacyanin

Any of a group of red pigments found mainly in plants of the order Chenopodiales, which includes the goosefoot, cactus, and portulaca families. They are nitrogen-containing glycosylated compounds responsible for the red colour of beetroot, for example, and are chemically distinct from the anthocyanins, which include many red and pink plant pigments. A group of yellow pigments, called betaxanthins , are chemically similar to betacyanins and are restricted to the same families. Both betacyanins and betaxanthins are classed as betalains .

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beta sheet (ß-pleated sheet)

A form of secondary structure in proteins in which extended polypeptide chains lie parallel to each other and are linked by hydrogen bonds between the N–H and C=O groups (see illustration). Beta sheets occur in many globular proteins and link polypeptides of the same type in certain fibrous proteins, including fibroin (the protein of silk). Compare alpha helix.

Beta sheet

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betaxanthin

See betacyanin.

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bicarbonate

See hydrogencarbonate.

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biceps

A muscle that runs along the large bone of the upper arm (humerus) and is connected to the radius at one end and the shoulder bone (scapula) at the other. Contraction of the biceps causes the arm to flex at the elbow joint (see flexor). It works antagonistically with the triceps, which contracts to extend the arm (see antagonism). See also voluntary muscle.

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bicuspid valve (left atrioventricular valve; mitral valve)

A valve, consisting of two flaps, situated between the left atrium and the left ventricle of the heart of birds and mammals. When the left ventricle contracts, forcing blood into the aorta, the bicuspid valve closes the aperture to the left atrium, thereby preventing any backflow of blood. The valve reopens to allow blood to flow from the atrium into the ventricle. Compare tricuspid valve.

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biennial

A plant that requires two growing seasons to complete its life cycle. During the first year it builds up food reserves, which are used during the second year in the production of flowers and seeds. Examples are carrot and parsnip.

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bilateral symmetry

A type of arrangement of the parts and organs of an animal in which the body can be divided into two halves that are mirror images of each other along one plane only (usually passing through the midline at right angles to the dorsal and ventral surfaces). Bilaterally symmetrical animals are characterized by a type of movement in which one end of the body always leads. In botany this type of symmetry is usually called zygomorphy when applicable to flowers (e.g. foxglove and antirrhinum flowers are zygomorphic). Compare radial symmetry.

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bile (gall)

A bitter-tasting greenish-yellow alkaline fluid produced by the liver, stored in the gall bladder, and secreted into the duodenum of vertebrates. It assists the digestion and absorption of fats by the action of bile salts , which chemically reduce fatty substances and decrease the surface tension of fat droplets so that they are broken down and emulsified. Bile may also stimulate gut muscle contraction (peristalsis). Bile also contains the bile pigments , bilirubin and biliverdin , which are produced by the breakdown of the blood pigment haemoglobin.

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bile duct

The tube through which bile passes from the liver or (when present) the gall bladder to the duodenum.

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bilirubin

See bile.

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biliverdin

See bile.

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binary fission

See fission.

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binding site

An area on the surface of a molecule that combines with another molecule. Binding sites on enzymes can be active sites or allosteric sites.

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binocular vision

The ability, found only in animals with forward-facing eyes, to produce a focused image of the same object simultaneously on the retinas of both eyes. This permits three-dimensional vision and contributes to distance judgment.

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binomial nomenclature

The system of naming organisms using a two-part Latinized (or scientific) name that was devised by the Swedish botanist Linnaeus (Carl Linné); it is also known as the Linnaean system . The first part is the generic name (see genus), the second is the specific epithet or name (see species). The Latin name is usually printed in italics, starting with a capital letter. For example, in the scientific name of the common frog, Rana temporaria, Rana is the generic name and temporaria the specific name. The name of the species may be followed by an abbreviated form of the name of its discoverer; for example, the common daisy is Bellis perennis L. (for Linnaeus). There are several International Codes of Taxonomic Nomenclature that lay down the rules for naming organisms. See also classification; taxonomy.

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bioaccumulation

An increase in the concentration of chemicals, such as pesticides, in organisms that live in environments contaminated by a wide variety of organic compounds. These compounds are not usually decomposed in the environment (i.e. they are not biodegradable) or metabolized by the organisms, so that their rate of absorption and storage is greater than their rate of excretion. The chemicals are normally stored in fatty tissues. DDT is known as a persistent pesticide , as it is not easily broken down and bioaccumulates along food chains, so that increasing concentrations occur in individual organisms at each trophic level.

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bioactivation

A metabolic process in which a product that is chemically reactive is produced from a relatively inactive precursor. See also prodrug.

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bioassay (biological assay)

A controlled experiment for the quantitative estimation of a substance by measuring its effect in a living organism. For example, the amount of the plant hormone auxin can be estimated by observing its effect on the curvature of oat coleoptiles – the concentration of the hormone is proportional to the curvature of the coleoptile.

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biochemical evolution (molecular evolution)

The changes that occur at the molecular level in organisms over a period of time. These range from deletions, additions, or substitutions of single nucleotides, through the rearrangement of parts of genes, to the duplication of entire genes or even whole genomes. Such mutations may result in functional changes to the proteins encoded by the genes, or even the evolution of novel genes and proteins.

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biochemical fuel cell

A system that exploits biological reactions for the conversion of biomass (chemical energy) to electricity (electrical energy). One potential application is the generation of electricity from industrial waste and sewage. Methyltrophic organisms (i.e. organisms that use methane or methanol as their sole carbon sources) are being investigated for their potential use in biochemical fuel cells.

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biochemical oxygen demand (BOD)

The amount of oxygen taken up by microorganisms that decompose organic waste matter in water. It is therefore used as a measure of the amount of certain types of organic pollutant in water. BOD is calculated by keeping a sample of water containing a known amount of oxygen for five days at 20°C. The oxygen content is measured again after this time. A high BOD indicates the presence of a large number of microorganisms, which suggests a high level of pollution.

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biochemical taxonomy

See molecular systematics.

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biochemistry

The study of the chemistry of living organisms, especially the structure and function of their chemical components (principally proteins, carbohydrates, lipids, and nucleic acids). Biochemistry has advanced rapidly with the development, from the mid-20th century, of such techniques as chromatography, X-ray diffraction, radioisotopic labelling, and electron microscopy. Using these techniques to separate and analyse biologically important molecules, the steps of the metabolic pathways in which they are involved (e.g. glycolysis and the Krebs cycle) have been determined. This has provided some knowledge of how organisms obtain and store energy, how they manufacture and degrade their biomolecules, how they sense and respond to their environment, and how all this information is carried and expressed by their genetic material. Biochemistry forms an important part of many other disciplines, especially physiology, nutrition, and genetics, and its discoveries have made a profound impact in medicine, agriculture, industry, and many other areas of human activity.

BIOCHEMISTRY

1833

French chemist Anselme Payen (1795–1871) discovers diastase (the first enzyme to be discovered).

1836

Theodor Schwann discovers the digestive enzyme pepsin.

c.1860

Louis Pasteur demonstrates fermentation is caused by ‘ferments’ in yeasts and bacteria.

1869

German biochemist Johann Friedrich Miescher (1844–95) discovers nucleic acid.

1877

Pasteur's ‘ferments’ are designated as enzymes.

1890

German chemist Emil Fischer (1852–1919) proposes the ‘lock-and-key’ mechanism to explain enzyme action.

1901

Japanese chemist Jokichi Takamine (1854–1922) isolates adrenaline (the first hormone to be isolated).

1903

German biologist Eduard Buchner (1860–1917) discovers the enzyme zymase (causing fermentation).

1904

British biologist Arthur Harden (1865–1940) discovers coenzymes.

1909

Russian-born US biochemist Phoebus Levene (1869–1940) identifies ribose in RNA.

1921

Canadian physiologist Frederick Banting (1891–1941) and US physiologist Charles Best (1899–1978) isolate insulin.

1922

Alexander Fleming discovers the enzyme lysozyme.

1925

Russian-born British biologist David Keilin (1887–1963) discovers cytochrome.

1926

US biochemist James Sumner (1877–1955) crystallizes urease (the first enzyme to be isolated).

1929

German chemist Hans Fischer (1881–1945) determines the structure of haem (in haemoglobin).

K. Lohman isolates ATP from muscle.

1930

US biochemist John Northrop (1891–1987) isolates the enzyme pepsin.

1932

Swedish biochemist Hugo Theorell (1903–82) isolates the muscle protein myoglobin.

1937

Hans Krebs discovers the Krebs cycle.

1940

German-born US biochemist Fritz Lipmann (1899–1986) proposes that ATP is the carrier of chemical energy in many cells.

1943

US biochemist Britton Chance (1913– ) discovers how enzymes work (by forming an enzyme–substrate complex).

1952

US biologist Alfred Hershey (1908– ) proves that DNA carries genetic information.

1953

Francis Crick and James Watson discover the structure of DNA.

1955

Frederick Sanger discovers the amino acid sequence of insulin.

1956

US biochemist Arthur Kornberg (1918– ) discovers DNA polymerase.

US molecular biologist Paul Berg (1926– ) identifies the nucleic acid later known as transfer RNA.

1957

British biologist Alick Isaacs (1921–67) discovers interferon.

1959

Austrian-born British biochemist Max Perutz (1914– ) determines the structure of haemoglobin.

1960

South African-born British molecular biologist Sydney Brenner (1927– ) and French biochemist François Jacob (1920– ) discover messenger RNA.

1961

British biochemist Peter Mitchell (1920–92) proposes the chemiosmotic theory.

Brenner and Crick discover that the genetic code consists of a series of base triplets.

1969

US biochemist Gerald Edelman (1929– ) discovers the amino acid sequence of immunoglobulin G.

1970

US virologists Howard Temin (1934–94) and David Baltimore (1938– ) discover the enzyme reverse transcriptase.

1970

US molecular biologist Hamilton Smith (1931– ) discovers restriction enzymes.

1973

US biochemists Stanley Cohen (1935– ) and Herbert Boyer (1936– ) use restriction enzymes to produce recombinant DNA.

1977

Sanger determines the complete base sequence of DNA in bacteriophage X174.

1985

British biochemist Alec Jeffreys (1950– ) devises DNA fingerprinting.

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biocoenosis

All the organisms living in a particular place at a particular time. It is the equivalent of a biome, and is used especially in the ecological literature of eastern Europe.

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biodegradable

See pollution.

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biodiversity (biological diversity)

The existence of a wide variety of species (species diversity ) or other taxa of plants, animals, and microorganisms in a natural community or habitat, or of communities within a particular environment (ecological diversity ), or of genetic variation within a species (genetic diversity ). The maintenance of a high level of biodiversity is important for the stability of ecosystems. Certain habitats, especially rainforests, have a rich species diversity, which is threatened by the continued destruction of habitats (see deforestation; desertification). Such ecosystems typically support large numbers of rare species, and population sizes of individual species tend to be small; they are therefore especially vulnerable to habitat destruction. Biodiversity in natural habitats also represents an important pool of species and genetic material of potential use to human societies. For example, wild plants continue to be used as a source of new drugs and other products, and the development of new strains and varieties of crop plants with increased disease resistance usually depends on incorporating genetic material from wild plants.

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biodiversity gradient

The gradual reduction in biomass and species numbers that occurs with increasing latitude. There are several theories to explain why life is more abundant in the tropics than in cooler regions. The simplest explanation is that the greater surface area of the planet at the equator, compared to the poles, provides more space in which species can evolve. Another theory suggests that the relative environmental stability of the tropics enables species to specialize to a greater extent, so that more can be packed into any given ecosystem. Further, the greater input of solar energy in the tropics increases available resources, resulting in greater biomass and population sizes compared to colder regions.

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bioelement

Any chemical element that is found in the molecules and compounds that make up a living organism. In the human body the most common bioelements (in decreasing order of occurrence) are oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Other bioelements include sodium, potassium, magnesium, and copper. See essential element.

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bioenergetics

The study of the flow and the transformations of energy that occur in living organisms. Typically, the amount of energy that an organism takes in (from food or sunlight) is measured and divided into the amount used for growth of new tissues; that lost through death, wastes, and (in plants) transpiration; and that lost to the environment as heat (through respiration).

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bioengineering

1 The use of artificial tissues, organs, and organ components to replace parts of the body that are damaged, lost, or malfunctioning, e.g. artificial limbs, heart valves, and heart pacemakers.

2 The application of engineering knowledge to medicine and zoology.
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biofeedback

The technique whereby a subject can learn to control certain body functions, such as heart rate or blood pressure, that are usually unconsciously regulated by the autonomic nervous system. It is facilitated by the use of monitoring devices, such as pulse monitors, electroencephalographs, and electromyographs, and can be useful in treating high blood pressure, migraine, epilepsy, and other disorders.

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biofuel

A gaseous, liquid, or solid fuel that contains an energy content derived from a biological source. The organic matter that makes up living organisms provides a potential source of trapped energy that is beginning to be exploited to supply the ever-increasing energy demand around the world. An example of a biofuel is rapeseed oil, which can be used in place of diesel fuel in modified engines. The methyl ester of this oil, rapeseed methyl ester (RME), can be used in unmodified diesel engines and is sometimes known as biodiesel . Other biofuels include biogas and gasohol.

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biogas

A mixture of methane and carbon dioxide resulting from the anaerobic decomposition of such waste materials as domestic, industrial, and agricultural sewage. The decomposition is carried out by methanogenic bacteria (see methanogen); these obligate anaerobes produce methane, the main component of biogas, which can be collected and used as an energy source for domestic processes, such as heating, cooking, and lighting. The production of biogas is carried out in special digesters , which are widely used in China and India. As well as providing a source of fuel, these systems also enable sewage, which contains pathogenic bacteria, to be digested, thereby removing the danger to humans that could otherwise result from untreated domestic and agricultural waste.

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biogenesis

The principle that a living organism can only arise from other living organisms similar to itself (i.e. that like gives rise to like) and can never originate from nonliving material. Compare spontaneous generation.

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biogeochemical cycle (nutrient cycle)

The cyclical movement of elements between living organisms (the biotic phase) and their nonliving (abiotic) surroundings (e.g. rocks, water, air). Examples of biogeochemical cycles are the carbon cycle, nitrogen cycle, oxygen cycle, phosphorus cycle, and sulphur cycle.

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biogeography

The branch of biology that deals with the geographical distribution of plants and animals. See plant geography; zoogeography.

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biolistics

A technique for introducing genetic material into living cells, especially plant cells, in which DNA-coated microscopic particles are fired into the cell using a special gun. The microprojectiles, typically 1 µm in diameter, are accelerated to high velocity by a specially modified small-calibre gun and penetrate the cell walls and plasma membrane with minimal damage. Hence novel DNA can be inserted into intact plant cells (see genetically modified organisms). The technique can also be used for delivering DNA through the double membranes of intact chloroplasts and mitochondria.

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biological clock

The mechanism, presumed to exist within many animals and plants, that produces regular periodic changes in behaviour or physiology. Biological clocks underlie many of the circadian, tidal, and annual rhythms seen in organisms (e.g. hibernation in animals). They continue to run even when conditions are kept artificially constant, but eventually drift out of step with the natural environment without the specific signals that normally keep them synchronized.

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biological control

The control of pests by biological (rather than chemical) means. This may be achieved, for example, by breeding disease-resistant crops or by introducing a natural enemy of the pest, such as a predator or a parasite. This technique, which may offer substantial advantages over the use of pesticides or herbicides, has been employed successfully on a number of occasions. Examples include the control of the prickly pear cactus (Opuntia) in Australia by introducing the cactus moth (Cactoblastis cactorum), whose caterpillars feed on the plant's growing shoots, and the use of the ladybird to prey upon the scale insect (Icerya), which kills citrus fruit trees. Insect pests have also been subjected to genetic control, by releasing large numbers of males of the pest species that have been sterilized by radiation: infertile matings subsequently cause a decline in the pest population. This method has been used to control the screw worm fly (Cochliomyia hominivora), which lays its eggs in the open wounds of domestic cattle. Biological control is considered to reduce a number of the problems associated with chemical control using pesticides, but care should be taken to avoid upsetting the natural ecological balance; for example, a particular predator may also destroy harmless or beneficial species.

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biological rhythm

See biorhythm.

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biological warfare

The military use of microorganisms, such as bacteria, viruses, fungi, and other microorganisms, including the agents for anthrax and botulism, to induce disease or death among humans, livestock, and crop plants. Though officially banned in most countries, research continues with the aim of developing virulent strains of existing microorganisms, using genetic engineering and other techniques.

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biology

The study of living organisms, which includes their structure (gross and microscopical), functioning, origin and evolution, classification, interrelationships, and distribution.

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bioluminescence

The emission of light without heat by living organisms. The phenomenon occurs in glow-worms and fireflies, bacteria and fungi, and in many deep-sea fish (among others); in animals it may serve as a means of protection (e.g. by disguising the shape of a fish) or species recognition or it may provide mating signals. The light is produced during the oxidation of a compound called luciferin (the composition of which varies according to the species), the reaction being catalysed by an enzyme, luciferase . Bioluminescence may be continuous (e.g. in bacteria) or intermittent (e.g. in fireflies). See also photophore.

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biomarker

A normal metabolite that, when present in abnormal concentrations in certain body fluids, can indicate the presence of a particular disease or toxicological condition. For example, abnormal concentrations of glucose in the blood can be indicative of diabetes mellitus (see insulin).

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biomass

The total mass of all the organisms of a given type and/or in a given area; for example, the world biomass of trees, or the biomass of elephants in the Serengeti National Park. It is normally measured in terms of grams of dry mass per square metre. See also pyramid of biomass.

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biome

A major ecological community or complex of communities that extends over a large geographical area characterized by a dominant type of vegetation. The organisms of a biome are adapted to the climate conditions associated with the region. There are no distinct boundaries between adjacent biomes, which merge gradually with each other. Examples of biomes are tundra, tropical rainforest, taiga, chaparral, grassland (temperate and tropical), and desert.

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biomechanics

The application of the principles of mechanics to living systems, particularly those living systems that have coordinated movements. Biomechanics also deals with the properties of biological materials, such as blood and bone. For example, biomechanics would be used to analyse the stresses on bones in animals, both when the animals are static and when they are moving. Other types of problems in biomechanics include the fluid mechanics associated with swimming in fish and the aerodynamics of birds flying. It is sometimes difficult to perform realistic calculations in biomechanics because of complexity in the shape of animals or the large number of parts involved (for example, the large number of muscles involved in the movement of a human leg).

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biomolecule

Any molecule that is involved in the maintenance and metabolic processes of living organisms (see metabolism). Biomolecules include carbohydrate, lipid, protein, nucleic acid, and water molecules; some biomolecules are macromolecules.

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biophysics

The study of the physical aspects of biology, including the application of physical laws and the techniques of physics to study biological phenomena.

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biopoiesis

The development of living matter from complex organic molecules that are themselves nonliving but self-replicating. It is the process by which life is assumed to have begun. See origin of life.

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biopsy

The removal of a small section of tissue from a potentially diseased organ or tissue in a living organism. The biopsied tissue is usually analysed by microscopic techniques in order to identify the nature of the disease.

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bioreactor (industrial fermenter)

A large stainless steel tank used to grow producer microorganisms in the industrial production of enzymes and other chemicals. After the tank is steam-sterilized, an inoculum of the producer cells is introduced into a medium that is maintained by probes at optimum conditions of temperature, pressure, pH, and oxygen levels for enzyme production. An agitator mixes the medium, which is constantly aerated. It is essential that the culture medium is sterile and contains the appropriate nutritional requirements for the microorganism. When the nutrients have been utilized the product is separated; if the product is an extracellular compound the medium can be removed during the growth phase of the microorganisms, but an intracellular product must be harvested when the batch culture growth stops. Some bioreactors are designed for continuous culture.

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biorhythm (biological rhythm)

A roughly periodic change in the behaviour or physiology of an organism that is generated and maintained by a biological clock. Well-known examples are the annual and circadian rhythms occurring in many animals and plants. The term biorhythm has unfortunately also become associated with a pseudoscientific cult postulating that human physical, emotional, and intellectual behaviour is influenced by three biological cycles with periodicities of exactly 23 days, 28 days, and 33 days respectively. There is no reliable evidence to support this theory.

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biosensor

A device that uses an immobilized agent to detect or measure a chemical compound. The agents include enzymes, antibiotics, organelles, or whole cells. A reaction between the immobilized agent and the molecule being analysed is transduced into an electronic signal. This signal may be produced in response to the presence of a reaction product, the movement of electrons, or the appearance of some other factor (e.g. light). Biosensors are being used increasingly in diagnostic tests: these allow quick, sensitive, and specific analysis of a wide range of biological products, including antibiotics, vitamins, and other important biomolecules (such as glucose), as well as the determination of certain xenobiotics, such as synthetic organic compounds.

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biosphere

The whole of the region of the earth's surface, the sea, and the air that is inhabited by living organisms.

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biostratigraphy

The characterization of rock strata on the basis of the fossils they contain. This involves identifying and establishing the distribution and succession of various fossil groups in order to define biozones , containing particular fossils or fossil assemblages that can generally be correlated with rock strata of a particular type in different locations. Ideally, a fossil used in biostratigraphical zoning has a limited range over geological time, so its occurrence is restricted to rock strata of a fairly narrow vertical range in the sequence. For example, the succession of numerous different ammonite species provides an important means of zoning rocks of the Mesozoic era throughout the world. Biozones thus form the basic biostratigraphy units. There are several types: for example, an assemblage zone is defined by the coincident and overlapping ranges of a particular group of fossil taxa, whereas an acme zone is defined by the exceptional abundance of one group or species.

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biosynthesis

The production of molecules by a living cell, which is the essential feature of anabolism.

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biosystematics

See systematics.

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biota

All the organisms living in a particular region, including plants, animals, and microorganisms. See also community.

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biotechnology

The development of techniques for the application of biological processes to the production of materials of use in medicine and industry. For example, the production of antibiotics, cheese, and wine rely on the activity of various fungi and bacteria. Genetic engineering can modify bacterial cells to synthesize completely new substances, e.g. hormones, vaccines, monoclonal antibodies, etc., or introduce novel traits into plants or animals.

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biotic factor

Any of the factors of an organism's environment that consist of other living organisms and together make up the biotic environment . These factors may affect an organism in many ways; for example, as competitors, predators, parasites, prey, or symbionts. In time, the distribution and abundance of the organism will be affected by its interrelationships with the biotic environment. Compare abiotic factor.

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biotin

A vitamin in the vitamin B complex. It is the coenzyme for various enzymes that catalyse the incorporation of carbon dioxide into various compounds. Adequate amounts are normally produced by the intestinal bacteria in animals although deficiency can be induced by consuming large amounts of raw egg white. This contains a protein, avidin, that specifically binds biotin, preventing its absorption from the gut. Other sources of biotin include cereals, vegetables, milk, and liver.

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biotope

A region that has a characteristic set of environmental conditions and consequently a particular type of fauna and flora (biota).

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biotype

1 A group of individuals within a species that are genetically very similar or identical.

2 A physiological race.
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biozone

See biostratigraphy.

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bipedalism

The ability to maintain a standing position, and to move about, using only two legs. Birds and humans are bipedal. The development of bipedalism was an important factor in hominid evolution.

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bipolar cell

See retina.

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bipolar neurone

A neurone that has two processes, an axon and a dendron, extending in different directions from its cell body. Many sensory neurones are bipolar neurones. Compare multipolar neurone; unipolar neurone.

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biramous appendage

A type of appendage that is characteristic of arthropods of the phylum Crustacea. It forks from the basal protopodite to form two branches, the inner endopodite and the outer exopodite (see illustration). Each of these branches can be composed of either one or more segments. There are many variations on this generalized structure; the branches often possess highly specialized extensions – the epipodite (on the coxopodite), the exite (on the exopodite), and the endite (on the endopodite). Compare uniramous appendage.

A generalized biramous appendage

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birds

See Aves.

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birth

See parturition.

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birth control (contraception)

The intentional avoidance of pregnancy by methods that do not normally hinder sexual activity. The methods used can be `natural' or `artificial'. Natural methods, often used because of religious or moral objections to artificial methods, include the rhythm method , in which sexual intercourse is avoided during times when ovulation occurs; and coitus interruptus , an unreliable method in which the penis is withdrawn from the vagina before ejaculation. The rhythm method requires a monitoring of the woman's menstrual cycle and may be unsuitable in those women with irregular cycles. Artificial methods use devices or other agents (contraceptives ) to prevent pregnancy. They include the condom , a rubber sheath placed over the penis to trap the sperm; and the diaphragm , a rubber cap placed over the cervix. Contraceptives that prevent implantation include the intrauterine device (IUD), a metal or plastic coil placed in the uterus by a doctor (which may cause unacceptable bleeding in some women); and the `morning-after pill', taken within three days after sexual intercourse. Other oral contraceptives prevent ovulation. Sterilization is usually considered to be irreversible but attempts at reversing the process are possible. For casual relationships, or relationships involving a partner whose sexual history is not known, health workers advise the use of condoms with all other forms of contraception as this provides the safest means of reducing the risk of infection by sexually transmitted diseases.

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birth rate (natality)

The rate at which a particular species or population produces offspring. The birth rate of a species is used to measure its fecundity (reproductive capability). It is also an important factor in controlling the size of a population. Compare death rate.

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bisexual

See hermaphrodite.

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biuret test

A biochemical test to detect proteins in solution, named after the substance biuret (H_2NCONHCONH_2), which is formed when urea is heated. Sodium hydroxide is mixed with the test solution and drops of 1% copper(II) sulphate solution are then added slowly. A positive result is indicated by a violet ring, caused by the reaction of peptide bonds in the proteins or peptides. Such a result will not occur in the presence of free amino acids.

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bivalent

(in genetics) See pairing.

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Bivalvia (Pelecypoda; Lamellibranchia)

A class of aquatic molluscs (the bivalves) that include the oysters, mussels, and clams. They are characterized by a laterally flattened body and a shell consisting of two hinged shells (i.e. a bivalved shell). The enlarged gills are covered with cilia and have the additional function of filtering microscopic food particles from the water flowing over them. Bivalves live on the sea bed or lake bottom and are sedentary, so the head and foot are reduced.

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bladder

1 (in anatomy)
a A hollow muscular organ in most vertebrates, also known as the urinary bladder , in which urine is stored before being discharged. In mammals urine is conveyed from the kidneys to the bladder by the ureters and is discharged to the outside through the urethra.
b Any of various other saclike organs in animals for the storage of liquid or gas. See gall bladder; swim bladder.

2 (in botany)
a A modified submerged leaf of certain aquatic insectivorous plants, such as the bladderwort (Utricularia). It forms a hollow with a single opening that is sealed by a valve to trap small aquatic invertebrates after they have been sucked in.
b An air-filled cavity in the thallus of certain seaweeds, such as the bladderwrack (Fucus vesiculosus).
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bladderworm (cysticercus)

A larval stage of some tapeworms (see Cestoda), consisting of a fluid-filled sac containing an inverted scolex. It develops in the muscle of the secondary host and matures into an adult worm in a primary host that eats this infected tissue.

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blastocoel

See blastula.

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blastocyst

See blastula; implantation.

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blastoderm

The layer of cells that surrounds the central cavity (blastocoel) of a blastula. In yolky eggs, such as those of insects, the blastoderm forms a layer covering the yolk. See also imaginal disc.

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blastopore

See archenteron.

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blastula

The stage of development of an animal embryo that results from cleavage of a fertilized egg. This stage generally resembles a hollow ball with the dividing cells (blastomeres ) of the embryo forming a layer (blastoderm ) around a central cavity (blastocoel ). In vertebrates the blastula forms a disc (blastodisc ) on the surface of the yolk. In mammals the blastula stage is known as a blastocyst . See also gastrula.

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blending inheritance

The early theory that assumed that hereditary substances from parents merge together in their offspring. Mendel showed that this does not occur (see Mendel's laws). In breeding experiments an appearance of blending may result from co-dominant alleles (see co-dominance) and polygenes but close study shows that the alleles retain their identity through successive generations. Compare particulate inheritance.

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blind spot

The portion of the retina at which blood vessels and nerve fibres enter the optic nerve. There are no rods or cones in this area, so no visual image can be transmitted from it.

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blood

A fluid body tissue that acts as a transport medium within an animal. It is contained within a blood vascular system and in vertebrates is circulated by means of contractions of the heart. Oxygen and food are carried to tissues, and carbon dioxide and chemical (nitrogenous) waste are transported from tissues to excretory organs for disposal (excretion). In addition blood carries hormones and also acts as a defence system. Blood consists of a liquid (see blood plasma) containing blood cells (see erythrocyte; leucocyte) and platelets (see illustration).

Composition of mammalian blood

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blood–brain barrier

The mechanism that controls the passage of substances from the blood to the cerebrospinal fluid bathing the brain and spinal cord. It takes the form of a semipermeable lipid membrane permitting the passage of solutions but excluding particles and large molecules. This barrier provides the central nervous system with a constant environment, while not interfering with the transport of essential metabolites.

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blood capillary

See capillary.

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blood cell (blood corpuscle)

Any of the cells that are normally found in the blood plasma. These include red cells (see erythrocyte) and white cells (see leucocyte).

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blood clotting (blood coagulation)

The production of a mass of semisolid material at the site of an injury that closes the wound, helping to prevent further blood loss and bacterial invasion. The clot is formed by the action of clotting factors and platelets. The cascade of reactions that culminate in the formation of a blood clot is initiated by thromboplastin , a membrane glycoprotein found in tissue cells. When tissue is damaged, this forms a cell-surface complex that, with phospholipid and calcium ions, converts a plasma glycoprotein, Factor X, to Factor Xa, which in turn converts prothrombin in the blood to its enzymically active form thrombin . Thrombin catalyses the formation of the insoluble protein fibrin from soluble fibrinogen ; the fibrin forms a fibrous network in which blood cells become enmeshed, producing a clot.

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blood groups

The many types into which an individual's blood may be classified, based on the presence or absence of certain antigenic proteins (agglutinogens) on the surface of the red blood cells. Blood of one group contains antibodies in the serum that react against the agglutinogens on the cells of other groups. Incompatibility between groups results in clumping of cells (agglutination), so knowledge of blood groups is important for blood transfusions. In humans, the two most important blood group systems are the ABO system and the system involving the rhesus factor.

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blood pigment

Any one of a group of metal-containing coloured protein compounds whose function is to increase the oxygen-carrying capacity of blood. See respiratory pigment.

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blood plasma

The liquid part of the blood (i.e. excluding blood cells). It consists of water containing a large number of dissolved substances, including proteins, salts (especially sodium and potassium chlorides and bicarbonates), food materials (glucose, amino acids, fats), hormones, vitamins, and excretory materials. See also blood serum; lymph.

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blood platelet

See platelet.

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blood pressure

The pressure exerted by the flow of blood through the major arteries of the body. This pressure is greatest during the contraction of the ventricles of the heart (systolic pressure ; see systole), which forces blood into the arterial system. Pressure falls to its lowest level when the heart is filling with blood (diastolic pressure ; see diastole). Blood pressure is measured in millimetres of mercury using an instrument called a sphygmomanometer . Normal blood pressure for a young average adult human is in the region of 120/80mmHg (the higher number is the systolic blood pressure; the lower number the diastolic blood pressure), but individual variations are common. Abnormally high blood pressure (hypertension ) may be associated with disease or it may occur without an apparent cause.

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blood serum

Blood plasma from which the fibrin and clotting factors have been removed by centrifugation or vigorous stirring, so that it cannot clot. Serum containing a specific antibody or antitoxin may be used in the treatment or prevention of certain infections. Such serum is generally derived from a nonhuman mammal (e.g. a horse).

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blood vascular system

The tissues and organs of an animal that transport blood through the body. In vertebrates it consists of the heart and blood vessels. See vascular system.

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blood vessel

A tubular structure through which the blood of an animal flows. See artery; arteriole; capillary; venule; vein.

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blue-green bacteria (blue-green algae)

See Cyanobacteria.

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B lymphocyte

See B cell.

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BOD

See biochemical oxygen demand.

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body cavity

The internal cavity of the body of an animal, which is present in most invertebrates and all vertebrates and contains the major organs. The body cavity of vertebrates and many invertebrates is the coelom. In vertebrates the body cavity is divided by a transverse septum just posterior to the heart into the abdominal and thoracic cavities (see abdomen; thorax). In mammals the septum is the diaphragm.

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body fluid

Any of the fluids found within animals, including blood, lymph, tissue fluid, urine, bile, sweat, and synovial fluid. Body fluids are generally involved with the processes of transport, excretion, or lubrication. They allow the distribution of oxygen and nutrients to the tissues and organs and the transport of waste products from the tissues, enabling their elimination from the body.

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bog

See hydrosere.

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Bohr effect

The effect of pH on the dissociation of oxygen from haemoglobin, first discovered by the Danish physiologist Christian Bohr (1855–1911). An increase in carbon dioxide concentration makes the blood more acidic and decreases the efficiency of the uptake of oxygen by haemoglobin molecules. This shifts the oxygen dissociation curve to the right and increases the tendency of haemoglobin to release oxygen (see haemoglobinic acid). Thus in actively respiring tissues, where the concentration of carbon dioxide in the blood is high, haemoglobin readily releases its oxygen, while in the lungs, where blood carbon dioxide is low (due to its continual diffusion into the alveoli), haemoglobin readily binds oxygen.

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bolus

The ball of chewed food bound together with saliva that is formed in the mouth by the action of the tongue. The bolus is shaped to a size that allows it to pass into the oesophagus after being swallowed (see deglutition).

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bomb calorimeter

An apparatus used for measuring heats of combustion. It consists of a strong container in which the sample is sealed with excess oxygen and ignited electrically. The heat of combustion at constant volume can be calculated from the resulting rise in temperature. The apparatus is used to calculate the calorific values of foods and the energy content of a sample of biomass, required for constructing a pyramid of energy.

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bone

The hard connective tissue of which the skeleton of most vertebrates is formed. It comprises a matrix of collagen fibres (30%) impregnated with bone salts (70%), mostly calcium phosphate (hydroxyapatite, Ca_10(PO_4)6(OH)2), in which are embedded bone cells: osteoblasts (which secrete the matrix) and osteocytes. Bone generally replaces embryonic cartilage and is of two sorts – compact bone and spongy bone. The outer compact bone is formed as concentric layers (lamellae ) that surround small holes (Haversian canals): see illustration. The inner spongy bone is chemically similar but forms a network of bony bars. The spaces between the bars may contain bone marrow or (in birds) air for lightness. See also cartilage bone; membrane bone; periosteum.

Structure of compact bone

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bone marrow

A soft tissue contained within the central cavity and internal spaces of a bone. At birth and in young animals the marrow of all bones is concerned with the formation of blood cells: it contains haemopoietic tissue and is known as red marrow . In mature animals the marrow of the long bones ceases producing blood cells and is replaced by fat, being known as yellow marrow .

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bony fishes

See Osteichthyes.

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bony labyrinth

See labyrinth.

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borax carmine

A red dye, used in optical microscopy, that stains nuclei and cytoplasm pink. It is frequently used to stain large pieces of animal tissue.

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botany

The scientific study of plants, including their anatomy, morphology, physiology, biochemistry, taxonomy, cytology, genetics, ecology, evolution, and geographical distribution.

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botulinum toxin

A toxin produced by the bacterium Clostridium botulinum, which can cause fatal food poisoning. It is the most toxic substance known to man.

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bovine spongiform encephalopathy (BSE)

A degenerative disease of the brain that affects cattle and is caused by an abnormal form of a cellular protein (see prion). Known colloquially as `mad cow disease', it results in a build-up of amyloid tissue in the brain. The infective agent can be transmitted to other cattle via feed containing offal derived from infected animals. It can also, under certain circumstances, be transmitted to other species. Humans infected by eating contaminated beef or beef products have developed a variant form of Creutzfeldt–Jakob disease.

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Bowman's capsule (renal capsule)

The cup-shaped end of a kidney nephron. Its epithelium contains podocytes, which facilitate the passage of glomerular filtrate from the blood into the nephron. It is named after its discoverer, the British physician Sir William Bowman (1816–92).

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bp

Symbol for base pair(s), used as a unit at the molecular level for measuring distances along a duplex polynucleotide and corresponding to the number of paired bases in a particular segment of DNA (or duplex RNA). See also kilobase.

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Brachiopoda

A phylum of marine invertebrates – the lamp shells. They live in shallow waters, attached to a firm substratum by means of a flexible stalk (peduncle ), and are protected by a bivalved shell consisting of dorsal and ventral valves. A food-gathering lophophore protrudes from the shell. Brachiopods thrived in Palaeozoic times but are now much less numerous; living brachiopods include Terebratella, with an articulated shell; and Lingula, in which the shell valves are held together by muscles only.

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bract

A modified leaf with a flower or inflorescence in its axil. Bracts are often brightly coloured and may be mistaken for the petals of a flower. For example the showy `flowers' of poinsettia and Bougainvillea are composed of bracts; the true flowers are comparatively inconspicuous. See also involucre.

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bracteole

A reduced leaf that arises from the stalk of an individual flower.

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bradycardia

A decrease in heart rate. See tachycardia.

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bradykinin

See kinin.

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bradymetabolism

Metabolism that is sustained at a relatively low rate. It is characteristic of `cold-blooded' animals (see ectotherm, and generally excludes any specific heat-generating mechanism. Compare tachymetabolism.

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brain

1 The enlarged anterior part of the vertebrate central nervous system, which is encased within the cranium of the skull. Continuous with the spinal cord, the brain is surrounded by three membranes (see meninges) and bathed in cerebrospinal fluid, which fills internal cavities (ventricles). It functions as the main coordinating centre for nervous activity, receiving information (in the form of nerve impulses) from sense organs, interpreting it, and transmitting `instructions' to muscles and other effectors. It is also the seat of intelligence and memory. The embryonic vertebrate brain is in three sections (see forebrain; hindbrain; midbrain), which become further differentiated during development into specialized regions. The main parts of the adult human brain are a highly developed cerebrum in the form of two cerebral hemispheres, a cerebellum, medulla oblongata, and hypothalamus (see illustration).

The human brain

2 A concentration of nerve ganglia at the anterior end of an invertebrate animal.
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brain death

The permanent absence of vital functions of the brain, which is marked by cessation of breathing and other reflexes (including the pupillary reflex) controlled by the brainstem and by a zero reading on an electroencephalogram. Organs may be removed for transplantation when brain death is established, which may not necessarily be associated with permanent absence of heart beat.

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brainstem

The part of the brain comprising the medulla oblongata, the midbrain, and the pons. It resembles and is continuous with the spinal cord. The midbrain controls and integrates reflex activities (such as respiration) that originate in higher centres of the brain via a network of nerve pathways (the reticular formation ).

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branchial

Of or relating to the gills. Most fish possess five pairs of branchial arches , skeletal supports for the gills, which are contained in a cavity known as the branchial chamber . Water is drawn into the branchial chamber through narrow gill slits (or branchial clefts ) in the wall of the pharynx and passes over the gills, where gaseous exchange takes place. Blood is transported to and from the gills in afferent and efferent branchial arteries , respectively.

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brassin (brassinosteroid)

Any of a group of steroid derivatives that occur at very low concentrations in plant tissues and may have hormone-like effects. Brassins have been found in pollen, leaves, stems, and flowers from various species. Extracts behave in a manner similar to auxins, for example stimulating elongation of hypocotyl and epicotyl tissue from seedlings when applied at concentrations as low as 10–7 moldm–3. However, their precise role in plants remains unknown.

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breathing

See expiration; inspiration; respiratory movement.

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breed

A domesticated variety of an animal or, rarely, a cultivated variety of plant. Cultivated plants are more often simply called varieties or, more correctly, cultivars. Examples of animal breeds are Friesian cattle and Shetland sheepdogs.

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breeding

The process of sexual reproduction and bearing offspring. Selective breeding of both plants and animals is used in agriculture to produce offspring that possess the beneficial characters of both parents (see also artificial insemination). Inbreeding is the production of homozygous phenotypically uniform offspring by mating between close relatives. Plants that self-fertilize, such as wheat and tomatoes, are inbreeders. Outbreeding is the production of heterozygous phenotypically variable offspring by mating between unrelated organisms.

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breeding season (mating season)

A specific season of the year in which many animals, including mammals and birds, mate, which ensures that offspring are produced only at a certain time of the year. This timing is important as it enables animals to give birth at a time of the year when environmental conditions and food supply are at their optimum. The breeding season of most animals is in the spring or summer. The stimulus to mate is the result of a photoperiodic response (see photoperiodism), which is thought to be controlled by day length.

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brewing

The process by which beer is made. Fermentation of sugars from barley grain by the yeasts Saccharomyces cerevisiae and S. uvarum (or S. carlsbergenesis) produces alcohol (ethanol). In the first stage the barley grain is soaked in water, a process known as malting . The grain is then allowed to germinate and the natural enzymes of the grain (the amylases and the maltases) convert the starch to maltose and then to glucose. The next stage is kilning or roasting , in which the grains are dried and crushed. The colour of a beer depends on the temperature used for this process: the higher the temperature, the darker the beer. In the next stage, mashing , the crushed grain is added to water at a specific temperature and any remaining starch is converted to sugar; the resultant liquid is the raw material of brewing, called wort . The yeast is then added to the wort to convert the sugar to alcohol, followed by hops, which give beer its characteristic flavour. Hops are the female flowers of the vine Humulus lupulus; they contain resins (humulones, cohumulones, and adhumulones) that give beer its distinctive bitter taste.

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bronchiole

A fine respiratory tube in the lungs of reptiles, birds, and mammals. It is formed by the subdivision of a bronchus and in reptiles and mammals it terminates in a number of alveoli.

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bronchus (bronchial tube)

(pl. bronchi) One of the major air tubes in the lung. The trachea divides into two main bronchi, one for each lung, which split into smaller bronchi and then into bronchioles. The walls of the bronchi are stiffened by rings of cartilage.

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Brown, Robert

(1773–1858)
British botanist, born in Scotland. After serving as an army medical officer he met botanist Joseph Banks (1743–1820) in 1798. Three years later Banks recommended him as naturalist on a survey of the Australian coast, during which he collected 4000 plant specimens; it took him five years to classify them. During this work he was the first to distinguish between gymnosperms and angiosperms. Then in 1827, while observing pollen grains in water, he discovered Brownian movement.

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brown algae

See Phaeophyta.

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brown fat

A darker coloured region of adipose tissue found in newborn and hibernating animals (in which it may also be called the hibernating gland ). Compared to normal white fat, deposits of brown fat are more richly supplied with blood vessels and have a higher proportion of unsaturated fatty acids. They can also be more rapidly converted to heat energy, especially during arousal from hibernation and during cold stress in young animals. Since the deposits are strategically placed near major blood vessels, the heat they generate warms the blood returning to the heart.
It has been said that some types of obesity in humans may be linked to a lack of brown fat in affected individuals: in normal individuals, an intake of excess food is converted into brown fat and rapidly metabolized.

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Brownian movement

The continuous random movement of microscopic solid particles (of about 1 micrometre in diameter) when suspended in a fluid medium. First observed by Robert Brown in 1827 when studying pollen particles, it was originally thought to be the manifestation of some vital force. It was later recognized to be a consequence of bombardment of the particles by the continually moving molecules of the liquid. The smaller the particles the more extensive is the motion. It can be observed in the particles of a colloidal solution and in the cytoplasm and nucleoplasm of dead cells.

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Brunner's glands (duodenal glands)

Glands in the submucosa of the duodenum that secrete mucus and an alkaline fluid that neutralizes the acidic chyme leaving the stomach. They are named after Swiss anatomist J. C. von Brunner (1653–1727).

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brush border

A region of surface epithelium that possesses densely packed microvilli (see microvillus), rather like the bristles of a brush. This greatly increases the surface area of the epithelium and facilitates the absorption of materials. Brush borders are found in the convoluted tubules of the kidney and in the lining of the small intestine.

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Bryophyta

A phylum of simple plants possessing no vascular tissue and rudimentary rootlike organs (rhizoids). They grow in a variety of damp habitats, from fresh water to rock surfaces. Some use other plants for support. Mosses show a marked alternation of generations between gamete-bearing forms (gametophytes) and spore-bearing forms (sporophytes): they possess erect or prostrate leafy stems (the gametophyte generation, which is haploid); these give rise to leafless stalks bearing capsules (the sporophyte generation, which is diploid), the latter being dependent on the former for water and nutrients. Spores formed in the capsules are released and grow to produce new plants.
Formerly, this phylum also included the liverworts and hornworts, now regarded as separate phyla (see Hepatophyta; Anthocerophyta) and the mosses were classified as a class (Musci) of the Bryophyta. The term `bryophytes' is still used informally to refer to members of all three phyla.

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Bryozoa (Ectoprocta)

A phylum of aquatic, mainly marine, invertebrates comprising the moss animals and sea mats. Bryozoans live in colonies, 50 cm or more across, which are attached to rocks, seaweeds, or shells. The individuals (zooids ) making up the colonies are about 1 mm long and superficially resemble cnidarian polyps, with a mouth surrounded by ciliated tentacles of the lophophore that trap minute particles of organic matter in the water. Some have a horny or calcareous outer skeleton into which the body can be withdrawn. Compare Entoprocta.

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BSE

See bovine spongiform encephalopathy.

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buccal cavity (oral cavity)

The mouth cavity: the beginning of the alimentary canal, which leads to the pharynx and (in vertebrates) to the oesophagus. In vertebrates it is separated from the nasal cavity by the palate. In mammals it contains the tongue and teeth, which assist in the mechanical breakdown of food, and the openings of the salivary gland ducts.

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bud

1 (in botany) A condensed immature shoot with a short stem bearing small folded or rolled leaves. The outer leaves of a bud are often scalelike and protect the delicate inner leaves. A terminal (or apical ) bud exists at the tip of a stem or branch while axillary (or lateral ) buds develop in the axils of leaves. However, in certain circumstances buds can be produced anywhere on the surface of a plant. Some buds remain dormant, but may become active if the terminal bud is removed. It is common gardening practice to remove the terminal buds of some shoots to induce the development of lateral shoots from axillary buds. See also apical dominance.

2 (in biology) An outgrowth from a parent organism that breaks away and develops into a new individual in the process of budding.
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budding

1 (in biology) A method of asexual reproduction in which a new individual is derived from an outgrowth (bud ) that becomes detached from the body of the parent. In animals the process is also called gemmation ; it is common in cnidarians (e.g. Hydra) and also occurs in some sponges and other invertebrates. Among fungi, budding is characteristic of the yeasts.

2 (in horticulture) A method of grafting in which a bud of the scion is inserted onto the stock, usually beneath the bark.
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buffer

A solution that resists change in pH when an acid or alkali is added or when the solution is diluted. Acidic buffers consist of a weak acid with a salt of the acid. The salt provides the negative ion A–, which is the conjugate base of the acid HA. An example is carbonic acid and sodium hydrogencarbonate, in which molecules H_2CO_3 and ions HCO_3– are present. When acid is added most of the extra protons are removed by the base:
HCO_3– + H+ H_2CO_3

When base is added, most of the extra hydroxide ions are removed by reaction with undissociated acid:
OH– + H_2CO_3 HCO_3– + H_2O

Thus, the addition of acid or base changes the pH very little. Basic buffers have a weak base and a salt of the base (to provide the conjugate acid). An example is ammonia solution with ammonium chloride.
Natural buffers occur in living organisms, where the biochemical reactions are very sensitive to change in pH (see acid–base balance). The main natural buffers are H_2CO_3/HCO_3– and H_2PO_4–/HPO_42– (see also haemoglobinic acid). Buffer solutions are also used in the laboratory (e.g. to keep microscopical preparations at their original pHs in order to prevent the formation of artefacts), in medicine (e.g. in intravenous injections), in agriculture, and in many industrial processes (e.g. dyeing, fermentation processes, and the food industry).

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bugs

See Hemiptera.

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bulb

An underground plant organ that enables a plant to survive from one growing season to the next. It is a modified shoot with a short flattened stem. A terminal bud develops at the centre of its upper surface, surrounded by swollen leaf bases that contain food stored from the previous growing season. Papery brown scale leaves cover the outside of the bulb. The stored food is used in the growing season when the terminal bud produces foliage leaves and flowers. The new leaves photosynthesize and some of the manufactured food passes into the leaf bases forming a new bulb (see illustration). If more than one bud develops, then additional bulbs form, resulting in vegetative propagation. Examples of bulb-forming plants are daffodil, onion, and tulip. Compare corm.

Development of a bulb

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bulbil

A small bulblike organ that may develop in place of a flower, from an axillary bud, or at the base of a stem in certain plants. If it becomes detached it develops into a new plant.

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bulbourethral glands

See Cowper's glands.

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bulla

The rounded hollow projection of bone from the skull that encloses the middle ear in mammals.

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bundle of His

The specialized cardiac muscle fibres in the mammalian heart that receive electrical stimuli from the atrioventricular node and transmit them throughout the network of Purkyne fibres. This allows the excitation to reach all parts of the ventricles rapidly and initiates a wave of contraction to expel blood into the aorta and pulmonary artery. The fibres are named after Swiss anatomist Wilhelm His (1831–1904).

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bundle sheath cells

A layer of cells in plant leaves and stems that forms a sheath surrounding the vascular bundles. In C_4 plants (see C4 pathway) the bundle sheath cells contain chloroplasts and are the site of the Calvin cycle. The initial fixation of carbon dioxide to form malic acid takes place in the palisade mesophyll cells, which in C_4 plants form a circle around the bundle sheath. This arrangement, known as Kranz anatomy or structure (after the German Kranz, wreath), ensures that the palisade cells are in close contact with the bundle sheath cells so that the malic acid can easily pass to the bundle sheath. It also means that the products of photosynthesis can be quickly transferred from the bundle sheath to the adjacent phloem tissue for transport to other parts of the plant.

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Burnet, Sir Frank Macfarlane

(1899–1985)
Australian virologist, who spent his working life at the Walter and Eliza Hall Institute in Melbourne. In the early 1930s he developed a method of growing influenza virus in chick embryos. He later discovered that immunological tolerance required repeated exposure to the antigen. For this work he shared the 1960 Nobel Prize for physiology or medicine with Sir Peter Medawar. He also proposed the clonal selection theory.

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bursa

(pl. bursae) A sac of fibrous connective tissue lined with synovial membrane and filled with synovial fluid. Bursae are found between bones and other tissues, such as skin, ligaments, tendons, and muscles, where they reduce friction when one part moves over another.

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butanedioic acid

See succinate.

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butterflies

See Lepidoptera.

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buttress root

See prop root.

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C

caecum

A pouch in the alimentary canal of vertebrates between the small intestine and colon. The caecum (and its appendix) is large and highly developed in herbivorous animals (e.g. rabbits and cows), in which it contains a large population of bacteria essential for the breakdown of cellulose. In humans the caecum is a vestigial organ and is poorly developed.

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Cainozoic

See Cenozoic.

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calciferol

See vitamin D.

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calcitonin (thyrocalcitonin)

A hormone in vertebrates that lowers the concentration of calcium (and phosphate) in the blood. It operates in opposition to parathyroid hormone. Calcitonin is produced by the C cells, which in mammals are located in the thyroid gland.

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calcium

Symbol Ca. A soft grey metallic element that is an essential element for living organisms, being required for normal growth and development. In animals it is an important constituent of bones and teeth and is present in the blood, being required for muscle contraction and other metabolic processes. In plants it is a constituent (in the form of calcium pectate) of the middle lamella.

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calcium cyclamate

An organic calcium salt, (C_6H_11NHSO_3)Ca.2H_2O, that has a very sweet taste and was formerly used as a sweetening agent. Its use was banned in the UK in 1969 as it was suspected of causing cancer.

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calliclone

A colony derived from a single plant cell that has been removed from a suspended culture and plated out in a semisolid medium.

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callus

1 (in botany) A protective tissue, consisting of parenchyma cells, that develops over a cut or damaged plant surface. Callus tissue can also be induced to form in cell cultures by hormone treatment.

2 (in pathology) A thick hard area of skin that commonly forms on the palms of the hands and soles of the feet as a result of continuous pressure or friction.
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3 (in physiology) Hard tissue formed round bone ends following a fracture, which is gradually converted to new bone.
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calmodulin

A protein, consisting of 148 amino-acid residues, that acts as a receptor for calcium ions in many calcium-regulated processes in both animal and plant cells. Calmodulin mediates reactions catalysed by many enzymes, including adenylate cyclase, phosphorylase kinase, and cyclic nucleotide phosphodiesterase. During the contraction of smooth muscle calmodulin binds calcium ions and subsequently activates the enzyme myosin light chain kinase . This enzyme phosphorylates the head of the myosin molecule, enabling it to bind to actin. Calmodulin may also regulate the functioning of the spindle observed during mitosis.

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calorie

The quantity of heat required to raise the temperature of 1 gram of water by 1°C (1K). The calorie, a c.g.s. unit, is now largely replaced by the joule, an SI unit. 1 calorie = 4.1868 joules.

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Calorie (kilogram calorie; kilocalorie)

1000 calories. This unit is still in limited use in estimating the energy value of foods, but is obsolescent.

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calorific value

The heat per unit mass produced by complete combustion of a given substance. Calorific values are used to express the energy values of fuels; usually these are expressed in megajoules per kilogram (MJkg–1). They are also used to measure the energy content of foodstuffs; i.e. the energy produced when the food is oxidized in the body. The units here are kilojoules per gram (kJg–1), although Calories (kilocalories) are often still used in nontechnical contexts.

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calorimeter

Any of various devices used to measure thermal properties, such as calorific value. See bomb calorimeter.

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Calvin, Melvin

(1911–97)
US biochemist. After World War II, at the Lawrence Radiation Laboratory, Berkeley, he investigated the dark reactions of photosynthesis. Using radioactive carbon-14 to label carbon dioxide he discovered the Calvin cycle, for which he was awarded the 1961 Nobel Prize for chemistry.

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Calvin cycle (photosynthetic carbon reduction cycle)

The metabolic pathway of the dark stage of photosynthesis, which occurs in the stroma of the chloroplasts. The pathway was elucidated by Melvin Calvin and his coworkers and involves the fixation of carbon dioxide and its subsequent reduction to carbohydrate. During the cycle, carbon dioxide combines with ribulose bisphosphate, through the mediation of the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco), to form an unstable six-carbon compound that breaks down to form two molecules of the three-carbon compound glycerate 3-phosphate. This is converted to glyceraldehyde 3-phosphate, which is used to regenerate ribulose bisphosphate and to produce glucose and fructose.

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calyptra

1 A layer of cells that covers the developing sporophyte of mosses, liverworts, clubmosses, horsetails, and ferns. In mosses it forms a hood over the capsule and in liverworts it forms a sheath at the base of the capsule stalk.

2 See root cap.
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calyptrogen

The region within the root apical meristem that divides to produce the root cap (calyptra).

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calyx

The sepals of a flower, collectively, forming the outer whorl of the perianth. It encloses the petals, stamens, and carpels and protects the flower in bud. See also pappus.

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CAM

See crassulacean acid metabolism.

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cambium (lateral meristem)

A plant tissue consisting of actively dividing cells (see meristem) that is responsible for increasing the girth of the plant, i.e. it causes secondary growth. The two most important cambia are the vascular (or fascicular ) cambium and the cork cambium. The vascular cambium occurs in the stem and root; it divides to produce secondary xylem and secondary phloem (new food- and water-conducting tissues). In mature stems the vascular cambium is extended laterally to form a complete ring: the sections of this ring between the vascular bundles comprises the interfascicular cambium . Compare apical meristem.

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Cambrian

The earliest geological period of the Palaeozoic era. It is estimated to have begun about 570 million years ago and lasted for some 100 million years. During this period marine animals with mineralized shells made their first appearance and Cambrian rocks are the first to contain an abundance of fossils. Cambrian fossils are all of marine animals; they include trilobites, which dominated the Cambrian seas, echinoderms, brachiopods, molluscs, and primitive graptolites (from the mid Cambrian). Trace fossils also provide evidence for a variety of worms.

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camouflage

A high degree of similarity between an animal and its visual environment, which enables it to be disguised or concealed. By blending into the background the animal can elude predators or remain invisible to potential prey. See also cryptic coloration; masquerade; mimicry. Compare warning coloration.

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Canada balsam

A yellow-tinted resin used for mounting specimens in optical microscopy. It has similar optical properties to glass.

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canaliculus

(pl. canaliculi) A very small channel that occurs between the cells of the liver and bone. In the liver the bile canaliculi carry bile to the bile ducts; in bone, canaliculi connect lacunae, the cavities containing bone cells.

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cancer

Any disorder of cell growth that results in invasion and destruction of surrounding healthy tissue by abnormal cells. Cancer cells arise from normal cells whose nature is permanently changed. They multiply more rapidly than healthy body cells and do not seem subject to normal control by nerves and hormones. They may spread via the bloodstream or lymphatic system to other parts of the body, where they produce further tissue damage (metastases ). Malignant tumour is another name for cancer. A cancer that arises in epithelium is termed a carcinoma ; one that arises in connective tissue is called a sarcoma . Leukaemia is cancer of white blood cells; lymphoma is cancer of lymphoid tissue; and myeloma is cancer of plasma cells of the bone marrow. Causative agents (carcinogens ) include various chemicals (including those in tobacco smoke), ionizing radiation, silica and asbestos particles, and oncogenic viruses. Hereditary factors and stress may also play a role.

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cane sugar

See sucrose.

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canine tooth

A sharp conical tooth in mammals that is large and highly developed in carnivores (e.g. dogs) for tearing meat. There are two canines in each jaw, each situated between the second incisor and the first premolar. In some animals (e.g. herbivores, such as giraffes and rabbits) canine teeth are absent.

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capacitation

The final stage in the maturation process of a spermatozoon. This takes place inside the genital tract as the sperm penetrates the ovum.

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capillarity

See surface tension.

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capillary (blood capillary)

The narrowest type of blood vessel in the vertebrate circulatory system. Capillaries conduct blood from arterioles to all living cells: their walls are only one cell layer thick, so that oxygen and nutrients can pass through them into the surrounding tissues. Capillaries also transport waste material (e.g. urea and carbon dioxide) to venules for ultimate excretion. Capillaries can be constricted or dilated, according to local tissue requirements.

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capitulum

A type of flowering shoot (see racemose inflorescence) characteristic of plants of the family Compositae (Asteraceae), e.g. daisy and dandelion. The tip of the shoot is flattened and bears many small stalkless flowers (florets ) surrounded by an involucre (ring) of bracts. This arrangement gives the appearance of a single flower.

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capsid

The protein coat of a virus, which is made up of units called capsomeres. The chemical nature of the capsid is important in stimulating the body's immune response against the invading virus.

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capsomere (capsomer)

Any of the protein units that make up the regularly organized outer coat (capsid) of the viruses. The capsid of most viruses contains more than one type of protein molecule.

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capsule

1 (in botany)
a A dry fruit that releases its seeds when ripe; it is formed from several fused carpels and contains many seeds. The seeds may be dispersed through pores (as in the poppy), through a lid (as in plantain), or by the splitting and separation of the individual carpels (as in the crocus). Various other forms of capsules include the silicula and siliqua.
b The part of the sporophyte of mosses and liverworts in which the haploid spores are produced. It is borne on a long stalk (seta ) and sheds its spores when mature (see peristome).

2 (in microbiology) A thick relatively rigid gelatinous layer completely surrounding the cell wall of certain bacteria (see glycocalyx). It appears to have a protective function, making ingestion of the bacterial cell by phagocytes more difficult and preventing desiccation.
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3 (in animal anatomy)
a The membranous or fibrous envelope that surrounds certain organs, e.g. the kidneys, spleen, and lymph nodes.
b The ligamentous sheath of connective tissue that surrounds various skeletal joints.
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carapace

1 The dorsal part of the exoskeleton of some crustaceans (e.g. crabs), which spreads like a shield over several segments of the head and thorax.

2 The domed dorsal part of the shell of tortoises and turtles, formed of bony plates fused with the ribs and vertebrae and covered by a horny epidermal layer. The ventral part of the shell (plastron ) is similar but flatter.
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carbamates

Salts or esters of carbamic acid, H_2NCOOH, or their derivatives.They include various insecticides, such as aldicarb, methiocarb, and propoxur. These are active against a wide range of insects upon contact or ingestion; they work by inhibiting cholinesterase enzymes, which are essential for nervous system function. Hence carbamates are also toxic to other animals, including humans. However, their persistence in the environment is relatively low.

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carbamide

See urea.

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carbohydrate

One of a group of organic compounds based on the general formula Cx(H_2O)y. The simplest carbohydrates are the sugars (saccharides), including glucose and sucrose. Polysaccharides are carbohydrates of much greater molecular weight and complexity; examples are starch, glycogen, and cellulose. Carbohydrates perform many vital roles in living organisms. Sugars, notably glucose, and their derivatives are essential intermediates in the conversion of food to energy. Starch and other polysaccharides serve as energy stores in plants, particularly in seeds, tubers, etc., which provide a major energy source for animals, including humans. Cellulose, lignin, and others form the supporting cell walls and woody tissue of plants. Chitin is a structural polysaccharide found in the body shells of many invertebrate animals. Carbohydrates also occur in the surface coat of animal cells and in bacterial cell walls.

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carbon

Symbol C. A nonmetallic element that occurs in all organic compounds and is therefore fundamental to the structure of all living organisms. It is an essential element for plants and animals, being ultimately derived from atmospheric carbon dioxide assimilated by plants during photosynthesis (see carbon cycle). The ubiquitous nature of carbon in living organisms is due to its unique ability to form stable covalent bonds with other carbon atoms and also with hydrogen, oxygen, nitrogen, and sulphur atoms, resulting in the formation of a variety of compounds containing chains and rings of carbon atoms.
There are two stable isotopes of carbon (proton numbers 12 and 13) and four radioactive ones (10, 11, 14, 15). Carbon–14 is used in carbon dating.

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carbon assimilation

The incorporation of carbon from atmospheric carbon dioxide into organic molecules. This process occurs during photosynthesis. See carbon cycle.

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carbon cycle

One of the major cycles of chemical elements in the environment (see biogeochemical cycle). Carbon (as carbon dioxide) is taken up from the atmosphere and incorporated into the tissues of plants in photosynthesis. It may then pass into the bodies of animals as the plants are eaten (see food chain). During the respiration of plants, animals, and organisms that bring about decomposition, carbon dioxide is returned to the atmosphere. The combustion of fossil fuels (e.g. coal and peat) also releases carbon dioxide into the atmosphere. See illustration.

The carbon cycle in nature

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carbon dating (radiocarbon dating)

A method of estimating the ages of archaeological specimens of biological origin. As a result of cosmic radiation a small number of atmospheric nitrogen nuclei are continuously being transformed by neutron bombardment into radioactive nuclei of carbon–14. Some of these radiocarbon atoms find their way into living trees and other plants in the form of carbon dioxide, as a result of photosynthesis. When the tree is cut down photosynthesis stops and the ratio of radiocarbon atoms to stable carbon atoms begins to fall as the radiocarbon decays. The ratio 14C/12C in the specimen can be measured and enables the time that has elapsed since the tree was cut down to be calculated. The method has been shown to give consistent results for specimens up to some 40000 years old, though its accuracy depends upon assumptions concerning the past intensity of the cosmic radiation. The technique was developed by Willard F. Libby (1908–80) and his coworkers in 1946–47.

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carbon dioxide

A colourless odourless gas, CO_2, which dissolves in water to give carbonic acid. It occurs in the atmosphere (0.04% by volume) but has a short residence time in this phase as it is both consumed by plants during photosynthesis and produced by respiration and by combustion.
The level of carbon dioxide in the atmosphere has increased by some 12% in the last 100 years, mainly because of extensive burning of fossil fuels and the destruction of large areas of rainforest. This has been postulated as the main cause of the average increase of 0.5°C in global temperatures over the same period, through the greenhouse effect. Steps are now being taken to prevent further increases in atmospheric CO_2 concentration and subsequent global warming.

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carbonic acid

A weak acid, H_2CO_3, formed in solution when carbon dioxide is dissolved in water:
CO_2 + H_2O H_2CO_3

The acid is in equilibrium with dissolved carbon dioxide, and also dissociates into hydrogencarbonate and hydrogen ions. These reactions, catalysed by carbonic anhydrase, take place in the red blood cells when carbon dioxide diffuses into them from the surrounding tissue cells. See also hydrogencarbonate.

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carbonic anhydrase

An enzyme, present in red blood cells and kidney cells, that catalyses the reaction between carbon dioxide and water to form carbonic acid, which subsequently dissociates:
CO_2 + H_2O H_2CO_3

H_2CO_3 H+ + HCO_3–

This reaction is one of the fastest known and controls the elimination of carbon dioxide from the body and the pH of urine. It also facilitates the transfer of carbon dioxide from the tissues to the blood and from the blood to the alveoli (air sacs) of the lungs. See also chloride shift; haemoglobinic acid.

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Carboniferous

A geological period in the Palaeozoic era. It began about 360 million years ago, following the Devonian period, and extended until the beginning of the Permian period, about 285 million years ago. In Europe the period is divided into the Lower and Upper Carboniferous, which roughly correspond to the Mississippian and Pennsylvanian periods, respectively, of North America. During the Lower Carboniferous a marine transgression occurred and the characteristic rock of this division – the Carboniferous limestone – was laid down in the shallow seas. Fauna included foraminiferans, corals, ectoprocts, brachiopods, blastoids, and other invertebrates. The Upper Carboniferous saw the deposition of the millstone grit, a mixture of shale and sandstone formed in deltaic conditions, followed by the coal measures, alternating beds of coal, sandstone, shale, and clay. The coal was formed from the vast swamp forests composed of seed ferns, lycopsids, and other plants. During the period fishes continued to diversify and amphibians became more common.

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carbon monoxide

A colourless odourless gas, CO. It is formed by the incomplete combustion of carbon and is present in car-exhaust gases. Carbon monoxide is able to bond with metals; this accounts for its toxicity, which is due to the binding of the CO to the iron in haemoglobin, thereby blocking the uptake of oxygen. See carboxyhaemoglobin.

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carboxyhaemoglobin

The highly stable product formed when haemoglobin combines with carbon monoxide. Carbon monoxide competes with oxygen for haemoglobin, with which it binds strongly: the affinity of haemoglobin for carbon monoxide is 250 times greater than that for oxygen. This reduces the availability of haemoglobin for combination with (and transport of) oxygen and accounts for the toxic effects of carbon monoxide on the respiratory system.

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carboxylase

Any enzyme involved in the transfer of carbon dioxide or carboxyl groups.

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carboxyl group

The organic group –CO.OH, present in carboxylic acids.

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carboxylic acids

Organic compounds containing the group –CO.OH (the carboxyl group ; i.e. a carbonyl group attached to a hydroxyl group). Many long-chain carboxylic acids occur naturally as esters in fats and oils and are therefore also known as fatty acids. See also glycerides.

Carboxylic acid structure

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carboxypeptidase

Any enzyme that catalyses the hydrolysis of amino acid residues from the carboxyl terminus of a peptide or polypeptide (see exopeptidase). Pancreatic juice contains a carboxypeptidase that is secreted into the duodenum. The enzyme is secreted as an inactive precursor, procarboxypeptidase , which is activated by another pancreatic protease, trypsin. See also chymotrypsin.

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carboxysome

An organelle found in certain autotrophic bacteria that contains the carbon-fixing enzyme ribulose bisphosphate carboxylase (see ribulose bisphosphate).

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carcerulus

A dry fruit that is a type of schizocarp. It consists of a number of one-seeded fragments (mericarps ) that adhere to a central axis. It is characteristic of mallow.

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carcinogen

Any agent that produces cancer, e.g. tobacco smoke, certain industrial chemicals, and ionizing radiation (such as X-rays and ultraviolet rays).

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carcinoma

See cancer.

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cardiac

1 Relating to the heart.

2 Relating to the part of the stomach nearest to the oesophagus.
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cardiac cycle

The sequence of events that occurs in the heart during one full heartbeat. These events comprise contraction (see systole) and relaxation (see diastole) of the chambers of the heart, associated with opening and closing of the heart valves. When both the atria and the ventricles are relaxed, pressure in the heart is low and blood flows from the vena cava and pulmonary vein into the atria and through to the ventricles. The aortic and pulmonary semilunar valves, at the junction between the left ventricle and aorta and the right ventricle and pulmonary artery, respectively, are closed; therefore, blood can enter but not leave the heart, which increases the pressure in the chambers. As the pressure in the heart increases, the atria begin to contract, forcing the blood into the ventricles and closing the tricuspid valve and the bicuspid valve. A wave of ventricular contraction follows, expelling the blood into the aorta and pulmonary artery to complete the cardiac cycle. At a resting heart rate, the human cardiac cycle lasts approximately 0.85 second.

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cardiac muscle

A specialized form of muscle that is peculiar to the vertebrate heart. The muscle fibres are branched and interlock (see Purkyne fibres) and the muscle itself shows spontaneous contraction and does not need nervous stimulation (see pacemaker). The vagus nerve to the heart can, however, affect the rate of contraction (see tachycardia).

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cardiac output

The volume of blood pumped per minute by each ventricle, which is also the total blood flow through the pulmonary circuit. At rest, normal human cardiac output is approximately 5 litres per minute, rising to 22 litres per minute during maximum physical exertion. The cardiac output can be calculated from heart rate (number of beats per minute) and stroke volume (volume of blood expelled from the heart per beat).

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cardiovascular centre

One of the areas in the brain that are responsible for the modification of the cardiovascular system based upon the integration of sensory information from the autonomic nervous system. These centres influence the heart rate via the sympathetic and parasympathetic nerves and by the action of certain hormones.

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carnassial teeth

Molar and premolar teeth modified for shearing flesh by having cusps with sharp cutting edges. They are typical of animals of the order Carnivora (e.g. tigers, wolves), in which they are the first molars in the lower jaw and the last premolars in the upper.

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Carnivora

An order of mainly flesh-eating mammals that includes the dogs, wolves, bears, badgers, weasels, and cats. Carnivores typically have very keen sight, smell, and hearing. The hinge joint between the lower jaw and skull is very tight, allowing no lateral movement of the lower jaw. This – together with the arrangement of jaw muscles – enables a very powerful bite. The teeth are specialized for stabbing and tearing flesh: canines are large and pointed and some of the cheek teeth are modified for shearing (see carnassial teeth).

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carnivore

An animal that eats meat, especially a member of the order Carnivora (e.g. tigers, wolves). Carnivores are specialized by having strong powerful jaws and well-developed canine teeth. They may be predators or carrion eaters. See also consumer. Compare herbivore; omnivore.

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carnivorous plant (insectivorous plant)

Any plant that supplements its supply of nitrates in conditions of nitrate deficiency by digesting small animals, especially insects. Such plants are adapted in various ways to attract and trap the insects and produce proteolytic enzymes to digest them. Venus' fly trap (Dionaea), for example, has spiny-margined hinged leaves that snap shut on an alighting insect. Sundews (Drosera) trap and digest insects by means of glandular leaves that secrete a sticky substance, and pitcher plants (families Nepenthaceae and Sarraceniaceae) have leaves modified as pitchers into which insects fall, drowning in the water and digestive enzymes at the bottom.

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carotene

A member of a class of carotenoid pigments. Examples are ß-carotene and lycopene, which colour carrot roots and ripe tomato fruits respectively. - and ß-carotene yield vitamin A when they are broken down during animal digestion; ß-carotene has antioxidant properties.

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carotenoid

Any of a group of yellow, orange, red, or brown pigments chemically related to terpenes. Carotenoids are responsible for the characteristic colour of many plant organs, such as ripe tomatoes, carrots, and autumn leaves. They also occur in certain algae and other photosynthesizing organisms (such as phototrophic bacteria), in which they function as accessory pigments in the light reaction of photosynthesis (see photosynthetic pigments). See carotene; xanthophyll.

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carotid artery

The major artery that supplies blood to the head. A pair of common carotid arteries arise from the aorta (on the left) and the innominate artery (on the right) and run up the neck; each branches into an external and an internal carotid artery , which supply the head.

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carotid body

One of a pair of tissue masses adjacent to the carotid sinus. Each contains receptors that are sensitive to oxygen and pH levels (acidity) in the blood. High levels of carbon dioxide in the blood lower the pH (i.e. increase the acidity). By responding to fluctuations in pH, the carotid body coordinates reflex changes in respiration rate. See also ventilation centre.

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carotid sinus

An enlarged region of the carotid artery at its major branching point in the neck. Its walls contain many receptors that are sensitive to changes in pressure and it regulates blood pressure by initiating reflex changes in heart rate and dilation of blood vessels.

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carpal (carpal bone)

One of the bones that form the wrist (see carpus) in terrestrial vertebrates.

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carpel

The female reproductive organ of a flower. Typically it consists of a stigma, style, and ovary. It is thought to have evolved by the fusion of the two edges of a flattened megasporophyll (see sporophyll). Each flower may have one carpel (monocarpellary ) or many (polycarpellary ), either free (apocarpous ) or fused together (syncarpous ). See also pistil.

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carpogonium

(pl. carpogonia) The female gametangium of the red algae (see Rhodophyta). Carpogonia, which are found at the tips of the female gametophyte, are flask-shaped structures from which protrudes a slender elongation called a trichogyne . The nonmotile male spermatia, released from the antheridia, attach to the trichogyne during sexual reproduction. After fertilization the carpogonium develops into the cystocarp , which contains the carpospores.

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carpospore

Any of the spores of the red algae (see Rhodophyta), which are produced in the fertilized carpogonium. After release the spores develop into a sporophyte (see tetraspore) or a new gametophyte.

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carpus

The wrist (or corresponding part of the forelimb) in terrestrial vertebrates, consisting of a number of small bones (carpals ). The number of carpal bones varies with the species. The rabbit, for example, has two rows of carpals, the first (proximal) row containing three bones and the second (distal) row five. In humans there are also eight carpals. This large number of bones enables flexibility at the wrist joint, between the hand and forelimb. See also pentadactyl limb.

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carr

See hydrosere.

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carrageenan (carrageen)

A naturally occurring polysaccharide isolated from red algae (Rhodophyta). The polymer is composed of D-galactose units, many of which are sulphated. K-carrageenan is a gelling agent and stabilizing agent used in foods, cosmetics, and pharmaceuticals.

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carrier

1 (in medicine) An individual who harbours a particular disease-causing microorganism without ill-effects and who can transmit the microorganism to others. Compare vector.

2 (in genetics) An individual with an allele for some defective condition that is masked by a normal dominant allele. Such individuals therefore do not suffer from the condition themselves but they may pass on the defective allele to their offspring. In humans, women may be carriers of such conditions as red–green colour blindness and haemophilia, the alleles for which are carried on the X chromosomes (see sex linkage).
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3 (in biochemistry) See carrier molecule; hydrogen carrier.
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carrier molecule

1 A molecule that plays a role in transporting electrons through the electron transport chain. Carrier molecules are usually proteins bound to a nonprotein group; they can undergo oxidation and reduction relatively easily, thus allowing electrons to flow through the system. There are four types of carrier: flavoproteins (e.g. FAD), cytochromes, iron–sulphur proteins (e.g. ferredoxin), and ubiquinone.

2 A lipid-soluble molecule that can bind to lipid-insoluble molecules and transport them across membranes. Carrier molecules have specific sites that interact with the molecules they transport. Several different molecules may compete for transport by the same carrier.
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carrying capacity

Symbol K. The maximum population of a particular species that can be supported indefinitely by a given habitat or area without damage to the environment. It can be manipulated by human intervention. For example, the carrying capacity for grazing mammals could be increased by boosting the yield of their grassland habitat by the application of fertilizer. See also K selection.

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cartilage (gristle)

A firm flexible connective tissue that forms the adult skeleton of cartilaginous fish (e.g. sharks). In other vertebrates cartilage forms the skeleton of the embryo, being largely replaced by bone in mature animals (although it persists in certain areas). Cartilage comprises a matrix consisting chiefly of a glycosaminoglycan (mucopolysaccharide) called chondroitin sulphate secreted by cells (chondroblasts ) that become embedded in the matrix as chondrocytes . It also contains collagenous and elastic fibres. Hyaline cartilage consists largely of glycosaminoglycan, giving it a shiny glasslike appearance; this type of cartilage gives flexibility and support at the joints. Fibrocartilage , in which bundles of collagen fibres predominate, is stronger and less elastic than hyaline cartilage; it is found in such areas as the intervertebral discs. Elastic cartilage has a yellow appearance due to the presence of numerous elastic fibres (see elastin). This cartilage maintains the shape of certain organs, such as the pinna of the ear.

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cartilage bone (replacing bone)

Bone that is formed by replacing the cartilage of an embryo skeleton. The process, called endochondral ossification , is brought about by the cells (osteoblasts) that secrete bone. Compare membrane bone.

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cartilaginous fishes

See Chondrichthyes.

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caruncle

A small outgrowth from the testa of a seed that develops from the placenta, funicle, or micropyle. Examples include the warty outgrowth from the castor-oil seed and the tuft of hairs on the testa of the seed of willowherb. See also aril.

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caryopsis

A dry single-seeded indehiscent fruit that differs from an achene in that the fruit wall is fused to the testa of the seed. It is the grain of cereals and grasses.

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casein

One of a group of phosphate-containing proteins (phosphoproteins) found in milk; it is also the principal protein of cheese (see curd). Caseins are easily digested by the enzymes of young mammals and represent a major source of phosphorus. See rennin.

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Casparian strip

A band of suberin, an impermeable substance, found in the endodermal cell walls of plant roots; it was named after R. Caspary. Movement of water through the apoplast pathway is diverted from the cell wall to the cytoplasm, where it then follows the symplast pathway. The endodermal cells actively secrete salts into the vascular tissue. This results in a low (more negative) water potential, allowing water to move down a water potential gradient from the endodermis into the vascular tissue. The Casparian strip prevents water from returning to the cortex; consequently a positive hydrostatic pressure is established in the vascular tissue – the phenomenon of root pressure.

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caste

A division found in eusocial insects, such as the Hymenoptera (ants, bees, wasps) and the Isoptera (termites), in which the individuals are structurally and physiologically specialized to perform a particular function. For example, in honeybees there are queens (fertile females), workers (sterile females), and drones (males). There are several different castes of workers (all sterile females) among ants.

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casual

See alien.

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catabolism

The metabolic breakdown of large molecules in living organisms to smaller ones, with the release of energy. Respiration is an example of a catabolic series of reactions. See metabolism. Compare anabolism.

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catalase

An enzyme, found in peroxisomes, that catalyses the decomposition of hydrogen peroxide, which results from oxidation reactions in the cell (see also superoxide dismutase), into water and oxygen. High concentrations of catalase are found in the liver. Catalase is the fastest known enzyme; it is employed in the rubber industry to form oxygen, which converts latex to foam rubber.

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catalysis

The process of changing the rate of a chemical reaction by use of a catalyst.

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catalyst

A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. The catalyst provides an alternative pathway by which the reaction can proceed, in which the activation energy is lower. It thus increases the rate at which the reaction comes to equilibrium, although it does not alter the position of the equilibrium. Enzymes are the catalysts in biochemical reactions; they are highly specific in the type of reaction they catalyse.

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catalytic activity

The increase in the rate of a specified chemical reaction caused by an enzyme or other catalyst under specified assay conditions. It is measured in katals or in moles per second.

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catalytic RNA

See ribozyme.

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catecholamine

Any of a class of amines that possess a catechol (C_6H_4(OH)2) ring. Including dopamine, adrenaline, and noradrenaline, they function as neurotransmitters and/or hormones.

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category

See rank.

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cathepsins

Protein-digesting enzymes that are found in lysosomes.

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cation

A positively charged ion, such as the sodium ion (Na+). Compare anion.

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catkin

A type of flowering shoot (see racemose inflorescence) in which the axis, which is often long, bears many small stalkless unisexual flowers. Usually the male catkins hang down from the stem; the female catkins are shorter and often erect. Examples include birch and hazel. Most plants with catkins are adapted for wind pollination, the male flowers producing large quantities of pollen; willows are an exception, having nectar-secreting flowers and being pollinated by insects.

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caudal vertebrae

The bones (see vertebra) of the tail, which articulate with the sacral vertebrae. The number of caudal vertebrae varies with the species. Rabbits, for example, have 15 caudal vertebrae, while in man these vertebrae are fused to form a single bone, the coccyx.

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caveola

(pl. caveolae) A small indentation in the plasma membrane of a cell that can trap extracellular material and transfer it to the cell interior or convey material across the cell. In the latter case the caveola forms an enclosed membranous vesicle that conveys the material to the opposite side of the cell, where it fuses with the plasma membrane and delivers the material to the exterior. Caveolae are believed to play a part in atherosclerosis of blood vessels. They may contain receptors that bind cholesterol-containing low-density lipoproteins and transport these across the cells of the vessel endothelium, encouraging the build-up of atherosclerotic plaques beneath the blood vessel lining and hence narrowing of the vessel.

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C cell (parafollicular cell)

Any one of a group of cells in vertebrates that are derived from the terminal pair of gill pouches. In mammals these cells are incorporated into the thyroid and parathyroid glands. In other vertebrates they occur mainly in the lung and the paired ultimobranchial bodies .

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CCK

See cholecystokinin.

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CD (cluster of differentiation)

Any group of antigens that is associated with a specific subpopulation of human T cells. The differentiation antigens expressed by a T cell vary with its stage of development and thus with its role in the immune response. Hence, for example, CD4 antigens are expressed by helper T cells, whereas CD8 antigens are expressed by cytotoxic and suppressor T cells. The antigens are glycoproteins and are characterized using monoclonal antibodies.

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cDNA

See complementary DNA.

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cecidium

See gall.

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cecropin

Any of a group of peptides that have potent activity against certain Gram-negative bacteria, such as E. coli. Produced by cecropia moths in response to bacterial infection, they disrupt the bacterial cell membranes.

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cell

The structural and functional unit of most living organisms (compare coenocyte; syncytium). Cell size varies, but most cells are microscopic (average diameter 0.01–0.1 mm). Cells may exist as independent units of life, as in bacteria and certain protoctists, or they may form colonies or tissues, as in all plants and animals. Each cell consists of a mass of protein material that is differentiated into cytoplasm and a nucleus, which contains DNA. The cell is bounded by a cell membrane, which in the cells of plants, fungi, algae, and bacteria is surrounded by a cell wall. There are two main types of cell. Prokaryotic cells (bacteria) are the more primitive. The nuclear material is not bounded by a membrane and chemicals involved in cell metabolism are associated with the cell membrane. Reproduction is generally asexual and involves simple cell cleavage. In eukaryotic cells the nucleus is bounded by a nuclear membrane and the cytoplasm is divided by membranes into a system of interconnected cavities and separate compartments (organelles ), e.g. mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and ribosomes (see illustration). Reproduction can be either asexual (see mitosis) or sexual (see meiosis). Plants and animals consist of eukaryotic cells but plant cells possess chloroplasts and other plastids and bear a rigid cellulose cell wall.

CELL BIOLOGY

1665

English physicist Robert Hooke (1635–1703) coins the word ‘cell’.

1831

Robert Brown discovers the nucleus in plant cells.

1838

German botanist Matthias Schleiden (1804–81) proposes that plants are composed of cells.

1839

Theodor Schwann states that animals are composed of cells and concludes that all living things are made up of cells.

1846

German botanist Hugo von Mohl (1805–72) coins the word ‘protoplasm’ for the living material of cells.

1858

German pathologist Rudolf Virchow (1821–1902) postulates that all cells arise from other cells.

1865

German botanist Julius von Sachs (1832–97) discovers the chlorophyll-containing bodies in plant cells later named chloroplasts.

1876–80

German cytologist Eduard Strasburger (1844–1912) describes cell division in plants and states that new nuclei arise from division of existing nuclei.

1882

German cytologist Walther Flemming (1843–1905) describes the process of cell division in animal cells, for which he coins the term ‘mitosis’. Strasburger coins the words ‘cytoplasm’ and ‘nucleoplasm’.

1886

German biologist August Weismann (1834–1914) proposes his theory of the continuity of the germ plasm.

1887

Belgian cytologist Edouard van Beneden (1846–1910) discovers that the number of chromatin-containing threadlike bodies (subsequently named chromosomes) in the cells of a given species is always the same and that the sex cells contain half this number.

1888

German anatomist Heinrich von Waldeyer (1836–1921) coins the word ‘chromosome’.

1898

Camillo Golgi discovers the Golgi apparatus.

1901

US biologist Clarence McClung (1870–1946) discovers the sex chromosomes.

1911

Thomas Hunt Morgan produces the first chromosome map.

1949

Canadian geneticist Murray Barr (1908–95) discovers Barr bodies.

1955

Belgian biochemist Christian de Duve (1917– ) discovers lysosomes and peroxisomes.

1956

Romanian-born US physiologist George Palade (1912– ) discovers the role of microsomes (later renamed ribosomes).

Generalized eukaryotic cells

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cell body (perikaryon)

The part of a neurone that contains the nucleus. The cell processes that are involved in the transmission and reception of nervous impulses (the axon and the dendrites respectively) develop as extensions from the cell body.

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cell culture

The culturing of cells from plants, animals, or microorganisms on a culture medium in vitro. See culture.

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cell cycle

The sequence of stages that a cell passes through between one cell division and the next. The cell cycle can be divided into four main stages: (1) the M phase , which consists of mitosis (nuclear division) and cytokinesis (cytoplasmic division); (2) the G_1 phase , in which there is a high rate of biosynthesis and growth; (3) the S phase , in which the DNA content of the cell doubles and the chromosomes replicate; (4) the G_2 phase , during which the final preparations for cell division are made. Interphase consists of the G_1, S, and G_2 phases, which comprise about 90% (16–24 hours) of the total time of the cell cycle in rapidly dividing cells. The M phase lasts about 1–2 hours. A point is reached in the G_1 phase, known as the restriction point , after which the cell becomes committed to passing through the remainder of the cell cycle regardless of the external conditions. See illustration. See also cyclin.

Cell cycle

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cell division

The formation of two or more daughter cells from a single mother cell. The nucleus divides first; this is followed by division of the cytoplasm (cytokinesis ) with the formation of a cell membrane between the daughter nuclei. Mitosis produces two daughter nuclei that are identical to the original nucleus; in plants a cell plate forms between the new cells. Meiosis results in four daughter nuclei each with half the number of chromosomes in the mother cell nucleus. See also cell cycle.

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cell fusion (somatic cell hybridization)

The technique of combining two cells from different tissues or species in a cell culture. The cells are fused (see chemical fusogen) and coalesce but their nuclei generally remain separate. However, during cell division a single spindle is formed so that each daughter cell has a single nucleus containing sets of chromosomes from each parental line. Subsequent division of the hybrid cells often results in the loss of chromosomes (and therefore genes), so that absence of a gene product in the culture can be related to the loss of a particular chromosome. Thus the technique is used to determine the control of characteristics exerted by specific chromosomes. Hybrid cells (see hybridoma) resulting from cell fusion have also been used to produce monoclonal antibodies.

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cell membrane (plasmalemma; plasma membrane)

The partially permeable membrane forming the outer limit of a cell. It consists mostly of protein and lipid (see lipid bilayer; fluid mosaic model). The membrane regulates the flow of materials in and out of the cell and plays a role in the production or assembly of the cell wall or capsule (when this is present).

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cell plate

The structure that forms in a dividing plant cell at the end of mitosis; it separates the cytoplasm of the two daughter protoplasts. The cell plate is formed from vesicles deposited by the Golgi apparatus in the equatorial region of the spindle. These vesicles contain pectin, cellulose, and hemicellulose, which contribute to the middle lamella and the primary wall of the new cell wall. The cell plate eventually fuses with the cell wall of the parent cell, dividing it into two daughter cells.

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cell sap

The solution that fills the vacuoles of plant cells. It contains sugars, amino acids, waste substances (such as tannins), and mineral salts.

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cell theory

The theory that was born of the findings of Matthias Schleiden in 1838 and Theodor Schwann in 1839, who postulated, respectively, that plants and animals were made up of cells and that these units were basic to the structure and function of all organisms. Previously, in 1665, Robert Hooke (1635–1703), while examining cork under the microscope, had observed that its structure consisted of hollow boxlike units, which he called `cells'. At the time, however, he did not realize the significance of these units, which were in fact dead cells.

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cellular respiration

See respiration.

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cellulase

A carbohydrate-digesting enzyme (a carbohydrase ) that hydrolyses cellulose to sugars, including cellobiose (a disaccharide consisting of two ß-(1,4) linked molecules of glucose) and glucose. Cellulase breaks the ß-glycosidic links that join the constituent sugar units of cellulose. It plays an essential part in abscission in plants, while microbial cellulases enable the digestion of plant material by herbivores. See also Ruminantia.

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cellulolytic

Able to digest cellulose. For example, cellulolytic bacteria, in the stomach of ruminants, digest cellulose by means of the enzyme cellulase.

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cellulose

A polysaccharide that consists of a long unbranched chain of glucose units (see formula). It is the main constituent of the cell walls of all plants, many algae, and some fungi and is responsible for providing the rigidity of the cell wall. It is an important constituent of dietary fibre. The fibrous nature of extracted cellulose has led to its use in the textile industry for the production of cotton, artificial silk, etc.

cellulose

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cell wall

A rigid outer layer that surrounds the cell membrane of plant, fungal, algal, and bacterial (but not animal) cells. It protects and/or gives shape to a cell, and in herbaceous plants provides mechanical support for the plant body. The cell walls in most plants and algae are composed of the polysaccharide cellulose and plant cell walls may be secondarily thickened by the addition of lignin. The cell walls of fungi consist mainly of chitin. Bacterial cell walls consist of complex polymers of polysaccharides and amino acids (see peptidoglycan; Gram's stain).

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Celsius scale

A temperature scale in which the fixed points are the temperatures at standard pressure of ice in equilibrium with water (0°C) and water in equilibrium with steam (100°C). The scale, between these two temperatures, is divided in 100 degrees. The degree Celsius (°C) is equal in magnitude to the kelvin. This scale was formerly known as the centigrade scale ; the name was officially changed in 1948 to avoid confusion with a hundredth part of a grade. It is named after the Swedish astronomer Anders Celsius (1701–44), who devised the inverted form of this scale (ice point 100°, steam point 0°) in 1742.

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cement (cementum)

A thin layer of bony material that fixes teeth to the jaw. It covers the dentine of the root of a tooth, below the level of the gum, and is attached to the periodontal membrane lining the tooth socket in the jawbone.

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Cenozoic (Cainozoic; Kainozoic)

The geological era that began about 65 million years ago and extends to the present. It followed the Mesozoic era and is subdivided into the Tertiary and Quaternary periods. The Cenozoic is often known as the Age of Mammals as these animals evolved to become an abundant, diverse, and dominant group. Birds and flowering plants also flourished. The era saw the formation of the major mountain ranges of the Himalayas and the Alps.

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centi-

Symbol c. A prefix used in the metric system to denote one hundredth. For example, 0.01 metre = 1 centimetre (cm).

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centigrade scale

See Celsius scale.

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centipedes

See Chilopoda.

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Central Dogma

The basic belief originally held by molecular geneticists, that flow of genetic information can only occur from DNA to RNA to proteins. It is now known, however, that information contained within RNA molecules of viruses can also flow back to DNA, for example in the replication of retroviruses. See also genetic code.

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central nervous system (CNS)

The part of the nervous system that coordinates all neural functions. In invertebrates it may comprise simply a few nerve cords and their associated ganglia. In vertebrates it consists of the brain and the spinal cord. The vertebrate CNS contains reflex arcs, which produce automatic and rapid responses to particular stimuli.

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centre

(in neurology) A part of the nervous system, consisting of a group of nerve cells, that coordinates a particular process. An example is the respiratory centre in the vertebrate brainstem, which controls breathing movements. The stimulation of a centre will initiate the process, while destruction of the centre will prevent or impair it.

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centrifuge

A device in which solid or liquid particles of different densities are separated by rotating them in a tube in a horizontal circle. The denser particles tend to move along the length of the tube to a greater radius of rotation, displacing the lighter particles to the other end.

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centriole

A structure associated with the centrosome and found mainly in animal cells. It consists of two short cylinders, orientated at right angles to each other and composed of microtubules. When present, the centriole replicates during the nondividing phase of the cell cycle, and during the prophase of mitosis a centriole migrates with each centrosome to lie at opposite poles of the cell. It was formerly believed that the centrioles were involved in assembly of the spindle microtubules, but this role is now in doubt – they are not present in the cells of most higher plants, and their removal from cells does not affect spindle formation. See also undulipodium.

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centromere (kinomere; spindle attachment)

The part of a chromosome that attaches to the spindle during cell division (see meiosis; mitosis). The position of the centromere is a distinguishing feature of individual chromosomes: a chromosome with the centromere at its centre is described as metacentric ; one with the centromere towards one end is acrocentric ; and a chromosome with the centromere at either end is telocentric . The centromere usually appears as a constriction when chromosomes contract during cell division; centromeres are often associated with repetitive DNA. See also kinetochore.

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centrosome (cell centre; centrosphere)

A specialized region of a living cell, situated next to the nucleus, where microtubules are assembled and broken down during cell division. The centrosomes of most animal cells contain a pair of centrioles. During metaphase of mitosis and meiosis, the centrosome separates into two regions, each containing one of the centrioles (where present). The two regions move to opposite ends of the cell and a spindle of microtubules forms between them.

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centrum

See vertebra.

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cephalization

The tendency among animal groups for the major sense organs, mouth, and brain to be grouped together at the front (anterior) end of the body. These are usually contained in a specialized cephalic region – the head.

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Cephalochordata

A subphylum or phylum of chordates that contains only the lancelets, a group of small invertebrate marine fishlike animals. There are 23 species in two genera – Branchiostoma (Amphioxus) and Epigonichthys. About 5–15 cm long, lancelets have gill slits and a notochord that persist into adulthood, the notochord extending the whole length of the body and providing the main skeletal support. The pharynx is modified for filter feeding (see endostyle).

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Cephalopoda

The most advanced class of molluscs, containing the squids, cuttlefishes, octopuses, and the extinct ammonites. Cephalopods have a highly concentrated central nervous system within a protective cartilaginous case. The eye has a well-developed retina and is comparable to that of vertebrates. All cephalopods are predacious carnivores capable of swimming by jet propulsion; they have highly mobile tentacles for catching and holding prey.

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cephalothorax

The fused head and thorax of many crustaceans and of arachnids (see prosoma), which is connected to the abdomen. See also tagma.

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cercaria

A tadpole-like larval stage of trematode worms (flukes) that develops in the body of a mollusc (the secondary host). It subsequently infects a primary host, in which it matures into an adult worm.

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cerci

A pair of many-jointed appendages that are present on the last segment of the abdomen in certain insects, such as mayflies, earwigs, and cockroaches.

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cerebellum

The part of the vertebrate brain concerned with the coordination and regulation of muscle activity and the maintenance of muscle tone and balance. In mammals it consists of two connected hemispheres, composed of a core of white matter and a much-folded outer cortex of grey matter containing numerous Purkyne cells, and it is situated above the medulla oblongata and partly beneath the cerebrum.

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cerebral cortex (pallium)

The layer of grey matter that forms the outer layer of the hemispheres of the cerebrum in many vertebrates. It is most highly developed in mammals. The cortex is responsible for the control and integration of voluntary movement and the senses of vision, hearing, touch, etc.; it also contains centres concerned with memory, language, thought, and intellect.

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cerebral hemisphere

Either of the two halves of the vertebrate cerebrum.

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cerebroside

Any one of a class of glycolipids in which a single sugar unit is bound to a sphingolipid (see phospholipid). The most common cerebrosides are galactocerebrosides , containing the sugar group galactose; they are found in the plasma membranes of neural tissue and are abundant in the myelin sheaths of neurones.

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cerebrospinal fluid (CSF)

The fluid, similar in composition to lymph, that bathes the central nervous system of vertebrates. It is secreted by the choroid plexus into the ventricles of the brain, filling these and other cavities in the brain and spinal cord, and is reabsorbed by veins on the brain surface. Its function is to protect the central nervous system from mechanical injury.

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cerebrum

The largest part of the vertebrate brain. It consists of two cerebral hemispheres , which develop from the embryonic forebrain. The hemispheres have an outer convoluted layer of grey matter – the cerebral cortex – which contains an estimated ten billion nerve cells. Underneath this is white matter. The two halves of the cerebrum are linked by the corpus callosum. The function of the cerebrum is to integrate complex sensory and neural functions. The cerebrum is thought to play a critical role in the process of learning, which involves both short-term and long-term memory.

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cerumen

Ear wax, which is secreted by ceruminous glands in the auditory meatus of the outer ear. Cerumen protects the delicate lining of the meatus and helps to prevent microorganisms from entering the ear.

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cervical vertebrae

The vertebrae of the neck. The number of cervical vertebrae varies: for example, rabbits have 7 and humans have 12. Their main functions are to support the head and to provide articulating surfaces against which it can move relative to the backbone. See atlas; axis.

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cervix

A narrow or necklike part of an organ. The cervix of the uterus (cervix uteri ) leads to the vagina. Glands in its walls produce mucus, whose viscosity changes according to the oestrous cycle. During labour, the cervix enlarges greatly to allow passage of the fetus.

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Cestoda

A class of flatworms (see Platyhelminthes) comprising the tapeworms – ribbon-like parasites within the gut of vertebrates. Tapeworms are surrounded by partially digested food in the host gut so they are able to absorb nutrients through their whole body surface. The body consists of a scolex (head), bearing suckers and hooks for attachment, and a series of proglottids (or proglottides ), which contain male and female reproductive systems. The life cycle of a tapeworm requires two hosts, the primary host usually being a predator of the secondary host. Taenia solium has man for its primary host and the pig as its secondary host. Mature proglottids, containing thousands of fertilized eggs, leave the primary host with its faeces and develop into embryos and then larvae that continue the life cycle in the gut and other tissues of a secondary host (see bladderworm).

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Cetacea

An order of marine mammals comprising the whales, which includes what is probably the largest known animal – the blue whale (Balaenoptera musculus), over 30m long and over 150 tonnes in weight. The forelimbs of whales are modified as short stabilizing flippers and the skin is very thin and almost hairless. A thick layer of blubber insulates the body against heat loss and is an important food store. Whales breathe through a dorsal blowhole, which is closed when the animal is submerged. The toothed whales (suborder Odontoceti), such as the dolphins and killer whale, are carnivorous; whalebone whales (suborder Mysticeti), such as the blue whale, feed on plankton filtered by whalebone plates.

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CFCs

See chlorofluorocarbons.

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c.g.s. units

A system of units based on the centimetre, gram, and second. Derived from the metric system, it was badly adapted to use with thermal quantities (based on the inconsistently defined calorie) and with electrical quantities (in which two systems, based respectively on unit permittivity and unit permeability of free space, were used). For scientific purposes c.g.s. units have now been replaced by SI units.

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chaeta

(pl. chaetae) A bristle, made of chitin, occurring in annelid worms. In the earthworm they occur in small groups projecting from the skin in each segment and function in locomotion. The chaetae of polychaete worms (e.g. ragworm) are borne in larger groups on paddle-like appendages (parapodia ).

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Chaetognatha

A phylum of marine coelomate invertebrates, the arrow worms, in which the head bears hooks for catching prey and the trunk and tail support paired lateral and tail fins. They lack organs for excretion, circulation, and respiration and are hermaphrodite.

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Chain, Sir Ernst Boris

(1906–79)
German-born British biochemist, who began his research career at Cambridge University in 1933. Two years later he joined Florey at Oxford, where they isolated and purified penicillin. They also developed a method of producing the drug in large quantities and carried out its first clinical trials. The two men shared the 1945 Nobel Prize for physiology or medicine with penicillin's discoverer, Alexander Fleming.

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chalaza

1 A twisted strand of fibrous albumen in a bird's egg that is attached to the membrane at either end of the yolk and thus holds the yolk in position in the albumen.

2 The part of a plant ovule where the nucellus and integuments merge.
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chalcone

Any of a class of flavonoids having the basic C_6–C_3–C_6 arrangement but in which the middle three carbon atoms do not form a closed ring, in contrast to other flavonoids. Chalcones are important precursors in the synthesis of anthocyanin pigments and other flavonoids in plants; their formation is catalysed by the enzyme chalcone synthase .

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chalk

A very fine-grained white rock composed of the fossilized skeletal remains of marine plankton known as coccoliths and consisting largely of calcium carbonate (CaCO_3). It is the characteristic rock of the Cretaceous period. It should not be confused with blackboard `chalk', which is made from calcium sulphate.

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chalone

Any one of a group of compounds that inhibit mitosis. This action was originally demonstrated in vitro, using preparations of mammalian cells and tissues; chalones are now thought to be inhibitory growth factors.

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channel

(in cell biology) A pore formed by a protein molecule in a cell membrane that aids the diffusion of certain substances into and out of the cell. These substances are usually charged ions or lipid-insoluble molecules.

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chaparral

A type of stunted (scrub) woodland found in temperate regions with little summer rainfall. It is dominated by drought-resistant evergreen shrubs, forming dense thickets, interspersed with dwarfed trees, such as oaks and eucalyptus. It is the typical vegetation found in the western United States and the Mediterranean region (where it is called maquis ).

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character

A distinctive inherited feature of an organism. Organisms in a population may display different aspects of a particular character, e.g. the A, B, and O human blood groups (see ABO system) are different aspects of the blood group character.

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Chargaff, Erwin

(1905–)
Ukrainian-born US biochemist who became professor of biochemistry at Columbia University (1952–74). Stimulated by Oswald Avery's identification of DNA as the basis of heredity in pneumococcus bacteria, he discovered that the constitution of DNA is consistent within a species but that there are as many types of DNA as there are species. However, his discovery that the number of purine bases is always equal to the number of pyrimidine bases, and that a similar relationship exists between adenine and thymine bases and between cytosine and guanine bases, provided Watson and Crick with an important clue in their elucidation of the chemical basis of heredity.

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chela

The terminal segment of an arthropod appendage when this can be opposed to the segment that precedes it. The chela is often enlarged and modified to act as a pincer, as in the lobster. Any appendage possessing a chela is described as chelate .

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chelicerae

The first pair of appendages on the head of arachnids and other arthropods of the phylum Chelicerata. These appendages take the form of pincers or claws and are used for grasping or tearing food.

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Chelicerata

A phylum of invertebrate animals that includes arthropods belonging to the classes Arachnida (spiders and scorpions), Merostomata (horseshoe crabs), and Pycnogonida (sea spiders). The body of these animals is divided into an anterior prosoma and a posterior opisthosoma and bears six pairs of appendages, typically comprising chelicerae, pedipalps, and four pairs of walking legs.

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chemical bond

A strong force of attraction holding atoms together in a molecule or crystal. In general, atoms combine to form molecules by sharing or transferring electrons in their outer shells. Typically chemical bonds have energies of about 1000 kJmol–1 and are distinguished from the much weaker forces between molecules. See also covalent bond; electrovalent bond; hydrogen bond.

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chemical control

The use of chemicals to kill pests (see pesticide). Compare biological control.

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chemical dating

An absolute dating technique that depends on measuring the chemical composition of a specimen. Chemical dating can be used when the specimen is known to undergo slow chemical change at a known rate. For instance, phosphate in buried bones is slowly replaced by fluoride ions from the ground water. Measurement of the proportion of fluorine present gives a rough estimate of the time that the bones have been in the ground. Another, more accurate, method depends on the fact that amino acids in living organisms are L-optical isomers. After death, these racemize and the age of bones can be estimated by measuring the relative amounts of D- and L-amino acids present.

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chemical fossil

Any of various organic compounds found in ancient geological strata that appear to be biological in origin and are assumed to indicate that life existed when the rocks were formed. The presence of chemical fossils in Archaean strata indicates that life existed over 3500 million years ago, perhaps even as much as 3800 million years ago.

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chemical fusogen

Any chemical that is used in the fusing of two cells or protoplasts (see cell fusion). Polyethylene glycol (PEG) is used in the formation of hybridomas, and sodium nitrate in the fusion of plant protoplasts in solution.

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chemical reaction

A change in which one or more chemical elements or compounds (the reactants ) form new compounds (the products ). All reactions are to some extent reversible ; i.e. the products can also react to give the original reactants. However, in many cases the extent of this back reaction is negligibly small, and the reaction is regarded as irreversible . See also endergonic reaction; exergonic reaction.

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chemiosmotic theory

A theory postulated by the British biochemist Peter Mitchell (1920–92) to explain the formation of ATP in the mitochondrial electron transport chain. As electrons are transferred along the electron carrier system in the inner mitochondrial membrane, hydrogen ions (protons) are actively transported (by proton pumps) into the space between the inner and outer mitochondrial membranes, which thus contains a higher concentration of protons than the matrix. This creates an electrochemical gradient across the inner membrane, down which protons move back into the matrix. This movement occurs through special channels associated with ATP synthetase, the enzyme that catalyses the conversion of ADP to ATP, and is coupled with the phosphorylation of ADP (see illustration). A similar gradient is created across the thylakoid membranes of chloroplasts during the light reaction of photosynthesis (see photophosphorylation).

ATP production in mitochondria according to the chemiosmotic theory

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chemoautotroph

See autotrophic nutrition; chemosynthesis.

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chemolithotroph

Any bacterium that is able to obtain its energy by the oxidation of inorganic compounds, including those of iron, nitrogen, and sulphur.

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chemoorganotroph

An organism, especially a microorganism, that obtains its energy by the oxidation of organic compounds. Compare chemolithotroph.

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chemoreceptor

A receptor that detects the presence of particular chemicals and (in multicellular organisms) transmits this information to sensory nerves. Examples include the taste buds and the receptors in the carotid body.

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chemosynthesis

A type of autotrophic nutrition in which organisms (called chemoautotrophs ) synthesize organic materials using energy derived from the oxidation of inorganic chemicals, rather than from sunlight. Most chemoautotrophs are bacteria, including Nitrosomonas, which oxidizes ammonium to nitrite; and Thiobacillus, which oxidizes sulphur to sulphate.

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chemosystematics

See systematics.

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chemotaxis

See taxis.

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chemotaxonomy

The classification of plants and microorganisms based on similarities and differences in their natural products and the biochemical pathways involved in their manufacture. See also taxonomy.

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chemotherapy

The use of chemicals, especially drugs, in the treatment of disease. The term is often used specifically to denote drug therapy for cancer, as distinct from treatments with radiation (radiotherapy).

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chemotropism

The growth or movement of a plant or plant part in response to a chemical stimulus. An example is the growth of a pollen tube down the style during fertilization in response to the presence of sugars in the style.

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chiasma

(pl. chiasmata) The point at which paired homologous chromosomes remain in contact as they begin to separate during the first prophase of meiosis, forming a cross shape. A number of chiasmata can usually be identified and at these points crossing over occurs.

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chill haze

The precipitation that occurs when beer is stored at cold temperatures. Chill haze consists of proteins that can be broken down by using protease enzymes.

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Chilopoda

A class of wormlike terrestrial arthropods belonging to the phylum Uniramia and comprising the centipedes. These are characterized by a distinct head, bearing a single pair of relatively long antennae and one pair of poison jaws, and 15–177 body segments, each bearing one pair of similar legs. Centipedes are fast-moving predators found in damp environments. See also Myriapoda.

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chimaera

An organism composed of tissues that are genetically different. Chimaeras can develop if a mutation occurs in a cell of a developing embryo. All the cells arising from it have the mutation and therefore produce tissue that is genetically different from adjacent tissue, e.g. brown patches in otherwise blue eyes in humans. Graft hybrids are examples of plant chimaeras.

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Chiroptera

An order of flying mammals comprising the bats. Their membranous wings are supported by very elongated forelimbs and digits and stretch along the sides of the body to the hindlimbs and tail. Whenever bats rest they allow their body temperature to fall, hibernating in winter when food is scarce. Most bats are nocturnal; their ears are enlarged and specialized for echolocation, which they use to hunt prey and avoid obstacles. Bats feed variously on insects, fruit, nectar, or blood.

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chitin

A polysaccharide comprising chains of N-acetyl-D-glucosamine, a derivative of glucose. Chitin is structurally very similar to cellulose and serves to strengthen the supporting structures of various invertebrates. It also occurs in fungi.

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chitinase

An enzyme that catalyses the hydrolysis of chitin, a major constituent of fungal cell walls and of the exoskeleton of insects and other arthropods. It is synthesized by certain insect-eating animals, such as frogs, and also by certain plants as part of their defence against fungal infection (see hypersensitivity). Transfer of the chitinase gene between different plant species has been successfully undertaken in an attempt to enhance the resistance of susceptible species to pathogenic fungi.

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chlamydospore

A thick-walled asexual spore that is produced from a fungal hypha. It is a resting spore, capable of withstanding unfavourable growing conditions.

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chlorenchyma

Parenchyma tissue that contains chloroplasts and is photosynthetic. Chlorenchyma makes up the mesophyll tissue of plant leaves and is also found in the stems of certain plant species. Compare collenchyma; sclerenchyma.

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chloride secretory cell

Any of the cells in the gills of marine bony fish that remove chloride from the blood and secrete it into the sea, thereby maintaining the osmotic composition of the body. The excretion of chloride is accompanied by movement of sodium ions from the blood to the sea, which keeps the electrochemical balance of the fish's body neutral.

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chloride shift

The movement of chloride ions (Cl–) into red blood cells. Carbon dioxide reacts with water to form carbonic acid in the red blood cells (see carbonic anhydrase). The carbonic acid then dissociates into hydrogencarbonate ions (HCO_3–) and hydrogen ions (H+). The cell membrane is relatively permeable to negative ions. Therefore the hydrogencarbonate ions diffuse out of the cell into the plasma, leaving the hydrogen ions, which create a net positive charge; this is neutralized by the diffusion of chloride ions from the plasma into the cell.

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chlorocruorin

A greenish iron-containing respiratory pigment that occurs in the blood of polychaete worms. It closely resembles haemoglobin.

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chlorofluorocarbons (CFCs)

Compounds obtained by replacing the hydrogen atoms of hydrocarbons by chlorine and fluorine atoms. Their high stability to temperature makes them suitable for a variety of uses, including aerosol propellants, oils, polymers, etc. They are often known as freons . Their widespread use in aerosols and refrigerator coolants has led to increased concentrations in the upper atmosphere, where photochemical reactions cause them to break down and react with ozone, which results in depletion of the ozone layer. For this reason CFCs are now being replaced with less damaging alternatives. See also pollution.

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chlorophyll

Any one of a class of pigments found in all photosynthetic organisms; the most important members are chlorophyll a (see formula) and chlorophyll b, which occur in all land plants and are responsible for their green colour. Chlorophyll molecules are the principal sites of light absorption in the light reaction of photosynthesis (see photosystems I and II). They are magnesium-containing porphyrins, chemically related to cytochrome and haemoglobin. See also bacteriochlorophyll.

Chlorophyll a

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Chlorophyta (green algae)

A large phylum of algae, the members of which possess chlorophylls a and b, store food reserves as starch, and have cellulose cell walls. In these respects they resemble plants more closely than do any of the other algal phyla, although they are still classified as protoctists. The Chlorophyta are widely distributed and diverse in form. Unicellular forms may occur singly (sometimes with undulipodia (flagella) for motility) or in colonies, while multicellular forms may be filamentous (e.g. Spirogyra) or platelike (e.g. Ulva).

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chloroplast

Any of the chlorophyll-containing organelles (see plastid) that are found in large numbers in those plant and algal cells undergoing photosynthesis. Plant chloroplasts are typically lens-shaped and bounded by a double membrane. They contain membranous structures called thylakoids, which are piled up into stacks (see granum), surrounded by a gel-like matrix (stroma ). The light reaction of photosynthesis occurs on the thylakoid membranes while the dark reaction takes place in the stroma.

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chlorosis

The abnormal condition in plant stems and leaves in which synthesis of the green pigment chlorophyll is inhibited, resulting in a pale yellow coloration. This may be caused by lack of light, mineral deficiency, infection (particularly by viruses), or genetic factors.

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chloroxybacteria (grass-green bacteria)

See Cyanobacteria.

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choanae (internal nares)

See nares.

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cholecalciferol

See vitamin D.

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cholecystokinin (CCK; pancreozymin)

A hormone, produced by the duodenal region of the small intestine, that induces the gall bladder to contract and eject bile into the intestine and stimulates the pancreas to secrete its digestive enzymes. Cholecystokinin output is stimulated by contact with the contents of the stomach.

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cholesterol

A sterol (see also steroid) occurring widely in animal tissues and also in some plants and algae. It can exist as a free sterol or esterified with a long-chain fatty acid. Cholesterol is absorbed through the intestine or manufactured in the liver. It serves principally as a constituent of blood plasma lipoproteins and of the lipid–protein complexes that form cell membranes. It is also important as a precursor of various steroids, especially the bile acids, sex hormones, and adrenocorticoid hormones. The derivative 7-dehydrocholesterol is converted to vitamin D_3 by the action of sunlight on skin. Increased levels of dietary and blood cholesterol have been associated with atherosclerosis.

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choline

An amino alcohol, CH_2OHCH_2N(CH_3)3OH. It occurs widely in living organisms as a constituent of certain types of phospholipids – the lecithins and sphingomyelins – and in the neurotransmitter acetylcholine. It is sometimes classified as a member of the vitamin B complex.

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cholinergic

Describing a nerve fibre that either releases acetylcholine when stimulated or is itself stimulated by acetylcholine. Compare adrenergic.

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cholinesterase (acetylcholinesterase)

An enzyme that hydrolyses the neurotransmitter acetylcholine to choline and acetate. Cholinesterase is secreted by nerve cells at synapses and by muscle cells at neuromuscular junctions. Organophosphorus insecticides (see pesticide) act by inhibiting the action of cholinesterase.

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Chondrichthyes

A class of vertebrates comprising the fishes with cartilaginous skeletons. The majority belong to the subclass Elasmobranchii (skates, rays, and sharks – see Selachii). Most cartilaginous fishes are marine carnivores with powerful jaws. Unlike bony fishes, they have no swim bladder, and therefore avoid sinking only by constant swimming with the aid of an asymmetrical (heterocercal ) tail. There is no operculum covering the gill slits, the first of which is modified as a spiracle. Fertilization is internal so the few eggs produced are consequently yolky, large, and well-protected. Some cartilaginous fishes show viviparous development of the young (see viviparity).

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chondrin

The matrix of cartilage, which is made up of chondrocytes embedded in chondroitin sulphate.

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chondrocyte

The cells that make up the matrix of cartilage.

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Chordata

A phylum of animals characterized by a hollow dorsal nerve cord and, at some stage in their development, a flexible skeletal rod (the notochord) and gill slits opening from the pharynx. There are four subphyla: the Urochordata (sea squirts), Cephalochordata (lancelets), Agnatha (jawless chordates), and Gnathostomata (jawed chordates). In the Agnatha and Gnathostomata, commonly known as vertebrates or craniates, the notochord is present only in the embryo or larva and becomes replaced by the vertebral column (backbone) before birth or metamorphosis. This has permitted the vertebrates a greater degree of movement and subsequent improvement in the sense organs and enlargement of the brain, which is enclosed in a skeletal case, the cranium.
In some classifications the two nonvertebrate subphyla are elevated to the status of phyla and the jawed and jawless chordates are included together in a third phylum, Craniata, containing a single subphylum, Vertebrata. The old subphyla Agnatha and Gnathostomata are then regarded as superclasses of the Vertebrata.

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chordotonal organs

Sensory receptors that detect alterations in the tension of insect muscles.

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chorion

1 A membrane enclosing the embryo, yolk sac, and allantois of reptiles, birds, and mammals. In mammals a section of the chorion becomes the embryonic part of the placenta. See extraembryonic membranes.

2 The protective shell of an insect egg, produced by the ovary. It is pierced by a small pore (micropyle ) that allows the entry of spermatozoa for fertilization. See also egg membrane.
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chorionic gonadotrophin

See gonadotrophin.

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choroid

A pigmented layer, rich in blood vessels, that lies between the retina and the sclerotic of the vertebrate eye. At the front of the eye the choroid is modified to form the ciliary body and the iris.

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choroid plexus

A membrane rich in blood vessels that lines the ventricles of the brain. It is an extension of the pia mater and secretes cerebrospinal fluid into the ventricles; it also controls exchange of materials between the blood and cerebrospinal fluid.

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chromaffin tissue

A group of cells in the adrenal medulla (see adrenal glands) that synthesizes noradrenaline. Chromaffin tissue also contains the enzyme that converts noradrenaline into adrenaline.

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chromatid

A threadlike strand formed from a chromosome during the early stages of cell division. Each chromosome divides along its length into two chromatids, which are at first held together at the centromere. They separate completely at a later stage. The DNA of the chromosome reproduces itself exactly so that each chromatid has the complete amount of DNA and becomes a daughter chromosome with exactly the same genes as the original chromosome from which it was formed.

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chromatin

The substance of which eukaryotic chromosomes are composed. It consists of proteins (principally histones), DNA, and small amounts of RNA (see nucleosome). In a metabolically inactive nucleus, chromatin is mainly in a condensed form, heterochromatin , but in an active nucleus most of the chromatin is in an expanded form, euchromatin . Messenger RNA molecules are formed in euchromatic regions.

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chromatogram

A record obtained by chromatography. The term is applied to the developed records of paper chromatography and thin-layer chromatography and also to the graphical record produced in gas–liquid chromatography.

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chromatography

A technique for analysing or separating mixtures of gases, liquids, or dissolved substances, such as mixtures of amino acids or chlorophyll pigments. The original technique (invented by the Russian botanist Mikhail Tsvet (1872–1919) in 1906) is a good example of column chromatography . A vertical glass tube is packed with an adsorbing material, such as alumina. The sample is poured into the column and continuously washed through with a solvent (a process known as elution ). Different components of the sample are adsorbed to different extents and move down the column at different rates. In Tsvet's original application, plant pigments were used and these separated into coloured bands in passing down the column (hence the name chromatography). The usual method is to collect the liquid (the eluate ) as it passes out from the column in fractions.
In general, all types of chromatography involve two distinct phases – the stationary phase (the adsorbent material in the column in the example above) and the moving phase (the solution in the example). The separation depends on competition for molecules of sample between the moving phase and the stationary phase. The form of column chromatography above is an example of adsorption chromatography , in which the sample molecules are adsorbed on the alumina. In partition chromatography , a liquid (e.g. water) is first absorbed by the stationary phase and the moving phase is an immiscible liquid. The separation is then by partition between the two liquids. In ion-exchange chromatography the process involves competition between different ions for ionic sites on the stationary phase (see ion exchange). Gel filtration is another chromatographic technique in which the size of the sample molecules is important.
See also affinity chromatography; gas–liquid chromatography; paper chromatography; thin-layer chromatography.

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chromatophore

1 A pigment-containing cell found in the skin of many lower vertebrates (e.g. chameleon) and in the integument of crustaceans. Concentration or dispersion of the pigment granules in the cytoplasm of the cell causes the colour of the animal to alter to match its surroundings. A common type of chromatophore is the melanophore , which contains the pigment melanin.

2 A membrane-bound structure in photosynthetic bacteria that contains photosynthetic pigments. See bacteriochlorophyll.
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chromophore

Any group of atoms in a molecule that includes an unsaturated chemical group (such as C=C) capable of absorbing ultraviolet radiation; the chromophore is responsible for the colour of the compound. For example, retinal is the chromophore of rhodopsin, the light-sensitive pigment in the rod cells of the retina.

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chromoplast

Any of various organelles in plant cells that contain pigments. Red, orange, and yellow chromoplasts contain carotenoid pigments, while green chromoplasts (called chloroplasts) contain the pigment chlorophyll and provide the green coloration of most plants. Compare leucoplast. See plastid.

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chromosome

A threadlike structure several to many of which are found in the nucleus of plant and animal (eukaryotic) cells. Chromosomes are composed of chromatin and carry the genes in a linear sequence; these determine the individual characteristics of an organism. When the nucleus is not dividing, individual chromosomes cannot be identified with a light microscope. During the first stage of nuclear division, however, the chromosomes contract and, when stained, can be clearly seen under a microscope. Each consists of two chromatids held together at the centromere (see also meiosis; mitosis). The number of chromosomes in each cell is constant for and characteristic of the species concerned. In the normal body cells of diploid organisms the chromosomes occur in pairs (see homologous chromosomes); in the gamete-forming germ cells, however, the diploid number is halved and each cell contains only one member of each chromosome pair. Thus in humans each body cell contains 46 chromosomes (22 matched pairs and one pair of sex chromosomes) and each germ cell 23. Abnormalities in the number or structure of chromosomes may give rise to abnormalities in the individual; Down's syndrome is the result of one such abnormality. See chromosome mutation.
Bacterial and viral cells contain only a single chromosome; it differs from the eukaryotic chromosome in being much simpler, lacking histones and consisting simply of a single or double strand of DNA or (in some viruses) RNA.

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chromosome diminution

A phenomenon seen in embryonic nematode worms in which the germ cells become separated from the somatic cells at an early stage of development. The chromosomes of the somatic cells tend to break down, with only a few remaining intact, whereas the integrity of the germ-cell chromosomes is maintained.

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chromosome jumping

See chromosome walking.

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chromosome map (linkage map)

A plan showing the relative positions of genes along the length of the chromosomes of an organism. It is constructed by making crosses and observing whether certain characteristics tend to be inherited together. The closer together two allele pairs are situated on homologous chromosomes, the less often will they be separated and rearranged as the reproductive cells are formed (see chiasma; crossing over). The proportion of offspring that show recombination of the alleles concerned thus reflects their spacing and is used as a unit of length in mapping chromosomes (see crossover value). The information obtained from such a classical linkage map can now be combined with a restriction map, which is a linkage map of sites cleaved by restriction enzymes (see restriction mapping), providing a huge number of potential marker sites for genes of interest. Ultimately, a physical map of a chromosome, or chromosome fragment, may be constructed, which gives the base sequence of the chromosomal DNA. This can be achieved by taking cloned segments of DNA from a gene library of the chromosome and fitting them together to form a series of overlapping, or contiguous, segments, called contigs . Once the correct order of contigs has been established, the base sequence of each can be determined, and hence the overall base sequence of the chromosomal DNA established. See also Human Genome Project.

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chromosome mutation

A change in the gross structure of a chromosome, which usually causes severely deleterious effects in the organism. Chromosome mutations often occur due to an error in pairing during the crossing over stage of meiosis. The main types of chromosome mutation include translocation, duplication, deletion, and inversion. Compare point mutation. See also mutation.

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chromosome walking

A technique used to select contiguous overlapping clones from a gene library and thus reconstruct the order of genes along a segment of chromosome. For example, one can effectively `walk' in either direction from a known marker gene to identify adjacent genes, a technique that is useful in fine-structure mapping of a genome. Essentially, the initial clone containing the marker gene is fragmented and each fragment subcloned for use as a gene probe to identify other clones containing adjacent and overlapping segments. In turn, these adjacent segments are fragmented and subcloned and used to probe for further overlaps, and so on. The cloned segments can then be placed in order corresponding to that on the chromosome. In a refinement of the technique, called chromosome jumping , only the ends of segments are identified, allowing the investigator to `jump' over the middle regions. This speeds up the process, and also is a way of bypassing stretches of repetitive DNA, which are not amenable to chromosome walking.

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chrysalis

See pupa.

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Chrysomonada (Chrysophyta; golden-brown algae)

A large phylum of mostly freshwater algae that possess carotenoid pigments (responsible for their colour), in addition to chlorophylls. Their main storage products are oils and the polysaccharide chrysolaminarin . Most chrysophytes are unicellular, with two unequal-sized undulipodia (flagella), but some form colonies or are filamentous; one group of marine chrysophytes construct tests of silica.

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chyle

A milky fluid consisting of lymph that contains absorbed food materials (especially emulsified fats). Most chyle occurs in the lymphatic ducts (lacteals) in the villi of the small intestine during the absorption of fat.

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chylomicron

Any of the lipoprotein particles synthesized by intestinal epithelial cells and consisting mainly of triglycerides. Chylomicrons are the form in which dietary fat is transported in the circulatory system.

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chyme

The semisolid and partly digested food that is discharged from the stomach into the duodenum.

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chymosin

See rennin.

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chymotrypsin

An endopeptidase enzyme in pancreatic juice that is secreted into the duodenum. The enzyme is secreted as an inactive precursor, chymotrypsinogen , which is activated by another pancreatic protease, trypsin.

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chymotrypsinogen

See chymotrypsin.

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ciliary body

The circular band of tissue surrounding and supporting the lens of the vertebrate eye. It contains the ciliary muscles , which bring about changes in the shape of the lens (see also accommodation). The ciliary body produces the aqueous humour.

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ciliary feeding

A method of feeding used by lancelets and many other aquatic invertebrates. The movement of cilia causes a current of water to be drawn towards and through the animal, and microorganisms in the water are filtered out by the cilia.

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ciliary muscle

See ciliary body.

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ciliated epithelium

A region of epithelium consisting of columnar or cuboidal cells bearing hairlike appendages (see cilium) that are capable of beating rapidly. Ciliated epithelium performs the function of moving particles or fluid over the epithelial surface in such structures as the trachea, bronchial tubes, and nasal cavities. It often occurs in the vicinity of mucus-secreting goblet cells.

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Ciliophora

A phylum of the Protoctista containing ciliated protozoa – ciliates – (including Paramecium) that possess two types of nuclei, a micronucleus and macronucleus (see nucleus). The cilia are used for feeding and locomotion. Ciliates reproduce sexually by conjugation.

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cilium

(pl. cilia) A short minute hairlike structure (up to 10 µm long) present on the surface of many cells, notably in certain protozoans and some types of vertebrate epithelium. Cilia usually occur in large groups and are shorter than eukaryotic flagella, although both organelles have the same structure and are collectively termed undulipodia (see undulipodium). Beating of cilia can produce cell movement or create a current in fluid surrounding a cell. See also axoneme.

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circadian rhythm (diurnal rhythm)

Any 24-hour periodicity in the behaviour or physiology of animals or plants. Examples are the sleep/activity cycle in many animals and the growth movements of plants. Circadian rhythms are generally controlled by biological clocks.

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circulation

The mass flow of fluid (e.g. blood or lymph) through the tissues and organs of an animal, allowing the transport and exchange of such materials as oxygen, nutrients, and waste products (see also vascular system; lymphatic system). Smaller animals (e.g. arthropods and most molluscs) have an open circulation , i.e. the blood is pumped into the body cavity, in which the internal organs are suspended. In open circulatory systems the tissues are in direct contact with the blood and materials are exchanged directly by diffusion. In a closed circulation , found in larger animals, the blood flows in vessels, which usually contain a series of one-way valves to maintain the flow in one direction. See also double circulation; single circulation.

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circulatory system

The heart, blood vessels, blood, lymphatic vessels, and lymph, which together serve to transport materials throughout the body. See also double circulation; single circulation; vascular system.

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cisterna

(pl. cisternae) Any of the membrane-bound sacs that form the branches of the endoplasmic reticulum (ER) and Golgi apparatus. The cisternae of the ER are continuous with each other and with the nuclear envelope; in rough ER the cisternae are flattened sacs, whereas in smooth ER they are more tubular in shape.

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cis-trans test

A test used to determine whether two independent mutations, affecting the same phenotypic function, occur within the same cistron or on different cistrons. A pair of homologous chromosomes are brought together in the same cell (normally a bacterial cell). Mutations located on the same chromosome are described as being in the cis position; if they occur one on each homologue they are in the trans position. If the cell is phenotypically normal for both cis and trans positions of the mutations, these mutations are considered to be present in different cistrons. If the resultant cell is normal when the mutations are in the cis position but mutant when the mutations are in the trans position, the two mutations are considered to lie in the same cistron. See illustration.

Cis-trans test

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cistron

A length of DNA that contains the information for coding a specific polypeptide chain. A cistron codes for a messenger RNA (mRNA) molecule; it does not include sections of DNA that may have noncoding functions or code for ribosomal or transfer RNA. The cistron as a unit of function was defined by the cis-trans test: mutations occurring within a cistron affect the same function.

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citric acid

A white crystalline hydroxy carboxylic acid, HOOCCH_2C(OH)(COOH)CH_2COOH. It is present in citrus fruits and is an intermediate in the Krebs cycle in plant and animal cells.

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citric-acid cycle

See Krebs cycle.

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CJD

See Creutzfeldt–Jakob disease.

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clade

A group of organisms that share a common ancestor. See cladistics.

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cladistics

A controversial method of classification in which animals and plants are placed into taxonomic groups called clades strictly according to their evolutionary relationships. These relationships are deduced on the basis of certain shared homologous characters (known as homologies ) that are thought to indicate common ancestry (see monophyletic). Implicit in this is the assumption that two new species are formed suddenly, by splitting from a common ancestor, and not by gradual evolutionary change. A diagram indicating these relationships (called a cladogram ) therefore consists of a system of dichotomous branches: each point of branching represents divergence from a common ancestor, as shown in the diagram. Thus the species A to F form a clade as they share the common ancestor X, and species A to D form a clade of a different taxonomic rank, sharing the ancestor X_2. Species C to F do not form a clade, since the latter must include all the descendants of a common ancestor.

A cladogram showing the relationships of six species (A-F)

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cladode (cladophyll)

A flattened stem or internode that resembles and functions as a leaf. It is an adaptation to reduce water loss, since it contains fewer stomata than a leaf. An example of a plant with cladodes is asparagus.

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cladogram

See cladistics.

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cladophyll

See cladode.

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class

A category used in the classification of organisms that consists of similar or closely related orders. Similar classes are grouped into a phylum. Examples include Mammalia (mammals), Aves (birds), and Dicotyledonae (dicots).

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classification

The arrangement of organisms into a series of groups based on physiological, biochemical, anatomical, or other relationships. An artificial classification is based on one or a few characters simply for ease of identification or for a specific purpose; for example, birds are often arranged according to habit and habitat (seabirds, songbirds, birds of prey, etc.) while fungi may be classified as edible or poisonous. Such systems do not reflect evolutionary relationships. A natural classification is based on resemblances and is a hierarchical arrangement. The smallest group commonly used is the species. Species are grouped into genera (see genus), the hierarchy continuing up through tribes, families, orders, classes, and phyla (see phylum) to kingdom. (In traditional systems of plant classification the phylum was replaced by the division.) Higher up in the hierarchy the similarities between members of a group become fewer. Present-day natural classifications try to take into account as many features as possible and in so doing aim to reflect evolutionary relationships (see cladistics). Natural classifications are also predictive. Thus if an organism is placed in a particular genus because it shows certain features characteristic of the genus, then it can be assumed it is very likely to possess most (if not all) of the other features of that genus. See also binomial nomenclature; taxonomy.

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clathrin

A protein that is the major constituent of the `coat' of the coated pits and coated vesicles formed during endocytosis of materials at the surface of cells (see endosome). The clathrin molecules are arranged in a localized polyhedral lattice on the membrane, which subsequently invaginates to form the coated pit and vesicle. When the coated vesicle has delivered its contents within the cell, the clathrin-coated membrane is recycled and returns to the cell surface. A similar process also tranfers materials between membranous organelles within the cell.

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clavicle

A bone that forms part of the pectoral (shoulder) girdle, linking the scapula (shoulder blade) to the sternum (breastbone). In humans it forms the collar bone and serves as a brace for the shoulders.

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clay

An inorganic constituent of soils consisting chiefly of clay minerals (mainly hydrous silicates of aluminium) in the form of particles less than 0.002mm in diameter. See also flocculation.

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cleavage

(in embryology) The series of cell divisions by which a single fertilized egg cell is transformed into a multicellular body, the blastula. Characteristically no growth occurs during cleavage, the shape of the embryo is unchanged except for the formation of a central cavity (the blastocoel), and the ratio of nuclear material (DNA) to cytoplasm increases.

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cleistothecium

See ascocarp.

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climate

The characteristic pattern of weather elements in an area over a period. The weather elements include temperature, rainfall, humidity, solar insolation, wind, etc. The climate of a large area is determined by several climatic controls: (1) the latitude of the area, which accounts for the amount of solar radiation it receives; (2) the distribution of land and sea masses; (3) the altitude and topography of the area; and (4) the location of the area in relation to the ocean currents. Weather elements are important abiotic factors.

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climax community

A relatively stable ecological community that is achieved at the end of a succession.

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cline

A gradual variation in the characteristics of a species or population over its geographical range. It occurs in response to varying environmental factors, such as soil type or climate.

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clinostat

A mechanical device that rotates whole plants (usually seedlings), so removing the effect of any stimulus that normally acts in one particular direction. It is most often used to study the growth of plant organs when the influence of gravity has been removed.

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clitellum

A thickened glandular region of the body in certain annelid worms that is prominent when the worm becomes sexually mature. The clitellum possesses long slender chaetae. Goblet cells in the clitellum secrete mucus, which forms the cocoon in which fertilized eggs develop.

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clitoris

An erectile rod of tissue in female mammals (and also some reptiles and birds) that is the equivalent of the male penis. It lies in front of the urethra and vagina.

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cloaca

The cavity in the pelvic region into which the terminal parts of the alimentary canal and the urinogenital ducts open in most vertebrates. Placental mammals, however, have a separate anus and urinogenital opening.

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clonal selection theory

A theory explaining how the cells of the immune system produce large quantities of the right antibody at the right time, i.e. when the appropriate antigen is encountered. It proposes that there is a pre-existing pool of lymphocytes (B cells) consisting of numerous small subsets. Each subset carries a unique set of surface antibody molecules with its own particular binding characteristics. If a cell encounters and binds the corresponding antigen it is `selected' – stimulated to divide repeatedly and produce a large clone of identical cells, all secreting the antibody. The involvement of helper T cells (see T cell) is essential for activation of the B cell. A form of clonal selection is also invoked to explain the development of immunological tolerance.

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clone

1 A group of cells, an organism, or a population of organisms arising from a single ancestral cell. All members of a particular clone are genetically identical. In nature clones are produced by asexual reproduction, for example by the formation of bulbs and tubers in plants or by parthenogenesis in certain animals. New techniques of cell manipulation and tissue culture have enabled the cloning of many plants and some animals. A wide range of commercially important plant species, including potatoes, tulips, and certain forest trees, are now cloned by micropropagation, resulting in more uniform crops. Cloning in animals is more complex, but has been accomplished successfully in sheep and cattle. The first mammal ever to be cloned experimentally from the body cell of an adult was a sheep (`Dolly') born in 1997 after over 200 previous failed attempts. The nucleus containing DNA was extracted from an udder cell (which had been deprived of nutrients) and inserted into an `empty' egg cell (from which the nucleus had been removed). This reconstituted egg cell was then stimulated to divide by an electric shock and implanted into the uterus of a surrogate mother ewe, who subsequently gave birth to a clone of the original sheep. This breakthrough offers the prospect of producing exact replicas of animals with certain genetically engineered traits, for example to manufacture drugs in their milk or provide organs for human transplantation.

2 (gene clone) An exact replica of a gene. See gene cloning.
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cloning vector

See vector.

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closed circulation

See circulation.

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clotting factors (coagulation factors)

A group of substances present in blood plasma that, under certain circumstances, undergo a series of chemical reactions leading to the conversion of blood from a liquid to a solid state (see blood clotting). Although they have specific names, most coagulation factors are referred to by an agreed set of Roman numerals (e.g. Factor VIII, Factor IX). Lack of any of these factors in the blood results in the inability of the blood to clot. See also haemophilia.

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clubmoss

See Lycophyta.

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cluster of differentiation

See CD.

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Cnidaria

A phylum of aquatic invertebrates (sometimes known as coelenterates) that includes Hydra, jellyfish, sea anemones, and corals. A cnidarian's body is diploblastic, with two cell layers of the body wall separated by mesoglea, and shows radial symmetry. The body cavity (gastrovascular cavity ) is sac-shaped, with one opening acting as both mouth and anus. This opening is surrounded by tentacles bearing thread cells. Cnidarians exist both as free-swimming medusae (e.g. jellyfish) and as sedentary polyps. The latter may be colonial (e.g. corals) or solitary (e.g. sea anemones and Hydra). In many cnidarians the life cycle alternates between these two forms (see alternation of generations). The phylum contains the classes Hydrozoa (e.g. Hydra, Obelia), most members of which show alternation of generations; Scyphozoa (jellyfish), in which the medusa phase is dominant; and Anthozoa (corals and sea anemones), in which medusae are absent. See also coelenterate; Ctenophora.

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cnidoblast

See thread cell.

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CNS

See central nervous system.

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CoA

See coenzyme A.

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coacervate

An aggregate of macromolecules, such as proteins, lipids, and nucleic acids, that form a stable colloid unit with properties that resemble living matter. Many are coated with a lipid membrane and contain enzymes that are capable of converting such substances as glucose into more complex molecules, such as starch. Coacervate droplets arise spontaneously under appropriate conditions and may have been the prebiological systems from which living organisms originated.

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coadaptation

The mutual adaptation of two species that occurs during coevolution.

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coagulation

The process in which colloidal particles come together irreversibly to form larger masses. Coagulation can be brought about by adding ions to change the ionic strength of the solution and thus destabilize the colloid (see flocculation). Ions with a high charge are particularly effective (e.g. alum, containing Al3+, is used in styptics to coagulate blood). Alum and iron(III) sulphate are also used for coagulation in sewage treatment. Heating is another way of coagulating certain colloids (e.g. boiling an egg coagulates the albumin). See also blood clotting.

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coal

A brown or black carbonaceous deposit derived from the accumulation and alteration of ancient vegetation, which originated largely in swamps or other moist environments. As the vegetation decomposed it formed layers of peat, which were subsequently buried (for example, by marine sediments following a rise in sea level or subsidence of the land). Under the increased pressure and resulting higher temperatures the peat was transformed into coal. Two types of coal are recognized: humic (or woody ) coals , derived from plant remains; and sapropelic coals , which are derived from algae, spores, and finely divided plant material.
As the processes of coalification (i.e. the transformation resulting from the high temperatures and pressures) continue, there is a progressive transformation of the deposit: the proportion of carbon relative to oxygen rises and volatile substances and water are driven out. The various stages in this process are referred to as the ranks of the coal. In ascending order, the main ranks of coal are: lignite (or brown coal ), which is soft, brown, and has a high moisture content; subbituminous coal , which is used chiefly by generating stations; bituminous coal , which is the most abundant rank of coal; semibituminous coal ; semianthracite coal , which has a fixed carbon content of between 86% and 92%; and anthracite coal , which is hard and black with a fixed carbon content of between 92% and 98%.
Most deposits of coal were formed during the Carboniferous and Permian periods. More recent periods of coal formation occurred during the early Jurassic and Tertiary periods. Coal deposits occur in all the major continents; the leading producers include the USA, China, Ukraine, Poland, UK, South Africa, India, Australia, and Germany. Coal is used as a fuel and in the chemical industry; by-products include coke and coal tar.

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cobalamin (vitamin B_12)

See vitamin B complex.

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cobalt

Symbol Co. A light-grey metallic element that is a trace element (see essential element) required by animals. It is present in foods of animal origin and is a constituent of vitamin B_12. Cobalt is also a micronutrient for plants.

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coccus

(pl. cocci) Any spherical bacterium. Cocci may occur singly, in pairs, in groups of four or more, in cubical packets, in grapelike clusters (Staphylococcus), or in chains (Streptococcus). Staphylococci and streptococci include pathogenic species. They are generally nonmotile and do not form spores.

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coccyx

The last bone in the vertebral column in apes and humans (i.e. tailless primates). It is formed by the fusion of 3–5 caudal vertebrae.

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cochlea

Part of the inner ear of mammals, birds, and some reptiles that transforms sound waves into nerve impulses. In mammals it is coiled, resembling a snail shell, and is divided by membranes into three parallel canals (see illustration): the vestibular canal (scala vestibule), cochlear duct (scala media), and tympanic canal (scala tympani). The cochlear duct is filled with a fluid (see endolymph) and contains the organ of Corti, which houses the sound receptors. The other two canals also contain a fluid (see perilymph). Sound-induced vibrations of the oval window are transmitted through the perilymph and endolymph and stimulate hair cells in the organ of Corti. These in turn stimulate nerve cells that transmit information, via the auditory nerve, to the brain for interpretation of the sounds.

Diagram of the cochlea (coiling reduced for simplicity)

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cockroaches

See Dictyoptera.

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cocoon

A protective covering for eggs and/or larvae produced by many invertebrates. For example, the larvae of many insects spin a cocoon in which the pupae develop (that of the silkworm moth produces silk), and earthworms secrete a cocoon for the developing eggs (see clitellum).

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codeine

A pain-relieving drug that is derived from the plant Papaver somniferum. See opiate; analgesic.

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coding strand (plus strand; sense strand)

The strand in duplex DNA that by convention contains the same sequence of bases as that in the messenger RNA (mRNA) transcribed from the DNA (except that the RNA has uracil substituting for thymine). It is complementary to the other anticoding strand, which is actually the one used as a template for mRNA assembly during transcription.

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co-dominance

The condition that arises when both alleles in a heterozygous organism are dominant and are fully expressed in the phenotype. For example, the human blood group AB is the result of two alleles, A and B, both being expressed. A is not dominant to B, nor vice versa. Compare incomplete dominance.

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codon

A triplet of nucleotides within a molecule of messenger RNA that functions as a unit of genetic coding (the triplet code ), usually by specifying a particular amino acid during the synthesis of proteins in a cell (see genetic code). A few codons specify instructions during this process (see start codon; stop codon). The term codon may also refer to any of the corresponding nucleotide triplets of DNA that are transcribed into codons. See also reading frame. Compare anticodon.

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coelacanth

A bony fish of the genus Latimeria, which was believed to have been extinct until 1938, when the first modern specimen of L. chalumnae was discovered in the Indian Ocean around the Comoros Islands, off the SE coast of Africa. A second species, L. menadoensis, was discovered in 1999 in the Celebes Sea, SE Asia. The coelacanth belongs to the same order (Crossopterygii – lobe-finned fishes) as the ancestors of the amphibians. It is a large fish, 1–2m long and weighing 80kg or more, with a three-lobed tail fin. The body is covered in rough heavy scales and the pectoral fins can be used like crutches to help movement across the sea bed. The young are born alive. Fossil coelacanths are most abundant in deposits about 400 million years old and no fossils less than 70 million years old have been found.

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coelenterate

An invertebrate animal with a radially symmetrical body, a body wall consisting of two cell layers, and a body cavity (gastrovascular cavity ) that communicates with the outside via a single opening, the mouth. Coelenterates include hydras, jellyfish, sea anemones, and corals (see Cnidaria) and also the comb jellies (see Ctenophora).

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coelom

A fluid-filled cavity that forms the main body cavity of vertebrate and most invertebrate animals. It is formed by the splitting of the mesoderm. Ciliated ducts (coelomoducts ) connect the coelom to the exterior allowing the exit of waste products and gametes; in higher animals these are specialized as oviducts, etc. The coelom is large and often subdivided in annelid worms (in which it functions as a hydrostatic skeleton) and vertebrates. In arthropods it is restricted to the cavities of the gonads and excretory organs, the body cavity being a blood-filled haemocoel.

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coelomoduct

See coelom.

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coenobium

(pl. coenobia) A loose association of unicellular organisms that live in a colony and may be surrounded by a common membrane. Often the cells are held together in a jelly excreted by all individuals in the colony. Both algae and bacteria form coenobia. For example, Volvox (a green alga) forms a hollow sphere in which 20000 cells may live; some of these are reproductive and others are concerned with photosynthesis. The cells in this coenobium are interconnected by protoplasmic strands.

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coenocyte

A mass of cytoplasm surrounding many nuclei and enclosed by a cell wall. It is found in certain algae and fungi. Compare cell; plasmodium; syncytium.

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coenzyme

An organic nonprotein molecule that associates with an enzyme molecule in catalysing biochemical reactions. Coenzymes usually participate in the substrate–enzyme interaction by donating or accepting certain chemical groups. Many vitamins are precursors of coenzymes. See also cofactor.

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coenzyme A (CoA)

A complex organic compound that acts in conjunction with enzymes involved in various biochemical reactions, notably the oxidation of pyruvate via the Krebs cycle and fatty-acid oxidation and synthesis (see acetyl coenzyme A). It comprises principally the B vitamin pantothenic acid, the nucleotide adenine, and a ribose–phosphate group (see formula).

Coenzyme A

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coenzyme Q

See ubiquinone.

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coevolution

The evolution of complementary adaptations in two species caused by the selection pressures that each exerts on the other. It is common in symbiotic associations (see symbiosis). For example, many insect-pollinated plants have evolved flowers whose shapes, colours, etc., make them attractive to particular insects; at the same time the pollinating insects have evolved sense organs and mouthparts specialized for quickly locating, and extracting nectar from, particular species of plants.

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cofactor

A nonprotein component essential for the normal catalytic activity of an enzyme. Cofactors may be organic molecules (coenzymes) or inorganic ions. They may activate the enzyme by altering its shape or they may actually participate in the chemical reaction.

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cohesion

1 The force of attraction between like molecules. Cohesion provides the force that holds up a column of water in the xylem tissue of plants without it breaking. The cohesion–tension theory offers an explanation for the continual flow of water upwards through the xylem of a plant. Water is removed from the plant by the process of transpiration, which creates a tension that pulls the water in the xylem upwards as a single column held together by cohesive forces.

2 (in botany) The union of like parts, such as the fusion of petals that occurs in some flowers.
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coitus

See sexual intercourse.

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colchicine

An alkaloid derived from the autumn crocus, Colchicum autumnale. It inhibits spindle formation in cells during mitosis so that chromosomes cannot separate during anaphase, thus inducing multiple sets of chromosomes (see polyploid). Colchicine is used in genetics, cytology, and plant breeding research and also in cancer therapy to inhibit cell division.

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cold-blooded animal

See ectotherm.

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Coleoptera

An order of insects comprising the beetles and weevils and containing about 330000 known species – the largest order in the animal kingdom. The forewings are hardened and thickened to form elytra , which meet at a precise mid-dorsal line and protect the underlying pair of hindwings and abdomen. The mouthparts are generally modified for biting and in some species assume antler-like proportions. Beetles occur in a wide variety of terrestrial and aquatic habitats; many feed on decaying organic matter, some eat living vegetation, while others prey on other arthropods. A number of beetles and weevils are economically important pests of stored grain, timber, and crops. The young emerge as larvae and generally undergo metamorphosis via a pupal stage to form the adult beetle.

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coleoptile

A protective sheath that covers the young shoot of the embryo in plants of the grass family. It bursts open when the first leaves develop. Experiments investigating growth movements of the oat coleoptile led to the discovery of the plant growth substance indoleacetic acid (IAA).

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coleorhiza

A protective sheath that covers the young root of the embryo in plants of the grass family.

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coliform bacteria

A group of Gram-negative rod-shaped bacteria that are found in the vertebrate gastrointestinal tract; their presence in water is an indicator of faecal pollution. They obtain their energy by aerobic respiration or fermentation; some of them can ferment lactose. Well-known coliform bacteria include Escherichia coli and Salmonella.

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collagen

An insoluble fibrous protein found extensively in the connective tissue of skin, tendons, and bone. The polypeptide chains of collagen (containing the amino acids glycine and proline predominantly) form triple-stranded helical coils that are bound together to form fibrils, which have great strength and limited elasticity. Collagen accounts for over 30% of the total body protein of mammals.

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collecting duct

Any of the ducts in the mammalian kidney that drains into the renal pelvis, which leads to the ureter. They are the main sites of water reabsorption from the glomerular filtrate, which drains into the ducts from the distal convoluted tubules of the nephrons. The cells of the collecting ducts are relatively impermeable to water. However, the influence of antidiuretic hormone increases the permeability of the collecting ducts, allowing the reabsorption of water and controlling the final urine concentration according to the body's state of hydration.

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Collembola

An order of small wingless insects, the springtails, less than 10 mm long, which leap by means of a specialized forked organ (furcula ) that is attached on the underside of the abdomen by a special catch (retinaculum ) and acts as a spring. The mouthparts are largely concealed within folds of the head. Most springtails are scavengers but some are pests of leguminous plants. Although some authorities place the Collembola in the subclass Apterygota, with other wingless insects, others regard them as constituting a separate class within the superclass Hexapoda, most closely affiliated with the Protura.

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collenchyma

A plant tissue (see ground tissues) that consists of living cells with additional cellulose thickening in their walls, providing support and protection for young stems and leaves. It is most commonly found in the stem cortex. Compare parenchyma; sclerenchyma.

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colloblast

See lasso cell.

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colloids

Colloids were originally defined by Thomas Graham (1805–69) in 1861 as substances, such as starch or gelatin, which will not diffuse through a membrane. He distinguished them from crystalloids (e.g. inorganic salts), which would pass through membranes. Later it was recognized that colloids were distinguished from true solutions by the presence of particles that were too small to be observed with a normal microscope yet were much larger than normal molecules. Colloids are now regarded as systems in which there are two or more phases, with one (the dispersed phase ) distributed in the other (the continuous phase ). Moreover, at least one of the phases has small dimensions (in the range 10–9–10–6 m). Colloids are classified in various ways.
Sols are dispersions of small solid particles in a liquid. The particles may be macromolecules or clusters of small molecules. Lyophobic sols are those in which there is no affinity between the dispersed phase and the liquid. An example is silver chloride dispersed in water. In such colloids the solid particles have a surface charge, which tends to stop them coming together. Lyophobic sols are inherently unstable and in time the particles aggregate and form a precipitate (see flocculation). Lyophilic sols , on the other hand, are more like true solutions in which the solute molecules are large and have an affinity for the solvent. Starch in water is an example of such a system. Association colloids are systems in which the dispersed phase consists of clusters of molecules that have lyophobic and lyophilic parts. Soap in water is an association colloid (see micelle).
Emulsions are colloidal systems in which the dispersed and continuous phases are both liquids, e.g. oil-in-water or water-in-oil. Such systems require an emulsifying agent to stabilize the dispersed particles.
Gels are colloids in which both dispersed and continuous phases have a three-dimensional network throughout the material, so that it forms a jelly-like mass. Gelatin is a common example. One component may sometimes be removed (e.g. by heating) to leave a rigid gel (e.g. silica gel).
Other types of colloid include aerosols (dispersions of liquid or solid particles in a gas, as in a mist or smoke) and foams (dispersions of gases in liquids or solids).

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colon

The section of the vertebrate large intestine that lies between the caecum and the rectum. Its prime function is to absorb water and minerals from indigestible food residues passing from the small intestine, which results in the formation of faeces.

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colony

1 (in zoology) A group of animals of the same species living together and dependent upon each other. Some, such as the corals and sponges, are physically connected and function as a single unit. Others, such as insect colonies, are not physically joined but show a high level of social organization with members specialized for different functions (see caste).

2 (in microbiology) A group of microorganisms, usually bacteria or yeasts, that are considered to have developed from a single parent cell. Colonies that grow on agar plates differ in shape, colour, surface texture, and translucency and can therefore be used as a means of identification.
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colony-stimulating factor (CSF)

One of several molecules that regulate the development of blood cells. GM-CSF is a glycoprotein manufactured in all tissues, large amounts being found in the lungs. It induces haemopoietic stem cells (the precursors of mature blood cells) to develop into granulocytes or monocytes (hence GM). G-CSF is a protein that induces stem cells to form granulocytes only. M-CSF , a glycoprotein, causes stem cells to form monocytes only.

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colostrum

A liquid with a high content of nitrogen, antibodies, and vitamins that is secreted from the mammary glands before and just after giving birth. The change of secretion from colostrum to proper milk takes place gradually during the days after birth.

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colour blindness

Any disorder of vision in which colours are confused. The most common type is red–green colour blindness. This is due to a recessive gene carried on the X chromosome (see sex linkage), and therefore men are more likely to show the defect although women may be carriers. It results in absence or malfunctioning of one or more of the three types of cone cell responsible for colour vision. In protanopia the individual lacks cones sensitive to red light; in deuteranopia cones sensitive to green light are absent. Tritanopia is a rare form of colour blindness in which the individual cannot distinguish between blue and green due to a lack of cones sensitive to blue light.

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colour vision

The ability of the eye to detect different wavelengths of light and to distinguish between these different wavelengths and their corresponding colours. In the mammalian eye this is achieved by the cone cells, which are located in and around the fovea near to the centre of the retina. The cone cells contain the light-sensitive pigment iodopsin , which – according to the trichromatic theory – exists in three forms, each form occurring in a different cone cell. Each form of iodopsin is sensitive to either red, blue, or green light. The relative stimulation of each type of cone will determine the colour that is interpreted by the brain. For example, if red cones and green cones are stimulated to an equal extent, the colour interpreted by the brain is yellow. See also colour blindness.
The compound eye of certain insects is also capable of colour vision.

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columnar epithelium

See epithelium.

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commensalism

An interaction between two animal or plant species that habitually live together in which one species (the commensal ) benefits from the association while the other is not significantly affected. For example, the burrows of many marine worms contain commensals that take advantage of the shelter provided but do not affect the worm.

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communicating junction

See gap junction.

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communication

An interaction between two organisms in which information is conveyed from one to the other. Communication can occur between individuals of the same species (intraspecific communication ) or between members of different species (interspecific communication ). It generally involves the transmission of a signal from one organism to another; signals can be visual, chemical, or tactile or they can take the form of sounds. Visual signals between members of the same species are widely used by animals in such activities as defining and protecting territories and finding suitable mates (see courtship; display behaviour; bioluminescence). Chemical and tactile signals also play an important role in these activities (see pheromone). Social species rely heavily on all three types of signalling, the classic example being provided by the dance of the bees, in which information about the distance and direction of a food source is conveyed to other members of the colony. Visual signals, in the form of body coloration, are the principal means of communication between animals of different species (see mimicry; warning coloration). Sounds are more effective than visual signals for intraspecific communication over long distances and at night. Certain insects produce sounds by stridulation, while birdsong and language are sophisticated examples of sound signals in birds and humans, respectively.
Among plants, visual and chemical signals are important in communication. Flowering plants whose flowers are pollinated by insects or other animals depend on the colour, shape, and scent of their flowers to attract suitable pollinating agents. Some plants produce chemical signals to deter competitors and predators (see allelopathy).

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community

A naturally occurring assemblage of plant and animal species living within a defined area or habitat. Communities are named after one of their dominant species (e.g. a pine community) or the major physical characteristics of the area (e.g. a freshwater pond community). Members of a community interact in various ways (e.g. through food chains and competition). Large communities may be divided into smaller component communities. See association.

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companion cell

A type of cell found within the phloem of flowering plants. Each companion cell is usually closely associated with a sieve element. Its function is uncertain, though it appears to regulate the activity of the adjacent sieve element. In gymnosperms a similar function is attributed to albuminous cells , which are found closely associated with gymnosperm sieve elements.

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compass plant

A plant that has its leaves permanently orientated in a north–south direction. Such an arrangement enables the plant to take full advantage of morning and evening sun, while avoiding the stronger midday sunlight. An example is the compass plant of the prairies (Silphium laciniatum).

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compensation point

The point reached in a plant when the rate of photosynthesis is equal to the rate of respiration. This means that the carbon dioxide released from respiration is equivalent to that which is taken up during photosynthesis. The compensation point is reached as light intensity increases. If the light intensity is increased beyond the compensation point, the rate of photosynthesis increases proportionally until the point of light saturation is reached, beyond which the rate of photosynthesis is no longer affected by light intensity.

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competent

Describing embryonic tissue that is capable of developing into a specialized tissue when suitably stimulated. See induction; evocation.

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competition

The interaction that occurs between two or more organisms, populations, or species that share some environmental resource when this is in short supply. Competition is an important force in evolution: plants, for example, become tall to compete for light, and animals evolve various foraging methods to compete for food. There may be a direct confrontation between competitors, as occurs between barnacles competing for space on a rock, or the numbers or fecundity of the competitors are indirectly reduced through joint dependence on limited resources. Competition occurs both between members of a species (intraspecific competition ) and between different species (interspecific competition ). Interspecific competition often results in the dominance of one species over another (see dominant). Since competition ultimately results in the displacement by one competitor of the others, it is to the advantage of the competitors to avoid one another wherever possible. Thus in time the competitors become separated from each other geographically or ecologically, which promotes evolutionary change. Competition for mates may lead to sexual selection.

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competitive exclusion principle

A rule, derived by G. F. Gause in 1934, stating that two species that occupy the same habitat cannot also occupy the same ecological niche. Any two species that occupy the same niche will compete with each other to the detriment of one of the species, which will thus be excluded.

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competitive inhibition

See inhibition.

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complement

A group of proteins present in blood plasma and tissue fluid that aids the body's defences following an immune response; the genes encoding it form part of the major histocompatibility complex. Following an antibody–antigen reaction, complement is activated chemically and becomes bound to the antibody–antigen complex (complement fixation ). It can cause lysis of certain types of bacteria, or it can render the target cell more susceptible to phagocytosis – a process called opsonization . The complement reactions attract scavenging white blood cells (phagocytes) to the area of conflict in the body.

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complemental males

The small males of certain animals that live in or on the females and are usually more or less degenerate apart from the reproductive organs. They occur in certain crustaceans (e.g. some barnacles), in which the normal individuals are hermaphrodite but the complemental males have suppressed ovaries, lose their alimentary canal, and lead a semiparasitic existence in the mantle cavity of the larger partner. This may ensure that cross fertilization occurs.

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complementary DNA (cDNA)

A form of DNA prepared in the laboratory using messenger RNA (mRNA) as template, i.e. the reverse of the usual process of transcription in cells; the synthesis is catalysed by reverse transcriptase. cDNA thus has a base sequence that is complementary to that of the mRNA template; unlike genomic DNA, it contains no noncoding sequences (introns). cDNA is used in gene cloning for the expression of eukaryote genes in prokaryote host cells, or as a gene probe to locate particular base sequences in genomic DNA.

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complementary genes

Two (or more) genes that are interdependent, such that the dominant allele from either gene can only produce an effect on the phenotype of an organism if the dominant allele from the other gene is also present.

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composite fruit

A type of fruit that develops from an inflorescence rather than from a single flower. See pseudocarp; sorosis; strobilus; syconus.

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compost

A mixture of decaying organic matter, such as vegetation and manure, that is used as a fertilizer. The organic material is decomposed by aerobic saprotrophic organisms, mostly fungi and bacteria. Some decomposition is also carried out by detritivores. Compost is used mainly on a domestic scale.

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compound eye

The eye of insects and crustaceans, which consists of numerous visual units, the ommatidia . Each ommatidium consists of an outer cuticle covering a lens, beneath which are 6–8 retinal cells surrounding a light-sensitive rhabdom . Adjacent ommatidia are separated by pigment cells. The eye is convex, with nerve fibres from the retinal cells converging onto the optic nerve. There are two types of compound eye. In apposition eyes , typical of diurnal insects, each ommatidium focuses rays parallel to its long axis so that each gives an image of a minute part of the visual field, producing a detailed mosaic image. In superposition eyes , typical of nocturnal insects, the pigment separating ommatidia migrates to the ends of the cells, so that each ommatidium receives light from a larger part of the visual field and the image may overlap with those received by many neighbouring ommatidia. This produces an image that is bright but lacks sharpness of detail.

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compound microscope

See microscope.

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concanavalin

See lectin.

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concentration

The quantity of dissolved substance per unit quantity of solvent in a solution. Concentration is measured in various ways. The amount of substance dissolved per unit volume (symbol c) has units of moldm–3 or moll–1. It is now called `concentration' (formerly molarity ). The mass concentration (symbol ) is the mass of solute per unit volume of solvent. It has units of kgdm–3, gcm–3, etc. The molal concentration (or molality ; symbol m) is the amount of substance per unit mass of solvent, commonly given in units of molkg–1.

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concentration gradient (diffusion gradient)

The difference in concentration between a region of a solution or gas that has a high density of particles and a region that has a relatively lower density of particles. By random motion, particles will move from the area of high concentration towards the area of low concentration, by the process of diffusion, until the particles are evenly distributed in the solution or gas.

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conceptacle

A flask-shaped cavity with a small opening (the ostiole ) that is found in the swollen tip of certain brown algae, such as Fucus. It contains the sex organs.

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conception

The fertilization of a mammalian egg cell by a sperm cell, which occurs in the fallopian tube. Conception is followed by implantation.

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condensation reaction

A chemical reaction in which two molecules combine to form a larger molecule with the elimination of a small molecule (H_2O in biological systems). See esterification; glycosidic bond; peptide.

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conditional response (conditioned reflex)

A learned response that develops to an initially ineffective stimulus in classical conditioning.

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conditioning

A process by which animals learn about a relation between two events. In classical (or Pavlovian) conditioning , repeated presentations of a neutral stimulus (e.g. the sound of a bell or buzzer) are followed each time by a biologically important stimulus (such as food or electric shock), which elicits a response (e.g. salivation). Eventually the neutral stimulus presented by itself produces a response (the conditional response , or conditioned reflex ) similar to that originally evoked by the biologically important stimulus. For example, Pavlov's dogs learned to salivate in response to the sound of a metronome that preceded the presentation of food. In instrumental (or operant ) conditioning the animal is rewarded (or punished) each time it makes a particular response; this eventually causes the frequency of the response to increase (or decrease). For example, a rat will learn to press a lever in order to obtain food. See learning; reinforcement.

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condyle

A smooth round knob of bone that fits into a socket on an adjoining bone, forming a joint. Such a joint permits up-and-down or side-to-side movement but does not allow rotation. There are condyles where the lower jawbone (mandible) is attached to the skull, which permits chewing movements. See also occipital condyle.

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cone

1 (in botany) A reproductive structure occurring in gymnosperms, known technically as a strobilus . It consists of sporophylls bearing the spore-producing sporangia. Gymnosperms produce different male and female cones. The large woody female cones of pines, firs, and other conifers are made up of structures called ovuliferous scales , which bear the ovules. Cones are also produced by clubmosses and horsetails.

2 (in animal anatomy) A type of light-sensitive receptor cell, found in the retinas of all diurnal vertebrates. Cones are specialized to transmit information about colour (see colour vision) and are responsible for the visual acuity of the eye. They function best in bright light. They are not evenly distributed on the retina, being concentrated in the fovea and absent on the margin of the retina. Compare rod.
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conformer

Any organism whose internal environment is highly influenced by external factors. Many marine invertebrates are conformers: they have no need to control their internal environment since the external environment is fairly constant in terms of temperature, oxygen tension, and nutrients. Compare regulator.

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congenital

Present at birth. Congenital disorders of the body may be due to genetic factors, e.g. Down's syndrome, or caused by injury or environmental factors, e.g. drugs (such as thalidomide), chemicals (such as dioxin), and infections (such as those caused by Listeria and cytomegalovirus).

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conidiospore

See conidium.

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conidium (conidiospore)

A spore of certain fungi, such as mildews and moulds, that is produced by the constriction of the tip of a specialized hypha, the conidiophore . Chains of conidia may be formed in this way; they are cut off, one at a time, from the tip of the hypha.

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Coniferophyta

A phylum of seed-bearing plants comprising the conifers, including the pines, firs, and spruces. Conifers have an extensive fossil record going back to the late Devonian. The gametes are carried in male and female cones, fertilization usually being achieved by wind-borne pollen. The ovules and the seeds into which they develop are borne unprotected (rather than enclosed in a carpel, as are those of the Anthophyta). Internal tissue and cell structure of these species is not as advanced as in the angiosperms. Conifers are typically evergreen trees inhabiting cool temperate regions and have leaves reduced to needles or scales. The wood of conifers, which is called softwood in contrast to the hardwood of angiosperm trees, is widely used for timber and pulp. See also gymnosperm.

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conjugation

1 The fusion of two reproductive cells, particularly when these are both the same size (see isogamy).

2 A form of sexual reproduction seen in some algae (e.g. Spirogyra), some bacteria (e.g. Escherichia coli), and ciliate protozoans. Two individuals are united by a tube formed by outgrowths from one or both of the cells. Genetic material from one cell (designated the male) then passes through the tube to unite with that in the other (female) cell. In bacteria conjugation is initiated and directed by sex factors.
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conjunctiva

The delicate membrane that covers the cornea and lines the inside of the eyelid of a vertebrate eye. It is kept clean by secretions of the lacrimal (tear) gland and the reflex blink mechanism.

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connective tissue

An animal tissue consisting of a small number of cells (e.g. fibroblasts and mast cells) and fibres (see collagen; elastin) and a large amount of intercellular material (matrix , or ground substance). It is widely distributed and has many functions, including support, packing, defence, and repair. The individual constituents vary, depending on the function of the tissue. Different types of connective tissue include mesenchyme in the embryo, adipose tissue, areolar connective tissue, blood, lymph, cartilage, and bone.

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consensual

Of or relating to movements that take place through the action of the nervous system independently of the will. These movements generally involve the involuntary reaction being correlated with a voluntary action. For example, the reflex action of both pupils contracting will occur even if only one retina has been stimulated by bright light (see pupillary reflex).

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consensus sequence

A sequence of nucleotides found in comparable regions of DNA or RNA, e.g. in the promoter regions (see operon) of different genes, in which certain bases occur with a frequency significantly greater than that expected by chance. Although such sequences may vary from case to case, it is possible to derive the most likely sequence overall. An example is the Pribnow box of prokaryote promoters. The term is also applied to sequences of amino acids in polypeptides.

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conservation

The sensible use of the earth's natural resources in order to avoid excessive degradation and impoverishment of the environment (see desertification). It should include the search for alternative food and fuel supplies when these are endangered (as by deforestation and overfishing); an awareness of the dangers of pollution; and the maintenance and preservation of natural habitats and the creation of new ones (e.g. nature reserves, national parks, and SSSIs).

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conservative replication

A hypothesis suggesting that DNA replication occurs by one DNA molecule initiating the synthesis of a new molecule while remaining intact. The preferred theory (see semiconservative replication) proposes that the DNA molecule divides to provide two templates for synthesizing the other half of the molecule. Compare dispersive replication.

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conserved sequence

A sequence consisting of exactly the same nucleotides in comparable segments of DNA or RNA, or of exactly the same amino acids in corresponding segments of different polypeptides. For example, the active site of a particular enzyme generally consists of exactly the same amino acid residues, regardless of which species it is taken from.

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consociation

A climax plant community that is dominated by one particular species, e.g. a pine forest. See dominant. Compare association.

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consumer

An organism that feeds upon those below it in a food chain (i.e. at the preceding trophic level). Herbivores, which feed upon green plants, are primary consumers ; a carnivore that feeds only upon herbivores is a secondary consumer ; a tertiary consumer is a carnivore that feeds on other carnivores. The consumer at the end of a food chain is known as the top carnivore . Compare producer.

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contact insecticide

Any insecticide (see pesticide) that kills its target insect by being absorbed through the cuticle or by blocking the spiracles, rather than by being ingested.

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contig

See chromosome map.

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continental drift

The theory that the earth's continents once formed a single mass and have since moved relative to each other. It was first postulated by A. Snider in 1858 and greatly developed by Alfred Wegener (1880–1930) in 1912. He used evidence, such as the fit of South America into Africa and the distribution of rock types, flora, fauna, and geological structures, to suggest that the present distribution of the continents results from the breaking up of one or two greater land masses. The original land mass was named Pangaea and it was suggested that this broke up into the northerly Laurasia and the southerly Gondwanaland (see illustration). The theory was not accepted for about 50 years by the majority of geologists but during the early 1960s, the seafloor-spreading hypothesis of Harry Hess (1906–69) and the subsequent development of plate tectonics produced a mechanism to explain the drift of the continents.

Continental drift

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continuous culture

A technique used to grow microorganisms or cells continually in a particular phase of growth. For example, if a constant supply of cells is required, a cell culture maintained in the log phase is best; the conditions must therefore be continually monitored and adjusted accordingly so that the cells do not enter the stationary phase (see bacterial growth curve). Growth may also have to be maintained in a particular growth phase if an enzyme or chemical product is produced only during that phase.

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continuous replication

See discontinuous replication.

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continuous variation (quantitative variation)

The range of differences that can be observed in many characteristics in a population. Characteristics resulting from polygenic inheritance show continuous variation, e.g. the wide range of foot sizes in an adult human population. Compare discontinuous variation.

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contour feathers

Feathers that are arranged in regular rows on a bird's body, giving the body its streamlined shape. Each has a central horny shaft (the rachis ) with a flattened vane on each side. Each vane is composed of two rows of filament-like barbs, which are connected to each other by means of hooked barbules to form a smooth surface. There is often a small second vane, the aftershaft , near the base of the feather.

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contraception

See birth control.

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contractile root

Any of the modified adventitious roots that develop from the base of the stem of a bulb or corm. The new bulb or corm develops at a higher level in the soil than the old one. The contractile roots shorten and pull it down to a suitable level.

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contractile vacuole

A membrane-surrounded cavity in a cell that periodically expands, filling with water, and then suddenly contracts, expelling its contents to the cell's exterior. It is thus an organ of osmoregulation and excretion. Contractile vacuoles are common in freshwater sponges and typical of freshwater protoctists, such as Amoeba (which has one spherical vacuole) and Paramecium (in which a number of accessory vacuoles are attached to a main vacuole).

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contraction

(in animal physiology) The shortening of muscle fibres in order to exert a force on a tissue or organ of the body. In striated muscle contraction is brought about by interaction of actin and myosin filaments (see sarcomere; sliding filament theory; voluntary muscle): it provides a force for locomotion and plays a role in maintaining the balance and posture of the animal. See also involuntary muscle.

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control

1 The part of an experiment that acts as a standard by which to compare experimental observations.

2 The natural regulation of biological processes. See control mechanism.
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3 See biological control; chemical control.
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control mechanism

Any mechanism that regulates a biological process, such as a metabolic pathway or enzyme-controlled reaction, or that helps to maintain the internal environment (see homeostasis). See also feedback.

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conus arteriosus

A small thick-walled chamber of the heart of vertebrate embryos that receives blood from the single ventricle and leads to the ventral aorta. It is retained as a distinct structure in adult fishes and (with modifications) amphibians, but in higher vertebrates it is incorporated into the roots of the aorta and pulmonary arteries.

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convergent evolution

The development of superficially similar structures in unrelated organisms, usually because the organisms live in the same kind of environment. Examples are the wings of insects and birds and the streamlined bodies of whales and fish. Compare adaptive radiation.

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convoluted tubule

See distal convoluted tubule; proximal convoluted tubule; nephron.

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coomassie blue

A biological dye used for staining proteins.

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cooperation

An association between two or more members of the same species (intraspecific cooperation ), or between individuals of different species (interspecific cooperation ), in which all members benefit. An example of interspecific cooperation is the relationship formed between ants and aphids: the aphids gain protection by living in the ant colonies, while the ants feed on secretions from the aphids. Interspecific cooperation is a looser association than mutualism.

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coordinate bond

See covalent bond.

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coordination

(in animal physiology) The processes involved in the reception of sensory information, the integration of that information, and the subsequent response of the organism. Coordination is controlled by regions of the brain that deal with specific functions, such as locomotion and breathing, and is carried out by the nervous system.

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Copepoda

A class of crustaceans occurring in marine and freshwater habitats. Copepods are usually 0.5–2mm long and lack both a carapace and compound eyes. Copepods are important members of plankton: some are free-living, feeding on microscopic organisms; others are parasitic. A familiar freshwater genus is Cyclops, so named because the members have a single median eye.

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coprophagy

The ingestion of faeces as a means of obtaining nutrients. Coprophagous animals include dung beetles, which eat cow dung, and rabbits, which ingest their own faeces.

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copulation

See sexual intercourse.

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coral

Any of a group of sedentary colonial marine invertebrates belonging to the class Anthozoa of the phylum Cnidaria. A coral colony consists of individual polyps within a protective skeleton that they secrete: this skeleton may be soft and jelly-like, horny, or stony. The horny skeleton secreted by corals of the genus Corallium, especially C. rubrum, constitutes the red, or precious, coral used as a gemstone. The skeleton of stony, or true, corals consists of almost pure calcium carbonate and forms the coral reefs common in tropical seas.

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cork (phellem)

A protective waterproof plant tissue produced by the cork cambium. It develops in plants undergoing secondary growth and replaces the epidermis. Its cells, whose walls are impregnated with suberin, are arranged in radial rows and fit closely together except where the cork is interrupted by lenticels. Some cork cells become air-filled while others contain deposits of lignin, tannins, and fatty acids, which give the cork a particular colour. The cork oak (Quercus suber) produces cork that can be used commercially.

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cork cambium (phellogen)

A type of cambium arising within the outer layers of the stems of woody plants, usually as a complete ring surrounding the inner tissues. The cells of the cork cambium divide to produce an outer corky tissue (cork or phellem ) and an inner secondary cortex (phelloderm ). Cork, cork cambium, and phelloderm together make up the periderm , an impermeable outer layer that protects the inner stem tissues if the outer tissues split as the stem girth increases with age. It thus takes over the functions of the epidermis.

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corm

An underground organ formed by certain plants, e.g. crocus and gladiolus, that enables them to survive from one growing season to the next (see illustration). It consists of a short swollen food-storing stem surrounded by protective scale leaves. One or more buds in the axils of scale leaves produce new foliage leaves and flowers in the subsequent season, using up the food stored in the stem. Compare bulb.

Development of a corm

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cornea

A transparent layer of tissue, continuous with the sclerotic, that forms the front part of the vertebrate eye, over the iris and lens. The cornea refracts light entering the eye onto the lens, thus assisting in the focusing of images onto the retina. See also astigmatism.

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cornification

See keratinization.

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corolla

The petals of a flower, collectively, forming the inner whorl of the perianth. It encircles the stamens and carpels. The form of the corolla is very variable. The petals may either be free (polypetalous ) or united to form a tube (gamopetalous or sympetalous ).

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coronary vessels

Two pairs of blood vessels (the coronary arteries and coronary veins) that supply the muscles of the heart itself. The coronary arteries arise from the aorta and divide into branches that encircle the heart. A blood clot in a coronary artery (coronary thrombosis ) is one of the causes of a `heart attack'.

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corpus allatum

(pl. corpora allata) See juvenile hormone.

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corpus callosum

The sweeping band (commissure) of white matter that provides a connection between the two halves of the cerebrum in the brain. It enables the transfer of information from one cerebral hemisphere to the other.

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corpus luteum (yellow body)

The yellowish mass of tissue that forms from the granulosa cells in the cavity of a Graafian follicle in the ovary of a mammal after the release of the egg cell. It secretes the hormone progesterone. Some species of sharks, reptiles, and birds have similar structures in their ovaries but the function of these is less well understood.

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corpus striatum

See basal ganglia.

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cortex

1 (in botany) The tissue between the epidermis and the vascular system in plant stems and roots. It is composed of parenchyma cells and shows little or no structural differentiation. Cortex is produced by activity of the apical meristem. See also endodermis.

2 (in zoology) The outermost layer of tissue of various organs, including the adrenal glands (adrenal cortex ), kidneys (renal cortex ), and cerebral hemispheres (see cerebral cortex).
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Corti cell

Any of the cells found in the organ of Corti in the inner ear. Corti cells are epithelial cells from which hairlike structures project; they play a role in the detection of sound.

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corticosteroid

Any of several hormones produced by the cortex of the adrenal glands. Glucocorticoids regulate the use of carbohydrates, proteins, and fats in the body and include cortisol and cortisone. Mineralocorticoids regulate salt and water balance (see aldosterone).

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corticotrophin

See ACTH.

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cortisol (hydrocortisone)

A hormone (see corticosteroid), produced by the adrenal glands, that promotes the synthesis and storage of glucose and is therefore important in the normal response to stress, suppresses or prevents inflammation, and regulates deposition of fat in the body. It is used as treatment for various allergies and for rheumatic fever, certain skin conditions, and adrenal failure (Addison's disease).

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cortisone

A corticosteroid that is itself biologically inactive and is formed naturally in the adrenal gland from the active hormone cortisol, which is structurally very similar to it. Cortisone is reconverted to the active hormone by metabolism in the liver and other organs. It may be administered therapeutically as an inactive precursor (prodrug) of cortisol.

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corymb

A type of flowering shoot (see racemose inflorescence) in which the lower flower stalks are longer than the higher ones, resulting in a flat-topped cluster of flowers. Examples are candytuft and wallflower.

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cosmid

A hybrid vector, used in gene cloning, that includes the cos gene (from the lambda bacteriophage). It also contains drug resistance marker genes and other plasmid genes. Cosmids can incorporate larger DNA fragments than either phage or plasmid vectors alone and are especially suitable for cloning large mammalian genes or multigene fragments.

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cosmoid scale

See scales.

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cotyledon (seed leaf)

A part of the embryo in a seed plant. The number of cotyledons is an important feature in classifying plants. Among the flowering plants, the class known as Monocotyledonae have a single cotyledon and Dicotyledonae have two. Conifers have either two cotyledons, as in Taxus (yews), or five to ten, as in Pinus (pines). In seeds without an endosperm, e.g. garden pea and broad bean, the cotyledons store food, which is used in germination. In seeds showing epigeal germination, e.g. runner bean, they emerge above the soil surface and become the first photosynthetic leaves.

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coumarin

Any of a group of organic compounds found widely in plants and derived from the amino acid phenylalanine. They include coumarin itself, which gives new-mown hay its characteristic fragrance, and umbelliferone , which is characteristic of members of the carrot family. Under certain conditions, coumarin is converted by fungi to the toxic substance dicoumarol . If ingested this causes haemorrhaging and other symptoms of bleeding disorder, by disrupting vitamin K metabolism and preventing the formation of prothrombin and certain other clotting factors by the liver. Dicoumarol and another coumarin derivative, warfarin, are widely used as rodenticides.

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countercurrent heat exchange

A counterflow mechanism that enables fluids at different temperatures flowing in channels in opposite directions to exchange their heat content without mixing. An example of countercurrent heat exchange occurs in the feet of penguins, in which heat from blood in the arteries supplying the feet is transferred to blood returning to the body's core in veins that lie close to these arteries. This helps to maintain the core temperature in freezing conditions.

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countercurrent multiplier system

An active process occurring in the loops of Henle in the kidney, which is responsible for the production of concentrated urine in the collecting ducts of the nephrons. Sodium and chloride ions are actively pumped from the ascending limb of the loop but water is retained, since the ascending limb is impermeable to water. This creates a concentration gradient in the medulla in which the concentration of sodium and chloride is greatest in the region of the bend of the loop. Fluid passing from the loop of Henle to the distal tubule is less concentrated than that entering the loop, but because of the high osmotic pressure in the medulla water diffuses out of the collecting ducts, producing a concentrated urine. See illustration.

Countercurrent multiplier system in the kidney

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counterflow

The flow of two fluids in apposed vessels in opposite directions. In biological systems such an arrangement enables the efficient transfer of heat, ions, molecules, etc., from fluids that are rich in these resources to fluids that are deficient in them.

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courtship

Behaviour in animals that plays a part in the initial attraction of a mate or as a prelude to copulation. Courtship often takes the form of displays that have evolved through ritualization; some are derived from other contexts (e.g. food begging in some birds). Chemical stimuli (see pheromone) are also important in many mammals and insects.
As well as ensuring that the prospective mate is of the same species, the male's courtship performance allows females to choose between different males. The later stages of courtship may involve both partners in an alternating series of displays that inhibit aggression and fear responses and ensure synchrony of sexual arousal.

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COV

See crossover value.

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covalent bond

A type of chemical bond in which atoms are held together in a molecule by sharing one or more pairs of electrons in their outer shells. For example, in the water molecule (H_2O) each hydrogen atom forms a covalent bond by sharing its only electron with one of the six electrons in the outer shell of the oxygen atom. Coordinate (or dative ) bonds are covalent bonds in which one of the atoms supplies both the electrons. Single bonds are those in which one pair of electrons is shared; in double or triple bonds two or three pairs, respectively, are shared.

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Cowper's glands (bulbourethral glands)

A pair of pea-sized glands that lie beneath the prostate gland, named after the English surgeon William Cowper (1660–1709). Cowper's glands secrete an alkaline fluid that forms part of the semen. This fluid neutralizes the acidic environment of the urethra, thereby protecting the sperm. See also seminal vesicle.

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coxa

The first (basal) segment, attached to the thorax, of the leg of an insect, arachnid, or of certain other arthropods. See also femur; trochanter.

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coxal glands

Paired ducts (coelomoducts) in arthropods that lead from the coelom to the exterior and are normally involved in excretion. In arachnids one or two pairs of coxal glands in the cephalothorax open at the bases (coxae) of one or two pairs of legs.

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C_3 pathway

The metabolic pathway followed in the dark stage of photosynthesis by most plants of temperate regions, in which the first product is the three-carbon compound glycerate 3-phosphate. This is formed when carbon dioxide combines with ribulose bisphosphate in the first reaction of the Calvin cycle. Plants that follow this pathway are referred to as C_3 plants . Compare C4 pathway.

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C_4 pathway (Hatch–Slack pathway)

The metabolic pathway followed in the dark stage of photosynthesis by tropical plants, such as sugar cane and maize, and by plants that live in arid environments; these plants are known as C_4 plants . The initial fixation of atmospheric carbon dioxide occurs with phosphoenolpyruvate (PEP), through the mediation of the enzyme PEP carboxylase, to form the four-carbon compound oxaloacetic acid (see illustration; compare C3 pathway). Oxaloacetic acid is then converted into malic acid; this breaks down to give carbon dioxide, which passes to the Calvin cycle, and pyruvic acid, which generates more PEP, thereby ensuring a constant supply of this compound for further carbon dioxide fixation. The C_4 pathway allows photosynthesis to occur at very low concentrations of carbon dioxide as PEP carboxylase has an extremely high affinity for carbon dioxide. This pathway also works well at high temperatures and light intensity, enabling efficient photosynthesis in tropical plants. In addition, the malic acid formed can be stored before being used, to be later broken down to carbon dioxide when required in the Calvin cycle. This is important in desert plants, which need to close their stomata during the day to reduce water loss (see crassulacean acid metabolism). The anatomy of the C_4 plant leaf is adapted to suit this metabolic pathway (see bundle sheath cells).

C_4 pathway

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cranial nerves

Ten to twelve pairs of nerves in vertebrates that emerge directly from the brain. They supply the sense organs and muscles of the head, neck, and viscera. Examples of cranial nerves include the optic nerve (II) and the vagus nerve (X). With the spinal nerves, the cranial nerves form an important part of the peripheral nervous system.

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cranial reflex

See reflex.

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Craniata

See Chordata.

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cranium (brain case)

The part of the vertebrate skull that encloses and protects the brain. It is formed by the fusion of several flattened bones, which have immovable joints (sutures) between them.

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crassulacean acid metabolism (CAM)

Photosynthesis by the C4 pathway in which carbon dioxide is taken up during the night, when the plant's stomata are open, and fixed into malic acid. During the day, when the stomata are closed, carbon dioxide is released from malic acid for use in the Calvin cycle. This is important for plants that live in arid conditions as it enables them to keep their stomata closed during the day to reduce water loss from evaporation. Crassulacean acid metabolism is common in succulent plants of desert regions, including cacti and spurges, and in certain ferns. It is so named because it was originally studied in plants belonging to the family Crassulaceae, which includes stonecrops and houseleeks.

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creatine

A compound, synthesized from the amino acids arginine, glycine, and methionine, that occurs in muscle. In the form of creatine phosphate (or phosphocreatine ), it is an important reserve of energy for muscle contraction, which is released when creatine phosphate loses its phosphate and is converted to creatinine , which is excreted in the urine (at a rate of 1.2–1.5 g/day in humans). See also phosphagen.

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creatinine

See creatine.

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creationist

A proponent of the theory of special creation.

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cremocarp

A dry fruit that is a type of schizocarp formed from two one-seeded carpels. The carpels remain separate and form indehiscent mericarps that are attached to a central supporting strand (carpophore ) for some time before dispersal. It is characteristic of the Umbelliferae (Apiaceae; carrot family).

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crenation

The shrinkage of cells that occurs when the surrounding solution is hypertonic to the cellular cytoplasm. Water leaves the cells by osmosis, which causes the plasma membrane to wrinkle and the cellular contents to condense.

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Cretaceous

The final geological period of the Mesozoic era. It extended from about 144 million years ago, following the Jurassic, to about 65 million years ago, when it was succeeded by the Palaeocene epoch. The name of the period is derived from creta (Latin: chalk) and the Cretaceous was characterized by the deposition of large amounts of chalk in western Europe. The Cretaceous was the time of greatest flooding in the Mesozoic. Angiosperm plants made their first appearance on land and in the early Cretaceous Mesozoic reptiles reached their peak. At the end of the period there was a widespread extinction of the dinosaurs, flying reptiles, and ammonites, the cause of which may be related to environmental changes resulting from collisions of the earth with large meteorites (see iridium anomaly).

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cretinism

The condition that results from inadequate secretion of thyroid hormones during fetal life or early infancy. The brain and skeleton fail to develop properly, resulting in mental retardation and dwarfism.

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Creutzfeldt–Jakob disease (CJD)

A disease of humans characterized by dementia and destruction of brain tissue. It is now known to be caused by an abnormal prion protein and is transmissible, although there is also an inherited familial form. This rare disease typically affects middle-aged and elderly people and leads to rapid mental deterioration and death. The abnormal prion interferes with the structure of normal prion protein in brain tissue, resulting in accumulations of the protein and consequent tissue damage. In most cases the source of infection is unknown. However, it is well established that infection can result, for example, via injections of growth hormone derived from infected human cadavers. During the 1990s a novel form of the disease emerged, so-called `new variant CJD', which typically affects young healthy individuals. This is thought to be caused by consumption of beef products derived from cattle infected with bovine spongiform encephalopathy.

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Crick, Francis Harry Compton

(1916–)
British molecular biologist, who in 1951 teamed up with James Watson at Cambridge University to try to find the structure of DNA. This they achieved in 1953, using the X-ray diffraction data of Rosalind Franklin (1920–58) and Maurice Wilkins (1916– ). Crick went on to investigate codons and the role of transfer RNA. Crick, Watson, and Wilkins shared a Nobel Prize in 1962.

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crista

1 See semicircular canals.

2 See mitochondrion.
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critical group

A large group of related organisms that, although variations exist between them, cannot be divided into smaller groups of equivalent taxonomic rank to the parent group. Critical groups are found among plants that reproduce by apomixis; for example, the 400 or so species of Rubus (brambles, etc.) are regarded as a critical group.

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Cromagnon man

The earliest form of modern man (Homo sapiens), which is believed to have appeared in Europe about 35000 years ago and possibly earlier in Africa and Asia. Fossils indicate that these hominids were taller and more delicate than Neanderthal man, which they replaced. They used intricately worked tools of stone and bone and left the famous cave drawings at Lascaux in the Dordogne. The name is derived from the site at Cromagnon, France, where the first fossils were found in 1868.

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crop

1 A plant that is cultivated for the purpose of harvesting its seeds, roots, leaves, or other parts that are useful to humans. See agriculture.

2 An enlarged portion of the anterior section of the alimentary canal in some animals, in which food may be stored and/or undergo preliminary digestion. The term is most commonly applied to the thin-walled sac in birds between the oesophagus and the proventriculus. In female pigeons the crop contains glands that secrete crop milk , used to feed nestlings.
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crop rotation

An agricultural practice in which different crops are cultivated in succession on the same area of land over a period of time so as to maintain soil fertility and reduce the adverse effects of pests. Legumes are important in the rotation as they are a source of nitrogen for the soil (see nitrogen fixation; root nodule). In the UK, other crops that may be included in a typical four-stage rotation are wheat, barley, and root crops. However, the use of pesticides enables the monoculture of crops in modern farming systems (see agriculture).

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cross

1 A mating between two selected individuals. Controlled crosses are made for many reasons, e.g. to investigate the inheritance of a particular characteristic or to improve a livestock or crop variety. See also back cross; reciprocal cross; test cross.

2 An organism resulting from such a mating.
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cross-fertilization

See fertilization.

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crossing over

An exchange of portions of chromatids between homologous chromosomes. As the chromosomes begin to move apart at the end of the first prophase of meiosis, they remain in contact at a number of points (see chiasma). At these points the chromatids break and rejoin in such a way that sections are exchanged (see illustration). Crossing over thus alters the pattern of genes in the chromosomes. See recombination.

Crossing over at two chiasmata in a pair of homologous chromosomes

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crossover value (COV)

The percentage of linked genes (see linkage) that are exchanged during the process of crossing over during the first prophase of meiosis. The COV can be calculated by the percentage of offspring that show recombination and is used to map the genes on a chromosome (see chromosome map). A small COV for a given pair of genes indicates that the genes are situated close together on the chromosome.

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cross-pollination

See pollination.

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Crustacea

A phylum of arthropods containing over 35000 species distributed worldwide, mainly in freshwater and marine habitats, where they constitute a major component of plankton. Crustaceans include shrimps, crabs, lobsters, etc. (see Decapoda) and the terrestrial woodlice, all of which belong to the class Malacostraca; the barnacles (class Cirripedia); the water fleas (see Daphnia), fairy shrimps, and tadpole shrimps (class Branchiopoda); and the copepods (see Copepoda). The segmented body usually has a distinct head (bearing compound eyes, two pairs of antennae, and various mouthparts), thorax, and abdomen, and is protected by a shell-like carapace. Each body segment may bear a pair of biramous appendages used for locomotion, as gills, and for filtering food particles from the water. Appendages in the head region are modified to form jaws and in the abdominal region are often reduced or absent. Typically, the eggs hatch to produce a free-swimming nauplius larva. This develops either by a series of moults or undergoes metamorphosis to the adult form.

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cryobiology

The study of the effects of very low temperatures on organisms, tissues, and cells. The ability of some animal tissues to remain viable in a frozen state enables them to be preserved by freezing for future use as grafts.

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cryophyte

An organism that can live in ice and snow. Most cryophytes are algae, including the green alga Chlamydomonas nivalis and some diatoms, but they also include dinomastigotes, certain mosses, bacteria, and fungi.

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cryoprotectant

A substance that helps to protect an organism's tissues from the effects of freezing. Cryoprotectants are vital to the survival of those organisms living in cold climates that adopt a strategy of tolerating a limited amount of freezing of their tissue fluids. Such organisms include certain insects, molluscs, and nematodes, and even some frogs, lizards, and turtles. One group of these substances consists of antifreeze molecules, notably glycerol and related polyhydric alcohols (polyols). Another group of cryoprotectants bind to membranes within cells and prevent the binding of water molecules, so preserving the integrity of cell structures. Examples in insects include proline and trehalose. See also ice-nucleating agent.

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cryptic coloration

The type of colouring or marking of an animal that helps to camouflage it in its natural environment. It may enable the animal to blend with its background or, like the stripes of zebras and tigers, help to break up the outline of its body.

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cryptobiosis (anhydrobiosis)

A state of apparent suspended animation entered by certain invertebrate animals in order to survive desiccation or other extreme stresses. It is best documented among the rotifers, nematodes, collembolans, tardigrades, and other minute inhabitants of mosses and lichens, where the water film essential for active life is transient and sporadic. When the film dries out these animals appear to be dead for periods of days, weeks, or even years until moisture returns, when they `come back to life' and resume their normal activities. Entering cryptobiosis involves various processes. The animal typically retracts its legs and other appendages, or curls up into a ball to minimize its surface area. Biochemical changes in the cuticle or the secretion of wax ensure that at least some water is retained, although this may be only some 5% of the normal content, and the body becomes contracted and shrivelled. Sugars, such as trehalose, manufactured by the body cells protect the integrity of the cell membranes and also convert the cytoplasm to a glasslike state. When reverting to its normal state the animal absorbs water, swells, and becomes active in a few hours.

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cryptochrome

A plant pigment that absorbs mainly blue and ultraviolet (UV-A) light and is thought to be involved in various plant responses to light, including phototropism and hypocotyl elongation. The chemical nature of cryptochromes has proved elusive (hence the name, which means `hidden pigment'), but recently a blue receptor in Arabidopsis has been characterized as a protein containing two light-absorbing centres; one is the riboflavin derivative FAD, and the other is a pterin. It is thought that other cryptochromes may also be flavoproteins.

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Cryptomonada (Cryptophyta)

A phylum of the Protoctista comprising unicellular organisms with flattened ovoid cells and two undulipodia (flagella) arising from an obliquely situated gullet (crypt ). Cryptomonads include freshwater and marine algae, which contain chlorophyll and phycobiliprotein photosynthetic pigments; and heterotrophic protozoa, which may be carnivorous or parasitic (in animal intestines).

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cryptonephridial system

An arrangement of the excretory system in certain beetles and lepidopteran larvae in which the distal blind endings of the Malpighian tubules are held against the wall of the rectum by a perinephric membrane. A high concentration of ions is established in the tubules, which draws water from the gut by osmosis. This creates a highly efficient mechanism for water conservation in species, such as mealworms (e.g. Tenebrio spp.), that feed on very dry materials.

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cryptozoic

Describing animals that live mainly within soil, litter, or detritus, being rarely seen in the `open air'. Most are invertebrates, such as earthworms, woodlice, centipedes, and numerous insect larvae, although the term may be extended to include burrowing or hole-dwelling vertebrates, for example some lizards, snakes, and rodents.

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crypts of Lieberkühn (intestinal glands)

Tubular glands that lie between the finger-like projections (see villus) of the inner surface of the small intestine. The cells of these glands (called Paneth cells ) secrete intestinal juice as they gradually migrate along the side of the crypt and the villus; they are eventually shed into the lumen of the intestine. The glands are named after German anatomist J. N. Lieberkühn (1711–56).

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CSF

1 See cerebrospinal fluid.

2 See colony-stimulating factor.
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ctenidia

The gills of aquatic molluscs, which are present on both sides of the mantle cavity and are held in place by specialized membranes. The gills are involved in both filter feeding and the exchange of respiratory gases.

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Ctenophora

A phylum of marine invertebrates that contains the comb jellies (e.g. Pleurobrachia). Like the closely related Cnidaria they are coelenterates; they possess tentacles armed with lasso cells, for catching prey, and many hundreds of thousands of cilia, which are fused at their bases and grouped together into longitudinal rows (comb plates or ctenes ). The beating of the cilia enables these animals to swim among the plankton.

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CT scanner (computerized tomography scanner)

See tomography.

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cuboidal epithelium

See epithelium.

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cultivar

A plant that has been developed and maintained by cultivation as a result of agricultural or horticultural practices. The term is derived from cultivated variety.

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cultivation

The planting and breeding of crop plants in agriculture and horticulture. It involves the investigation of new means of increasing crop yield and quality.

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culture

A batch of cells, which can be microorganisms or of animal or plant origin, that are grown under specific conditions of nutrient levels, temperature, pH, oxygen levels, osmotic factors, light, pressure, and water content. Cultures of cells are prepared in the laboratory for a wide spectrum of scientific research. A culture medium provides the appropriate conditions for growth. See also batch culture; continuous culture; monolayer culture; organ culture; suspension culture; tissue culture.

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culture medium

A nutrient material, either solid or liquid, used to support the growth and reproduction of microorganisms or to maintain tissue or organ cultures. See also agar.

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cupula

See ampulla.

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cupule

1 A hard or membranous cup-shaped structure formed from bracts and enclosing various fruits, such as the hazelnut and acorn.

2 A structure in club mosses (Lycopodium species) that protects the gemma (resting bud) during its development. It is composed of six leaflike structures.
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3 The bright red tissue around the seed of yew (Taxus), forming the yew `berry'.
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curare

A resin obtained from the bark of South American trees of the genera Strychnos and Chondrodendron that causes paralysis of voluntary muscle. It acts by blocking the action of the neurotransmitter acetylcholine at neuromuscular junctions. Curare is used as an arrow poison by South American Indians and was formerly used as a muscle relaxant in surgery.

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curd

The solid component produced by the coagulation of milk during the manufacture of cheese. After being pasteurized, milk is cooled down and a culture of lactic acid bacteria is added to ferment the milk sugar, lactose, to lactic acid. The resulting decrease in pH causes casein, a milk protein, to coagulate, a process known as curdling . The solid curds are then separated from the liquid component, known as whey , and inoculated with different types of microbes to produce different cheeses.

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cusp

1 A sharp raised protuberance on the surface of a molar tooth. The cusps of opposing molars (i.e. on opposite jaws) are complementary to each other, which increases the efficiency of grinding food during chewing.

2 A flap forming part of a valve.
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cuticle

1 (in botany) The continuous waxy layer that covers the aerial parts of a plant. Composed mainly of cutin, it is secreted by the epidermis and its primary function is to prevent water loss.

2 (in zoology) A layer of horny noncellular material covering, and secreted by, the epidermis of many invertebrates. It is usually made of a collagen-like protein or of chitin and its main function is protection. In arthropods it is also strong enough to act as a skeleton (see exoskeleton) and in insects it reduces water loss. Growth is allowed by moulting of the cuticle (see ecdysis).
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cuticularization

The secretion by the outer (epidermal) layer of cells of plants and many invertebrates of substances that then harden to form a cuticle.

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cutin

A polymer of long-chain fatty acids that forms the main constituent of the cuticle of epidermal plant cells. The cutin polymers are cross-linked forming a network, which is embedded in a matrix of waxes. The deposition of cutin (cutinization ) reduces water loss by the plant and helps prevent the entry of pathogens. See also suberin.

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cutinization

The deposition of cutin in plant cell walls, principally in the outermost layers of leaves and young stems.

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cutis

See dermis.

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cutting

A part of a plant, such as a bud, leaf, or a portion of a root or shoot, that, when detached from the plant and inserted in soil, can take root and give rise to a new daughter plant. Taking or striking cuttings is a horticultural method for propagating plants. See also vegetative propagation.

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Cuvier, George Léopold Chrétien Frédéric Dagobert, Baron

(1769–1832)
French comparative anatomist, who became professor at the Collège de France in 1799, moving in 1802 to the Jardin de Plantes. Cuvier extended the classification system of Linnaeus, adding the category phylum (of which he recognized four in the animal kingdom) and concentrating on the taxonomy of fishes. He also initiated the classification of fossils and established the science of palaeontology.

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Cyanobacteria

A phylum consisting of two groups of photosynthetic eubacteria: the blue-green bacteria (formerly known as blue-green algae, or Cyanophyta), which comprise the vast majority of members, and the recently discovered grass-green bacteria, or chloroxybacteria. Both groups obtain their food by photosynthesis in a manner very similar to that of green plants and true algae, producing oxygen in the process. All the blue-green bacteria contain the photosynthetic pigment chlorophyll a, plus accessory pigments called phycobiliproteins. The blue colour is caused by one class of these pigments, the phycocyanins; some also have red pigments (phycoerythrins). Blue-green bacteria are unicellular but sometimes become joined in colonies or filaments by a sheath of mucilage. They occur in all aquatic habitats. A few species fix atmospheric nitrogen and thus contribute to soil fertility (see heterocyst; nitrogen fixation). Others exhibit symbiosis (see lichens). The chloroxybacteria have been found in marine and freshwater habitats. The first to be discovered, Prochloron, lives as a symbiont on tunicates and other invertebrate animals. Chloroxybacteria differ from the blue-green bacteria in containing chlorophyll a and chlorophyll b, but no phycobiliproteins – a combination like that found in plant chloroplasts. This prompted the hypothesis that ancestors of these bacteria were the symbiotic organisms that became the chloroplasts of plants (see endosymbiont theory).

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cyanocobalamin

See vitamin B complex.

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Cycadofilicales (Pteridospermales; seed ferns)

An extinct order of gymnosperms that flourished in the Carboniferous period. They possessed characteristics of both the ferns and the seed plants in reproducing by means of seeds and yet retaining fernlike leaves. Their internal anatomy combined both fern and seed-plant characteristics.

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Cycadophyta

A phylum of seed plants (see gymnosperm) that contains many extinct species; the few modern representatives of the group include Cycas and Zamia. Cycads inhabit tropical and subtropical regions, sometimes growing to a height of 20m. The stem bears a crown of fernlike leaves. These species are among the most primitive of living seed plants.

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cyclamates

Salts of the acid C_6H_11.NH.SO_3H, where C_6H_11– is a cyclohexyl group. Sodium and calcium cyclamates were formerly used as sweetening agents in soft drinks, etc, until their use was banned when they were suspected of causing cancer.

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cyclic AMP

A derivative of ATP that is widespread in animal cells as a second messenger in many biochemical reactions induced by hormones. Upon reaching their target cells, the hormones activate adenylate cyclase, the enzyme that catalyses cyclic AMP production. Cyclic AMP ultimately activates the enzymes of the reaction induced by the hormone concerned. Cyclic AMP is also involved in controlling gene expression and cell division, in immune responses, and in nervous transmission.

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cyclic phosphorylation (cyclic photophosphorylation)

See photophosphorylation; photosystems I and II.

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cyclin

Any of a family of proteins that help control the various phases of the cell cycle. Their concentrations fluctuate in step with the cycle, providing the cues for a progression from mitosis to the G_1, S, and G_2 phases during each complete cycle. They act in conjunction with cyclin-dependent protein kinases, which are proteins that phosphorylate other proteins. For example, in all eukaryotes mitosis (M phase) is initiated by high levels of cyclin B, which combines with a protein kinase to form the mitosis-promoting factor (MPF). By the end of the M phase, cyclin B is at a very low concentration; thereafter it rises steadily again to peak just before M phase.

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cyclomorphosis (seasonal polyphenism)

The occurrence of seasonal changes in the phenotype of an organism through successive generations. It occurs in small aquatic invertebrates that reproduce by parthenogenesis and give rise to several generations annually, such as rotifers and cladoceran crustaceans. Cyclomorphic species of Daphnia, for example, undergo changes in the shape of the head during the year; it is rounded from about midsummer to spring, and thereafter becomes helmet-shaped, reverting to the rounded shape at midsummer. Also, summer generations tend to be smaller and more transparent than at other times. Such changes are thought to be caused by the interaction of environmental cues with the organism's genes, thereby altering the course of development. The modifications are associated with improved survival of the organism, for instance by reducing the likelihood of predation.

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Cyclostomata

See Agnatha.

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cyme

See cymose inflorescence.

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cymose inflorescence (cyme; definite inflorescence)

A type of flowering shoot (see inflorescence) in which the first-formed flower develops from the growing region at the top of the flower stalk (see illustration). Thus no new flower buds can be produced at the tip and other flowers are produced from lateral buds beneath. In a monochasial cyme (or monochasium ), the development of the flower at the tip is followed by a new flower axis growing from a single lateral bud. Subsequent new flowers may develop from the same side of the lateral shoots, as in the buttercup, or alternately on opposite sides, as in forget-me-not. In a dichasial cyme (or dichasium ), the development of the flower at the apex is followed by two new flower axes developing from buds opposite one another, as in plants of the family Caryophyllaceae (such as stitchwort). Compare racemose inflorescence.

Types of cymose inflorescence

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cypsela

A dry single-seeded fruit that does not split open during seed dispersal and is formed from a double ovary in which only one ovule develops into a seed. It is similar to an achene and characteristic of members of the family Compositae (Asteraceae), such as the dandelion. See also pappus.

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cysteine

See amino acid.

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cysticercus

See bladderworm.

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cystine

A molecule resulting from the oxidation reaction between the sulphydryl (–SH) groups of two cysteine molecules (see amino acid). This often occurs between adjacent cysteine residues in polypeptides. The resultant disulphide bridges (–S–S–) are important in stabilizing the structure of protein molecules.

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cystocarp

See carpogonium.

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cytidine

A nucleoside comprising one cytosine molecule linked to a D-ribose sugar molecule. The derived nucleotides, cytidine mono-, di-, and triphosphate (CMP, CDP, and CTP respectively), participate in various biochemical reactions, notably in phospholipid synthesis.

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cytochrome

Any of a group of proteins, each with an iron-containing haem group, that form part of the electron transport chain in mitochondria and chloroplasts. Electrons are transferred by reversible changes in the iron atom between the reduced Fe(II) and oxidized Fe(III) states. See also cytochrome oxidase.

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cytochrome oxidase

An enzyme complex comprising the terminal two cytochromes (cytochromes a and a_3) of the respiratory chain in the mitochondria (see electron transport chain). It is responsible for the reduction of oxygen, receiving two electrons from cytochrome c (which precedes it in the chain) and combining them with two hydrogen ions and an oxygen atom to form water.

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cytogenetics

The study of inheritance in relation to the structure and function of cells. For example, the results of breeding experiments can be explained in terms of the behaviour of chromosomes during the formation of the reproductive cells.

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cytokine

Any soluble factor secreted by cells of the lymphoid system that acts as a signal to other lymphoid cells. There are two categories: lymphokines, secreted by lymphocytes; and monokines , secreted by macrophages. However, certain cytokines, notably interferons and interleukins, are secreted by both lymphocytes and macrophages.

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cytokinesis

See cell cycle; cell division.

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cytokinin (kinin)

Any of a group of plant growth substances chemically related to the purine adenine. Cytokinins stimulate cell division in the presence of auxin and have also been found to delay senescence, overcome apical dominance, and promote cell expansion. Zeatin is a naturally occurring cytokinin. Carefully formulated mixtures of cytokinins and auxins are used in micropropagation to generate cloned plantlets from undifferentiated callus tissue.

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cytology

The study of the structure and function of cells. The development of the light and electron microscopes has enabled the detailed structure of the nucleus (including the chromosomes) and other organelles to be elucidated. Microscopic examination of cells, either live or as stained sections on a slide, is also used in the detection and diagnosis of various diseases, especially cancer.

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cytolysis

The breakdown of cells, usually as a result of destruction or dissolution of their outer membranes.

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cytomegalovirus

A virus belonging to the herpes group (see herpesvirus). In humans it normally causes symptoms that are milder than the common cold, but it can produce more serious symptoms in those whose immune response is disturbed (e.g. cancer patients). Infection in pregnant women may cause congenital handicap in their children.

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cytoplasm

The material surrounding the nucleus of a cell. It consists of a matrix (see cytosol) in which the cell's organelles are suspended. The cytoplasm may be differentiated into dense outer ectoplasm , which is concerned primarily with cell movement, and less dense endoplasm , which contains most of the cell's structures.

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cytoplasmic inheritance

The inheritance of genes contained in the cytoplasm of a cell, rather than the nucleus. Only a very small number of genes are inherited in this way. The phenomenon occurs because certain organelles, the mitochondria and (in plants) the chloroplasts, contain their own genes and can reproduce independently. The female reproductive cell (the egg) has a large amount of cytoplasm containing many such organelles, which are consequently incorporated into the cytoplasm of all the cells of the embryo. The male reproductive cells (sperm or pollen), however, consist almost solely of a nucleus. Cytoplasmic organelles are thus not inherited from the male parent. In plants, male sterility can be inherited via the cytoplasm. The inheritance of any such factors does not follow Mendelian laws.

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cytoplasmic streaming

The directional movement of cytoplasm in certain cells, which allows movement of substances through the cell, especially around the cell's periphery. It has been observed most clearly in large cells, such as plant sieve elements and unicellular algae, in which simple diffusion is ineffective as a means of local transport in the cell. The exact mechanism of streaming is unknown but it is thought to involve the formation of microfilaments parallel to the direction of flow. A similar streaming of cytoplasm is responsible for amoeboid movement.

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cytosine

A pyrimidine derivative. It is one of the principal component bases of nucleotides and the nucleic acids DNA and RNA.

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cytoskeleton

A network of fibres permeating the matrix of living eukaryotic cells that provides a supporting framework for organelles, anchors the cell membrane, facilitates cellular movement, and provides a suitable surface for chemical reactions to take place. The fibres are composed of microtubules, intermediate filaments, and microfilaments.

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cytosol

The semifluid soluble part of the cytoplasm of cells, which contains the components of the cytoskeleton and in which the cell's organelles are suspended.

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cytostome

A mouthlike structure of certain protoctists, through which particulate food is ingested. The cytostome is typically located at the base of an indentation in the cell.

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cytotaxonomy

See taxonomy.

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cytotoxic

Destructive to living cells. The term is applied particularly to a class of drugs that inhibit cell division and are therefore used to destroy cancer cells and to a group of T cells that destroy virus-infected cells.

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D

2,4-D

2,4-dichlorophenoxyacetic acid (2,4-dichlorophenoxyethanoic acid): a synthetic auxin frequently used as a weedkiller of broad-leaved weeds. See also pesticide.

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dance of the bees

A celebrated example of communication in animals, first investigated by Karl von Frisch (1886–1982). Honeybee workers on returning to the hive after a successful foraging expedition perform a `dance' on the comb that contains coded information about the distance and direction of the food source. For example the waggle dance , characterized by tail-wagging movements, indicates the direction of a food source at a distance of more than 100 m. Other workers, sensing vibrations from the dance, follow the instructions to find the food source.

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Daphnia

A genus of crustaceans belonging to the class Branchiopoda and order Cladocera (water fleas). Daphnia species have a transparent carapace and a protruding head with a pair of highly branched antennae for swimming and a single median compound eye. The five pairs of thoracic appendages form an efficient filter-feeding mechanism. Reproduction can take place without mating, i.e. by parthenogenesis. Some species exhibit cyclomorphosis.

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dark adaptation

The changes that need to take place in the eye when an animal moves from a brightly lit environment to a relatively dark one to enable objects to be seen clearly. On moving to a darker environment, the pupils dilate and rhodopsin – the pigment in the rod cells that is broken down in bright light – is regenerated from its constituents.

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dark period

(in botany) The period considered to be critical in the responses of plants to changes in day length (see photoperiodism). It is believed that such responses, which include the onset of flowering, are determined by the length of the period of darkness that occurs between two periods of light.

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dark reaction (light-independent reaction)

See photosynthesis.

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Darwin, Charles

(1809–82)
British naturalist, who studied medicine before studying geology at Cambridge University. In 1831 he joined HMS Beagle as naturalist on a round-the-world voyage. After his return five years later he published works on the geology he had observed. He was also formulating his theory of evolution by natural selection, but it was to be 20 years before he published The Origin of Species (1859), prompted by similar views expressed by Alfred Russel Wallace. Among his later works was The Descent of Man (1871). See also Darwinism.

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Darwinism

The theory of evolution proposed by Charles Darwin in On the Origin of Species (1859), which postulated that present-day species have evolved from simpler ancestral types by the process of natural selection acting on the variability found within populations. On the Origin of Species caused a furore when it was first published because it suggested that species are not immutable nor were they specially created – a view directly opposed to the doctrine of special creation. However, the wealth of evidence presented by Darwin gradually convinced most people and the only major unresolved problem was to explain how the variations in populations arose and were maintained from one generation to the next. This became clear with the rediscovery of Mendel's work on classical genetics in the 1900s and led to the present theory of neo-Darwinism.

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Darwin's finches (Galapagos finches)

The 14 species of finch, unique to the Galapagos Islands, that Charles Darwin studied during his journey on HMS Beagle. Each is adapted to exploit a different food source. They are not found on the mainland because competition there for these food sources from other birds is fiercer. Darwin believed all the Galapagos finches to be descendants of a few that strayed from the mainland, and this provided important evidence for his theory of evolution. See also adaptive radiation.

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dating techniques

Methods of estimating the age of rocks, palaeontological specimens, archaeological sites, etc. Relative dating techniques date specimens in relation to one another; for example, stratigraphy is used to establish the succession of fossils. Absolute (or chronometric ) techniques give an absolute estimate of the age and fall into two main groups. The first depends on the existence of something that develops at a seasonally varying rate, as in dendrochronology and varve dating. The other uses some measurable change that occurs at a known rate, as in chemical dating, radioactive (or radiometric ) dating (see carbon dating; fission-track dating; potassium–argon dating; rubidium–strontium dating; uranium–lead dating), and thermoluminescence.

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day-neutral plant

A plant in which flowering can occur irrespective of the day length. Examples are cucumber and maize. See photoperiodism. Compare long-day plant; short-day plant.

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DDT

Dichlorodiphenyltrichloroethane; a colourless organic crystalline compound, (ClC_6H_4)2CH(CCl_3), made by the reaction of trichloromethanal with chlorobenzene. DDT is the best known of a number of chlorine-containing pesticides used extensively in agriculture in the 1940s and 1950s. The compound is stable, accumulates in the soil, and concentrates in fatty tissue, reaching dangerous levels in carnivores high in the food chain. Restrictions are now placed on the use of DDT and similar pesticides.

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deacetylation

The removal of an acetyl group (–COCH_3) from a molecule. Deacetylation is an important reaction in several chemical pathways, including the Krebs cycle.

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deamination

The removal of an amino group (–NH_2) from a compound. Enzymatic deamination occurs in the liver and is important in amino-acid metabolism, especially in their degradation and subsequent oxidation (see also oxidative deamination). The amino group is removed as ammonia and excreted, either unchanged or as urea or uric acid.

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death

The point at which the processes that maintain an organism alive no longer function. In humans it is diagnosed by permanent cessation of the heartbeat; however, the heart can continue beating after a large part of the brain ceases to function (see brain death). The death of a cell due to external damage or the action of toxic substances is known as necrosis. This must be distinguished from programmed cell death (see apoptosis), which is a normal part of the developmental process.

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death phase

See bacterial growth curve.

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death rate (mortality)

The rate at which a particular species or population dies, whatever the cause. The death rate is an important factor in controlling the size of a population. Compare birth rate.

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deca-

Symbol da. A prefix used in the metric system to denote ten times. For example, 10 hertz = 1 decahertz (daHz).

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Decapoda

An order of crustaceans of the class Malacostraca that are distributed worldwide, mainly in marine habitats. Decapods comprise swimming forms (shrimps and prawns) and crawling forms (crabs, lobsters, and crayfish). All are characterized by five pairs of walking legs, the first pair of which are highly modified in crawling forms to form powerful grasping pincers. The carapace is fused with the thorax and head forming a cephalothorax. The antennae are especially long in shrimps and prawns, which also possess several pairs of well-developed swimming appendages (pleopods ) posterior to the walking legs. Following fertilization by the male, females usually carry the eggs until they hatch. The larvae undergo several transformations before attaining adult form.

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decarboxylation

The removal of carbon dioxide from a molecule. Decarboxylation is an important reaction in many biochemical processes, such as the Krebs cycle and the synthesis of fatty acids. See also oxidative decarboxylation.

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decay

See decomposition.

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deci-

Symbol d. A prefix used in the metric system to denote one tenth. For example, 0.1 metre = 1 decimetre (dm).

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decibel

A unit used to compare two power levels, usually applied to sound or electrical signals. Although the decibel is one tenth of a bel , it is the decibel, not the bel, that is invariably used. Two power levels P and P_0 differ by n decibels when n = 10log_10P/P_0. If P is the level of sound intensity to be measured, P_0 is a reference level, usually the intensity of a note of the same frequency at the threshold of audibility.
The logarithmic scale is convenient as human audibility has a range of 1 (just audible) to 10^12 (just causing pain) and one decibel, representing an increase of some 26%, is about the smallest change the ear can detect.

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deciduous

Describing plants in which all the leaves are shed at the end of each growing season, usually the autumn in temperate regions or at the beginning of a dry season in the tropics. This seasonal leaf fall helps the plant retain water that would otherwise be lost by transpiration from the leaves. Examples of deciduous plants are rose and horse chestnut. Compare evergreen.

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deciduous teeth (milk teeth)

The first of two sets of teeth of a mammal. These teeth are smaller than those that replace them (the permanent teeth) and fewer in number, since there are no deciduous molars. See also diphyodont.

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decomposer

An organism that obtains energy from the chemical breakdown of dead organisms or animal or plant wastes. Decomposers, most of which are bacteria and fungi, secrete enzymes onto dead matter and then absorb the breakdown products (see saprotroph). Many decomposers (e.g. nitrifying bacteria) are specialized to break down organic materials that are difficult for other organisms to digest. Decomposers fulfil a vital role in the ecosystem, returning the constituents of organic matter to the environment in inorganic form so that they can again be assimilated by plants. Compare detritivore. See also carbon cycle; nitrogen cycle.

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decomposition (decay)

The chemical breakdown of organic matter into its constituents by the action of decomposers.

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defecation

The expulsion of faeces from the rectum due to contractions of muscles in the rectal wall. A sphincter muscle, which is under voluntary control, is situated at the end of the rectum (the anus); relaxation of this muscle allows defecation to occur. In babies control of the anal sphincter muscle has not been developed and defecation occurs automatically as a reflex response to the presence of faeces in the rectum.

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deficiency disease

Any disease caused by an inadequate intake of an essential nutrient in the diet, primarily vitamins, minerals, and amino acids. Examples are scurvy (lack of vitamin C), rickets (lack of vitamin D), and iron-deficiency anaemia. See also mineral deficiency.

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definite inflorescence

See cymose inflorescence.

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deforestation

The extensive cutting down of forests for the purpose of extracting timber or fuel wood or to clear the land for mining or agriculture. Forests are often situated in upland areas and are important in trapping rainwater. Deforestation in these areas, particularly in India and Bangladesh, has resulted in the flooding of low-lying plains; it has also led to an increase in soil erosion and hence desert formation (see desertification), resulting in crop loss and economic problems for local communities. The felling and burning of trees releases large amounts of carbon dioxide, thereby increasing global carbon dioxide levels and possibly contributing to the greenhouse effect. Rainforests, particularly those of South America, are rich in both fauna and flora; their removal leads to an overall decrease in biodiversity and the loss of plant species that have potentially beneficial pharmaceutical effects. Despite movements to reduce deforestation, economic pressures ensure that the process still continues.

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degeneration

1 Changes in cells, tissues, or organs due to disease, etc., that result in an impairment or loss of function and possibly death and breakdown of the affected part.

2 The reduction in size or complete loss of organs during evolution. The human appendix has undergone this process and performs no obvious function in humans. Degeneration of external organs may cause animals to appear to be more primitive than they really are; for example, early zoologists believed whales were fish rather than mammals because of the degeneration of their limbs. See also vestigial organ.
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deglutition (swallowing)

A reflex action initiated by the presence of food in the pharynx. During deglutition, the soft palate is raised, which prevents food from entering the nasal cavity; the epiglottis closes, which blocks the entrance to the windpipe; and the oesophagus starts to contract (see peristalsis), which ensures that food is conveyed to the stomach.

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dehiscence

The spontaneous and often violent opening of a fruit, seed pod, or anther to release and disperse the seeds or pollen. Examples are the splitting of laburnum pods and primrose capsules; such structures are described as dehiscent (compare indehiscent).

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dehydrogenase

Any enzyme that catalyses the removal of hydrogen atoms (dehydrogenation) in biological reactions. Dehydrogenases occur in many biochemical pathways but are particularly important in driving the electron transport chain reactions of cell respiration. They work in conjunction with the hydrogen-accepting coenzymes NAD and FAD.

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dehydrogenation

A chemical reaction in which hydrogen is removed from a compound. Dehydrogenation of organic compounds converts single carbon–carbon bonds into double bonds. In biological systems it is usually effected by dehydrogenases.

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deletion

(in genetics) 1 A point mutation involving the removal of one or more base pairs in the DNA sequence.

2 A frequently lethal chromosome mutation that arises from an inequality in crossing over during meiosis such that one of the chromatids loses more genetic information than it receives.
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deme

A group of organisms in the same taxon. The term is used with various prefixes that denote how the group differs from other groups. For example, an ecodeme occurs in a particular ecological habitat, cytodemes differ from each other cytologically, and genodemes differ genetically.

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denature

To produce a structural change in a protein or nucleic acid that results in the reduction or loss of its biological properties. Denaturation involves unfolding of the polypeptide chains of proteins and of the double helix of nucleic acids, with loss of secondary and tertiary structure; it is caused by heat (thermal denaturation ), chemicals, and extremes of pH. The differences between raw and boiled eggs are largely a result of denaturation. Compare renaturation.

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dendrite

Any of the slender branching processes that arise from the dendrons of the cell body of a motor neurone. A dendrite forms connections (see synapse) with the axons of other neurones and transmits nerve impulses from these to the cell body.

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dendrochronology

An absolute dating technique using the growth rings of trees. It depends on the fact that trees in the same locality show a characteristic pattern of growth rings resulting from climatic conditions. Thus it is possible to assign a definite date for each growth ring in living trees, and to use the ring patterns to date fossil trees or specimens of wood (e.g. used for buildings or objects on archaeological sites) with lifespans that overlap those of living trees. The bristlecone pine (Pinus aristata), which lives for up to 5000 years, has been used to date specimens over 8000 years old. Fossil specimens accurately dated by dendrochronology have been used to make corrections to the carbon dating technique. Dendrochronology is also helpful in studying past climatic conditions. Analysis of trace elements in sections of rings can also provide information on past atmospheric pollution.

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dendrogram

A diagram, similar to a family tree, that indicates some type of similarity between different organisms. Dendrograms can be based on phenetic or phylogenetic similarities; a cladogram shows similarities according to the system of cladistics.

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dendron

Any of the major cytoplasmic processes that arise from the cell body of a motor neurone. A dendron usually branches into dendrites.

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denitrification

A chemical process in which nitrates in the soil are reduced to molecular nitrogen, which is released into the atmosphere. This process is effected by denitrifying bacteria (e.g. Pseudomonas denitrificans), which use nitrates as a source of energy for other chemical reactions in a manner similar to respiration in other organisms. Compare nitrification. See nitrogen cycle.

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de novo pathway

Any metabolic pathway in which a biomolecule is synthesized from simple precursor molecules. Nucleotide synthesis is an example.

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density-dependent factor

Any factor limiting the size of a population whose effect is dependent on the number of individuals in the population. For example, disease will have a greater effect in limiting the growth of a large population, since overcrowding facilitates its spread. See also environmental resistance.

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density-independent factor

Any factor limiting the size of a population whose effect is not dependent on the number of individuals in the population. An example of such a factor is an earthquake, which will kill all members of the population regardless of whether the population is small or large.

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dental caries

Tooth decay, which involves the destruction of the enamel layer of the tooth by acids produced by the action of bacteria on sugar. Bacteria can bind to teeth on dextran , a sticky substance derived from sucrose. The bacterial cells and other waste attached to dextran gives rise to plaque. If dental caries is not treated it can spread to the dentine and pulp of the tooth, which leads to infection and death of the tooth.

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dental formula

A representation of the dentition of an animal. A dental formula consists of eight numbers, four above and four below a horizontal line. The numbers represent (from left to right) the numbers of incisors, canines, premolars, and molars in either half of the upper and lower jaws. The total number of teeth in both jaws is therefore obtained by adding up all the numbers in the dental formula and multiplying by 2. Representative dental formulas are shown in the illustration. See also permanent teeth.

Representative dental formulas

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dentary

A membrane bone, present in the lower jaw of the vertebrates, that supports the teeth. In mammals the dentary is the sole bone of the lower jaw.

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denticle (placoid scale)

See scales.

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dentine

The bony material that forms the bulk of a tooth. Dentine is similar in composition to bone but is perforated with many tiny canals for nerve fibres, blood capillaries, and processes of the dentine-forming cells (odontoblasts). Ivory, the material that forms elephant tusks, is made of dentine.

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dentition

The type, number, and arrangement of teeth in a species. This can be represented concisely by a dental formula. See also permanent teeth; diphyodont; monophyodont; polyphyodont; heterodont; homodont.

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deoxyribonuclease

See DNAase.

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deoxyribonucleic acid

See DNA.

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deoxyribose (2-deoxyribose)

A pentose (five-carbon) sugar, a derivative of ribose, that is a component of the nucleotides (deoxyribonucleotides) that form the building blocks of DNA.

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depolarization

A reduction in the difference of electrical potential that exists across the cell membrane of a nerve or muscle cell. Depolarization of a nerve-cell membrane occurs during the passage of an action potential along the axon when the nerve is transmitting an impulse.

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derived trait

See apomorphy.

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dermal bone

See membrane bone.

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Dermaptera

An order of insects comprising the earwigs. Earwigs typically have long thin cylindrical bodies with biting mouthparts and a stout pair of curved forceps (cerci ) at the tip of the abdomen, used for catching prey and in courtship. Some species have a single pair of wings, which at rest are folded back over the abdomen like a fan; others are wingless. Most earwigs are nocturnal and omnivorous.

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dermis (corium; cutis)

The thicker and innermost layer of the skin of vertebrates, the other layer being the epidermis. The dermis consists of fibrous connective tissue in which are embedded blood vessels, sensory nerve endings, and (in mammals) hair follicles, sebaceous glands, and sweat ducts. Beneath the dermis lies the subcutaneous tissue.

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descending tracts

See spinal cord.

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desert

A major terrestrial biome characterized by low rainfall. Hot deserts, such as the Sahara and Kalahari deserts of Africa, have a rainfall of less than 25cm a year and extremely high daytime temperatures (up to 36°C). Vegetation is sparse, and desert plants are adapted to conserve water and take advantage of the rain when it falls. The perennials include xerophytic trees and shrubs (see xerophyte) and succulents, such as cacti; many succulents show crassulacean acid metabolism. Annual plants are ephemerals, lying dormant as seeds for most of the year and completing their life cycle in the brief rainy periods. Desert animals are typically nocturnal or active at dawn and dusk, thus avoiding the extreme daytime temperatures.

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desertification

The gradual conversion of fertile land into desert, usually as a result of human activities. Loss of topsoil leads to further soil erosion until the land can no longer be used to grow crops or support livestock. A major factor contributing to desertification is bad management of farmland. Overgrazing of livestock removes the plant cover and exposes the soil, making it vulnerable to erosion. Overintensive cultivation of crop plants, especially monoculture (see agriculture), depletes the soil of nutrients and organic matter, resulting in loss of fertility and increasing its susceptibility to erosion. In many Third World countries it is difficult to control the process of desertification as the livelihood of the people often depends on practices that contribute to soil erosion. Another major cause of desertification is deforestation.

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desiccation

A method of preserving organic material by the removal of its water content. Cells and tissues can be preserved by desiccation after lowering the samples to freezing temperatures; thereafter they can be stored at room temperature.

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desiccator

A container for drying substances or for keeping them free from moisture. Simple laboratory desiccators are glass vessels containing a drying agent, such as silica gel. They can be evacuated through a tap in the lid.

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desmids

Unicellular green algae of the class Desmidioideae. Like Spirogyra, they have an elaborate chloroplast. The cells of desmids are characteristically split into two halves joined by a narrow neck, each half being a mirror image of the other. The outer wall of the cell is patterned with various protuberances and covered with a mucilaginous sheath, which is thought to play a role in the cell's slow gliding movement. Desmids are found mainly in fresh water.

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desmosome (macula adherens)

A type of junction found between adjacent cells in epithelial tissues that helps strengthen the tissue by binding the cells together while allowing movement of materials within the intercellular space. It consists of a discrete cluster of fibres running across a space, some 25 nm wide, between the two cells. On either side, these fibres are anchored beneath the plasma membrane of each cell within a proteinaceous cytoplasmic plaque, from which more fibres extend into the cytoplasm of the cell. See also gap junction; tight junction.

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desorption

The removal of adsorbed atoms, molecules, or ions from a surface.

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desulphuration

The removal of sulphur from a compound, which can occur by a variety of metabolic pathways. Desulphuration has been implicated in the toxicity of a number of sulphur-containing organic compounds: it is postulated that the atomic sulphur released into a cell by desulphuration, which is highly electrophilic, can bind to proteins and thereby alter their function.

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determined

Describing embryonic tissue at a stage when it can develop only as a certain kind of tissue (rather than as any kind).

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detoxification (detoxication)

The process by which harmful compounds, such as drugs and poisons, are converted to less toxic compounds in the body. Detoxification is an important function of the liver. See also phase I metabolism; phase II metabolism.

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detritivore

An animal that feeds on detritus. Examples of detritivores are earthworms, blowflies, maggots, and woodlice. Detritivores play an important role in the breakdown of organic matter from decomposing animals and plants (see decomposer).

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detritus

Particles of organic material derived from dead and decomposing organisms, resulting from the activities of the decomposers. Detritus is the source of food for detritivores, which can themselves be eaten by carnivores in a detritus food chain :
detritus detritivore carnivore.

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detrusor muscle

The smooth muscle of the bladder wall, which is innervated with sympathetic fibres and contracts as a result of a reflex action when the bladder wall tension has reached a certain level.

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deuteranopia

See colour blindness.

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Deuteromycota

A taxon, usually given the rank of phylum, that is used in some classifications to include all fungi in which sexual reproduction is apparently absent. These fungi, described as `imperfect fungi' or `Fungi Imperfecti', are usually regarded as ascomycotes or basidiomycotes that have lost the ability to produce asci or basidia, respectively, and in some cases it may be possible to identify close sexual relatives, or even sexual stages of the same species, in either of these groups. For example, Penicillium, traditionally classified as a deuteromycote, is now known to have a sexual stage in the form of the ascomycote fungus Talaromyces.

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deutoplasm

The nutritional material found in the yolk of eggs.

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development

The complex process of growth and maturation that occurs in living organisms. Cell division and differentiation are important processes in development. In vertebrate animals there are three developmental stages: (1) cleavage, in which the zygote divides to form a ball of cells, the blastula; (2) gastrulation, in which the cells become arranged in three primary germ layers (see gastrula); (3) organogenesis (or organogeny ), in which further cell division and differentiation results in the formation of organs. The development of many invertebrates (e.g. insects) and amphibians involves the process of metamorphosis. See also morphogenesis; primary growth.

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Devonian

A geological period in the Palaeozoic era that extended from the end of the Silurian (about 408 million years ago) to the beginning of the Carboniferous (about 360 million years ago). It was named by Adam Sedgwick (1785–1873) and Roderick Murchison (1792–1871) in 1839. The Devonian is divided into seven stages based on invertebrate fossil remains, such as corals, brachiopods, ammonoids, and crinoids, found in marine deposits. There were also extensive continental deposits consisting of conglomerates, red silts, and sandstones, forming the Old Red Sandstone facies. Fossils in the Old Red Sandstone include fishes and the earliest land plants. Graptolites became extinct early in the Devonian and the trilobites declined.

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de Vries, Hugo Marie

(1848–1935) Dutch botanist, who became professor of botany at Amsterdam in 1878. He was the first to recognize the importance of genetic mutations in the evolution of living organisms. To investigate his theories of hereditary traits in plants, he began a series of plant breeding experiments in 1892. Within four years he had obtained evidence for the segregation of characters in the offspring of crosses only to discover that Mendel had published similar results 34 years earlier, which had been ignored.

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dextrin

An intermediate polysaccharide compound resulting from the hydrolysis of starch to maltose by amylase enzymes.

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dextrorotatory

Denoting a chemical compound that rotates the plane of polarization of plane-polarized light to the right (clockwise as observed by someone facing the oncoming radiation). See optical activity.

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dextrose

See glucose.

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d-form

See optical activity.

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diabetes

See antidiuretic hormone; insulin.

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diagenesis

See taphonomy.

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diakinesis

The period at the end of the first prophase of meiosis when the separation of homologous chromosomes is almost complete and crossing over has occurred.

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dialysis

A method by which large molecules (such as starch or protein) and small molecules (such as glucose or amino acids) in solution may be separated by selective diffusion through a semipermeable membrane. For example, if a mixed solution of starch and glucose is placed in a closed container made of a semipermeable substance (such as Cellophane), which is then immersed in a beaker of water, the smaller glucose molecules will pass through the membrane into the water while the starch molecules remain behind. The cell membranes of living organisms are partially permeable, and dialysis takes place naturally in the kidneys for the excretion of nitrogenous waste. An artificial kidney (dialyser ) utilizes the principle of dialysis by taking over the functions of diseased kidneys.

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diapause

A period of suspended development or growth occurring in many insects and other invertebrates during which metabolism is greatly decreased. Some long-lived species may undergo diapause as adults, but for many others the egg is the diapausal stage. Diapause is often triggered by seasonal changes and regulated by an inborn rhythm; it enables the animal to survive unfavourable environmental conditions so that its offspring may be produced in more favourable ones.

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diaphragm

The muscular membrane that divides the thorax (chest) from the abdomen in mammals. It plays an essential role in breathing (see also respiratory movement), being depressed during inspiration and raised during expiration.

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diaphysis

The shaft of a mammalian limb bone, which in immature animals is separated from the ends of the bone (see epiphysis) by cartilage.

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diastase

See amylase.

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diastema

The gap that separates the biting teeth from the grinding teeth in herbivores. It creates a space in which food can be held in readiness for the grinding action of the teeth. This space is filled by large canine teeth in carnivores.

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diastole

The phase of a heart beat that occurs between two contractions of the heart, during which the heart muscles relax and the ventricles fill with blood. Compare systole. See blood pressure.

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diatoms

See Bacillariophyta.

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dicarboxylic acid

A carboxylic acid having two carboxyl groups in its molecules. In systematic chemical nomenclature, dicarboxylic acids are denoted by the suffix -dioic; e.g. hexanedioic acid, HOOC(CH_2)4COOH.

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dichasium

See cymose inflorescence.

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2,4-dichlorophenoxyacetic acid

See 2,4-D.

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dichogamy

The condition in which the male and female reproductive organs of a flower mature at different times, thereby ensuring that self-fertilization does not occur. Compare homogamy. See also protandry; protogyny.

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dichotomous

1 Describing the type of branching in plants that results when the growing point (apical bud) divides into two equal growing points, which in turn divide in a similar manner after a period of growth, and so on. Dichotomous branching is common is ferns and mosses.

2 See key.
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Dicotyledonae

One of the two classes of flowering plants (see Anthophyta), distinguished by having two seed leaves (cotyledons) within the seed. The dicotyledons usually have leaf veins in the form of a net, a ring of vascular bundles in the stem, and flower parts in fours or fives or multiples of these. Dicotyledons include many food plants (e.g. potatoes, peas, beans), ornamentals (e.g. roses, ivies, honeysuckles), and hardwood trees (e.g. oaks, limes, beeches). Compare Monocotyledonae.

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dicoumarol

See coumarin.

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Dictyoptera

An order of insects (sometimes classified as Orthoptera) comprising the cockroaches (suborder Blattaria) and the mantids (suborder Mantodea), occurring mainly in tropical regions. Cockroaches are oval and flattened in shape; some have a single well-developed pair of wings, folded back over the abdomen at rest, while in others the wings may be reduced or absent. They are usually found in forest litter, feeding on dead organic matter, but some species, e.g. the American cockroach (Periplaneta americana), are major household pests, scavenging on starchy foods, fruits, etc. In most species the females produce capsules (oothecae ) containing 16–40 eggs. These are either deposited or carried by the female during incubation.

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diet

The food requirements of an organism. The foods that constitute the human diet should contain vitamins, mineral salts (see essential element), and dietary fibre as well as water, carbohydrates and fats (which provide energy), and proteins (required for growth and maintenance). A balanced diet contains of the correct proportions of these nutrients, which will vary depending on the age, sex, body size, and the level of activity of the individual. An inadequate supply of different food types in the diet can lead to malnutrition.

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dietary fibre

See fibre.

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differentiation

The changes from simple to more complex forms undergone by developing tissues and organs so that they become specialized for particular functions. Differentiation occurs during embryonic development, regeneration, and (in plants) meristematic activity (see meristem). The mechanisms involved in the differentiation of cells and organization of tissues in animals has been most intensively studied in experimental organisms, notably the fruit fly Drosophila. In this insect the general body plan is established in the early embryo by proteins called morphogens, which are encoded by maternal genes of follicle cells and diffuse into the developing embryo. The various morphogens set up a pattern of concentrations that activates genes in different zones of the embryo to different extents, creating the basic pattern of body segments. A class of genes within the embryo itself, the segment genes, further refine this pattern. Within each segment the differentiation of appendages, such as limbs, is controlled by homeotic genes.

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diffusion (passive transport)

The random movement of particles (e.g. molecules or ions) from an area of high concentration to an area of low concentration until an even distribution of particles (i.e. uniform concentration) is obtained. Small molecules and ions (such as oxygen and Na+) can diffuse across a plasma membrane.

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diffusion gradient

See concentration gradient.

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digestion

The breakdown by a living organism of ingested food material into chemically simpler forms that can be readily absorbed and assimilated by the body. This process requires the action of digestive enzymes and may take place extracellularly (i.e. in the alimentary canal), as is the case in most animals; or intracellularly (e.g. by engulfing phagocytic cells), as occurs in many protoctists and in cnidarians.

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digestive system

The system of organs that are involved in the process of digestion. The digestive system of mammals is divided into the gastrointestinal tract (see alimentary canal) and accessory structures, such as teeth, tongue, liver, pancreas, and gall bladder.

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digit

A finger or toe. In the basic limb structure of terrestrial vertebrates there are five digits (see pentadactyl limb). This number is retained in humans and other primates, but in some other species the number of digits is reduced. Frogs, for example, have four fingers and five toes, and in ungulate (hooved) mammals, the digits are reduced and their tips are enclosed in horn, forming hooves.

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digitalis

A preparation of the dried leaves or seeds of the foxglove (Digitalis), used historically as a heart stimulant. Modern clinically prescribed drugs derived from digitalis include digoxin and digitoxin , both of which belong to a class of drugs known as the cardiac glycosides. They are used to treat heart failure and some forms of arrhythmia because of their ability to increase the force of contraction of the heart muscle. Their toxic effects arise from their capacity to disturb the normal rhythm of the heart.

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digitigrade

Describing the gait of most fast-running animals, such as dogs and cats, in which only the toes are on the ground and the rest of the foot is raised off the ground. Compare plantigrade; unguligrade.

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dihybrid cross

A genetic cross between parents that differ in two characteristics, controlled by genes at different loci. Mendel performed a dihybrid cross using pea plants and the characteristics of seed colour and texture: the parental plants had either smooth yellow seeds (SSYY) – the dominant characteristics – or wrinkled green seeds (ssyy) – the recessive characteristics. All the offspring had smooth yellow seeds, being heterozygous (SsYy) for the two alleles. Crossing between these offspring produced an F_2 generation of plants with smooth yellow, smooth green, wrinkled yellow, and wrinkled green seeds in the ratio 9:3:3:1 (see illustration). Mendel used these results as the basis for his Law of Independent Assortment (see Mendel's laws). Compare monohybrid cross.

Dihybrid cross

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dikaryon

A cell of a fungal hypha or mycelium containing two haploid nuclei of different strains. The nuclei associate in pairs but do not fuse, therefore the cell is not truly diploid. Dikaryosis occurs in the Basidiomycota and Ascomycota. See heterokaryosis.

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dikaryosis

The most common form of heterokaryosis. See also dikaryon.

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dilation

See vasodilation.

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dimethylbenzenes (xylenes)

Three compounds with the formula (CH_3)2C_6H_4, each having two methyl groups substituted on the benzene ring. 1,2-dimethylbenzene is ortho-xylene, etc. A mixture of the isomers is obtained from petroleum and is used as a clearing agent in preparing specimens for optical microscopy.

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dimictic lake

A lake that is stratified by a thermocline that is not permanent but is turned over twice during one year. The thermocline is disrupted due to seasonal changes in the climate. A meromictic lake is one in which there is a permanent stratification.

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dimorphism

The existence of two distinctly different types of individual within a species. An obvious example is sexual dimorphism in certain animals, in which the two sexes differ in colouring, size, etc. Dimorphism also occurs in some lower plants, such as mosses and ferns, that show an alternation of generations.

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Dinomastigota (Dinoflagellata)

A phylum of mostly single-celled organisms classified in the kingdom Protoctista. They are abundant in the marine plankton; many are photoautotrophs, containing brown xanthophyll pigments in addition to chlorophyll. Dinoflagellates characteristically have two undulipodia (flagella) for locomotion and most have a rigid cell wall of cellulose encrusted with silica. Some species (e.g. Noctiluca miliaris) are bioluminescent.

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dinosaur

An extinct terrestrial reptile belonging to a group that constituted the dominant land animals of the Jurassic and Cretaceous periods, 190–65 million years ago. There were two orders. The Ornithischia were typically quadrupedal herbivores, many with heavily armoured bodies, and included Stegosaurus, Triceratops, and Iguanodon. They were all characterized by birdlike pelvic girdles. The Saurischia included many bipedal carnivorous forms, such as Tyrannosaurus (the largest known carnivore), and some quadrupedal herbivorous forms, such as Apatosaurus (Brontosaurus) and Diplodocus. They all had lizard-like pelvic girdles. Many of the herbivorous dinosaurs were amphibious or semiaquatic.

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dinucleotide

A compound consisting of two nucleotides.

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dioecious

Describing plant species that have male and female flowers on separate plants. Examples of dioecious plants are willows. Compare monoecious.

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dipeptide

A compound consisting of two amino acid units joined at the amino (–NH_2) end of one and the carboxyl (–COOH) end of the other. This peptide bond (see peptide) is formed by a condensation reaction that involves the removal of one molecule of water.

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diphyodont

Describing a type of dentition that is characterized by two successive sets of teeth: the deciduous (milk) teeth, which are followed by the permanent (adult) teeth. Mammals have a diphyodont dentition. Compare monophyodont; polyphyodont.

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diploblastic

Describing an animal with a body wall composed only of two layers, ectoderm and endoderm, sometimes with a noncellular mesoglea between them. Coelenterates (i.e. cnidarians and ctenophorans) are diploblastic. Compare triploblastic.

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diploid

Describing a nucleus, cell, or organism with twice the haploid number of chromosomes characteristic of the species. The diploid number is designated as 2n. Two sets of chromosomes are present, one set being derived from the female parent and the other from the male. In animals, all the cells except the reproductive cells are diploid.

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diplont

An organism that is at the diploid stage of its life cycle. Compare haplont.

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Diplopoda

A class of wormlike terrestrial arthropods belonging to the phylum Uniramia and comprising the millipedes. Diplopods are characterized by a distinct head, bearing a single pair of short antennae, and 20 to over 60 body segments each bearing two pairs of legs. Restricted to damp habitats, millipedes are slow moving and feed on decaying leaves. See also Myriapoda.

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diplotene

The period in the first prophase of meiosis when paired homologous chromosomes begin to move apart. They remain attached at a number of points (see chiasma).

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Diplura

An order of small to medium-sized wingless insects, sometimes known as 'two-pronged bristletails', with elongated bodies, prominent paired tail-like cerci, and slender paired antennae. There are some 800 known species, typically 2–5 mm long, exceptionally reaching 50 mm. They lack eyes, have partially concealed mouthparts, and are found in dark humid places, for example in soil or under bark, living mostly on decaying vegetation, although some species are predators. The number of body segments is fixed during development, and moulting continues throughout life. Although some authorities place the Diplura in the subclass Apterygota, with the other wingless insects, others regard them as a separate class within the superclass Hexapoda.

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Dipnoi

A subclass or order of bony fishes that contains the lungfishes, which have lungs and breathe air. They are found in Africa, Australia, and South America, where they live in freshwater lakes and marshes that tend to become stagnant or even dry up in summer. They survive in these conditions by burrowing into the mud, leaving a small hole for breathing air, and entering a state of aestivation, in which they can remain for six months or more. The Dipnoi date from the Devonian era (408–360 million years ago) and share many features with the modern Amphibia.

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Diptera

An order of insects comprising the true, or two-winged, flies. Flies possess only one pair of wings – the forewings; the hindwings are modified to form small clublike halteres that function as balancing organs. Typically fluid feeders, flies have mouthparts adapted for piercing and sucking or for lapping; the diet includes nectar, sap, decaying organic matter, and blood. Some species prey on insects; others are parasitic. Dipteran larvae (maggots ) are typically wormlike with an inconspicuous head. They undergo metamorphosis via a pupal stage to the adult form. Many flies or their larvae are serious pests, either by feeding on crops (e.g. fruit flies) or as vectors of disease organisms (e.g. the house fly (Musca domestica) and certain mosquitoes).

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directional selection

Natural selection that favours the establishment of one particular advantageous mutation within a population, resulting in a change in phenotype in that direction. An example of directional selection is the increase in darker forms of the peppered moth (Biston betularia) that occurred in industrial areas, where the moths with darker wing coloration are better camouflaged than those with lighter wings against polluted tree trunks (see industrial melanism). Compare disruptive selection; stabilizing selection.

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disaccharide

A sugar consisting of two linked monosaccharide molecules. For example, sucrose comprises one glucose molecule and one fructose molecule bonded together.

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discontinuous replication

The synthesis of a new strand of a replicating DNA molecule as a series of short fragments that are subsequently joined together. Only one of the new strands, the so-called lagging strand , is synthesized in this way. The other strand (leading strand ) is synthesized by continuous addition of nucleotides to the growing end, i.e. continuous replication . The difference arises because of the different orientations of the parent template strands. The template of the leading strand is oriented in the 3'5' direction (according to the numbering of atoms in the sugar residues), which means that the leading strand itself is oriented in the opposite 5'3' direction, providing an –OH group at the 3' end for the continual addition of nucleotides by DNA polymerase, which moves forwards as the template strands unwind at the replication fork. However, the template of the lagging strand is oriented in a 5'3' direction, so the lagging strand itself is oriented in the 3'5' direction, and hence the DNA polymerase complex must move backwards away from the replication fork. Synthesis of the lagging strand proceeds not continuously, as on the leading strand, but discontinuously in a series of repeated steps. Discontinuous replication produces a series of short DNA fragments (Okazaki fragments ) complementary to the template strand. These vary in length, being about 100–200 nucleotides in eukaryotes and 1000–2000 nucleotides in prokaryotes. The fragments are then covalently bound together by the enzyme DNA ligase, forming a continuous chain of nucleotides, thus completing replication of the lagging strand. See also DNA replication; primase.

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discontinuous variation (qualitative variation)

Clearly defined differences in a characteristic that can be observed in a population. Characteristics that are determined by different alleles at a single locus show discontinuous variation, e.g. garden peas are either wrinkled or smooth. Compare continuous variation.

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disease

A condition in which the normal function of some part of the body (cells, tissues, or organs) is disturbed. A variety of microorganisms and environmental agents are capable of causing disease. The functional disturbances are often accompanied by structural changes in tissue.

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disinfectant

Any substance that kills or inhibits the growth of disease-producing microorganisms and is in general toxic to human tissues. Disinfectants include cresol, bleaching powder, and phenol. They are used to cleanse surgical apparatus, sick-rooms, and household drains and if sufficiently diluted can be used as antiseptics.

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disinhibition

(in animal behaviour) The tendency to exhibit displacement behaviour at a point of equilibrium in situations where there are two or more conflicting drives. See displacement activity.

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dispersal

The dissemination of offspring of plants or sessile animals. Dispersal provides organisms that are not mobile with a better chance of survival by reducing competition among offspring and parents. It also promotes the colonization of new habitats. Flowering plants produce fruits or seeds that are dispersed by such agents as wind, water, or animals. Specialized structures have evolved in many species to aid dispersal (see fruit).

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dispersive replication

A form of DNA replication in which the original DNA chain breaks and recombines in a random fashion before the double helix structure unwinds and separates to act as a template for messenger RNA synthesis. There is no evidence that it occurs in nature. Compare conservative replication; semiconservative replication.

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displacement activity

An activity shown by an animal that appears to be irrelevant to its situation. Displacement activities are frequently observed when there is conflict between opposing drives. For example, birds in aggressive situations, in which there are simultaneous drives to attack and to flee, may preen their feathers as a displacement activity.

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display behaviour

Stereotyped movement or posture that serves to influence the behaviour of another animal. Many displays in courtship and aggression are conspicuous and characteristic of the species; special markings or parts of the body may be prominently exhibited (for example, the male peacock spreads its tail in courtship). Other displays are cryptic and make it harder for a predator to recognize the displaying animal as potential prey. For example, geometer moth caterpillars, which look like twigs, hold themselves on plant stems with one end sticking into the air. Alternatively, a prey species may deter a predator by means of a startle display.

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disruptive selection

Natural selection that favours the extremes of a phenotype in a population. It often operates when an environmental factor shows distinct variations, for example high temperatures in summer and low temperatures in winter, with no intermediate forms. In this case the population will be variously adapted to withstand both high and low temperatures.

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distal

Describing the part of an organ that is farthest from the organ's point of attachment to the rest of the body. For example, hands and feet are at the distal ends of arms and legs, respectively. Compare proximal.

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distal convoluted tubule (second convoluted tubule)

The part of a nephron that leads from the thick ascending limb of the loop of Henle and drains into a collecting duct. The main function of the distal tubule is to absorb sodium chloride and other inorganic salts while retaining water.

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disulphide bridge (sulphur bridge)

A covalent bond (S–S) formed between the thiol groups (–SH) of two cysteine residues, usually in the polypeptide chains of proteins. Easily hydrolysed and prone to rearrangement, these bonds contribute to the tertiary structure of proteins.

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diuresis

The production of watery urine in large quantities. Diuresis causes the osmotic pressure of the blood to rise and is counteracted by the antidiuretic hormone.

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diuretic

A drug or other agent that increases the rate of urine formation and hence the rate at which water and certain salts are lost from the body. Many diuretic drugs work by decreasing the reabsorption of sodium and chloride ions from the filtrate in the kidney tubules, so that less water is reabsorbed. They are used to treat fluid retention (oedema) arising from disorders of the heart, kidneys, or other organs, and are used in helping to reduce high blood pressure (hypertension). There are several groups of diuretic drugs, with different modes of action. The most powerful are loop diuretics , such as frusemide, which act primarily by blocking Na+/K+/Cl– carriers in cells of the loop of Henle. Another group consists of the thiazides, such as metolazone, which inhibit Na+/Cl– transport in the distal convoluted tubule. Spironolactone exerts its diuretic effect by blocking the binding of the hormone aldosterone to its receptors. The osmotic diuretics , such as mannitol, act by increasing the osmolarity of the filtrate, and hence increasing urine volume.

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diurnal

Daily; denoting an event that happens once every 24 hours.

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diurnal rhythm

See circadian rhythm.

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divergent evolution

See adaptive radiation.

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diverticulum

A saclike or tubular outgrowth from a tubular or hollow internal organ. Diverticula may occur as normal structures (e.g. the caecum and appendix in the alimentary canal) or abnormally, from a weakened area of the organ.

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division

A category used traditionally in the classification of plants that consists of one or several similar classes. An example is the Spermatophyta (seed-bearing plants). In modern classification systems the phylum has replaced the division.

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dizygotic twins

See fraternal twins.

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dl-form

See optical activity.

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DNA (deoxyribonucleic acid)

The genetic material of most living organisms, which is a major constituent of the chromosomes within the cell nucleus and plays a central role in the determination of hereditary characteristics by controlling protein synthesis in cells (see also genetic code). It is also found in organelles other than the nucleus, such as the mitochondria (see extranuclear genes; mitochondrial DNA). DNA is a nucleic acid composed of two chains of nucleotides in which the sugar is deoxyribose and the bases are adenine, cytosine, guanine, and thymine (compare RNA). The two chains are wound round each other and linked together by hydrogen bonds between specific complementary bases (see base pairing) to form a spiral ladder-shaped molecule (double helix ; see also supercoiling). See illustration.
When the cell divides, its DNA also replicates in such a way that each of the two daughter molecules is identical to the parent molecule (see DNA replication). See also complementary DNA.

Molecular structure of DNA

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DNAase (DNase; deoxyribonuclease)

An enzyme that catalyses the cleavage of DNA. DNAase I is a digestive enzyme, secreted by the pancreas, that degrades DNA into shorter nucleotide fragments. Many other endonucleases and exonucleases cleave DNA, including the restriction enzymes and enzymes involved in DNA repair and replication.

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DNA-binding proteins

Proteins that are able to bind to DNA in both eukaryotes and prokaryotes and act as either activators or repressors of gene expression by controlling the binding of RNA polymerase to DNA during the process of transcription (see transcription factor). They are also involved in DNA replication: by binding to the nucleotides in the single DNA strands of the template that have unwound, they stabilize these strands so that they do not rewind. DNA-binding proteins do not include the histones.

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DNA blotting

See Southern blotting.

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DNA cloning

See gene cloning.

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DNA-dependent RNA polymerase

See polymerase.

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DNA filter assay

A method used to determine the presence of recombinant DNA in cloned cells. This is necessary as a culture will contain many cells that have not been transformed. After the culture has been plated on Petri dishes a copy of the cell pattern is made using a special filter and the cells are then lysed. DNA from the cells binds to the filter as it is heated in a pattern equivalent to that of the cells on the Petri dish. The presence of recombinant DNA is revealed by the use of a gene probe and autoradiography.

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DNA fingerprinting (genetic fingerprinting)

A technique in which an individual's DNA is analysed to reveal the pattern of repetition of particular short nucleotide sequences (called variable number tandem repeats) throughout the genome. This pattern is claimed to be unique to the individual concerned, and the technique is therefore used for identification purposes in forensic science and paternity disputes, and in veterinary science. Sufficient DNA can be obtained from very small samples of body tissue, such as blood, semen, or hair, if necessary by using the polymerase chain reaction to amplify minute quantities of DNA. Restriction enzymes are used to cleave the DNA and the marker sequences are revealed by specific gene probes and Southern blotting.

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DNA hybridization

A method of determining the similarity of DNA from different sources. Single strands of DNA from two sources, e.g. different bacterial species, are put together and the extent to which double hybrid strands are formed is estimated. The greater the tendency to form these hybrid molecules, the greater the extent of complementary base sequences, i.e. gene similarity. The method is one way of determining the genetic relationships of species.

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DNA ligase

An enzyme that is able to join together two portions of DNA and therefore plays an important role in DNA repair. DNA ligase is also used in recombinant DNA technology (see genetic engineering) as it ensures that the foreign DNA (e.g. the complementary DNA used in gene cloning) is bound to the plasmid into which it is incorporated.

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DNA methylation

The addition of methyl groups to constituent bases of DNA. In both prokaryotes and eukaryotes certain bases of the DNA generally occur in a methylated form. In bacteria this methylation protects the cell's DNA from attack by its own restriction enzymes, which cleave foreign unmethylated DNA and thereby help to eliminate viral DNA from the bacterial chromosome. Methylation is also important in helping DNA repair enzymes to distinguish the parent strand from the progeny strand when repairing mismatched bases in newly replicated DNA, and it may also play some role in controlling the transcription of DNA.

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DNA photolyase

An enzyme found in bacteria and other organisms that repairs damage to DNA induced by exposure to ultraviolet (UV) light. In the dark it binds to thymine dimers formed by the UV light, and on subsequent exposure to blue light it absorbs the light energy and uses it to split the dimer and restore normal structure. This process is called photoreactivation ; the DNA photolyase of E. coli is encoded by the phr gene (for `photoreactivation').

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DNA polymerase

See polymerase.

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DNA probe

See gene probe.

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DNA profiling

See DNA fingerprinting.

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DNA repair

A variety of mechanisms that help to ensure that the genetic sequence, as expressed in the DNA, is maintained and that errors that occur during DNA replication, by mutation, are not allowed to accumulate. An error in the genetic sequence could cause cell death by interfering with the replication process. The mechanisms work because DNA is made up of two complementary strands. A damaged section of a strand, or a mismatched base, can be removed by enzymes and replaced by the correct form by DNA polymerases. The phosphodiester backbone is then sealed by DNA ligase. See excision repair; mismatch repair; postreplicative repair.

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DNA replication

The process whereby DNA makes exact copies of itself, which is controlled by the enzyme DNA polymerase. Replication occurs at rates of between 50 nucleotides per second (in mammals) and 500 nucleotides per second (in bacteria). The hydrogen bonds between the complementary bases on the two strands of the parent DNA molecule break and the strands unwind, each strand acting as a template for the synthesis of a new one complementary to itself (see DNA-binding proteins). DNA polymerases move down the two single strands linking free nucleotides to their complementary bases (see base pairing) on the templates. The process continues until all the nucleotides on the templates have joined with appropriate free nucleotides and two identical molecules of DNA have been formed. This process is known as semiconservative replication as each new molecule contains half of the original parent DNA molecule (compare conservative replication; dispersive replication). Sometimes mutations occur that may cause the exact sequence of the parent DNA not to be replicated. However, DNA repair mechanisms reduce this possibility. See also discontinuous replication.

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DNA sequencing

See gene sequencing.

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dodecanoic acid

See lauric acid.

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Domagk, Gerhard

(1895–1964)
German biochemist who went to work for I.G. Farbenindustrie to investigate new drugs. In 1935 he discovered the antibacterial properties of a dye, Prontosil, which became the first sulpha drug (see sulphonamides). He was offered the 1939 Nobel Prize for physiology or medicine but was forced by Hitler to refuse; he finally received the award in 1947.

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domain

1 (in biochemistry) A functional unit of the tertiary structure of a protein. It consists of chains of amino acids folded into alpha helices and beta sheets to form a globular structure. Different domains are linked together by relatively straight sections of polypeptide chain to form the protein molecule. Domains allow a degree of movement in the protein structure. See also finger domain.

2 (in taxonomy) In some classification systems, the highest taxonomic category, consisting of one or more kingdoms. Some authors divide living organisms into three domains: Archaea (archaebacteria), Bacteria (eubacteria; see bacteria), and Eukarya (eukaryotic organisms).
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dominance hierarchy

See dominant.

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dominant

1 (in genetics) Describing the allele that is expressed in the phenotype when two different alleles of a gene are present in the cells of an organism. For example, the height of garden peas is controlled by two alleles, `tall' (T) and `dwarf' (t). When both are present (Tt), i.e. when the cells are heterozygous, the plant is tall since T is dominant and t is recessive. See also incomplete dominance.

2 (in ecology) Describing the most conspicuously abundant and characteristic species in a community. The term is usually used of a plant species in plant ecology; for example, pine trees in a pine forest.
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3 (in animal behaviour) Describing an animal that is allowed priority in access to food, mates, etc., by others of its species because of its success in previous aggressive encounters. Less dominant animals frequently show appeasement behaviour towards a more dominant individual, so overt aggression is minimized. In a stable group there may be a linear dominance hierarchy or peck order (so called because it was first observed in domestic fowl), with each animal being subservient to those above it in the hierarchy and taking precedence over those below it.
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donor

An individual whose tissues or organs are transferred to another (the recipient ). Donors may provide blood for transfusion or a kidney or heart for transplantation.

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dopa (dihydroxyphenylalanine)

A derivative of the amino acid tyrosine. It is found in particularly high levels in the adrenal glands and is a precursor in the synthesis of dopamine, noradrenaline, and adrenaline. The laevorotatory form, L-dopa (levodopa ), is administered in the treatment of Parkinson's disease, in which brain levels of dopamine are reduced.

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dopamine

A catecholamine that is a precursor in the synthesis of noradrenaline and adrenaline. It also functions as a neurotransmitter, especially in the brain.

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dormancy

An inactive period in the life of an animal or plant during which growth slows or completely ceases. Physiological changes associated with dormancy help the organism survive adverse environmental conditions. Annual plants survive the winter as dormant seeds while many perennial plants survive as dormant tubers, rhizomes, or bulbs. Hibernation and aestivation in animals help them survive extremes of cold and heat, respectively.

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dorsal

Describing the surface of a plant or animal that is farthest from the ground or other support, i.e. the upper surface. In vertebrates, the dorsal surface is that down which the backbone runs. Thus in upright (bipedal) mammals, such as humans and kangaroos, it is the backward-directed (posterior) surface. Compare ventral.

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dorsal aorta

The artery in vertebrate embryos that transports blood from the aortic arches to the trunk and limbs. In adult fish it is a major artery that carries oxygenated blood from the efferent branchial arteries to branches that supply the body organs; in adult tetrapods it arises from the systemic arch (see aorta). Compare ventral aorta.

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dorsal root

The part of a spinal nerve that enters the spinal cord on the dorsal side and contains only sensory fibres. The cell bodies of these fibres form the dorsal root ganglion (see ganglion), a swelling in the root that lies just outside the cord. Compare ventral root.

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dose

A measure of the extent to which matter has been exposed to ionizing radiation. The absorbed dose is the energy per unit mass absorbed by matter as a result of such exposure. The SI unit is the gray, although it is often measured in rads (1 rad = 0.01 gray; see radiation units). The maximum permissible dose is the recommended upper limit of absorbed dose that a person or organ should receive in a specified period according to the International Commission on Radiological Protection.

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dot-blot

A method for detecting a specific sequence of nucleotides in a DNA or RNA molecule. The nucleic acid sample is adsorbed onto a nitrocellulose filter and an appropriate gene probe, which matches the specific sequence being investigated, is added. After a period of incubation any excess probe is washed off and the nucleotide sequence under investigation can be detected by autoradiography.

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double circulation

The type of circulatory system that occurs in mammals, in which the blood passes through the heart twice before completing a full circuit of the body (see illustration). Blood is pumped from the heart to the lungs and returns to the heart before being distributed to the other organs and tissues of the body. The heart is divided into two separate compartments to prevent oxygenated blood returning from the lungs from mixing with deoxygenated blood from the other parts of the body. See also pulmonary circulation; systemic circulation. Compare single circulation.

Double circulation in a mammal

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double fertilization

A process, unique to flowering plants, in which two male gamete nuclei, which have travelled down the pollen tube, separately fuse with different female nuclei in the embryo sac. The first male nucleus fuses with the egg cell to form the zygote; the second male nucleus fuses with the two polar nuclei to form a triploid nucleus that develops into the endosperm.

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double helix

See DNA.

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double recessive

An organism with two recessive alleles for a particular characteristic.

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down feathers (plumules)

Small soft feathers that cover and insulate the whole body of a bird. In nestlings they are the only feathers; in adults they lie between and beneath the contour feathers. Down feathers have a fluffy appearance as their barbs are not joined together to form a smooth vane.

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Down's syndrome

A congenital form of mental retardation due to a chromosome defect in which there are three copies of chromosome no. 21 instead of the usual two (see trisomy). The affected individual has a short broad face and slanted eyes (as in the Mongolian races), short fingers, and weak muscles. Down's syndrome can be detected before birth by amniocentesis. It is named after the British physician John Down (1828–96), who first studied the incidence of the disorder.

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dragonflies

See Odonata.

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drive

The motivation that results in an animal performing a particular activity. There are two types: primary drive , which arises as a direct result of tissue requirements (e.g. the need for food or water); and secondary drive , resulting from learned behaviour.

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drone

A fertile male in a colony of social bees, especially the honeybee (Apis mellifera). The drones die after mating with the queen bee as the male reproductive organs explode within the female.

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Drosophila

A genus of fruit flies often used in genetic and developmental biology research because the larvae possess giant chromosomes in their salivary glands (see polyteny). These chromosomes have conspicuous transverse bands, which can be studied microscopically to reveal chromosome mutations and gene activity. Fruit flies have a short life cycle and produce a large number of offspring, which also makes them a good model animal for genetic research.

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drug

Any chemical substance that alters the physiological state of a living organism. Drugs are widely used in medicine for the prevention, diagnosis, and treatment of diseases; they include analgesics, antibiotics, anaesthetics, antihistamines, and anticoagulants. Some drugs are taken solely for the pleasurable effects they induce; these include narcotics; stimulants, such as cocaine and amphetamine; hallucinogens, such as LSD; and some tranquillizers. Many of these drugs are habit-forming and their use is illegal.

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drupe (pyrenocarp)

A fleshy fruit that develops from either one or several fused carpels and contains one or many seeds. The seeds are enclosed by the hard protective endocarp (see pericarp) of the fruit. Thus the stone of a peach is the endocarp containing the seed. Plums, cherries, coconuts, and almonds are other examples of one-seeded drupes; holly and elder fruits are examples of many-seeded drupes. See also etaerio.

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dry mass

The mass of a biological sample after the water content has been removed, usually by placing the sample in an oven. The dry mass is used as a measure of the biomass of a sample.

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Dryopithecus

A genus of extinct apes, fossils of which have been found in India and dated as 30–40 million years old. Fossils of Dryopithecus and of the similar genus Proconsul are often referred to as dryopithecines . Dryopithecines are believed to be either the ancestors of the fossil hominids or ancestral to both the human and ape lines.

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duct

A tube or passage in an organism that is involved in the secretion or excretion of substances (see gland).

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ductless gland

See endocrine gland.

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ductus arteriosus

A channel that connects the pulmonary artery with the aorta in the mammalian fetus and therefore allows blood to bypass the inactive lungs of the fetus. Derived from the fifth aortic arch, it normally closes soon after birth.

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duodenum

The first section of the small intestine of vertebrates. It is the site where food from the stomach is subjected to the action of bile (from the bile duct) and pancreatic enzymes (from the pancreatic duct) as well as the enzymes secreted by digestive glands in the duodenum itself (see succus entericus), which are required in the breakdown of proteins, carbohydrates, and fats. By neutralizing the acidic secretions of the stomach, the duodenum provides an alkaline environment necessary for the action of the intestinal enzymes.

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duplex

Describing a biological molecule comprising two cross-linked polymeric chains oriented lengthways side by side. The term is applied particularly to the double-stranded structure of DNA.

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duplication

(in genetics) The doubling or repetition of part of a chromosome, which generally originates during the crossing over phase of meiosis. Occasionally this type of chromosome mutation may have beneficial effects on a population. For example, a beneficial duplication resulted in the evolution of four types of haemoglobin in humans and apes from a single form. One of these types of haemoglobin (gamma or fetal haemoglobin) has a greater affinity for oxygen and maximizes fetal uptake of oxygen from the mother's blood.

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dura mater

The outermost and toughest of the three membranes (meninges) that surround the central nervous system in vertebrates. It lies adjacent to the skull and its purpose is to protect the delicate inner meninges (the arachnoid membrane and the pia mater).

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duramen

See heartwood.

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dwarfism

See growth hormone.

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dynein

A protein, found in many eukaryotic cells, that possesses ATPase activity and binds to microtubules. Dynein is associated with the microtubules of undulipodia (cilia and flagella); it moves along the microtubules, using energy derived from the hydrolysis of ATP, causing the undulipodium to bend. Dynein in the cytoplasm of cells causes the movement of organelles along the microtubules.

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dysphotic zone

See euphotic zone.

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dystrophic

Describing a body of water, such as a lake, that contains large amounts of undecomposed organic matter derived from terrestrial plants. Dystrophic lakes are poor in dissolved nutrients and therefore unproductive; they are common in peat areas and may develop into peat bogs. Compare eutrophic; mesotrophic; oligotrophic.

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E

ear

The sense organ in vertebrates that is specialized for the detection of sound and the maintenance of balance. It can be divided into the outer ear and middle ear, which collect and transmit sound waves, and the inner ear, which contains the organs of balance and (except in fish) hearing (see illustration). The term ear is often used for the pinna of the mammalian outer ear.

Structure of the mammalian ear

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eardrum

See tympanum.

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ear ossicles

Three small bones – the incus (anvil ), malleus (hammer ), and stapes (stirrup ) – that lie in the mammalian middle ear, forming a bridge between the tympanum (eardrum) and the oval window. The function of the ossicles is to transmit (and amplify) vibrations of the tympanum across the middle ear to the oval window, which transfers them to the inner ear. Muscles of the middle ear constrict the movement of the ossicles. This serves to safeguard the ear from damage caused by excessively loud noise.

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earwigs

See Dermaptera.

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Eccles, Sir John Carew

(1903–1997)
Australian physiologist, who was educated in Melbourne and Oxford, and held appointments in Britain, Australia, New Zealand and, finally, the USA. While in Australia he carried out his best-known work, on the transmission of nerve impulses across synapses, which he attributed to a chemical neurotransmitter that either initiated conduction or inhibited it. He shared the 1963 Nobel Prize for physiology or medicine with the British physiologists Sir Alan Hodgkin (1914–98) and Sir Andrew Huxley (1917– ), who postulated the sodium pump as the mechanism of impulse propagation.

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eccrine secretion

See secretion.

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ecdysis (moulting)

1 The periodic loss of the outer cuticle of arthropods. It starts with the reabsorption of some materials in the inner part of the old cuticle and the formation of a new soft cuticle. The remains of the old cuticle then split; the animal emerges and absorbs water or swallows air and increases in size while the new cuticle is still soft. This cuticle is then hardened with chitin and lime salts. In insects and crustaceans ecdysis is controlled by the hormone ecdysone. See also eclosion hormone; ecdysis-triggering hormone.

2 The periodic shedding of the outer layer of the epidermis of reptiles (except crocodiles) to allow growth to occur.
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ecdysis-triggering hormone

A peptide hormone, produced by neurosecretory cells in insects, that works in conjunction with other hormones (e.g. ecdysone and eclosion hormone) to trigger moulting of the cuticle (see ecdysis).

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ecdysone

A steroid hormone, secreted by a pair of prothoracic glands in the thorax of insects and by Y organs in crustaceans, that stimulates moulting (see ecdysis) and metamorphosis. In insects its release is stimulated by prothoracicotropic hormone. It acts on specific gene loci, stimulating the synthesis of proteins involved in these bodily changes. Some plants contain phytoecdysones , which are structurally similar to the insect ecdysones. They may help to protect the plant by interfering with the moulting cycle of insect pests that consume plant tissue.

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ECG

See electrocardiogram.

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Echinodermata

A phylum of marine invertebrates that includes the sea urchins, starfish, brittlestars, and sea cucumbers. Echinoderms have an exoskeleton (test ) of calcareous plates embedded in the skin. In many species (e.g. sea urchins) spines protrude from the test. A system of water-filled canals (the water vascular system ) provides hydraulic power for thousands of tube feet : saclike protrusions of the body wall used for locomotion, feeding, and respiration. Echinoderms have a long history: fossils of primitive echinoderms are known from rocks over 500 million years old. Extant classes include the Crinoidea (sea lilies), Holothuroidea (sea cucumbers), Echinoidea (sea urchins), and Stelleroidea (starfish and brittlestars).

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echolocation

A method used by some animals (such as bats, dolphins, and certain birds) to detect objects in the dark. The animal emits a series of high-pitched sounds that echo back from the object and are detected by the ear or some other sensory receptor. From the direction of the echo and from the time between emission and reception of the sounds the object is located, often very accurately.

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eclosion hormone

A peptide hormone secreted by cells of the insect nervous system, especially the brain, that triggers the sequence of events leading to emergence of the adult from the pupa, i.e. eclosion. It is also involved, with other hormones (e.g. ecdysone), in moulting of the cuticle by immature stages (see ecdysis).

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E. coli

See Escherichia coli.

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ecological equivalents

Unrelated organisms that occupy similar habitats and resemble each other. Ecological equivalents result from convergent evolution. For example, sharks (fish) and dolphins (mammals) live in a marine habitat and superficially resemble each other.

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ecological niche

The status or role of an organism in its environment. An organism's niche is defined by the types of food it consumes, its predators, temperature tolerances, etc. Two species cannot coexist stably if they occupy identical niches.

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ecological pyramid

See pyramid of biomass; pyramid of energy; pyramid of numbers.

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ecology

The study of the interrelationships between organisms and their natural environment, both living and nonliving. For this purpose, ecologists study organisms in the context of the populations and communities in which they can be grouped and the ecosystems of which they form a part. The study of ecological interactions provides important information on the nature and mechanisms of evolutionary change. Advances made in ecology over the last 25 years have led to increased concern about the effects of man's activities on the environment (notably the effects of pollution), which has resulted in a greater awareness of the importance of conservation.

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ecosystem

A biological community and the physical environment associated with it. Nutrients pass between the different organisms in an ecosystem in definite pathways; for example, nutrients in the soil are taken up by plants, which are then eaten by herbivores, which in turn may be eaten by carnivores (see food chain). Organisms are classified on the basis of their position in an ecosystem into various trophic levels. Nutrients and energy move round ecosystems in loops or cycles (in the case above, for example, nutrients are returned to the soil via animal wastes and decomposition). See carbon cycle; nitrogen cycle.

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ectoderm

The external layer of cells of the gastrula, which will develop into the epidermis and the nervous system in the adult. See also germ layers.

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ectoparasite

A parasite that lives on the outside of its host's body. See parasitism.

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ectoplasm

See cytoplasm.

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Ectoprocta

See Bryozoa.

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ectotherm (poikilotherm)

An animal that maintains its body temperature by absorbing heat from the surrounding environment. All animals except mammals and birds are ectotherms; they are often described as being cold-blooded and are unable to regulate their body temperature metabolically. See poikilothermy. Compare endotherm; heterotherm.

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edaphic factor

An abiotic factor relating to the physical or chemical composition of the soil found in a particular area. For example, very alkaline soil may be an edaphic factor limiting the variety of plants growing in a region.

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EDTA

Ethylenediaminetetraacetic acid, (HOOCCH_2)2N(CH_2)2N(CH_2COOH)2: a compound that acts as a chelating agent, reversibly binding with iron, magnesium, and other metal ions. It is used in certain culture media bound with iron, which it slowly releases into the medium, and also in some forms of quantitative analysis.

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EEG

See electroencephalogram.

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effector

A cell or organ that produces a physiological response when stimulated by a nerve impulse. Examples include muscles and glands.

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effector neurone

A nerve cell, such as a motor neurone, that transmits impulses from the central nervous system to an effector in order to bring about a physiological response to changes in the environment.

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efferent

Carrying (nerve impulses, blood, etc.) away from the centre of a body or organ towards peripheral regions. The term is usually applied to types of nerve fibres or blood vessels. Compare afferent.

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egestion

The expulsion from the body of waste food materials that have never left the gut, particularly the expulsion of undigested materials from the gut through the anus (see defecation). Egestion should not be confused with excretion, in which the waste materials are produced by metabolic activity in the body's tissues.

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egg

1 The fertilized ovum (zygote) in egg-laying animals, e.g. birds and insects, after it emerges from the body. The egg is covered by egg membranes that protect it from environmental damage, such as drying.

2 (or egg cell) The mature female reproductive cell in animals and plants. See oosphere; ovum.
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egg membrane

The layer of material that covers an animal egg cell. Primary membranes develop in the ovary and cover the egg surface in addition to the normal cell membrane. The primary membrane is called the vitelline membrane in insects, molluscs, birds, and amphibians, the chorion in tunicates and fish, and the zona pellucida in mammals. Insects have a second thicker membrane, also called the chorion. Secondary membranes are secreted by the oviducts and parts of the genital system while the egg is passing to the outside. They include the jelly coat of frogs' eggs and the albumen and shell of birds' eggs.

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Ehrlich, Paul

(1854–1915)
German bacteriologist, who graduated as a physician in 1878. After working in a Berlin hospital for nine years he taught at Berlin University (unpaid because he was a Jew). In 1890 he went to work with Robert Koch (1843–1910) to study tuberculosis, cholera, and other diseases. In collaboration with the German immunologist Emil von Behring (1854–1917), Ehrlich developed a serum containing antitoxins against diphtheria, which provided immunity against the disease. In 1910 he discovered Salvarsan, an arsenical drug effective against syphilis. He was awarded the 1908 Nobel Prize for physiology or medicine for his work on serum therapy.

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eicosanoid

Any of a group of fatty acids or their related compounds having 20 carbon atoms and one or more double bonds. The eicosanoids include various biologically important molecules, notably arachidonic acid and its derivatives, the prostaglandins and leukotrienes.

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ejaculation

The propulsion of semen out of the erect penis due to powerful rhythmic contractions of the urethra. An ejaculation coincides with the peak of sexual excitement (orgasm ) and is accompanied by various physiological effects in the body, such as increased respiration rate and heart rate.

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elaiosome

See myrmecochory.

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Elasmobranchii

See Chondrichthyes.

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elastic cartilage

See cartilage.

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elastic fibres

See elastin.

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elastin

A fibrous protein that is the major constituent of the yellow elastic fibres of connective tissue. It is rich in glycine, alanine, proline, and other nonpolar amino acids that are cross-linked, making the protein relatively insoluble. Elastic fibres can stretch to several times their length and then return to their original size. Elastin is particularly abundant in elastic cartilage, blood-vessel walls, ligaments, and the heart.

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electric organ

An organ occurring on the body or tail of certain fish, such as the electric ray (Torpedo) and electric eel (Electrophorus electricus). It gives an electric shock when touched and is used either to stun prey or predators or, in some species, to maintain a weak electric field in the surrounding water that is used in navigation. The organ is composed of modified muscle cells (electroplate or electroplax cells ), nervous stimulation of which greatly increases the potential difference across the cell. The electroplates are in series so a high overall voltage can be achieved.

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electric potential

Symbol V. The energy required to bring unit electric charge from infinity to the point in an electric field at which the potential is being specified. The unit of electric potential is the volt. The potential difference (p.d.) between two points in an electric field or circuit is the difference in the values of the electric potentials at the two points, i.e. it is the work done in moving unit charge from one point to the other.

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electrocardiogram (ECG)

A tracing or graph of the electrical activity of the heart. Recordings are made from electrodes fastened over the heart and usually on both arms and a leg. Changes in the normal pattern of an ECG may indicate heart irregularities or disease.

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electroencephalogram (EEG)

A tracing or graph of the electrical activity of the brain. Electrodes taped to the scalp record electrical waves from different parts of the brain. The pattern of an EEG reflects an individual's level of consciousness and can be used to detect such disorders as epilepsy, tumours, or brain damage. See also brain death.

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electrolyte

A liquid that conducts electricity as a result of the presence of positive or negative ions. An example is a sodium chloride solution, which consists of free sodium (Na+) and chloride (Cl–) ions. In biology and medicine `electrolyte' usually refers to the ion itself.

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electromagnetic spectrum

The range of wavelengths over which electromagnetic radiation extends. The longest waves (10^5–10–3 metres) are radio waves, the next longest (10–3–10–6 m) are infrared waves, then comes the narrow band (4–7 × 10–7 m) of visible light, followed by ultraviolet radiation (10–7–10–9 m), X-rays (10–9–10–11 m), and gamma rays (10–11–10–14 m).

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electron

An elementary particle present in all atoms in groupings called shells around the nucleus. When electrons are detached from the atom they are called free electrons .

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electron flow

The transfer of electrons along a series of carrier molecules in the electron transport chain.

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electron microscope

A form of microscope that uses a beam of electrons instead of a beam of light (as in the optical microscope) to form a large image of a very small object, such as a cell organelle, a virus, or a DNA molecule. In optical microscopes the resolution is limited by the wavelength of the light. High-energy electrons, however, can be associated with a considerably shorter wavelength than light; for example, electrons accelerated to an energy of 10^5 electronvolts have a wavelength of 0.04 nanometre, enabling a resolution of 0.2–0.5 nm to be achieved. The transmission electron microscope (see illustration) has an electron beam, sharply focused by electron lenses (coils producing a magnetic field or electrodes between which an electric field is created), passing through a very thin metallized specimen (less than 50 nanometres thick) onto a fluorescent screen, where a visual image is formed. This image can be photographed. The scanning electron microscope can be used with thicker specimens and forms a perspective image, although the resolution and magnification are lower. It is used particularly for examining surface features of small objects, such as pollen grains. In this type of instrument a beam of primary electrons scans the specimen and those that are reflected, together with any secondary electrons emitted, are collected. This current is used to modulate a separate electron beam in a TV monitor, which scans the screen at the same frequency, consequently building up a picture of the specimen. The resolution is limited to about 10–20nm. See also field-emission microscope; field-ionization microscope.

Principle of transmission electron microscope

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electron transport chain (electron transport system)

A sequence of biochemical reduction–oxidation reactions that effects the transfer of electrons through a series of carriers. An electron transport chain, also known as the respiratory chain , forms the final stage of aerobic respiration. In the mitochondria, NADH or FADH_2, generated by the Krebs cycle, transfer their electrons through a chain of carrier molecules, including ubiquinone and a series of cytochromes, that undergo reversible reduction–oxidation reactions, accepting electrons and then donating them to the next carrier in the chain – a process known as electron flow . Cytochrome oxidase combines electrons and hydrogen ions with oxygen to form water (see illustration). This process is coupled to the formation of ATP (see chemiosmotic theory; oxidative phosphorylation). An electron transport chain also occurs in the thylakoid membranes of chloroplasts in photosynthesis; the carrier molecules include plastoquinone, plastocyanin, and ferredoxin. See photophosphorylation.

The mitochondrial electron transport chain

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electrophoresis (cataphoresis)

A technique for the analysis and separation of colloids, based on the movement of charged colloidal particles in an electric field. There are various experimental methods. In one the sample is placed in a U-tube and a buffer solution added to each arm, so that there are sharp boundaries between buffer and sample. An electrode is placed in each arm, a voltage applied, and the motion of the boundaries under the influence of the field is observed. The rate of migration of the particles depends on the field, the charge on the particles, and on other factors, such as the size and shape of the particles. More simply, electrophoresis can be carried out using an adsorbent, such as a strip of filter paper, soaked in a buffer with two electrodes making contact. The sample is placed between the electrodes and a voltage applied. Different components of the mixture migrate at different rates, so the sample separates into zones. The components can be identified by the rate at which they move. In gel electrophoresis the medium is a gel, typically made of polyacrylamide, agarose, or starch.
Electrophoresis, which has also been called electrochromatography , is used extensively in studying mixtures of proteins (see PAGE), nucleic acids, carbohydrates, enzymes, etc. In clinical medicine it is used for determining the protein content of body fluids.

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electroplax

One of the cells found in the electric organs of certain fish.

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electroreceptor

An organ specialized for detecting electric currents. Such organs are fairly common in marine fishes, which use them for detecting prey or potential attackers. The best known example is the ampulla of Lorenzini , groups of which are embedded in the head of sharks and rays. Each consists of a jelly-filled cuplike structure connected to the skin surface by a duct, sometimes several centimetres long, which is also filled with jelly. Sensory hair cells in the ampulla detect electric currents in the surrounding water, channelled inwards via the external pore and duct. The high sensitivity of these organs enables a shark, for example, to sense the very weak electric currents, perhaps just a few microamps, generated by the respiratory muscles of a resting plaice buried in the sand. Sharks and rays also use their ampullae of Lorenzini as magnetoreceptors to detect the earth's magnetic field. Similar organs occur in certain teleost fish, for example the marine catfish Plotosus. Some fish generate their own weak electric field as an alarm system or as a means of locating objects or communicating with other individuals of the same species. Disturbances in this field are detected by the fish's electroreceptors, warning of possible threats from intruders or receiving signals from conspecifics.

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electrovalent bond (ionic bond)

A type of chemical bond formed by the transfer of one or more electrons from one atom to another, so that oppositely charged ions are produced. For example, the bond between the sodium and chlorine atoms in sodium chloride (NaCl) is formed by the transfer of an electron from sodium to chlorine, creating Na+ and Cl– ions. The electrostatic attraction between these ions provides the bonding in NaCl.

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elicitor

A substance or other stimulus that triggers the hypersensitive response in a plant (see hypersensitivity). Most elicitors are polysaccharides, small proteins, or lipids associated with the fungal or bacterial cell wall. However, pectic fragments resulting from microbial damage to the plant's own cell walls may also act as elicitors. The elicitors interact with the plasma membrane of undamaged cells and trigger activation of genes involved in the defence response. The precise nature of this signal transduction mechanism is still unknown.

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ELISA (enzyme-linked immunosorbent assay)

A sensitive technique (see immunoassay) for accurately determining the amount of protein or other antigen in a given sample by means of an enzyme-catalysed colour change. Antibody specific to the test protein is adsorbed onto a solid substrate, such as a PVC sheet, and a measured amount of the sample is added; all molecules of the test protein in the sample are bound by the antibody. A second antibody specific for a second site on the test protein is added; this is conjugated with an enzyme, which catalyses a colour change in the fourth reagent, added finally to the sheet. The colour change can be measured photometrically and compared against a standard curve to give the concentration of protein in the sample. ELISA is widely used for diagnostic and other purposes.

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elongation

(in protein synthesis) The phase in which amino acids are linked together by sequentially formed peptide bonds to form a polypeptide chain (see translation). Elongation factors are proteins that – by binding to a tRNA–amino-acid complex – enable the correct positioning of this complex on the ribosome, so that translation can proceed.

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Elton, Charles Sutherland

(1900–91)
British zoologist and ecologist, who founded the Bureau of Animal Population at Oxford in 1932 and the same year became editor of the new Journal of Animal Ecology. The first zoologist to study animals in relation to their environment, he explored the nature of food chains and studied population fluctuations.

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elytra

The thickened horny forewings of the Coleoptera (beetles), which cover and protect the membranous hindwings when the insect is at rest.

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emasculation

The removal of the anthers of a flower in order to prevent self-pollination or the undesirable pollination of neighbouring plants.

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Embden–Meyerhof pathway

See glycolysis.

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embedding

A stage in the preparation of a sample for examination by microscopy that involves impregnation of the sample with wax or plastic following dehydration. The embedded sample can then be cut into extremely thin sections to reveal cellular and subcellular structure.

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embryo

1 An animal in the earliest stages of its development, from the time when the fertilized ovum starts to divide (see cleavage), while it is contained within the egg or reproductive organs of the mother, until hatching or birth. A human embryo (see illustration) is called a fetus after the first eight weeks of pregnancy.

A developing human embryo

2 The structure in plants that develops from the zygote prior to germination. In seed plants the zygote is situated in the embryo sac of the ovule. It divides by mitosis to form the embryonic cell and a structure called the suspensor , which embeds the embryo in the surrounding nutritive tissue. The embryonic cell divides continuously and eventually gives rise to the radicle (young root), plumule (young shoot), and one or two cotyledons (seed leaves). Changes also take place in the surrounding tissues of the ovule, which becomes the seed enclosing the embryo plant.
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embryology

The study of the development of animals from the fertilized egg to the new adult organism. It is sometimes limited to the period between fertilization of the egg and hatching or birth (see embryo).

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embryo mother cell

See megaspore mother cell.

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embryo sac

A large cell that develops in the ovule of flowering plants. It is equivalent to the female gametophyte of lower plants, although it is very much reduced. Typically, it contains eight nuclei formed by division of the megaspore mother cell (see illustration). The oosphere (egg cell), which is associated with two synergid cells to form the egg apparatus , is fertilized by a male nucleus and becomes the embryo. The two polar nuclei fuse with a second male nucleus to form a triploid nucleus that gives rise to the endosperm. The three remaining nuclei form the antipodal cells .

Embryo sac

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emulsification

(in digestion) The breakdown of fat globules in the duodenum into tiny droplets, which provides a larger surface area on which the enzyme pancreatic lipase can act to digest the fats into fatty acids and glycerol. Emulsification is assisted by the action of the bile salts (see bile).

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emulsion

A colloid in which small particles of one liquid are dispersed in another liquid. Usually emulsions involve a dispersion of water in an oil or a dispersion of oil in water. Dietary fats are reduced to an emulsion in the duodenum to facilitate their subsequent digestion (see emulsification).

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enamel

The material that forms a covering over the crown of a tooth (i.e. the part that projects above the gum). Enamel is smooth, white, and extremely hard, being rich in minerals containing calcium, especially apatite. It is produced by certain cells (ameloblasts ) of the oral epithelium and protects the underlying dentine of the tooth. Enamel may also be found in the placoid scales of certain fish, which demonstrates the common developmental origin of scales and teeth.

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enamel organ

A mass of soft pulpy tissue that develops on the surface of a dental papilla. It consists of a mesh of fibres, fluid albumin, and ameloblasts, which deposit a layer of protective enamel on the outer surface of the dentine.

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encephalin (enkephalin)

Any of a class of endorphins found principally in the brain. Two encephalins have so far been isolated. They bind to opiate receptors in the brain and their release controls levels of pain and other sensations.

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endangered species

A plant or animal species defined by the IUCN (International Union for the Conservation of Nature and Natural Resources) as being in immediate danger of extinction because its population numbers have reached a critical level or its habitats have been drastically reduced. If these causal factors continue the species is unlikely to survive. A list of endangered species is published by the IUCN, which also defines other categories of threatened species.

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endemic

1 Describing a plant or animal species that is restricted to one or a few localities in its distribution. Endemic species are usually confined to islands and are vulnerable to extinction.

2 Describing a disease or a pest that is always present in an area. For example, malaria is endemic in parts of Africa.
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endergonic reaction

A chemical reaction in which energy is absorbed. In an endothermic reaction the energy is in the form of heat. Compare exergonic reaction.

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endocardium

See myocardium.

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endocarp

See pericarp.

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endochondral ossification

See ossification.

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endocrine gland (ductless gland)

Any gland in an animal that manufactures hormones and secretes them directly into the bloodstream to act at distant sites in the body (known as target organs or cells ). Endocrine glands tend to control slow long-term activities in the body, such as growth and sexual development. In mammals they include the pituitary, adrenal, thyroid, and parathyroid glands, the ovary and testis, the placenta, and part of the pancreas (see islets of Langerhans). The activity of endocrine glands is controlled by negative feedback, i.e. a rise in output of hormone inhibits a further increase in its production, either directly or indirectly via the target organ or cell. See also neuroendocrine system. Compare exocrine gland.

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endocrinology

The study of the structure and functions of the endocrine glands and of the hormones they produce.

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endocytosis

The process by which materials enter a cell without passing through the cell membrane. The membrane folds around material outside the cell, resulting in the formation of a saclike vesicle into which the material is incorporated. This vesicle is then pinched off from the cell surface so that it lies within the cell (see also endosome). Both phagocytosis and pinocytosis are forms of endocytosis. Compare exocytosis.

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endoderm (entoderm)

The internal layer of cells of the gastrula, which will develop into the alimentary canal (gut) and digestive glands of the adult. See also germ layers.

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endodermis

The innermost layer of the root cortex of a plant, lying immediately outside the vascular tissue. Various modifications of the endodermal cell walls enable them to regulate the passage of materials both into and out of the vascular system (see Casparian strip). An endodermis may also be seen in the stems of some plants.

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endogamy

The fusion of reproductive cells from closely related parents, i.e. inbreeding. Compare exogamy.

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endogenous

Describing a substance, stimulus, organ, etc., that originates from within an organism. For example, growth rhythms not directed by environmental stimuli are termed endogenous rhythms. Lateral roots, which always grow from inside the main root rather than from its surface, are said to arise endogenously. Compare exogenous.

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endolymph

The fluid that fills the membranous labyrinth of the vertebrate inner ear. See cochlea; semicircular canals. Compare perilymph.

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endometrium

The mucous membrane that lines the uterus of mammals. It comprises an upper mucus-secreting layer, which is shed during menstruation, and a basal layer, which proliferates to form the upper layer. See also menstrual cycle.

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endomitosis

The replication of chromosomes in the absence of cell or nuclear division, resulting in numerous copies within each cell. It occurs notably in the salivary glands of Drosophila and other flies. Cells in these tissues contain giant chromosomes (see polyteny), each consisting of over a thousand intimately associated, or synapsed, chromatids.

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endonuclease

An enzyme that catalyses the internal cleavage of nucleic acids. See also restriction enzyme. Compare exonuclease.

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endoparasite

A parasite that lives inside its host's body. See parasitism.

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endopeptidase

A protein-digesting enzyme that cleaves a polypeptide chain at specific sites between amino acids. For example, chymotrypsin cleaves the chain next to aromatic amino acids, such as phenylalanine; trypsin cleaves the chain next to basic amino acids, such as lysine or arginine; and pepsin cleaves the chain next to tyrosine and phenylalanine. Compare exopeptidase.

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endoplasm

See cytoplasm.

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endoplasmic reticulum (ER)

A system of membranes within the cytoplasm of plant and animal cells. It forms a link between the cell and nuclear membranes (see cisterna) and is the site of protein synthesis. It is also concerned with the transport of proteins and lipids within the cell. Rough ER has ribosomes attached to its surface; proteins synthesized on the ribosomes are enclosed in vesicles and transported to the Golgi apparatus. Smooth ER lacks ribosomes; it is the site of important metabolic reactions, including phospholipid and fatty-acid synthesis.

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endopterygote

Any winged insect having holometabolous development, i.e. the eggs hatch into larvae that are dissimilar to the adults and lack wings. These develop internally before the final moult into a pupa, from which the adult emerges; metamorphosis is described as complete . Endopterygote insects include the beetles (Coleoptera), ants, bees, and wasps (Hymenoptera), flies (Diptera), and butterflies and moths (Lepidoptera). Compare exopterygote.

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end organ

The structure at the end of a peripheral nerve. Examples of end organs are the muscle end plate at the end of a motor neurone and the receptor at the end of a sensory neurone.

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endorphin

Any of a group of substances, found naturally in brain and other tissues, that have pain-relieving effects similar to those of morphine. They are all peptides or polypeptides and include the encephalins, which consist of five amino acids and occur principally in the brain. Several endorphins are associated with the pituitary, while others occur in various body tissues, including the placenta, adrenal glands, and pancreas.

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endoscopy

The study of the interior of hollow organs, such as the intestines and the reproductive organs, using a flexible probe (endoscope ), which contains optical fibres for transmitting images onto a screen.

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endoskeleton

A supporting framework that lies entirely within the body of an animal, such as the bony skeleton of vertebrates or the spicules of a sponge. The function of an endoskeleton is to support the body and in vertebrates it also protects the organs and provides a system of levers on which the muscles can act to produce movement. Compare exoskeleton.

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endosome

A vesicle formed within a cell during forms of endocytosis in which the material to be ingested first binds to receptor sites on the cell surface. These receptors occur in areas of the plasma membrane called coated pits , which are pinched off to become coated vesicles , surrounded by a protein coat (see clathrin) and containing the ingested material, within the cell. The vesicles then shed their coats and merge with other vesicles to form endosomes. The endosomes fuse with vesicles from the Golgi apparatus to form lysosomes.

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endosperm

A nutritive tissue, characteristic of flowering plants, that surrounds the developing embryo in a seed. It develops from nuclei in the embryo sac and its cells are triploid. In endospermic seeds it remains and increases in size; in nonendospermic seeds it disappears as the food is absorbed by the embryo, particularly the cotyledons. Many plants with endospermic seeds, such as cereals and oil crops, are cultivated for the rich food reserves in the endosperm.

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endospore

The resting stage of certain bacteria, formed in response to adverse conditions. The bacterial cell changes into a partially dehydrated core, enclosed in a multilayered protein coat. On return to favourable conditions the spore germinates and reverts to the normal vegetative form of the organism. Endospores can remain viable for long periods, perhaps several thousands of years. There are even reports of successful germination of endospores obtained from the guts of prehistoric bees preserved in amber for 25–40 million years.

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endostyle

A ciliated groove in the wall of the pharynx of lancelets (Cephalochordata), some tunicates, and larval lampreys that contains mucus-secreting glands. Mucus secreted by the endostyle traps food particles in water drawn into the pharynx by the beating action of the cilia.

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endosymbiont theory

A theory, devised principally by US biologist Lynn Margulis (1938– ), that eukaryotic organisms evolved from symbiotic associations between prokaryotic ancestors. Free-living aerobic bacteria and photosynthetic cyanobacteria became incorporated inside larger nucleated prokaryotic cells, where they acted as forerunners of the mitochondria and chloroplasts seen in modern eukaryotes. Such events are held to have occurred on several occasions, producing various lineages of both heterotrophic and phototrophic protoctists, from which evolved ancestors of animals, plants, and fungi. There is strong evidence for the theory, particularly the finding that mitochondria and chloroplasts have DNA similar in form to that of eubacteria, and that they contain prokaryotic-type ribosomes.

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endothelium

A single layer of thin platelike cells that line the inner surfaces of blood and lymph vessels and the heart. Endothelium is derived from the mesoderm. Compare epithelium; mesothelium.

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endotherm (homoiotherm)

An animal that can generate and maintain heat within its body independently of the environmental temperature. Mammals and birds are endotherms; they are often described as being warm-blooded . See homoiothermy. Compare ectotherm; heterotherm.

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endothermic

1 Of or relating to endotherms.

2 Denoting a chemical reaction that takes heat from its surroundings. See endergonic reaction. Compare exothermic.
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endotoxin

See toxin.

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end plate

The area of muscle cell membrane that lies immediately beneath a motor nerve ending at a neuromuscular junction. Release of a neurotransmitter at the end plate induces contraction of the muscle fibre.

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energy

A measure of a system's ability to do work. Like work itself, it is measured in joules. Energy is conveniently classified into two forms: potential energy is the energy stored in a body or system as a consequence of its position, shape, or state (this includes chemical energy in food substances, etc.); kinetic energy is energy of motion and is usually defined as the work that will be done by the body possessing the energy when it is brought to rest.

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energy flow

(in ecology) The flow of energy that occurs along a food chain. Energy enters the food chain at the level of the producers (usually plants) in the form of solar energy. The plants convert solar energy into chemical energy in the process of photosynthesis. Chemical energy is passed from one trophic level to the next through feeding. Since a large proportion of energy is lost at each trophic level, mostly in the form of heat energy due to respiration, a food chain does not normally consist of more than five trophic levels: the fifth trophic level does not contain enough energy to support further levels. Energy is also lost from the food chain in excretory products and the remains of dead organisms; this is converted into heat energy by the action of decomposers. See also productivity; pyramid of energy.

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enhancer

A region on a DNA molecule that can initiate the transcription of a gene that may be some distance away from the enhancer on the same chromosome. It is activated by DNA-binding proteins, which bind to the enhancer regions and in turn regulate the binding of RNA polymerase to the DNA molecule.

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enterokinase (enteropeptidase)

An enzyme in the small intestine that activates trypsinogen to trypsin.

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enteron (coelenteron)

See gastrovascular cavity.

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enthalpy

Symbol H. A thermodynamic property of a system defined by H = U + pV, where H is the enthalpy, U is the internal energy of the system, p its pressure, and V its volume. In a chemical reaction carried out in the atmosphere the pressure remains constant and the enthalpy of reaction, H, is equal to U + pV. For an exothermic reaction H is taken to be negative.

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entoderm

See endoderm.

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entomology

The study of insects.

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entomophily

Pollination of a flower in which the pollen is carried on an insect. Entomophilous flowers are usually brightly coloured and scented and often secrete nectar. In some species (e.g. primulas) there are structural differences between the flowers to ensure that cross-pollination occurs. Other examples of entomophilous flowers are orchids and antirrhinums. Compare anemophily; hydrophily.

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Entoprocta

A phylum of small invertebrate aquatic animals typically having a cup-shaped body bearing tentacles and attached to the substrate by means of a stalk. The entire animal is usually less than 10 mm long. There are about 150 known species, widely distributed and almost exclusively marine. Many species are colonial and form mats on seaweeds, rocks, shells, and other surfaces in coastal waters. Each individual has a ring of 4–36 ciliated tentacles (calyx ) on top of the body, enclosing both the mouth and anus. The tentacles set up a feeding current and trap minute plankton and other particles in mucus, transferring the particles to the mouth. Digestion occurs in the U-shaped gut and waste is discharged via the anus. There is no heart or blood vessels, and the nervous system consists of a single ganglion between the mouth and anus, from which nerves extend to the tentacles, body, and stalk. Dissolved nitrogenous waste is discharged by exocytosis from the stomach wall into the gut, and also collected by a ciliated flame cell, and discharged via pores. Reproduction is both asexual, by budding, and sexual. Most entoprocts are hermaphrodites, and produce free-swimming larvae that settle on a substrate before undergoing metamorphosis into the sessile adult form. The space between the body wall and the gut is filled with a jelly-like mesenchyme; this has been interpreted by some as a pseudocoelom, prompting the suggestion that entoprocts are allied with other pseudocoelomate animals, such as nematodes. Others argue that developmental similarities between entoprocts and bryozoans (see Bryozoa) point to a close relationship between these two groups.

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entropy

Symbol S. A measure of the unavailability of a system's energy to do work; an increase in entropy is accompanied by a decrease in energy availability. When a system undergoes a reversible change the entropy (S) changes by an amount equal to the energy (Q) absorbed by the system divided by the thermodynamic temperature (T) at which the energy is absorbed, i.e. S = Q/T. However, all real processes are to a certain extent irreversible changes and in any closed system an irreversible change is always accompanied by an increase in entropy.
In a wider sense entropy can be interpreted as a measure of a system's disorder; the higher the entropy the greater the disorder. As any real change to a closed system tends towards higher entropy, and therefore higher disorder, it follows that the entropy of the universe (if it can be considered a closed system) is increasing and its available energy is decreasing. This increase in the entropy of the universe is one way of stating the second law of thermodynamics.

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environment

(in ecology) The physical, chemical, and biological conditions of the region in which an organism lives. See also ecology; ecosystem.

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environmental resistance

The sum total of the factors that prevent populations from continually growing and therefore tend to keep populations at constant levels. These factors include predators, disease, and a shortage of any of the various requirements for survival, such as food, water, shelter, and light (which is particularly important for plants). See also population growth.

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environmental selection

Natural selection within a population resulting from the influence exerted by the environment. Environmental selection inevitably leads to a change in the composition of genes within a population. For example, a change in the environment resulting in a drop in temperature will favour animals with an increased ability to maintain their body heat and may eventually result in a population of animals with denser fur.

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enzyme

A protein that acts as a catalyst in biochemical reactions. Each enzyme is specific to a particular reaction or group of similar reactions. Many require the association of certain nonprotein cofactors in order to function. The molecule undergoing reaction (the substrate ) binds to a specific active site on the enzyme molecule to form a short-lived intermediate (see enzyme–substrate complex): this greatly increases (by a factor of up to 10^20) the rate at which the reaction proceeds to form the product. Enzyme activity is influenced by substrate concentration and by temperature and pH, which must lie within a certain range. Other molecules may compete for the active site, causing inhibition of the enzyme or even irreversible destruction of its catalytic properties.
Enzyme production is governed by a cell's genes. Enzyme activity is further controlled by pH changes, alterations in the concentrations of essential cofactors, feedback inhibition by the products of the reaction, and activation by another enzyme, either from a less active form or an inactive precursor (zymogen). Such changes may themselves be under the control of hormones or the nervous system. See also enzyme kinetics.
Enzymes are classified into six major groups, according to the type of reaction they catalyse: (1) oxidoreductases; (2) transferases; (3) hydrolases; (4) lyases; (5) isomerases; (6) ligases. The names of most individual enzymes also end in -ase, which is added to the names of the substrates on which they act. Thus lactase is the enzyme that acts to break down lactose; it is classified as a hydrolase.

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enzyme inhibition

See inhibition.

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enzyme kinetics

The study of the rates of enzyme-catalysed reactions. Rates of reaction are usually measured by using the purified enzyme in vitro with the substrate and then observing the formation of the product or disappearance of the substrate. As the concentration of the substrate is increased the rate of reaction increases proportionally up to a certain point, after which any further increase in substrate concentration no longer increases the reaction rate (see Michaelis–Menten curve). At this point, all active sites of the enzyme are saturated with substrate; any further increase in the rate of reaction will occur only if more enzyme is added. Reaction rates are also affected by the presence of inhibitors (see inhibition), temperature, and pH (see enzyme).

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enzyme-linked immunosorbent assay

See ELISA.

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enzyme–substrate complex

The intermediate formed when a substrate molecule interacts with the active site of an enzyme. Following the formation of an enzyme–substrate complex, the substrate molecule undergoes a chemical reaction and is converted into a new product. Various mechanisms for the formation of enzyme–substrate complexes have been suggested, including the induced-fit model and the lock-and-key mechanism.

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Eocene

The second geological epoch of the Tertiary period. It extended from the end of the Palaeocene epoch, about 54 million years ago, to the beginning of the Oligocene epoch, about 38 million years ago. The term was first proposed by Sir Charles Lyell (1797–1875) in 1833. In some classifications of geological time the Palaeocene is included as part of the Eocene. Mammals were dominant in the Eocene: rodents, artiodactyls, carnivores, perissodactyls (including early horses), and whales were among the groups to make their first appearance.

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eosin

One of a series of acidic dyes, used in optical microscopy, that colours cytoplasm pink and cellulose red. It is frequently used as a counterstain with haematoxylin for colouring tissue smears and sections of animal tissue.

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eosinophil

A type of white blood cell (leucocyte) that has a granular cytoplasm (see granulocyte). Its function involves the regulation of allergic responses and it also produces an enzyme capable of destroying parasites.

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ephemeral

1 (in botany) An annual plant that completes its life cycle in considerably less than one growing season. A number of generations can therefore occur in one year. Many troublesome weeds, such as groundsel and willowherb, are ephemerals. Certain desert plants are also ephemerals, completing their life cycles in a short period following rain.

2 (in zoology) A short-lived animal, such as a mayfly.
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Ephemeroptera

An order of exopterygote insects that comprises the mayflies, in which the adult stage lasts for only a few hours. The adults have two pairs of wings held vertically at rest, a pair of tail bristles (cerci), and vestigial mouthparts (they do not feed). The nymphs (naiads) live for up to a number of years; they are mainly herbivorous but some possess mandibles for feeding on animal prey.

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epicalyx

A ring of bracts below a flower that resembles a calyx. It is seen, for example, in the strawberry flower.

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epicarp

See pericarp.

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epicotyl

The region of a seedling stem above the stalks of the seed leaves (cotyledons) of an embryo plant. It grows rapidly in seeds showing hypogeal germination and lifts the stem above the soil surface. Compare hypocotyl.

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epidemic

An outbreak of a disease (especially an infectious disease) that affects a large number of individuals within a population at the same time. Compare endemic.

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epidemiology

The study of diseases that affect large numbers of people. Traditionally, epidemiologists have been concerned primarily with infectious diseases, such as typhoid and influenza, that arise and spread rapidly among the population as epidemics. However, today the discipline also covers noninfectious disorders, such as diabetes, heart disease, and back pain. Typically the distribution of a disease is charted in order to discover patterns that might yield clues about its mode of transmission or the susceptibility of certain groups of people. This in turn may reveal insights about the causes of the disease and possible preventive measures.

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epidermis

1 (in zoology) The outermost layer of cells of the body of an animal. In invertebrates the epidermis is normally only one cell thick and is covered by an impermeable cuticle. In vertebrates the epidermis is the thinner of the two layers of skin (compare dermis). It consists of a basal layer of actively dividing cells (see Malpighian layer), covered by layers of cells that become impregnated with keratin (see keratinization). The outermost layers of epidermal cells (the stratum corneum) form a water-resistant protective layer. The epidermis may bear a variety of specialized structures (e.g. feathers, hairs).

2 (in botany) The outermost layer of cells covering a plant. It is overlaid by a cuticle and its functions are principally to protect the plant from injury and to reduce water loss. Some epidermal cells are modified to form guard cells (see stoma) or hairs of various types (see piliferous layer). In woody plants the functions of the shoot epidermis are taken over by the periderm tissues (see cork cambium) and in mature roots the epidermis is sloughed off and replaced by the hypodermis.
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epididymis

A long coiled tube in which spermatozoa are stored in vertebrates. In reptiles, birds, and mammals it is attached at one end to the testis and opens into the sperm duct (vas deferens) at the other.

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epigamic

Serving to attract a mate. Epigamic characters include the bright plumage of some male birds.

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epigeal

Describing seed germination in which the seed leaves (cotyledons) emerge from the ground and function as true leaves. Examples of epigeal germination are seen in sycamore and sunflower. Compare hypogeal.

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epiglottis

A flexible flap of cartilage in mammals that is attached to the wall of the pharynx near the base of the tongue. During swallowing (see deglutition) it covers the glottis (the opening to the respiratory tract) and helps to prevent food from entering the trachea (windpipe), although it is not essential for this purpose.

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epigyny

A floral arrangement in which the ovary is completely enclosed by the receptacle so that the stamens and perianth arise above it, from the top of the receptacle; i.e. the ovary is inferior (see illustration). The perianth and stamens are said to be epigynous with respect to the ovary, as seen in the daffodil. Compare hypogyny; perigyny.

Epigyny

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epilimnion

The upper layer of water in a lake. Compare hypolimnion. See thermocline.

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epinephrine

See adrenaline.

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epipelagic zone

See euphotic zone.

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epiphysis

The terminal section of a growing bone (especially a long limb bone) in mammals. It is separated from the bone shaft (diaphysis ) by cartilage. New bone is produced on the side of the cartilage facing the diaphysis, while new cartilage is produced on the other side of the cartilage disc. When the bone reaches adult length the epiphysis merges with the diaphysis.

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epiphyte

A plant that grows upon another plant but is neither parasitic on it nor rooted in the ground. Epiphytes include many mosses and lichens and some tropical orchids.

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episome

A genetic element that can exist and replicate either independently of its host cell's chromosomes or as an integrated part of the chromosomes. Examples include certain bacterial plasmids.

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epistasis

A gene interaction in which one gene suppresses the effect of another gene that is situated at a different locus on the chromosome. For example, in guinea pigs the gene that controls the production of melanin is epistatic to the gene that regulates the deposition of melanin. A dominant allele (C) is responsible for the production of melanin, while the amount of melanin deposited is controlled by a second gene, which determines whether the coat colour is black or brown. If an animal is homozygous recessive (cc) for melanin production, the coat colour will be white regardless of the alleles that produce black or brown coloration.

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epithelium

A tissue in vertebrates consisting of closely packed cells in a sheet with little intercellular material. It forms a membrane over the outer surfaces of the body and walls of the internal cavities (coeloms). It also forms glands and parts of sense organs. Its functions are protective, absorptive, secretory, and sensory. The types of cell vary, giving rise to squamous epithelium; ciliated epithelium; cuboidal epithelium , with cube-shaped cells; and columnar epithelium , with rectangular cells (see also stratified epithelium). Epithelium is derived from ectoderm and endoderm. Compare endothelium; mesothelium.

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EPSP

See excitatory postsynaptic potential.

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Epstein–Barr virus

See herpesvirus.

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equifinality

The ability of certain structures or behaviour patterns to form by more than one route in a developing embryo or young animal.

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erepsin

An obsolete name for a mixture of protein-degrading enzymes secreted by the small intestine.

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ergocalciferol

See vitamin D.

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ergonomics

The study of the engineering aspects of the relationship between workers and their working environment.

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ergosterol

A sterol occurring in fungi, bacteria, algae, and plants. It is converted into vitamin D_2 by the action of ultraviolet light.

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ergot

The dark hard-walled mass of hyphae produced by the fungus Claviceps purpurea (see Ascomycota) in the grain of the cereals and other grasses that it parasitizes. Ergots are the sclerotia of this species – resting bodies that germinate in favourable conditions to produce mycelia or ascocarps. They contain alkaloids related to LSD, which cause blood vessels to constrict and are used therapeutically in the treatment of migraine and haemorrhage. Ingestion of infected grain can result in ergot poisoning (ergotism), with symptoms of gangrene and hallucinations – known as `St Anthony's fire' in the Middle Ages.

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erythroblast

Any of the cells in the myeloid tissue of red bone marrow that develop into erythrocytes (red blood cells). Erythroblasts have a nucleus and are at first colourless, but fill with haemoglobin as they develop. In mammals the nucleus disappears. See erythropoiesis.

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erythrocyte (red blood cell)

The most numerous type of blood cell, which contains the red pigment haemoglobin and is responsible for oxygen transport. Mammalian erythrocytes are disc-shaped and lack a nucleus; those of other vertebrates are oval and nucleated. In humans the number of erythrocytes in the blood varies between 4.5 and 5.5 million per cubic millimetre. They survive for about four months and are then destroyed in the spleen and liver. See also erythropoiesis. Compare leucocyte.

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erythropoiesis

The formation of red blood cells (erythrocytes), which occurs in the red bone marrow (see haemopoietic tissue). The earliest precursor that can be distinguished microscopically is the proerythroblast , which develops from a haemopoietic stem cell and gives rise successively to the early erythroblast , intermediate erythroblast , and the late erythroblast , in which most haemoglobin is synthesized. In mammals the nucleus is then forced from the cell, which assumes a biconcave shape and is known as a reticulocyte . Reticulocytes are released into the blood and develop into mature erythrocytes within two days. See also erythropoietin.

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erythropoietin

A hormone that is released from the kidney (and to a lesser extent from the liver) in response to low concentrations of oxygen in the tissues. It speeds up the process of erythropoiesis and is the means by which the rate of red cell production is controlled. It is now prepared using genetically engineered cell cultures and administered therapeutically in cases of kidney failure.

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Escherichia coli (E. coli)

A species of Gram-negative aerobic bacteria that is found in the intestine (see coliform bacteria) and is also widely used in microbiological and genetics research. The motile rod-shaped cells ferment lactose and are usually harmless commensals, although certain strains are pathogenic and can cause a severe form of food poisoning. Studies of E. coli laboratory cultures have revealed much about the genetics of prokaryotes; the species is also frequently used in genetic engineering, particularly as a host for gene cloning and the expression of recombinant foreign genes in culture.

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eserine (physostigmine)

An alkaloid, derived from the calabar bean plant, that inhibits cholinesterase by covalently binding with it (see inhibition). Eserine is used to treat the eye condition glaucoma.

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essential amino acid

An amino acid that an organism is unable to synthesize in sufficient quantities. It must therefore be present in the diet. In humans the essential amino acids are arginine, histidine, lysine, threonine, methionine, isoleucine, leucine, valine, phenylalanine, and tryptophan. These are required for protein synthesis and deficiency leads to retarded growth and other symptoms. Most of the amino acids required by humans are also essential for all other multicellular animals and for most protozoans.

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essential element

Any of a number of elements required by living organisms to ensure normal growth, development, and maintenance. Apart from the elements found in organic compounds (i.e. carbon, hydrogen, oxygen, and nitrogen), plants, animals, and microorganisms all require a range of elements in inorganic forms in varying amounts, depending on the type of organism. The major elements , present in tissues in relatively large amounts (greater than 0.005%), are calcium, phosphorus, potassium, sodium, chlorine, sulphur, and magnesium (see also macronutrient). The trace elements occur at much lower concentrations and thus requirements are much less. The most important are iron, manganese, zinc, copper, iodine, cobalt, selenium, molybdenum, chromium, and silicon (see also micronutrient). Each element may fulfil one or more of a variety of metabolic roles. Sodium, potassium, and chloride ions are the chief electrolytic components of cells and body fluids and thus determine their electrical and osmotic status. Calcium, phosphorus, and magnesium are all present in bone. Calcium is also essential to nerve and muscle activity, while phosphorus is a key constituent of the chemical energy carriers (e.g. ATP) and the nucleic acids. Sulphur is needed primarily for amino acid synthesis (in plants and microorganisms). The trace elements may serve as cofactors or as constituents of complex molecules, e.g. iron in haem and cobalt in vitamin B_12. See also mineral deficiency.

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essential fatty acids

Fatty acids that must normally be present in the diet of certain animals, including humans. Essential fatty acids, which include arachidonic, linoleic, and linolenic acids, all possess double bonds at the same two positions along their hydrocarbon chain and so can act as precursors of prostaglandins. Deficiency of essential fatty acids can cause dermatosis, weight loss, irregular oestrus, etc. An adult human requires 2–10g linoleic acid or its equivalent per day.

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essential oil

A natural oil with a distinctive scent secreted by the glands of certain aromatic plants. Terpenes are the main constituents. Essential oils are extracted from plants by steam distillation, extraction with cold neutral fats or solvents (e.g. alcohol), or pressing and used in perfumes, flavourings, and medicine. Examples are citrus oils, flower oils (e.g. rose, jasmine), and oil of cloves.

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ester

An organic compound formed by esterification. Esters formed from carboxylic acids have the general formula RCOOR'. Esters containing simple hydrocarbon groups are volatile fragrant substances used as flavourings in the food industry. Triesters, molecules containing three ester groups, occur in nature as oils and fats. See also glyceride.

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esterification

A condensation reaction of an alcohol with an acid to produce an ester and water; e.g.
CH_3OH + C_6H_5COOH CH_3OOCC_6H_5 + H_2O

The reaction is an equilibrium and is slow under normal conditions, but can be speeded up by addition of a strong acid catalyst.

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etaerio

A cluster of fruits formed from the unfused carpels of a single flower. For example, the anemone has an etaerio of achenes, larkspur an etaerio of follicles, and blackberry an etaerio of drupes.

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ethanedioic acid

See oxalic acid.

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ethanoate

See acetate.

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ethanoic acid

See acetic acid.

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ethanol (ethyl alcohol)

A colourless water-soluble alcohol, C_2H_5OH. It is the active principle in intoxicating drinks, in which it is produced by fermentation of sugar using yeast:
C_6H_12O_6 2C_2H_5OH + 2CO_2

The ethanol produced kills the yeast and fermentation alone cannot produce ethanol solutions containing more than 15% ethanol by volume. See also brewing.

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ethene

See ethylene.

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Ethiopian region

See faunal region.

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ethology

The study of the biology of animal behaviour. Central to the ethologist's approach is the principle that animal behaviour (like physical characteristics) is subject to evolution through natural selection. Ethologists therefore seek to explain how the behaviour of an animal in its natural environment may contribute to the survival of the maximum number of its relatives and offspring. This involves recognizing the stimuli that are important in nature (see sign stimulus) and how innate predispositions interact with learning in the development of behaviour (see instinct). Studies of this sort were pioneered by Konrad Lorenz and Niko Tinbergen.

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ethylene (ethene)

A colourless gaseous hydrocarbon, C_2H_4, that occurs naturally in plants and acts as a growth substance in a variety of physiological roles. It seems not to be required for normal growth, but is produced in response to stresses, such as water shortage, or when the concentration of auxins is elevated. The best known effect is the stimulation of fruit ripening: fruits such as bananas, apples, and avocados naturally produce ethylene during the later stages of ripening, and ethylene gas is used to promote the ripening of fruits, such as bananas, that are picked and shipped `green'. Ethylene generally suppresses flowering, except in members of the pineapple family (Bromeliaceae) – hence flowering of pineapples may be synchronized by releasing ethylene into the growing crop. Studies have shown varied and often contradictory effects of ethylene on vegetative growth. For example, in rice it acts with gibberellins to promote stem elongation, while in peas ethylene inhibits root and shoot elongation. Seed germination, bud opening, and root initiation may also be promoted by ethylene.

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etiolation

The abnormal form of growth observed when plants grow in darkness or severely reduced light. Such plants characteristically have blanched leaves and shoots, excessively long shoots, and reduced leaves and root systems.

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Eubacteria

A subkingdom (or kingdom) of prokaryotic organisms that comprise the vast majority of present-day bacteria. They include aerobic and anaerobic species and occur in virtually all habitats. Some live in or on the bodies of other organisms, and may cause disease. Eubacteria are now regarded by many authorities as being quite distinct, in evolutionary terms, from the other group of prokaryotes, the archaebacteria (see Archaea). This is based mainly on comparisons of the nucleotide sequences of ribosomal RNA (see molecular systematics). However, eubacteria have other defining characteristics. For example, virtually all have cell walls containing peptidoglycan, and their membrane lipids contain fatty acids in ester linkage to glycerol, whereas archaebacteria lack peptidoglycan, and have ether-linked lipids.

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eucarpic

Denoting a fungus in which the thallus is differentiated into vegetative and reproductive regions. Compare holocarpic.

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eucaryote

See eukaryote.

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euchromatin

See chromatin.

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eugenics

The study of methods of improving the quality of human populations by the application of genetic principles. Positive eugenics would seek to do this by selective breeding programmes. Negative eugenics aims to eliminate harmful genes (e.g. those causing haemophilia and colour blindness) by counselling any prospective parents who are likely to be carriers.

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Euglenida

A class of mostly unicellular protoctists (including Euglena) that move by means of undulipodia (flagella). Most euglenids are photosynthetic and inhabit fresh water, and in some classification systems are regarded as green algae (phylum Chlorophyta). However, they lack a cell wall, being covered with a proteinaceous pellicle, and some forms are colourless and thus ingest food, since they cannot photosynthesize. Euglenids are sometimes classified as a phylum, Euglenophyta, but they are now more usually included in a larger phylum, Discomitochondria, together with three other groups of organisms, on the basis of a common mitochondrial structure (characterized by disc-shaped cristae) and the absence of sexual reproduction.

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Eukarya

A superkingdom (or domain) containing all eukaryotic organisms, i.e. embracing protoctists, fungi, plants, and animals. In three-domain classification systems, the other two (prokaryotic) domains are Archaea (comprising the archaebacteria) and Bacteria (containing a single kingdom, Eubacteria).

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eukaryote (eucaryote)

An organism consisting of cells in which the genetic material is contained within a distinct nucleus. All organisms except bacteria are eukaryotes. See Eukarya. Compare prokaryote.

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Eumetazoa

See Metazoa.

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euphotic zone (epipelagic zone; photic zone)

The topmost layer of a lake or sea in which there is sufficient light for net primary production, i.e. where the energy fixed by photosynthesis exceeds that lost by respiration. The depth varies, depending on such factors as turbidity, supply of nutrients in the water, tidal turbulence, and temperature. For example, high nutrient levels will encourage a greater biomass of phytoplankton near the surface, which causes shading and consequent reduction in depth of the euphotic zone. It typically ranges from <1 m to about 30 m in lakes and coastal waters, and rarely reaches depths of more than 200 m in the open ocean. At depths between 200 and 1000 m blue light may still penetrate sufficiently to allow limited photosynthesis. This is sometimes referred to as the dysphotic zone or mesopelagic zone . Below this is the aphotic zone, where no light penetrates.

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euploid

Describing a nucleus, cell, or organism that has an exact multiple of the haploid number (n) of chromosomes. For example, diploid (2n), triploid (3n), and tetraploid (4n) nuclei or cells are all euploid. Compare aneuploid.

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eury-

A prefix denoting width of range. For example, euryhaline aquatic organisms can tolerate a wide range of saline conditions. Compare steno-.

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eusocial

Describing the most highly developed form of animal societies, such as those of colonial ants, termites, wasps, and bees (see Hymenoptera). Typically there is extensive division of labour and cooperation, with various castes each performing particular tasks, such as food-gathering, defence, or tending to the young. Reproduction is undertaken by an elite of fertile individuals, assisted by sterile workers. Moreover, there is an overlap of different generations within the colony at any one time, and continuity of the society is maintained from year to year. Some vertebrate animals may display equivalent levels of social organization. For example, colonies of naked mole rats have a caste system, with nonbreeding adult members that forage, maintain the burrow, or simply keep the other group members warm.

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Eustachian tube

The tube that connects the middle ear to the back of the throat (pharynx) in vertebrates. It is normally closed, but during swallowing it opens to allow air into the middle ear, which equalizes the pressure on each side of the tympanum (eardrum). It was named after the Italian anatomist Bartolomeo Eustachio (?1520–74).

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euthanasia

The act of ending the life of a person or animal in order to prevent further suffering, e.g. from an incurable and painful disease. This can be achieved by administering a lethal drug or by withholding vital treatment. In human medicine euthanasia is fraught with ethical and legal problems, and is illegal in most countries. Where it is practised, strict safeguards are enforced to ensure that the patient's wishes are determined and adhered to. Euthanasia is widely performed in veterinary medicine.

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Eutheria (Placentalia)

An infraclass of mammals in which the embryos are retained in a uterus in the mother's body and nourished by a placenta. The young are thus fully protected during their embryonic development and kept at a constant temperature. Placental mammals evolved during the Cretaceous period (about 100 million years ago). Modern placentals are a highly diverse group that occupy all types of habitat in all parts of the world. They include the orders Artiodactyla, Carnivora, Cetacea, Chiroptera, Insectivora, Perissodactyla, Primates, Proboscidea, and Rodentia. Compare Metatheria; Prototheria.

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eutrophic

Describing a body of water (e.g. a lake) with an abundant supply of nutrients and a high rate of formation of organic matter by photosynthesis. Pollution of a lake by sewage or fertilizers renders it eutrophic (a process called eutrophication ). This stimulates excessive growth of algae (see algal bloom); the death and subsequent decomposition of these increases the biochemical oxygen demand and thus depletes the oxygen content of the lake, resulting in the death of the lake's fish and other animals. Compare dystrophic; mesotrophic; oligotrophic.

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evergreen

(Describing) a plant that bears leaves throughout the year, each leaf being shed independently of the others after two or three years. The leaves of evergreens are often reduced or adapted in some way to prevent excessive water loss; examples are the needles of conifers and the leathery waxy leaves of holly. Compare deciduous.

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evocation

The ability of experimental stimuli (e.g. chemicals or tissue implants) to cause unspecialized embryonic tissue to develop into specialized tissue.

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evolution

The gradual process by which the present diversity of plant and animal life arose from the earliest and most primitive organisms, which is believed to have been continuing for at least the past 3000 million years. Until the middle of the 18th century it was generally believed that each species was divinely created and fixed in its form throughout its existence (see special creation). Lamarck was the first biologist to publish a theory to explain how one species could have evolved into another (see Lamarckism), but it was not until the publication of Darwin's On the Origin of Species in 1859 that special creation was seriously challenged. Unlike Lamarck, Darwin proposed a feasible mechanism for evolution and backed it up with evidence from the fossil record and studies of comparative anatomy and embryology (see Darwinism; natural selection). The modern version of Darwinism, which incorporates discoveries in genetics made since Darwin's time, probably remains the most acceptable theory of species evolution (see also punctuated equilibrium). More controversial, however, and still to be firmly clarified, are the relationships and evolution of groups above the species level. See also macroevolution; microevolution.

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exa-

Symbol E. A prefix used in the metric system to denote 10^18 times. For example, 10^18 metres = 1 exametre (Em).

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exaptation

A morphological or physiological character that predisposes an organism to adapt to a changed environment or lifestyle. For example, a hearing mechanism sensitive to low-frequency sound evolved in crickets, perhaps 250 million years ago, to aid intraspecific communication. When bats evolved, only about 50 million years ago, the cricket hearing mechanism served as a preadaptation, being modified to perceive the high-frequency sonar emitted by these night-flying predators, in addition to the low-frequency sounds of crickets.

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excision repair

A form of DNA repair found in living cells in which damaged or mismatched bases in one of the two strands of DNA are cut out and replaced with the correct bases using the other strand as a template. It is performed by various enzymes. Typically, a DNA glycosylase enzyme recognizes the damage and excises the affected base. Then endonuclease enzymes nick the damaged strand either side of the site where the base was removed, allowing an exonuclease to remove the affected region of the strand. The gap is then repaired by a DNA polymerase, and the new section of the strand is rejoined to the existing strands by a DNA ligase. See also mismatch repair.

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excitation

1 The act or process of stimulating a cell, especially a nerve cell.

2 The response of a cell that has been stimulated.
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excitatory postsynaptic potential (EPSP)

The electric potential that is generated in a postsynaptic neurone during the transmission of a nerve impulse (see synapse). It is caused by depolarization of the postsynaptic membrane when a neurotransmitter (such as acetylcholine), released from the presynaptic membrane, binds to the postsynaptic membrane. This will induce an action potential in the receiving neurone if the EPSP is large enough. See facilitation. Compare inhibitory postsynaptic potential.

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excretion

The elimination by an organism of the waste products that arise as a result of metabolic activity. These products include water, carbon dioxide, and nitrogenous compounds. Excretion plays an important role in maintaining the constancy of an organism's internal environment (see homeostasis). The simplest method of excretion, which occurs, for example, in plants, involves diffusion of waste products from the body, but many animals have specialized organs and organ systems devoted to this function (see Malpighian tubule; nephridium). Examples of excretory organs in vertebrates are the lungs (for carbon dioxide and water), and the kidneys (for nitrogenous waste and water). In addition, mammals excrete small amounts of urea, salts, and water from the skin in sweat.

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exercise

Increased muscular activity, which results in an increase in metabolic rate, heart rate, and oxygen uptake. Exercise also causes an increase in anaerobic respiration in order to compensate for the oxygen debt, which results in a build-up of lactic acid in the tissues.

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exergonic reaction

A chemical reaction in which energy is released (compare endergonic reaction). An exothermic reaction is an exergonic reaction in which energy is released in the form of heat.

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exhalation

See expiration.

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exine

See pollen.

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exocarp

See pericarp.

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exocrine gland

A gland that discharges its secretion into a body cavity (such as the gut) or onto the body surface. Examples are the sebaceous and sweat glands, the mammary glands, and part of the pancreas. Exocrine glands are formed in the embryo from the invagination of epithelial cells. Their secretions pass initially into a cavity (an alveolus or acinus) and then out through a duct or duct network, along which the secretion may become modified by exchange with the blood across the duct epithelium.

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exocytosis

The passage of material from the inside of the cell to the cell surface within membrane-bound vesicles. The membranes of the vesicles fuse with the plasma membrane, releasing their contents to the exterior. Exocytosis is used both for the removal of waste material from the cell and for secretion; for example of mucus by goblet cells. Compare endocytosis.

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exodermis

See hypodermis.

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exogamy

The fusion of reproductive cells from distantly related or unrelated organisms, i.e. outbreeding. Compare endogamy.

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exogenous

Describing substances, stimuli, etc., that originate outside an organism. For example, vitamins that cannot be synthesized by an animal are said to be supplied exogenously in the diet. Compare endogenous.

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exon

A nucleotide sequence in a gene that codes for part or all of the gene product or for some control function. In eukaryotes, exons are separated by noncoding sequences called introns.

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exon shuffling

See intron.

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exonuclease

An enzyme that catalyses the cleavage of nucleotides from the end of a nucleic acid molecule. Compare endonuclease.

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exopeptidase

A protein-digesting enzyme that cleaves amino acids from the ends of a polypeptide chain. Carboxypeptidase, which breaks down proteins in the small intestine, is an example of a exopeptidase. Compare endopeptidase.

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exopterygote

Any winged insect showing hemimetabolous development, i.e. the eggs hatch into young (called nymphs) that resemble the adults but lack wings; these develop gradually and externally in a series of stages (instars) until the final moult produces the adult insect. There is no pupal stage and metamorphosis is described as incomplete . Exopterygote insects include the mayflies (Ephemeroptera), dragonflies (Odonata), grasshoppers (Orthoptera), and bugs (Hemiptera). Compare endopterygote.

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exoskeleton

A rigid external covering for the body in certain animals, such as the hard chitinous cuticle of arthropods. An exoskeleton protects and supports the body and provides points of attachment for muscles. The cuticle of arthropods must be shed at intervals to allow growth to occur (see ecdysis). Other examples of exoskeletons are the shells of molluscs and the bony plates of tortoises and armadillos. Compare endoskeleton.

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exothermic

Denoting a chemical reaction that releases heat into its surroundings. See exergonic reaction. Compare endothermic.

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exotic

See alien.

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exotoxin

See toxin.

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experiment

A process or trial designed to test a scientific theory.

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experimental taxonomy

See systematics.

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expiration (exhalation)

The process by which gas is expelled from the lungs (see respiratory movement). In mammals, the volume of the thoracic cavity is reduced by contraction of the internal intercostal muscles and relaxation of the muscles of the diaphragm, assisted by upward pressure of the abdominal organs. As a result, pressure in the lungs exceeds atmospheric pressure and gas flows out of the lungs, allowing the pressures to equalize. Compare inspiration.

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expiratory centre

See ventilation centre.

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explantation

The removal of cells, tissues, or organs of animals and plants for observation of their growth and development in appropriate culture media. The removed parts are known as explants . See also tissue culture; organ culture.

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exponential growth

A form of population growth in which the rate of growth is related to the number of individuals present. Increase is slow when numbers are low but rises sharply as numbers increase. If population number is plotted against time on a graph a characteristic J-shaped curve results (see graph). In animal and plant populations, such factors as overcrowding, lack of nutrients, and disease limit population increase beyond a certain point and the J-shaped exponential curve tails off giving an S-shaped (sigmoid) curve.

Graph showing exponential growth of the human population

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expression vector

A vector used in genetic engineering that enables a particular gene to be not only cloned but also expressed in a host cell. The vector is constructed to contain appropriate regulatory sequences, such as a promoter and operator, so that the host-cell machinery can transcribe the gene and translate the resultant messenger RNA to synthesize the corresponding protein. Such vectors are thus essential for the manufacture of, for example, mammalian proteins by bacterial host cells. Expression vectors used in prokaryotes are typically based on plasmids or phages, or plasmid–phage hybrids (phagemids ); in eukaryotes, vectors based on baculovirus, a DNA virus of insects, have been used successfully in cultures of insect cells. Secretion vectors allow both expression and secretion of the novel protein by the host cell, by ensuring that the expressed protein carries a signal peptide that allows it to be transported across the cell membrane.

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extensor

Any muscle that causes a limb to extend. See voluntary muscle. Compare flexor.

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exteroceptor

Any receptor that detects external stimuli. Examples of exteroceptors are the thermoreceptors in the skin, which monitor the temperature of the external environment. Compare interoceptor.

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extinction

1 The irreversible condition of a species or other group of organisms of having no living representatives in the wild, which follows the death of the last surviving individual of that species or group. Extinction may occur on a local or global level; it can result from various human activities, including the destruction of habitats or the overexploitation of species that are hunted or harvested as a resource. Species at the top of a food chain (e.g. large birds of prey) will be more prone to extinction since they exist in relatively small numbers and will be affected by a deleterious change at any of the levels in the food chain.

2 The termination of a behaviour pattern that is no longer appropriate. For example, dogs can be conditioned to salivate when they hear a bell ring in the absence of a food stimulus (see conditioning). However, if the bell continues to be rung in the absence of food the dogs will gradually stop salivating on hearing the bell.
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extracellular

Located or occurring outside the cell. Cuticularization is an example of an extracellular process.

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extraembryonic membranes (embryonic membranes)

The tissues produced by an animal embryo for protection and nutrition but otherwise taking no part in its development. The four membranes, which are called fetal membranes in humans, are the chorion, amnion, allantois, and yolk sac.

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extrafusal

See intrafusal.

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extranuclear genes

Genes included in the DNA present in organelles other than the nucleus, such as the mitochondria and chloroplasts, some of which code for the synthesis of proteins. The DNA of these organelles is inherited by the offspring via the cytoplasm of the gametes (see cytoplasmic inheritance). In organisms in which one gamete is much larger than the other, so that the smaller gamete contains very little cytoplasm, one parent will contribute most or all of the extranuclear genes. For example, human mitochondrial DNA is passed from generation to generation through the maternal line in the ovum.

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extremophile

A bacterium that thrives under extreme conditions, e.g. at very high or very low temperatures, or in very salty or acidic environments. For example, certain archaebacteria (Archaea), termed hyperthermophiles , live in hot springs at temperatures near or even above 100°C. The enzymes of such organisms exhibit great stability and have been extracted for use in laboratory and commercial processes.

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eye

The organ of sight. The most primitive eyes are the eyespots of some unicellular organisms. More advanced eyes are the ocelli and compound eyes of arthropods (e.g. insects). The cephalopod molluscs (e.g. the octopus and squid) and vertebrates possess the most highly developed eyes (see illustration). These normally occur in pairs, are nearly spherical, and filled with fluid. Light is refracted by the cornea through the pupil in the iris and onto the lens, which focuses images onto the retina. These images are received by light-sensitive cells in the retina (see cone; rod), which transmit impulses to the brain via the optic nerve.

Structure of the vertebrate eye

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eye muscle

Any of the muscles associated with the functioning of the eye, which are divided into two groups. The intrinsic muscles , which are involuntary, are situated inside the eyeball and comprise the ciliary muscle (see ciliary body) and the iris. The extrinsic muscles , which comprise three pairs of voluntary muscles, are inserted on the sclera (outer surface) of the eyeball and control its movements.

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eyepiece (ocular)

The lens or system of lenses in an optical instrument that is nearest to the eye. It usually produces a magnified image of the previous image formed by the instrument.

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eyespot (stigma)

1 A structure found in some free-swimming unicellular algae and in plant reproductive cells that contains orange or red pigments (carotenoids) and is sensitive to light. It enables the cell to move in relation to a light source (see phototaxis).

2 A spot of pigment found in some lower animals, e.g. jellyfish.
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3 An eyelike marking on the wings of certain insects, which is revealed in startle displays.
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eye tooth

A canine tooth in the upper jaw.

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F

F_1 (first filial generation)

The first generation of offspring resulting from an arranged cross between homozygous parents in breeding experiments. See monohybrid cross.

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F_2 (second filial generation)

The second generation of offspring in breeding experiments, obtained by crosses between individuals of the F1 generation. See monohybrid cross.

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facilitated diffusion

The transport of molecules across the outer membrane of a living cell by a process that involves a specific carrier located within the cell membrane but does not require expenditure of energy by the cell. The carrier combines with a molecule on one side of the membrane, moves through the membrane, and releases the molecule on the other side. It enables the diffusion through the membrane of molecules that otherwise could not pass through. Compare active transport.

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facilitation

1 (in neurophysiology) The effect of successive stimuli on the postsynaptic membrane, which results in the generation of an excitatory postsynaptic potential (EPSP). Although a single impulse may fail to cross the junction between adjacent nerve cells, the synapse becomes more responsive to the following impulse.

2 (in ecology) The phenomenon observed during succession in which the presence of one species increases the likelihood or speed of colonization by a second species. The first species brings about some change in the environment that make it more favourable for the second. For example, a pre-existing plant may provide the germinating seeds and seedlings of another species with vital shelter from the wind or protection from herbivores. Alternatively, it may alter the nature of the soil, for example by changing the pH, sufficiently to permit growth of a new incoming species.
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Factor VIII (antihaemophilic factor)

One of the blood clotting factors. Factor VIII is a soluble protein that stimulates the activation of Factor X by Factor IXa, which in turn converts prothrombin to thrombin, thus causing the fibrin matrix of a blood clot to form. Haemophilia is due to a deficiency or defect of Factor VIII and is treated by administration of blood plasma or plasma concentrate containing the factor. Factor VIII can now be obtained from genetically engineered cell cultures. Such preparations avoid the risk of contamination with viruses, notably HIV (the AIDS virus).

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facultative anaerobes

Organisms, such as certain bacteria, fungi, and some internal parasites of animals, that are able to alter their metabolism to grow in either the presence or absence of oxygen. The best-known facultative anaerobe is Saccharomyces cerevisiae, the yeast used in brewing. See anaerobic respiration.

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FAD (flavin adenine dinucleotide)

A coenzyme important in various biochemical reactions. It comprises a phosphorylated vitamin B_2 (riboflavin) molecule linked to the nucleotide adenine monophosphate (AMP). FAD is usually tightly bound to the enzyme forming a flavoprotein . It functions as a hydrogen acceptor in dehydrogenation reactions, being reduced to FADH_2. This in turn is oxidized to FAD by the electron transport chain, thereby generating ATP (two molecules of ATP per molecule of FADH_2).

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faeces

Waste material that is eliminated from the alimentary canal through the anus. Faeces consist of the indigestible residue of food that remains after the processes of digestion and absorption of nutrients and water have taken place, together with bacteria and dead cells shed from the gut lining.

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Fahrenheit scale

A temperature scale in which (by modern definition) the temperature of boiling water is taken as 212 degrees and the temperature of melting ice as 32 degrees. It was invented in 1714 by the German scientist G. D. Fahrenheit (1686–1736), who set the zero at the lowest temperature he knew how to obtain in the laboratory (by mixing ice and common salt) and took his own body temperature as 96°F. The scale is no longer in scientific use. To convert to the Celsius scale the formula is
C = 5(F – 32)/9.

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fallopian tube (oviduct)

The tube that carries egg cells from the ovary to the womb in mammals. The eggs are carried by the action of muscles and cilia. It was named after Gabriel Fallopius.

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Fallopius, Gabriel

(1523–62)
Italian anatomist, who was professor of anatomy at Pisa (from 1548) and Padua (from 1551). Best known for his discoveries about the human skeletal and reproductive systems, he identified the tubes connecting the ovaries with the uterus, which are named after him (fallopian tubes).

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false fruit

See pseudocarp.

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family

A category used in the classification of organisms that consists of one or several similar or closely related genera. Similar families are grouped into an order. Family names end in -aceae or -ae in botany (e.g. Cactaceae) and -idae in zoology (e.g. Equidae). The names are usually derived from a type genus (Cactus and Equus in the examples above) that is characteristic of the whole family (see type specimen). In botany, families are sometimes called natural orders .

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farming

See agriculture.

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fascia

A sheet of fibrous connective tissue occurring beneath the skin and also enveloping glands, vessels, nerves, and forming muscle and tendon sheaths.

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fascicle

1 A small bundle of nerve or muscle fibres.

2 See vascular bundle.
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fascicular cambium

See cambium.

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fast green

A green dye used in optical microscopy that stains cellulose, cytoplasm, collagen, and mucus green. It is frequently used to stain plant tissues, with safranin as a counterstain. Unlike light green , a similar dye, it does not fade easily.

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fat

A mixture of lipids, chiefly triglycerides, that is solid at normal body temperatures. Fats occur widely in plants and animals as a means of storing food energy, having twice the calorific value of carbohydrates. In mammals, fat is deposited in a layer beneath the skin (subcutaneous fat) and deep within the body as a specialized adipose tissue (see also brown fat). The insulating properties of fat are also important, especially in animals lacking fur and those inhabiting cold climates (e.g. seals and whales).
Fats derived from plants and fish generally have a greater proportion of unsaturated fatty acids than those from mammals. Their melting points thus tend to be lower, causing a softer consistency at room temperatures. Highly unsaturated fats are liquid at room temperatures and are therefore more properly called oils.

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fat body

1 An abdominal organ in amphibians attached to the anterior of each kidney. It contains a reserve of fat that nourishes the gonads during the winter hibernation in readiness for the spring breeding season.

2 A mass of fatty tissue spreading throughout the body cavity of insects in which fats, proteins, and glycogen are stored as a reserve for hibernation or pupation.
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fat cell (adipocyte)

Any of the cells of adipose tissue, in which fats (triglycerides) are stored. Fat cells contain enzymes (lipases) that can break down fat into glycerol and fatty acids, which can be transported in the blood to the liver, where they are used in fatty-acid oxidation.

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fate map

A diagram used to depict embryological development, showing the adult structures that develop from the different parts of the embryo.

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fatigue

A decline in the level of response of tissues (such as muscle), cells, etc., to nervous stimulation, which occurs after prolonged and continued stimulation of these structures.

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fatty acid

An organic compound consisting of a hydrocarbon chain and a terminal carboxyl group (see carboxylic acids). Chain length ranges from one hydrogen atom (methanoic, or formic, acid, HCOOH) to nearly 30 carbon atoms. Ethanoic (acetic), propanoic (propionic), and butanoic (butyric) acids are important in metabolism. Long-chain fatty acids (more than 8–10 carbon atoms) most commonly occur as constituents of certain lipids, notably glycerides, phospholipids, sterols, and waxes, in which they are esterified with alcohols. These long-chain fatty acids generally have an even number of carbon atoms; unbranched chains predominate over branched chains. They may be saturated (e.g. palmitic acid and stearic acid) or unsaturated , with one double bond (e.g. oleic acid) or two or more double bonds, in which case they are called polyunsaturated fatty acids (e.g. linoleic acid and linolenic acid). See also essential fatty acids.
The physical properties of fatty acids are determined by chain length, degree of unsaturation, and chain branching. Short-chain acids are pungent liquids, soluble in water. As chain length increases, melting points are raised and water-solubility decreases. Unsaturation and chain branching tend to lower melting points.

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fatty-acid oxidation (ß-oxidation)

The metabolic pathway in which fats are metabolized to release energy. Fatty-acid oxidation occurs continually but does not become a major source of energy until the animal's carbohydrate resources are exhausted, for example during starvation. Fatty-acid oxidation occurs in the mitochondria. A series of reactions cleave off two carbon atoms at a time from the hydrocarbon chain of the fatty acid. These two-carbon fragments are combined with coenzyme A to form acetyl coenzyme A (acetyl CoA), which then enters the Krebs cycle. The formation of acetyl CoA occurs repeatedly until all the hydrocarbon chain has been used up. See also glyoxylate cycle.

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fauna

All the animal life normally present in a given habitat at a given time. See also macrofauna; microfauna. Compare flora.

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faunal region

A region of the earth with a distinct and characteristic assemblage of animal taxa. There are six commonly recognized faunal regions used in zoogeography: the Ethiopian region , consisting of Africa south of the Sahara and part of southern Arabia; the Oriental region , which covers tropical Asia and associated continental islands; the Palaearctic region , comprising Europe and Asia north of the tropics; the Nearctic region , consisting of North America except for tropical Mexico; the Neotropical region , consisting of South and Central America including tropical Mexico; and the Australian region , covering principally Australia and New Guinea. The regions are described mainly in terms of their terrestrial and freshwater vertebrate fauna, especially mammals and birds, although the invertebrate faunas may sometimes be included. The concept was originated by 19th century naturalists, such as P. L. Sclater and A. R. Wallace, and held as evidence of continental drift (see Wallace's line). However, another important factor in determining the nature of a region's fauna is the historical migration of species from their centres of origin, a process influenced by the presence of physical barriers, such as mountains. Climate, and hence latitude, is also an important factor.

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feathers

The body covering of birds, formed as outgrowths of the epidermis and composed of the protein keratin. Feathers provide heat insulation, they give the body its streamlined shape, and those of the wings and tail are important in flight. Basically a feather consists of a quill , which is embedded in the skin attached to a feather follicle and is continuous with the shaft (rachis ) of the feather, which carries the barbs. This basic structure is modified depending on the type of feather (see contour feathers; down feathers; filoplumes).

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fecundity

The number of offspring produced by an organism (in higher animals, generally of the female of the species) in a given time. Normally all organisms, assuming they reach reproductive age, are sufficiently fecund to replace themselves several times over. Darwin noted this, together with the fact that population numbers nevertheless tended to remain fairly constant: these observations led him to formulate his theory of evolution by natural selection. Compare fertility.

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feedback

The use of part of the output of a system to control its performance. In positive feedback , the output is used to enhance the input; in negative feedback , the output is used to reduce the input. Many biological processes rely on negative feedback. As the population of a species expands, so its food supply per individual is diminished; the result is that the population then begins to fall. Many biochemical processes are controlled by feedback inhibition. Feedback mechanisms play an important role in maintaining a state of equilibrium within an organism (see homeostasis).

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feeding

See ingestion.

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Fehling's test

A chemical test to detect reducing sugars and aldehydes in solution, devised by the German chemist H. C. von Fehling (1812–85). Fehling's solution consists of Fehlings A (copper(II) sulphate solution) and Fehling's B (alkaline 2,3-dihydroxybutanedioate (sodium tartrate) solution), equal amounts of which are added to the test solution. After boiling, a positive result is indicated by the formation of a brick-red precipitate of copper(I) oxide. Methanal, being a strong reducing agent, also produces copper metal; ketones do not react. The test is now little used, having been replaced by Benedict's test.

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female

1 Denoting the gamete (sex cell) that, during sexual reproduction, fuses with a male gamete in the process of fertilization. Female gametes are generally larger than the male gametes and are usually immotile (see oosphere; ovum).

2 (Denoting) an individual organism whose reproductive organs produce only female gametes. Compare hermaphrodite.
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femoral

Of or relating to the thigh or the femur (thigh bone). For example, the femoral artery runs down the front of the thigh.

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femto-

Symbol f. A prefix used in the metric system to denote 10–15. For example, 10–15 second = 1 femtosecond (fs).

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femur

1 The thigh bone of terrestrial vertebrates. It articulates at one end with the pelvic girdle at the hip joint and at the other (via two condyles) with the tibia.

2 The third segment of an insect's leg, attached to the trochanter. See also coxa.
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fen

See hydrosere.

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fenestra

Either of the two delicate membranes between the middle ear and the inner ear. The upper membrane is the fenestra ovalis (see oval window); the lower membrane is the fenestra rotunda (see round window).

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fenestration

The condition of tissues or other structures within an organism of being perforated by window-like slits. For example, the endothelium of the capillaries in the glomerulus within the kidneys is fenestrated in order to facilitate the filtration of substances.

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feral animal

Any domesticated animal, such as a dog or a cat, that has returned to live in wild conditions.

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fermentation

A form of anaerobic respiration occurring in certain microorganisms, e.g. yeasts. Alcoholic fermentation comprises a series of biochemical reactions by which pyruvate (the end product of glycolysis) is converted to ethanol and carbon dioxide. It is the basis of the baking and brewing industries (see baker's yeast). In lactic-acid fermentation , which occurs in many microorganisms and (when sugar is in short supply) in animal cells, the end product is lactic acid. Microorganisms display a range of fermentations, producing not only ethanol or lactic acid, but other products, such as propionic and butyric acids, acetate, and methane.

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ferns

See Filicinophyta.

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ferredoxin

A protein containing iron associated with sulphur that is a carrier molecule in the electron transport chain in photosynthesis. It accepts electrons from photosystem I and passes them on to NADP reductase, which donates them for the reduction of NADP+ (see photophosphorylation; photosystems I and II). Ferredoxin also takes part in nitrogen fixation in plants.

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ferritin

A protein, found predominantly in the tissues of the liver and spleen but also present in nearly all cells of the body, that is used for the storage of iron. A ferritin molecule, which is spherical and has a crystalline core, can store as many as 4000 iron atoms, which can be released when required for haemoglobin formation. Compare transferrin.

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fertility

1 The potential capability of an organism to reproduce itself. In sexually reproducing plants and animals it is the number of fertilized eggs produced in a given time. For practical purposes this usually cannot be measured, and the only reliable indicators are the numbers of mature seeds produced, eggs laid, or live offspring delivered. However, these measures are strictly referred to as fecundity, since they exclude fertilized embryos that have failed to develop.

2 The relative ability of a soil to support plant growth. It consists of both physical factors, e.g. particle size and moisture content, and chemical factors, e.g. concentration and availability of nutrients.
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fertilization (syngamy)

The union of male and female gametes (reproductive cells) during the process of sexual reproduction to form a zygote . It involves the fusion of the gametic nuclei (karyogamy ) and cytoplasm (plasmogamy ). As each gamete contains only half the correct number of chromosomes, fertilization and zygote formation results in a cell with the full complement of chromosomes, half of which are derived from each of the parents. In animals the process involves fusion of the nuclei of a spermatozoan and an ovum. In most aquatic animals (e.g. fish) this takes place in the surrounding water, into which the gametes are shed. Among most terrestrial animals (e.g. insects, many mammals) fertilization occurs in the body of the female, into which the sperms are introduced. In flowering plants, after pollination, the grain pollen produces a pollen tube, which grows down into the female reproductive organ (carpel) to enable a male gamete nucleus to fuse with the egg nucleus (see double fertilization).
In self-fertilization the male and female gametes are derived from the same individual. Among plants, self-fertilization (also called autogamy ) is common in many cultivated species, e.g. wheat and oats. However, self-fertilization is a form of inbreeding and does not allow for the mixing of genetic material; if it occurs over a number of generations it will result in offspring being less vigorous and productive than those resulting from cross-fertilization. In cross-fertilization (also called allogamy in plants) the gametes are derived from different individuals. In plants the pollen comes either from another flower of the same plant or from a different plant (see also incompatibility).

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fertilizer

Any substance that is added to soil in order to increase its productivity. Fertilizers can be of natural origin, such as composts, or they can be made up of synthetic chemicals, particularly nitrates and phosphates. Synthetic fertilizers can increase crop yields dramatically, but when leached from the soil by rain, which runs into lakes, they also increase the process of eutrophication (see algal bloom; eutrophic). Bacteria that can fix nitrogen are sometimes added to the soil to increase its fertility; for example, in tropical countries the cyanobacterium Anabaena is added to rice paddies to increase soil fertility.

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fetal membranes

See extraembryonic membranes.

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fetus (foetus)

The embryo of a mammal, especially a human, when development has reached a stage at which the main features of the adult form are recognizable. In humans the embryo from eight weeks to birth is called a fetus.

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Feulgen's test

A histochemical test in which the distribution of DNA in the chromosomes of dividing cell nuclei can be observed. It was devised by the German chemist R. Feulgen (1884–1955). A tissue section is first treated with dilute hydrochloric acid to remove the purine bases of the DNA, thus exposing the aldehyde groups of the sugar deoxyribose. The section is then immersed in Schiff's reagent, which combines with the aldehyde groups to form a magenta-coloured compound.

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fibre

1 An elongated plant cell whose walls are extensively (usually completely) thickened with lignin (see sclerenchyma). Fibres are found in the vascular tissue, usually in the xylem, where they provide structural support. The term is often used loosely to mean any kind of xylem element. The fibres of many species, e.g. flax, are of commercial importance.

2 Any of various threadlike structures in the animal body, such as a muscle fibre, a nerve fibre, a collagen fibre, or an elastic fibre.
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3 (or dietary fibre; roughage) The part of food that cannot be digested and absorbed to produce energy. Dietary fibre falls into four groups: cellulose , hemicelluloses , lignins , and pectins . Highly refined foods, such as sucrose, contain no dietary fibre. Foods with a high fibre content include wholemeal cereals and flour, root vegetables, nuts, and fruit. Dietary fibre is considered by some to be helpful in the prevention of many of the diseases of Western civilization, such as diverticulosis, constipation, appendicitis, obesity, and diabetes mellitus.
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fibre optics

See optical fibres.

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fibril

Any small fibre or threadlike structure. See also microfibril.

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fibrin

The insoluble protein that forms fibres at the site of an injury and is the foundation of a blood clot. See blood clotting.

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fibrinogen

The protein dissolved in the blood plasma that, when suitably activated, is converted to insoluble fibrin fibres. See blood clotting.

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fibrinolysis

The breakdown of the protein fibrin by the enzyme plasmin (fibrinase or fibrinolysin ), which occurs when blood clots are removed from the circulation.

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fibroblast

A cell that secretes fibres in the intercellular substance of connective tissue. The cells are long, flat, and star-shaped and lie close to collagen fibres.

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fibrocartilage

See cartilage.

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fibrous protein

See protein.

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fibula

The smaller and outer of the two bones between the knee and the ankle in terrestrial vertebrates. Compare tibia.

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field capacity

The amount of water that remains in a soil when excess has drained away. It is held by capillary forces of the soil pores and reflects the physical nature of the soil.

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field-emission microscope (FEM)

A type of electron microscope in which a high negative voltage is applied to a metal tip placed in an evacuated vessel some distance from a glass screen with a fluorescent coating. The tip produces electrons by field emission , i.e. the emission of electrons from an unheated sharp metal part as a result of a high electric field. The emitted electrons form an enlarged pattern on the fluorescent screen, related to the individual exposed planes of atoms. As the resolution of the instrument is limited by the vibrations of the metal atoms, it is helpful to cool the tip in liquid helium. Although the individual atoms forming the point are not displayed, individual adsorbed atoms of other substances can be, and their activity is observable.

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field-ionization microscope (field-ion microscope; FIM)

A type of electron microscope that is similar in principle to the field-emission microscope, except that a high positive voltage is applied to the metal tip, which is surrounded by low-pressure gas (usually helium) rather than a vacuum. The image is formed in this case by field ionization : ionization at the surface of an unheated solid as a result of a strong electric field creating positive ions by electron transfer from surrounding atoms or molecules. The image is formed by ions striking the fluorescent screen. Individual atoms on the surface of the tip can be resolved and, in certain cases, adsorbed atoms may be detected.

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filament

1 (in zoology) A long slender hairlike structure, such as any of the barbs of a bird's feather.

2 (in botany) The stalk of the stamen in a flower. It bears the anther and consists mainly of conducting tissue.
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3 (in cell biology) See intermediate filament.
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Filicinophyta (Pterophyta)

A phylum of mainly terrestrial vascular plants (see tracheophyte) – the ferns. Ferns are perennial plants bearing large conspicuous leaves (fronds : see megaphyll) usually arising from either a rhizome or a short erect stem. Bracken is a common example. Only the tree ferns have stems that reach an appreciable height. There is a characteristic uncurling of the young leaves as they expand into the adult form. Reproduction is by means of spores borne on the underside of specialized leaves (sporophylls).

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filoplumes

Minute hairlike feathers consisting of a shaft (rachis ) bearing a few unattached barbs. They are found between the contour feathers.

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filter feeding

A method of feeding in which tiny food particles are strained from the surrounding water by various mechanisms. It is used by many aquatic invertebrates, especially members of the plankton, and by some vertebrates, notably baleen whales. See also ciliary feeding; whalebone.

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filtrate

The clear liquid obtained by filtration.

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filtration

The process of separating solid particles using a filter. In vacuum filtration , the liquid is drawn through the filter by a vacuum pump. Ultrafiltration is filtration under pressure; for example, ultrafiltration of the blood occurs in the nephrons of the vertebrate kidney.

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finger domain

A finger-shaped structure produced in a protein when a series of the constituent amino acids combines with a metal atom. Finger domains are often found repeated in transcription factors. See also domain.

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fingerprinting

See DNA fingerprinting; peptide mapping.

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fins

The locomotory organs of aquatic vertebrates. In fish there are typically one or more dorsal and ventral fins (sometimes continuous), whose function is balance; a caudal fin around the tail, which is the main propulsive organ; and two paired fins: the pectoral fins attached to the pectoral (shoulder) girdle and the pelvic fins attached to the pelvic (hip) girdle, which are used in steering. These paired fins are homologous with the limbs of tetrapods. Fins are strengthened by a number of flexible fin rays, which may be cartilaginous, bony and jointed, horny, or fibrous and jointed.

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first convoluted tubule

See proximal convoluted tubule.

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fish

See Chondrichthyes (cartilaginous fish); Osteichthyes (bony fish); Pisces.

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fission

A type of asexual reproduction occurring in some unicellular organisms, e.g. diatoms, protozoans, and bacteria, in which the parent cell divides to form two (binary fission ) or more (multiple fission ) similar daughter cells.

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fission-track dating

A method of estimating the age of glass and other mineral objects by observing the tracks made in them by the fission fragments of the uranium nuclei that they contain. By irradiating the objects with neutrons to induce fission and comparing the density and number of the tracks before and after irradiation it is possible to estimate the time that has elapsed since the object solidified.

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fitness (in genetics)

Symbol W. A measure of the relative breeding success of an individual or genotype in a given population at a given time. Individuals that contribute the most offspring to the next generation are the fittest. Fitness therefore reflects how well an organism is adapted to its environment, which determines its survival. See also inclusive fitness; selection coefficient.

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fixation

1 The first stage in the preparation of a specimen for microscopical examination, in which the tissue is killed and preserved in as natural a state as possible by immersion in a chemical fixative . The fixative prevents the distortion of cell components by denaturing its constituent protein. Some commonly used fixatives are formaldehyde, ethanol, and Bouin's fluid (for light microscopy), and osmium tetroxide and gluteraldehyde (for electron microscopy). Fixation may also be brought about by heat.

2 See nitrogen fixation.
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fixed action pattern

See instinct.

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flaccid

(in botany) Describing plant tissue that has become soft and less rigid than normal because the cytoplasm within its cells has shrunk and contracted away from the cell walls through loss of water (see plasmolysis).

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flagellum

(pl. flagella) 1 (in prokaryotes) A long slender structure that protrudes from the cell surface of a bacterium. It rotates from its base and propels the bacterium along. Up to several micrometres in length, a flagellum is constructed of numerous subunits of the protein flagellin , while at the base a system of rings anchors the flagellum in the cell wall and cell membrane. Surrounding these rings are paired motor proteins, which impart a rotary motion to the filament, and switch proteins, which can reverse the direction of rotation. Flagella may be attached singly or in groups, for example at the poles of the bacterial cell, or scattered over the cell surface.

2 (in eukaryotes) See undulipodium.
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flame cells

Ciliated cells that form part of the excretory and osmoregulatory system of platyhelminths, rotifers, and nemertine worms. This system, known as a protonephridium , consists of branching tubules that open to the exterior through excretory pores; flame cells occur at the ends of the tubules, into which their cilia project. Fluid, containing water and nitrogenous waste products, diffuses into the flame cells and is directed through the tubules to the exterior by movements of the cilia, which resemble the flickering of flames. Flame cells that possess only one cilium are known as solenocytes ; these are found in the protonephridia of marine worms of the phylum Priapulida.

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flatworms

See Platyhelminthes.

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flavin adenine dinucleotide

See FAD.

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flavonoid

One of a group of naturally occurring phenolic compounds many of which are plant pigments. They include the anthocyanins, flavonols , and flavones . Patterns of flavonoid distribution have been used in taxonomic studies of plant species. See also chalcone.

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flavoprotein

See FAD.

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fleas

See Siphonaptera.

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Fleming, Sir Alexander

(1881–1955)
British bacteriologist, born in Scotland. He studied medicine at St Mary's Hospital, London, where he remained all his life. In 1922 he identified lysozyme, an enzyme that destroys bacteria, and in 1928 discovered the antibiotic penicillin. He shared the 1945 Nobel Prize for physiology or medicine with Florey and Chain, who first isolated the drug.

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flexor

A muscle that causes a limb to bend by bringing the two parts of the limb together. An example is the biceps. Flexors work antagonistically with extensors. See voluntary muscle.

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flies

See Diptera.

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flight

1 Any form of locomotion in air, which can be active or passive (gliding ). Mechanisms of flight have evolved mainly in birds, bats, and insects: these animals are adapted for flight by the presence of wings , which increases the ratio of surface area to body weight. Birds possess powerful flight muscles: the depressor muscle runs from the underside of the humerus to the sternum and is responsible for the downstroke of the wing; the levator muscle works antagonistically, producing the upstroke. Flight in insects works in a similar fashion but the muscles that control the wing movement are attached to the thorax. A few species of mammals, reptiles, and fish have developed flight to a lesser extent. For example, flying squirrels (order Dermoptera) possess a membrane attached to the limbs that can open and function as a parachute, allowing the animals to glide.

2 Part of a survival mechanism in an animal that is generated in response to a threatening situation. A potentially dangerous situation can induce the release of adrenaline, which prepares the animal for `fight or flight' by increasing the blood pressure and heart rate and diverting the blood flow to the muscles and heart. See alarm response.
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flip-flop

The movement (transverse diffusion ) of a lipid molecule from one surface of a lipid bilayer membrane to the other, which occurs at a very slow rate. This contrasts with the much faster rate at which lipid molecules exchange places with neighbouring molecules on the same surface of the membrane (lateral diffusion ).

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flocculation

The process in which particles in a colloid aggregate into larger clumps. Flocculation of clay particles in soil can be induced by the addition of calcium salts. Clay particles have an overall negative charge and therefore attract positive ions, such as Ca2+, which form bridges holding the particles together. Flocculation is also often observed in cultures of bacterial and yeast cells.

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flora

All the plant life normally present in a given habitat at a given time. See also microflora. Compare fauna.

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floral formula

A summary of the structure and components of a flower using symbols and numbers. The symbols representing symmetry include (for actinomorphy) and , ?, or ? (for zygomorphy). The parts of the flower are represented by K (for calyx), C (for corolla), P (for perianth), A (for androecium), and G (for gynoecium). These are followed by numbers indicating the number of parts in each whorl (e.g. K5 indicates a calyx of five sepals); indicates an indefinite number of parts (more than 12). If the parts are fused the number is in parentheses; if they are in separate groups or whorls the number is split (e.g. C2 + 2 indicates a corolla of two whorls each of two petals). The symbol G has a line above it to indicate an inferior ovary and a line beneath it to indicate a superior ovary. For example, the buttercup flower has the following floral formula:
K5 C5 A G,

i.e. it has a calyx of five sepals, a corolla of five petals, an androecium of many stamens, and a superior ovary consisting of many carpels.

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Florey, Howard Walter, Baron

(1898–1968)
Australian pathologist, who moved to Oxford in 1922. After working in Cambridge and Sheffield (studying lysozyme), he returned to Oxford in 1935. There he teamed up with Ernst Chain and by 1939 they succeeded in isolating and purifying penicillin. They also developed a method of producing the drug in large quantities and carried out its first clinical trials. The two men shared the 1945 Nobel Prize for physiology or medicine with penicillin's discoverer, Alexander Fleming.

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florigen

A hypothetical plant growth substance that is postulated to transmit the stimulus for flowering, which is a response to photoperiod (see photoperiodism), from the leaves to the apex of the plant. However, florigen has never been isolated and some plant physiologists question its existence.

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flower

The structure in angiosperms (flowering plants) that bears the organs for sexual reproduction. Flowers are very variable in form, ranging from the small green insignificant wind-pollinated flowers of many grasses to spectacular brightly coloured insect-pollinated flowers. Flowers are often grouped together into inflorescences, some of which (e.g. that of dandelion) are so compacted as to resemble a single flower. Typically flowers consist of a receptacle that bears sepals, petals, stamens, and carpels (see illustration). The flower parts are adapted to bring about pollination and fertilization resulting in the formation of seeds and fruits. The sepals are usually green and leaflike and protect the flower bud. The petals of insect-pollinated flowers are adapted in many ingenious ways to attract insects and, in some instances, other animals. For example, some flowers are adapted to attract short-tongued insects by having an open shallow corolla tube and nectar situated in an exposed position. Flowers adapted for pollination by long-tongued insects have a long corolla tube of fused petals with nectar in a concealed position. The tongue of the insect brushes against the anthers and stigma before reaching the nectar. Wind-pollinated flowers, in contrast, are inconspicuous. The anthers dangle outside the corolla and the stigmas have a feathery surface to catch the pollen grains.
Some species are adapted for self-pollination and have small flowers, no nectar, and stamens and carpels that mature simultaneously.

Section through a monocarpellary flower at the time of pollination

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flowering plants

See Anthophyta.

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fluence

A measure, widely used in studies of photosynthesis in plants, of the quantity of light. It is defined as the quantity of radiant energy falling on a small sphere divided by the cross-sectional area of the sphere, and can be expresssed in two ways. Photon fluence (units: molm–2) measures the number of photons falling on the sphere, whereas energy fluence (units: Jm–2) measures the energy incident on the sphere. Hence the corresponding rates of input are the photon fluence rate (units: molm–2s–1) and the energy fluence rate (units: Jm–2s–1). Many radiation-measuring instruments now used in plant biology record only the part of the spectrum utilized by plants, i.e. radiation of wavelength in the range 400–700 nm; this is termed photosynthetically active radiation (PAR), and fluence rates used in practice are often qualified in this way.

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fluid mosaic model

A widely accepted model, proposed by S. J. Singer and G. L. Nicholson in the 1970s, for the structure of biological membranes. It is based on a lipid bilayer forming the framework of the membrane, with proteins randomly embedded in it; these proteins may be restricted to either lipid layer (extrinsic proteins ) or they may span both (intrinsic proteins ), producing an asymmetric structure (see illustration). This structure allows the lateral movement of the constituent phospholipid and protein molecules of the membrane.

Fluid mosaic model of cell membranes

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flukes

See Trematoda.

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fluoridation

The process of adding very small amounts of fluorine salts (e.g. sodium fluoride, NaF) to drinking water to prevent tooth decay. The fluoride becomes incorporated into the fluoroapatite (see apatite) of the growing teeth and reduces the incidence of dental caries. However, some argue that there is an accompanying risk of fluoride toxicity.

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focusing

(in animal physiology) The process of directing and concentrating light from a source onto the retina of the eye, by means of the lens, in order to obtain a clear image of objects at a range of distances. See accommodation.

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foetus

See fetus.

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folacin

See folic acid.

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folic acid (folacin)

A vitamin of the vitamin B complex. In its active form, tetrahydrofolic acid, it is a coenzyme in various reactions involved in the metabolism of amino acids, purines, and pyrimidines. It is synthesized by intestinal bacteria and is widespread in food, especially green leafy vegetables. Deficiency causes poor growth and nutritional anaemia.

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follicle

1 (in animal anatomy) Any enclosing cluster of cells that protects and nourishes a cell or structure within. For example, follicles in the ovary contain developing egg cells, while hair follicles envelop the roots of hairs.

2 (in botany) A dry fruit that, when ripe, splits along one side to release its seeds. It is formed from a single carpel containing one or more seeds. Follicles do not occur singly but are grouped to form clusters (etaerios ). Examples include larkspur, columbine, and monk's hood.
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follicle-stimulating hormone (FSH; follitropin)

A hormone, secreted by the anterior pituitary gland in mammals, that stimulates, in female mammals, ripening of specialized structures in the ovary (Graafian follicles) that produce ova and, in males, the formation of sperm in the testis. It is a major constituent of fertility drugs, used to treat failure of ovulation and decreased sperm production. See also gonadotrophin.

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follicular phase

The phase of the oestrous cycle (menstrual cycle of women) during which the Graafian follicles mature and the lining of the uterus thickens under the influence of oestrogen.

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fontanelle

A gap in the bones of the skull. Newborn babies are born with a fontanelle, which disappears as the bones in the skull fuse; the fusion is complete at around 18 months of age.

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food

Any material containing nutrients, such as carbohydrates, proteins and fats, which are required by living organisms in order to obtain energy for growth and maintenance. Heterotrophic organisms, such as animals, ingest their food (see also diet); autotrophic organisms, such as plants, manufacture their food materials.

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food additive

A substance added to a food during its manufacture or processing in order to improve its keeping qualities, texture, appearance, or stability or to enhance its taste or colour. Additives are usually present in minute quantities; they include colouring materials, sweeteners, preservatives (see food preservation), antioxidants, emulsifiers, and stabilizers. In most countries the additives used must be selected from an approved list of such compounds, which have been tested for safety, and they must be listed on the food labels of individual products.

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food chain

The transfer of energy from green plants (the primary producers) through a sequence of organisms in which each eats the one below it in the chain and is eaten by the one above. Thus plants are eaten by herbivores, which are then eaten by carnivores. These may in turn be eaten by different carnivores. The position an organism occupies in a food chain is known as its trophic level. In practice, many animals feed at several different trophic levels, resulting in a more complex set of feeding relationships known as a food web. See bioenergetics; consumer; producer; pyramid of biomass; pyramid of energy; pyramid of numbers.

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food poisoning

An acute illness caused by food that may be naturally poisonous or contaminated by certain types of pathogenic microorganisms. The most common type of food poisoning in the UK is that caused by the bacteria belonging to the genus Salmonella, which inhabit the alimentary canal of livestock. Other food poisoning bacteria include Staphylococcus aureus, Clostridium perfringens, Campylobacter jejuni, Listeria monocytogenes, and pathogenic Escherichia coli. Freezing and other types of food preservation can prevent the growth of the bacteria and thorough cooking will kill the microorganisms before the meat is eaten. However, food poisoning can result if frozen meat is not completely thawed at its centre before cooking, as it may not reach sufficiently high temperatures to kill the bacteria during cooking. Another type of food poisoning, known as botulism , is caused by toxins produced by the bacterium Clostridium botulinum, which can grow in badly preserved canned foods.

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food preservation

Prevention of the spoilage of food, which is achieved by a variety of techniques. These aim to prevent bacterial and fungal decay and contamination of food, which can cause food poisoning. For example, dehydration removes the water from food, which prevents microorganisms from growing. Treating food with salt (salting) causes the microorganisms to lose water due to osmosis. Pickling involves treatment with vinegar (ethanoic acid), which reduces the pH and prevents bacteria from growing. Heating food (blanching ) to temperatures of 90°C denatures the enzymes that cause the breakdown of food and kills many bacteria. The food is then packed in air-tight containers, such as cans or bottles. Heating milk to high temperatures to kill the bacteria is the basis of pasteurization. Freezing food prevents the growth of bacteria but does not necessarily kill them; thorough cooking is therefore essential. In freeze drying, food is rapidly frozen and then dehydrated, usually in a vacuum. Preprepared food can be preserved by the addition of chemicals (see food additive), such as sodium benzenecarboxylate, proprionates, and sulphur dioxide, but some of these may have adverse side-effects. Irradiation is a recently developed method of food preservation in which the bacteria are killed by irradiating the food with gamma rays.

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food production

See agriculture.

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food reserves

Reserves of fat, carbohydrate, or (rarely) protein in cells and tissues that function as an important store of energy that can be released and used in ATP production when required by the organism. For example, in animals fat is stored in adipose tissue, and carbohydrate – in the form of the storage compound glycogen – is stored in liver and muscle cells. In plants starch is a major storage compound, being found in perennating organs (see perennation) and seeds (in which it is mobilized at germination), and oils are important storage materials in some species (e.g. in the seeds of the castor-oil plant).

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food supply

(in human ecology) The production of food for human consumption. See agriculture.

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food web

A system of food chains that are linked with one another. In a food web a particular organism may feed at more than one trophic level. For example, in a pond food web a freshwater mussel may feed directly on green algae, in which case it is a primary consumer. However, it can also feed on protozoa, which are themselves primary consumers, in which case the mussel is the secondary consumer. A food web does not usually include the decomposers, but these organisms are very important in the flow of energy through a food web (see energy flow).

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footprinting

A technique for detecting regions where a protein is bound to DNA; for example, where RNA polymerase is bound to the DNA of a gene. The protein protects the nucleic acid (i.e. the DNA) from digestion by a nuclease enzyme, so that after such treatment the undegraded DNA, the `footprint', can be isolated and characterized.

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foramen

An aperture in an animal part or organ, especially one in a bone or cartilage. For example, the foramen magnum is the opening at the base of the skull through which the spinal cord passes.

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forebrain (prosencephalon)

One of the three sections of the brain of a vertebrate embryo. The forebrain develops to form the cerebrum, hypothalamus, and thalamus in the adult. Compare hindbrain; midbrain.

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foregut

1 The anterior region of the alimentary canal of vertebrates, up to the anterior part of the duodenum.

2 The anterior part of the alimentary canal of arthropods. See also hindgut; midgut.
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forest

An area of vegetation in which the dominant plants are trees; forests constitute major biomes. Temperate forests have adequate or abundant rainfall and moderate temperatures. They may be dominated by deciduous trees (such as oak, ash, elm, beech, or maple), often growing together to form mixed deciduous forest, as in temperate regions of Europe, Asia, and North America; or by broad-leaved evergreens (such as southern beech, Nothofagus), as in Chile. Cold forests, of northern regions, are dominated by evergreen conifers (see taiga). Tropical forests include rainforest, characterized by regular heavy rainfall; monsoon forest, found in SE Asia and having heavy rainfall interspersed with periods of drought; and thorn forest, as in SW North America, SW Africa, and parts of Central and South America and Australia, which has sparse rainfall, is dominated by small thorny trees, and grades into savanna woodland (see grassland) and semidesert.

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form

1 A category used in the classification of organisms into which different types of a variety may be placed.

2 Any distinct variant within a species. Seasonal variants, e.g. the tawny brown (summer) and blue-white (winter) forms of the blue hare, may be called forms, as may the different types that constitute a polymorphism.
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fossil

The remains or traces of any organism that lived in the geological past. In general only the hard parts of organisms become fossilized (e.g. bones, teeth, shells, and wood) but under certain circumstances the entire organism is preserved. For example, virtually unaltered fossils of extinct mammals, such as the woolly mammoth and woolly rhinoceros, have been found preserved in ice in the Arctic. Small organisms or parts of organisms (e.g. insects, leaves, flowers) have been preserved in amber.
In the majority of fossils the organism has been turned to stone – a process known as petrification . This may take one of three forms. In permineralization , solutions originating underground fill the microscopic cavities in the organism. Minerals in these solutions (e.g. silica or calcite) may actually replace the original material of the organism so that even microscopic structures may be preserved; this process is known as replacement (or mineralization ). A third form of petrification – carbonization (or distillation ) – occurs in certain soft tissues that are composed chiefly of compounds of carbon, hydrogen, and oxygen (e.g. cellulose). After the organism has been buried, and in the absence of oxygen, carbon dioxide and water are liberated until only free carbon remains. This forms a black carbon film in the rock outlining the original organism. Moulds are formed when the original fossil is dissolved away leaving a mould of its outline in the solid rock. The deposition of mineral matter from underground solutions in a mould forms a cast . Palaeontologists often produce casts from moulds using such substances as dental wax. Moulds of thin organisms (e.g. leaves) are commonly known as imprints . Trace fossils are the fossilized remnants of the evidence of animal life, such as tracks, trails, footprints, burrows, and coprolites (fossilized faeces).
The ideal conditions for the formation of fossils occur in areas of rapid sedimentation, especially those parts of the seabed that lie below the zone of wave disturbance. See also chemical fossil; index fossil; microfossil; taphonomy.

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fossil fuel

Coal, oil, and natural gas, the fuels used by man as a source of energy. They are formed from the remains of living organisms and all have a high carbon or hydrogen content. Their value as fuels relies on the exothermic oxidation of carbon to form carbon dioxide (C + O_2 CO_2) and the oxidation of hydrogen to form water (H_2 + ½O_2 H_2O).

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fossil hominid

See hominid.

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founder effect

The phenomenon occurring when a population is founded by a small sample of the entire species, perhaps just a handful of individuals. Chance dictates that these founder members will be genetically unrepresentative of the species as a whole, and that the genetic make-up of the new population will differ markedly from the main species population. The founder effect thus increases the likelihood of evolutionary divergence from the main species, and eventually the formation of a new species.

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fovea (fovea centralis)

A shallow depression in the retina of the eye, opposite the lens, that is present in some vertebrates. This area contains a large concentration of cones with only a thin layer of overlying nerves. It is therefore specialized for the perception of colour and sharp intense images. The clarity is enhanced when light is focused on the foveae of both retinas simultaneously. See binocular vision.

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fragmentation

A method of asexual reproduction, occurring in some invertebrate animals, in which parts of the organism break off and subsequently differentiate and develop into new individuals. It occurs especially in certain cnidarians and annelids. In some, regeneration may occur before separation, producing chains of individuals budding from the parent.

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frameshift

An alteration in the sequence of DNA bases read as base triplets during transcription, caused by the deletion (or addition) of a single nucleotide in the DNA sequence. The missing (or additional) base results in an abnormal triplet and also causes each subsequent triplet group to be altered. A frameshift mutation therefore produces messenger RNA with corresponding changed codons and results in the synthesis of an abnormal protein. For example, the deletion of uracil (U) in the second codon of the following mRNA sequence:
AUU CAU CGG UAG ACC UGU AUG

results in the frameshifted sequence:
AUU CAC GGU AGA CCU GUA UG

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fraternal twins (dizygotic twins)

Two individuals that result from a single pregnancy, each having developed from a separate fertilized egg. The two egg cells contain different combinations of alleles and so do the two sperm that fertilize them. The twins therefore have no more genetic similarity than brothers or sisters from single births. Compare identical twins.

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free energy

A measure of a system's ability to do work. The Gibbs free energy (or Gibbs function ), G, is defined by G = H – TS, where G is the energy liberated or absorbed in a reversible process at constant pressure and constant temperature (T), H is the enthalpy and S the entropy of the system. Changes in Gibbs free energy, G, are useful in indicating the conditions under which a chemical reaction will occur. If G is positive the reaction will only occur if energy is supplied to force it away from the equilibrium position (i.e. when G = 0). If G is negative the reaction will proceed spontaneously to equilibrium.

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free radical

An atom or group of atoms with an unpaired valence electron. Free radicals are formed when a bond is broken without forming ions. Because of their unpaired electrons, most free radicals are extremely reactive. For example, superoxide free radicals, formed in the body during normal metabolic processes and as a reaction to toxins and infections, have damaging effects on cells and tissues. See also antioxidants; superoxide dismutase.

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freeze drying (lyophilization)

The removal of liquid from heat-sensitive materials. The material is frozen, placed under a high vacuum, and maintained at a low temperature (–40°C or below). The pressure generated by the vacuum causes the ice to turn from a solid to a gaseous form without passing through a liquid state. This allows the removal of water from the material without otherwise disturbing its composition. Freeze drying is used to preserve tissues (e.g. blood plasma) and foods and to concentrate solutions.

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freeze fracture

A method of preparing material for electron microscopy that allows the visualization of the interior of cell membranes and organelles. Cells are frozen at –196°C and cracked so that the plane of fracture runs through the middle of lipid bilayers, separating the two halves. The exposed surfaces are then coated with carbon and platinum and the organic material is digested with enzymes (freeze etching ), leaving a carbon–platinum replica of the fractured surface, which can be examined using the microscope.

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Frisch, Karl von

(1886–1982)
Austrian zoologist, who became director of the Zoological Institute in Munich in 1925. Frisch is remembered for his discovery that bees perform a `dance' to indicate to their fellows the location of a source of food (see dance of the bees). For this work he was awarded a share in the 1973 Nobel Prize for physiology or medicine, jointly with Konrad Lorenz and Niko Tinbergen.

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frogs

See Amphibia.

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frond

See megaphyll.

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frontal lobe

The anterior part of each cerebral hemisphere, which is associated with the higher mental functions, such as abstract thought.

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fructification

See sporophore.

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fructose (fruit sugar; laevulose)

A simple sugar, C_6H_12O_6, stereoisomeric with glucose (see monosaccharide). (Although natural fructose is the D-form, it is in fact laevorotatory.) Fructose occurs in green plants, fruits, and honey and tastes sweeter than sucrose (cane sugar), of which it is a constituent. Derivatives of fructose are important in the energy metabolism of living organisms. Some polysaccharide derivatives (fructans) are carbohydrate energy stores in certain plants.

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fructose 1,6-bisphosphate

An intermediate formed in the initial stage of glycolysis by the phosphorylation of glucose using ATP.

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fruit

The structure formed from the ovary of a flower, usually after the ovules have been fertilized (see also parthenocarpy). It consists of the fruit wall (see pericarp) enclosing the seed(s). Other parts of the flower, such as the receptacle, may develop and contribute to the structure, resulting in a false fruit (see pseudocarp). The fruit may retain the seeds and be dispersed whole (an indehiscent fruit ), or it may open (dehisce) to release the seeds (a dehiscent fruit ). Fruits are divided into two main groups depending on whether the ovary wall remains dry or becomes fleshy (succulent ). Succulent fruits are generally dispersed by animals and dry fruits by wind, water, or by some mechanical means. See illustration. See also composite fruit.

Different types of fruit and methods of seed dispersal

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fruit fly

See Drosophila.

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fruit sugar

See fructose.

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frustule

The cell wall of a diatom (see Bacillariophyta), which is impregnated with silica. The wall consists of two halves that overlap one another.

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FSH

See follicle-stimulating hormone.

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fucoxanthin

The major carotenoid pigment present, with chlorophyll, in the brown algae (see Phaeophyta).

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fugitive species

A species that is able to coexist with a competitively superior species due to its better dispersal capabilities. The fugitive species is quicker to exploit any vacant patches in the environment that become available (for example due to a fire or storm), colonizing and reproducing before the more competitive species occupies the patch and excludes it. For example, the sea palm (Postelsia palmaeformis, a brown alga) colonizes bare patches in mussel beds off the northwestern coast of the USA. When a patch is created by wave action, the alga attaches itself to the bare rock, only to be gradually excluded as the surrounding mussels encroach. Although it is eventually driven from each site, the fugitive alga can coexist with the mussels provided sufficient bare patches become available.

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fumaric acid

A carboxylic acid, HCOOHC:CHCOOH, that is an intermediate in the Krebs cycle, being formed by the dehydrogenation of succinic acid.

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fungi

A group of organisms formerly regarded as simple plants lacking chlorophyll but now classified in a separate kingdom, Fungi. They can either exist as single cells or make up a multicellular body called a mycelium, which consists of filaments known as hyphae. Most fungal cells are multinucleate and have cell walls composed chiefly of chitin. Fungi exist primarily in damp situations on land and, because of the absence of chlorophyll, are either parasites or saprotrophs on other organisms. The principal criteria used in classification are the nature of the spores produced and the presence or absence of cross walls within the hyphae (see Ascomycota; Basidiomycota; Deuteromycota; Zygomycota). See also lichens.

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fungicide

See pesticide.

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Fungi Imperfecti

See Deuteromycota.

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funicle

The stalk that attaches an ovule to the placenta in the ovary of a flowering plant. It contains a strand of conducting tissue leading from the placenta into the chalaza.

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furanose

A sugar having a five-membered ring containing four carbon atoms and one oxygen atom.

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fusion

The combining together of cells, nuclei, or cytoplasm. See cell fusion; fertilization.

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G

GABA

See gamma-aminobutyric acid.

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Gaia hypothesis

The theory, based on an idea put forward by the British scientist James Ephraim Lovelock (1919– ), that the whole earth, including both its biotic (living) and abiotic (nonliving) components, functions as a single self-regulating system. Named after the Greek earth goddess, it proposes that the responses of living organisms to environmental conditions ultimately bring about changes that make the earth better adapted to support life; the system would rid itself of any species that adversely affects the environment. The theory has found favour with many conservationists.

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galactose

A simple sugar, C_6H_12O_6, stereoisomeric with glucose, that occurs naturally as one of the products of the enzymic digestion of milk sugar (lactose) and as a constituent of gum arabic.

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ß-galactosidase

See lactase.

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gall (cecidium)

An abnormal growth of a plant tissue or organ elicited by a foreign organism. Galls can take a wide variety of forms, but most frequently occur as swellings or pits in stems, roots, leaves, and buds. Organisms responsible for their formation include bacteria, viruses, fungi, nematodes, mites, and insects. The gall structure can be quite complex, with several distinct cell layers, or relatively simple and undifferentiated, but is typically very distinct from surrounding normal tissue and often is characteristic of the eliciting organism. It can involve cell enlargement (hypertrophy) and/or cell proliferation (hyperplasia). The mechanisms underlying gall formation are known in only a few cases. The bacterium Agrobacterium tumefaciens, which is responsible for crown galls, induces a genetic change in infected host tissue by transfer of a plasmid bearing tumour-forming genes. Insects may secrete substances in their saliva that induce gall formation, or in some cases may transmit viruses or other genetic 'carriers' that affect the plant genome.

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gall bladder

A small pouch attached to the bile duct, present in most vertebrates. Bile, produced in the liver, is stored in the gall bladder and released when food (especially fatty substances) enters the duodenum.

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gallery forest

A type of forest that grows alongside rivers and streams running through tropical grassland (savanna). The increased soil moisture is sufficient to support tree growth, which may be dense and multilayered, especially on alluvial nutrient-rich soils of flood plains.

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gallstone

A hard ball of material formed in the gall bladder from surplus cholesterol, which precipitates in the bile. Gallstones may lodge in and obstruct the bile duct.

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GALP

See glyceraldehyde 3-phosphate.

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Galvani, Luigi

(1737–98)
Italian physiologist. In the late 1770s he observed that the muscles of a dead frog twitched when touched by two different metals. He concluded that the muscle was producing electricity, but this was later disproved. Galvani also invented galvanized iron and the galvanometer.

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gametangium

An organ that produces gametes. The term is usually restricted to the sex organs of algae, fungi, mosses, and ferns. See antheridium; archegonium; oogonium.

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gamete

A reproductive cell that fuses with another gamete to form a zygote. Examples of gametes are ova and spermatozoa. Gametes are haploid, i.e. they contain half the normal (diploid) number of chromosomes; thus when two fuse, the diploid number is restored (see fertilization). Gametes are formed by meiosis. Gametes often differ in size, the smaller (usually male) gamete being known as the microgamete and the larger (usually female) as the macrogamete . See also sexual reproduction.

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gametogenesis

The processes involved in the formation of gametes. Gametes are normally formed by meiosis but sometimes by mitosis (as in the gametophyte generation of the ferns). In mammals gametogenesis in the female is known as oogenesis and occurs in the ovaries; in the male it is known as spermatogenesis and occurs in the testes.

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gametophyte

The generation in the life cycle of a plant that bears the gamete-producing sex organs. The gametophyte is haploid. It is the dominant phase in the life cycle of mosses and liverworts, the sporophyte generation depending on it either partially or completely. In clubmosses, horsetails, and ferns it is the prothallus. In seed plants it is very much reduced. For example, in angiosperms the pollen grain is the male gametophyte and the embryo sac is the female gametophyte. See also alternation of generations.

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gamma-aminobutyric acid (GABA)

An inhibitory neurotransmitter in the central nervous system (principally the brain) that is capable of increasing the permeability of postsynaptic membranes (see inhibitory postsynaptic potential). GABA is synthesized by decarboxylation of the amino acid glutamate.

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gamma globulin

See globulin.

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gamopetalous

Describing a flower in which the petals are fused to form a corolla tube. Compare polypetalous.

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gamosepalous

Describing a flower that possesses a calyx consisting of fused sepals. Compare polysepalous.

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ganglion

A mass of nervous tissue containing many cell bodies and synapses, usually enclosed in a connective-tissue sheath. In vertebrates most ganglia occur outside the central nervous system; exceptions are the basal ganglia in the brain. In invertebrates ganglia occur along the nerve cords and the most anterior pair (cerebral ganglia ) are analogous to the vertebrate brain; invertebrate ganglia constitute a part of the central nervous system.

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ganglion cell

See retina.

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ganoid scale

See scales.

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gap junction (nexus)

A passage through the lipid bilayers of adjacent cell membranes that mediates the transfer of small molecules or ions between interacting cells. Gap junctions are abundant in epithelial tissues and cardiac muscle. They consist of hexagonally packed tubes, approximately 7nm in diameter, through which small molecules or ions may directly pass from the interior of one cell to the interior of the other. Gap junctions, together with chemical synapses (which function through neurotransmitters), are communicating junctions and comprise one of several types of intercellular junction (compare desmosome; tight junction).

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gas bladder

See swim bladder.

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gaseous exchange

The transfer of gases between an organism and the external environment in either direction. It occurs by diffusion across a concentration gradient and includes the exchange of oxygen and carbon dioxide in respiration and photosynthesis. Successful gaseous exchange requires a large surface area, as is provided by the alveoli of the lungs and the leaves of plants.

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gas–liquid chromatography

A technique for separating or analysing mixtures of gases by chromatography. The apparatus consists of a very long tube containing the stationary phase, a nonvolatile liquid, such as a hydrocarbon oil coated on a solid support. The sample is often a volatile liquid mixture (e.g. of fatty acids), which is vaporized and swept through the column by a carrier gas (e.g. hydrogen). The components of the mixture pass through the column at different rates and are detected as they leave, either by measuring the thermal conductivity of the gas or by a flame detector.
Gas chromatography is usually used for analysis; components can be identified by the time they take to pass through the column. It is also used for separating mixtures into their components, which are then directly injected into a mass spectrometer in the technique of gas chromatography–mass spectroscopy.

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gasohol

Alcohol that has been produced by fermentation of sugar cane by the yeast Saccharomyces cerevisiae. Gasohol has been produced on a large scale in Brazil, where it is manufactured in large distilleries and provides an alternative source of energy for motor vehicles. In Brazil all cars are adapted so as to be able to use gasohol or a mixture of gasohol and petrol. Gasohol is an example of a biofuel.

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gastric

Of or relating to the stomach.

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gastric gland

See gastric juice.

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gastric juice

An acidic mixture of inorganic salts, hydrochloric acid, mucus, and pepsinogens secreted by gastric glands in the stomach lining.

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gastric mill (proventriculus)

A type of gizzard occurring in many crustaceans. It is situated in the anterior region of the stomach and consists of a set of bones (ossicles) and muscles that grind food particles. The food particles are then filtered by bristles in the posterior section of the stomach.

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gastrin

A hormone, produced by G cells in the mucosa of the stomach and first part of the duodenum, that controls the release of gastric juice. The secretion of gastrin is stimulated by the presence of food in the stomach. It is one of the hormones that integrates and controls digestive processes (see also secretin).

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gastrodermis

The epithelial lining of the digestive tract of certain invertebrates, including the nematode worms and coelenterates.

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Gastropoda

A class of molluscs that includes the snails, whelks, limpets, land and sea slugs, and conches. Molluscs have a well-developed head with tentacles, a large flattened foot, and a coiled twisted shell. They occupy marine, freshwater, and terrestrial habitats; in the terrestrial and some freshwater gastropods the mantle cavity acts as a lung instead of enclosing gills.

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gastrovascular cavity (enteron; coelenteron)

The body cavity of the coelenterates, which has one opening functioning both as mouth and anus. See Cnidaria; Ctenophora.

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gastrula

The stage in the development of an animal embryo that succeeds the blastula. It begins with the production of the primary germ layers and the embryo becomes converted to a cup-shaped structure containing a cavity (the archenteron).

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G cells

See gastrin.

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gel

A lyophilic colloid that has coagulated to a rigid or jelly-like solid. In a gel, the disperse medium has formed a loosely held network of linked molecules through the dispersion medium. Examples of gels are silica gel and gelatin.

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gelatin(e)

A colourless or pale yellow water-soluble protein obtained by boiling collagen with water and evaporating the solution. It swells when water is added and dissolves in hot water to form a solution that sets to a gel on cooling. It is used in bacteriology for preparing culture media, in pharmacy for preparing capsules and suppositories, and in jellies and other foodstuffs.

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gel filtration

A type of column chromatography in which a mixture of liquids is passed down a column containing a gel. Small molecules in the mixture can enter pores in the gel and move slowly down the column; large molecules, which cannot enter the pores, move more quickly. Thus, mixtures of molecules can be separated on the basis of their size. The technique is used particularly for separating proteins but it can also be applied to cell nuclei, viruses, etc.

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gemmation

A type of vegetative propagation in which small clumps of undifferentiated cells (gemmae ) develop on the surface of a plant. These are shed and dispersed to other areas, where they grow to produce new individuals. Gemmation is found only in certain lower plants, such as mosses and liverworts.

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gene

A unit of heredity composed of DNA. In classical genetics (see Mendelism; Mendel's laws) a gene is visualized as a discrete particle, forming part of a chromosome, that determines a particular characteristic. It can exist in different forms called alleles, which determine which aspect of the characteristic is shown (e.g. tallness or shortness for the characteristic of height).
A gene occupies a specific position (locus) on a chromosome. In view of the discoveries of molecular genetics, it may be defined as the sequence of nucleotides of DNA (or RNA) concerned with a specific function, such as the synthesis of a single polypeptide chain or of a messenger RNA molecule, corresponding to a particular sequence of the genetic code. One or more of these structural genes , coding for protein, may be associated with other genes controlling their expression (see operon). See also cistron.

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gene amplification

The multiple replication of a section of the genome, which occurs during a single cell cycle and results in the production of many copies of a specific sequence of the DNA molecule. For example, in the oocytes of amphibians and other animals, in which large numbers of ribosomes are needed, the genes encoding ribosomal RNA are greatly amplified. Viral genes that cause the formation of tumours (see oncogene) are amplified in tumour cells.

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gene bank

See gene library.

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gene cloning (DNA cloning)

The production of exact copies (clones ) of a particular gene or genes using genetic engineering techniques. The DNA containing the target gene(s) is split into fragments using restriction enzymes. These fragments are then inserted into cloning vectors, such as bacterial plasmids or bacteriophages, which transfer the recombinant DNA to suitable host cells, such as the bacterium E. coli. Alternatively, complementary DNA is inserted into the vectors, or `naked' DNA fragments can be taken up directly by a host bacterium from its medium (this is less efficient than vector transfer).
Inside the host cell the recombinant DNA undergoes replication; thus, a bacterial host will give rise to a colony of cells containing the cloned target gene. Various screening methods may be used to identify such colonies, enabling them to be selected and cultured. Gene cloning facilitates gene sequencing; it also enables large quantities of a desired protein product to be produced: human insulin, for example, is now produced by bacteria containing the cloned insulin gene.

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gene expression

The manifestation of the effects of a gene by the production of the particular protein, polypeptide, or type of RNA whose synthesis it controls. Individual genes can be `switched on' (exert their effects) or `switched off' according to the needs and circumstances of the cell at a particular time. A number of mechanisms are thought to be responsible for the control of gene expression; the Jacob–Monod hypothesis postulates the mechanism operating in prokaryotes (see operon). Control of gene expression is known to be more complicated in eukaryotes, which possess various control mechanisms not seen in prokaryotes. For example, the methylation of cytosine bases of specific genes in eukaryotic DNA is observed in cells in which the gene is not expressed; if DNA methylation is prevented by the use of inhibitory chemicals, this can cause certain genes to be expressed. In multicellular organisms, expression of the right genes in the right order at the right times is particularly crucial during embryonic development and cell differentiation. This involves subtle and complex interplay of chemical signals with the embryo's genes, in patterns that vary between different types of organism (see differentiation). Abnormalities of gene expression may result in the death of cells, or their uncontrolled growth, as in cancer.

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gene family (multigene family)

A group of genes that have arisen by duplication of an ancestral gene. Such genes show similarities of nucleotide sequence that betray their common origin, and if they have evolved relatively recently they occur close together on the same chromosome. However, more distantly related members of a gene family may be found scattered widely on different chromosomes, reflecting chromosomal rearrangements during the evolution of the genome. Members of a gene family may be functionally very similar or differ widely. For example, members of the histone gene family all produce very similar proteins, whereas the serine protease family contains both trypsin, the proteolytic digestive enzyme, and haptoglobin, a protein that binds globin but has no proteolytic activity.

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gene flow

The exchange of genetic material by interbreeding between populations of the same species or between individuals within a population. Gene flow increases the variation in the genetic composition of a population.

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gene frequency

See allele frequency.

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gene imprinting

The differential expression of a single gene according to its parental origin. For example, the symptoms of Huntingdon's disease, an inherited neuromuscular disorder caused by a dominant mutant allele, first occur during adolescence if the mutant allele is inherited from the male parent but symptoms generally do not arise until middle age if the allele derives from the female parent.

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gene library (gene bank)

A collection of cloned DNA fragments representing the entire genetic material of an organism. This facilitates screening and isolation of any particular gene. Gene libraries are created by fractionating the genomic DNA into fragments using restriction enzymes and/or physical methods. These fragments are cloned (see gene cloning) and the host cells containing the recombinant fragments are centrifuged and frozen; alternatively, the phage vectors are maintained in culture. Individual genes in the library are identified using specific gene probes with the Southern blotting technique or, via their protein products, using Western blotting. Gene libraries are thus repositories of raw material for use in genetic engineering. A human gene library may contain some 500000 separate clones.

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gene manipulation

See biotechnology; genetic engineering.

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gene mutation

See point mutation.

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gene pool

All the genes and their different alleles that are present in a population of a particular species of organism. See also population genetics.

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gene probe (DNA probe)

A single-stranded DNA or RNA fragment used in genetic engineering to search for a particular gene or other DNA sequence. The probe has a base sequence complementary to the target sequence and will thus attach to it by base pairing. By labelling the probe with a radioactive isotope it can be identified on subsequent separation and purification. Probes of varying lengths, up to about 100 nucleotides, can be constructed in the laboratory. They are used in the Southern blotting technique to identify particular DNA fragments, for instance in conjunction with restriction mapping to diagnose gene abnormalities or to map certain sequences.

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generation

A group of organisms of approximately the same age within a population. Organisms that are crossed to produce offspring in a genetics study are referred to as the parental generation and their offspring as the first filial generation . See also F1; F2; P.

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generation time

The interval between the beginnings of consecutive cell divisions. It may be as short as 20 minutes in bacteria. See also interphase.

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generative nucleus

One of the two nuclei in an angiosperm pollen grain (compare tube nucleus). It divides to produce two male gamete nuclei (see double fertilization).

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generator potential

The local electrical charge that develops in the sensitive part of a receptor cell, known as the generator region . The generator potential is caused by depolarization of the membrane surrounding this part of the cell, brought about by an exchange of ions in response to a stimulus. The strength of the potential is proportional to the strength of the stimulus: when the potential exceeds a certain threshold, it fires an action potential. More action potentials are triggered as long as the generator potential exists.

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gene sequencing (DNA sequencing)

The process of elucidating the nucleotide sequence of a gene. Two techniques are used. The Maxam–Gilbert method (named after Allan Maxam and Walter Gilbert) involves cleaving the DNA with a restriction enzyme and labelling each of the resulting fragments with 32P-phosphate at one end. The fragments are subjected to four different sets of reactions, each set specifically cleaving DNA at a particular base or bases. The cleaved fragments are separated by electrophoresis according to their chain length and identified by autoradiography. The base (nucleotide) sequence is deduced from the position of bands in each of the four lanes in the gel.
The Sanger method (named after Frederick Sanger), also called the dideoxy method, involves synthesizing a new DNA strand using as template single-stranded DNA from the gene being sequenced. Synthesis of the new strand can be stopped at any of the four bases by adding the corresponding dideoxy (dd) derivative of the deoxyribonucleoside phosphates; for example, by adding ddATP the synthesis terminates at an adenosine; by adding ddGTP it terminates at a guanosine, etc. As in the first method, the fragments, which comprise radiolabelled nucleotides, are finally subjected to electrophoresis and autoradiography. A big advantage of the Sanger method is that it can easily be adapted to sequencing RNA, by making single-stranded DNA from the RNA template using the enzyme reverse transcriptase. This enables, for example, sequencing of ribosomal RNA for use in molecular systematics. Furthermore, by using fluorescent dyes as labels instead of radioisotopes, the Sanger method has been fully automated. After separation of the fragments, the products of all four reactions are detected by fluorescence spectroscopy and analysed by computer, which gives a printout of the base sequence. Gene sequencing is now employed on a major scale, for example in determining the nucleotide sequence of the entire human genome (see Human Genome Project).

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gene splicing

A stage in the processing of messenger RNA (see transcription), occurring only in eukaryote cells, in which noncoding introns are removed from the primary mRNA transcript and the coding exons are spliced together to form the functional mRNA molecule. Splicing is catalysed by a complex of small RNA molecules and proteins called a spliceosome . In some organisms, self-splicing of mRNA is known to occur.

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gene therapy

The application of genetic engineering techniques to alter or replace defective genes. A defective gene may result from an incorrect sequence of bases in the DNA molecule or an inability of the gene to code for the expression of a particular polypeptide. Techniques currently being investigated involve the transfer of normal genes into the genetic material of the cell to replace the defective gene. Retroviruses are often used as vectors for transferring genes into cells as part of the natural retrovirus life cycle involves the insertion of their own genetic material into the chromosomes of their host. Alternatively liposomes may be used. Gene therapy is being developed in an attempt to cure and prevent such genetic diseases as cystic fibrosis.

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genetically modified organisms (GMOs)

Organisms whose genomes incorporate and express genes from another species. Genetically modified (or transgenic) individuals are created by genetic engineering, using suitable vectors to insert the desired foreign gene into the fertilized egg or early embryo of the host.
Since the early 1980s developments in genetic engineering have made it possible to produce genetically modified organisms. A gene from one organism is isolated and transfered to cells of another organism, where it is incorporated into the recipient's chromosomes and expressed. Such transgenic organisms can exhibit quite novel characteristics. During the 1990s there was a dramatic growth in the commercial applications of this new technology, ranging from the production of human hormones in bacteria and vaccines in yeasts to the development of genetically modified (GM) crop plants. Techniques Various methods are used to introduce novel genes, depending on the nature of the recipient organism. Much of the work with genetic modification of plants involves protoplasts, cultured spherical cells from which the cell walls have been removed. The Ti plasmid (see illustration) of A. tume faciens has been used successfully as a vector with certain dicotyledons, including tobacco, tomato, potato, soyabean, and cotton. It works much less well with grasses, cereals, and other monocots. In these plants various other techniques are available, including:
• electroporation – treatment of cells by exposure to an electric field that renders them transiently permeable to DNA fragments;
• microinjection – injection of DNA directly into the cell nucleus;
• biolistics – ‘shooting’ a cell with a DNA-coated tungsten microprojectile.

To produce a transgenic animal the novel genes are inserted at a very early stage of development, e.g. the early embryo or the pronucleus of a fertilized egg, typically using microinjection. The recombinant embryos are then transferred to the uterus of a foster mother to complete their development. Applications Plants
• tolerance to herbicides
• improved insect resistance
• ‘vaccination’ against specific diseases
• longer ‘shelf life’ for fruit

Animals
• production of therapeutic proteins in milk
• potential for improved growth rates and milk yields
• potential for production of organs for human transplants

Risks The use of GM organisms in the environment poses certain potential problems. For example, genes for herbicide or insect resistance may spread from crop plants to wild plants, with possible serious consequences for both agriculture and natural ecosystems. Farmers may be faced with new ‘superweeds’, while insect populations could decline. Moreover, the products of GM crops have to be fully evaluated to ensure that they are safe to eat. Genetic modification of animals often has unforeseen side-effects and raises ethical issues about such treatments.

In plants one of the most useful cloning vectors is the Ti plasmid of the bacterium Agrobacterium tumefaciens, which is responsible for a plant tumour known as crown-gall disease. The plasmid is engineered by replacing its tumour-inducing genes (T-DNA) with the gene to be cloned and then returned to the bacterium, which is allowed to infect cultured plant protoplasts. Transformed cells are selected by the presence of marker genes and propagated with plant growth substances so they develop into complete plants.

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genetic code

The means by which genetic information in DNA controls the manufacture of specific proteins by the cell. The code takes the form of a series of triplets of bases in DNA, from which is transcribed a complementary sequence of codons in messenger RNA (see transcription). The sequence of these codons determines the sequence of amino acids during protein synthesis. There are 64 possible codes from the combinations of the four bases present in DNA and messenger RNA and 20 amino acids present in body proteins: some of the amino acids are coded by more than one codon, and some codons have other functions (see start codon; stop codon).

First base in codon

Second base in codon

Third base in codon

U

C

A

G

U

UUU Phe

UCU Ser

UAU Tyr

UGU Cys

U

UUC Phe

UCC Ser

UAC Tyr

UGC Cys

C

UUA Leu

UCA Ser

UAA (stop codon)

UGA (stop codon)

A

UUG Leu

UCG Ser

UAG (stop codon)

UGG Trp

G

C

CUU Leu

CCU Pro

CAU His

CGU Arg

U

CUC Leu

CCC Pro

CAC His

CGC Arg

C

CUA Leu

CCA Pro

CAA Gln

CGA Arg

A

CUG Leu

CCG Pro

CAG Gln

CGG Arg

G

A

AUU Ile

ACU Thr

AAU Asn

AGU Ser

U

AUC Ile

ACC Thr

AAC Asn

AGC Ser

C

AUA Ile

ACA Thr

AAA Lys

AGA Arg

A

AUG Met (start codon)

ACG Thr

AAG Lys

AGG Arg

G

G

GUU Val

GCU Ala

GAU Asp

GGU Gly

U

GUC Val

GCC Ala

GAC Asp

GGC Gly

C

GUA Val

GCA Ala

GAA Glu

GGA Gly

A

GUG Val

GCG Ala

GAG Glu

GGG Gly

G

The genetic code

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genetic drift (Sewall Wright effect)

The random change in the frequency of alleles in a population over successive generations due to sampling error in the gametes. Each new generation differs from its parental generation with regard to allele frequencies simply because of random variation in the distribution of gametes. Over time, this may lead to certain alleles becoming fixed and others being lost altogether. This process is more rapid in smaller populations, or when the alleles concerned confer no apparent benefit compared to their counterparts. Hence, genetic drift can ultimately cause loss of genetic diversity if there are no counteracting factors.

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genetic engineering (recombinant DNA technology)

The techniques involved in altering the characters of an organism by inserting genes from another organism into its DNA. This altered DNA (known as recombinant DNA ) is usually produced by gene cloning. Genetic engineering has many applications, ranging from the commercial production of insulin and other hormones, vaccines, etc., to the creation of transgenic animals and crop plants in agriculture (see genetically modified organisms). See also gene library; gene probe; gene therapy; monoclonal antibody.

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genetic fingerprinting

See DNA fingerprinting.

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genetic load

The concentration of deleterious genes carried within a population.

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genetic mapping

See chromosome map; restriction mapping.

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genetic marker

See marker gene.

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genetic polymorphism

See polymorphism.

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genetics

The branch of biology concerned with the study of heredity and variation. Classical genetics is based on the work of Gregor Mendel (see Mendelism). During the 20th century genetics has expanded to overlap with the fields of ecology and animal behaviour (see behavioural genetics; population genetics), and important advances in biochemistry and microbiology have led to clarification of the chemical nature of genes and the ways in which they can replicate and be transmitted, creating the field of molecular genetics .

GENETICS

1866

Gregor Mendel publishes his findings on inheritance in peas and his observation that characters are determined by discrete ‘factors’.

1875

German cytologist Oskar Hertwig (1849–1922) describes the process of fertilization and formation of the zygote.

1879–85

German cytologist Walther Flemming (1843–1905) describes the behaviour of chromosomes during cell division, which he terms ‘mitosis’.

1886

German biologist August Weismann (1834–1914) publishes a theory of continuity of the germ plasm through successive generations.

1887–92

German cytologist Theodor Boveri (1862–1915), Hertwig, and others describe meiosis, confirming Weismann's prediction of a ‘reduction division’.

1900

Hugo de Vries, German botanist Karl Correns (1894–1933), and Austrian botanist Erich von Tschermak (1871–1962) independently rediscover Mendel's work.

1903

US cytologist Walter S. Sutton (1877–1916) describes how the behaviour of chromosomes during meiosis explains Mendel's laws and suggests that genes are located on chromosomes.

1909

Dutch botanist Wilhelm Johannsen (1857–1927) coins the term ‘gene’. Frans-Alfons Janssens describes crossing over.

1910

Thomas Hunt Morgan discovers sex-linked traits in fruit flies.

1913

US geneticist Alfred Sturtevant (1891–1970) publishes the first genetic map – of fruit-fly genes.

1916

US geneticist Calvin Bridges (1889–1938) proves the chromosome theory of heredity.

1927

US geneticist Hermann Müller (1890–1967) demonstrates that X-rays can cause mutations.

1930

British statistician Ronald Fisher (1890–1962) publishes The Genetical Theory of Natural Selection, a key work in neo-Darwinism.

1941

George Beadle and Edward Tatum begin work with nutritional mutants of bread mould, leading to their ‘one gene–one enzyme’ hypothesis.

1944

Oswald Avery and colleagues demonstrate that DNA is the genetic material.

1947

Erwin Chargaff establishes the one-to-one ratio of purine and pyrimidine bases in DNA.

1953

James Watson and Francis Crick propose a molecular structure for DNA.

1960

French biochemists Jacques Monod (1910–76) and François Jacob (1920– ) introduce the term ‘operon’ for a functionally integrated group of genes.

1961–66

The genetic code is deciphered by US biochemists Marshall Nirenberg (1927– ), Phillip Leder (1934– ), and others.

1972

Paul Berg (1926– ) creates the first recombinant DNA molecule, based on a lambda phage.

1973

First experimental genetic manipulation of a bacterium takes place.

1977

Techniques for sequencing DNA devised by US biochemist Walter Gilbert (1932– ), Frederick Sanger, and colleagues.

1978

Human insulin is produced by genetically engineered bacteria.

1983

US biochemist Kary Mullis (1944– ) devises the polymerase chain reaction for amplifying DNA.

First transgenic plant is created.

1984

British geneticist Sir Alec Jeffreys (1950– ) develops DNA (or genetic) fingerprinting.

1988

First patent is awarded for a genetically engineered animal – a cancerprone mouse

Field trials of genetically modified tomatoes take place in the USA The Human Genome Project begins.

1993

Transgenic sheep are used to produce human proteins in their milk. Genetically modified tomatoes go on sale in the USA.

1997

British geneticist lan Wilmut and colleagues announce the birth of a lamb (‘Dolly’) – the first mammal to be cloned from an adult body cell.

1998

The first complete sequence of a genome for a complex animal, the nematode Caenorhabditis elegans, is published.

See also genetic engineering.

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genetic screening

The process by which the chromosomes of a human or other organism are analysed for genetic markers (see marker gene) that indicate the presence of particular genes, especially ones that cause or predispose to certain diseases. Increased knowledge of the human genome (see Human Genome Project) and technological advances have already simplified genetic screening in persons with a family history of certain inherited diseases, e.g. certain forms of breast cancer. However, in the future it may be feasible to screen routinely for many different genes, including ones that predispose to major degenerative disorders, such as cancer or heart disease. This has major implications for the insurance industry as well as for medicine. For example, some healthy individuals may be expected to pay a premium for life insurance because genetic screening reveals the presence of such genes.

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genetic variation

See variation.

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gene tracking

A method for determining the inheritance of a particular gene in a family. It is used in the diagnosis of genetic diseases, such as cystic fibrosis and Huntington's chorea. Restriction fragment length polymorphisms (RFLPs) situated in or near the locus of interest are identified using gene probes, and suitable marker RFLPs selected. These can then be traced through members of the family and used to detect the presence or absence of the disease locus prenatally in future at-risk pregnancies.

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genome

All the genes contained in a single set of chromosomes, i.e. in a haploid nucleus. Each parent, through its reproductive cells, contributes its genome to its offspring.

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genomics

The branch of molecular genetics concerned with studying the organization of genomes and determining the nucleotide sequences of the component genes.

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genotoxicity

The condition resulting from the interaction of toxic agents (genotoxins ) with DNA molecules in genes. Since the genes are passed down to the next generation, the toxicity induced by genotoxins is heritable. Genotoxins can induce mutations in chromosomes (clastogenesis ) or in a small number of base pairs (mutagenesis ). Genotoxic agents include X-rays, natural carcinogens, some man-made products (e.g. acridine and vinyl chloride), and viruses.

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genotype

The genetic composition of an organism, i.e. the combination of alleles it possesses. Compare phenotype.

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genotype frequency

The percentage of individuals in a population that possess a specific genotype. The genotype frequency can be calculated using the Hardy–Weinberg equation (see Hardy–Weinberg equilibrium).

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genus

(pl. genera) A category used in the classification of organisms that consists of a number of similar or closely related species. The common name of an organism (especially a plant) is sometimes similar or identical to that of the genus, e.g. Lilium (lily), Crocus, Antirrhinum. Similar genera are grouped into families. See also binomial nomenclature.

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geochronology

See varve dating.

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geographical isolation

The separation of two populations of the same species or breeding group by a physical barrier, such as a mountain or body of water. Geographical isolation may ultimately lead to the populations becoming separate species by adaptive radiation. See also allopatric; speciation.

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geological time scale

A time scale that covers the earth's history from its origin, estimated to be about 4600 million years ago, to the present. The chronology is divided into a hierarchy of time intervals: eons, eras, periods, epochs, ages, and chrons.

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geotaxis

The movement of a cell or microorganism in response to gravity. For example, certain cnidarian larvae that swim towards the seabed exhibit positive geotaxis. See taxis.

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geotropism (gravitropism)

The growth of plant organs in response to gravity. A main root is positively geotropic and a main stem negatively geotropic, growing downwards and upwards respectively, irrespective of the positions in which they are placed. For example, if a stem is placed in a horizontal position it will still grow upwards. It is thought that mobile membrane-bound clusters of starch grains (amyloplasts) sink to the bottom of a plant cell, under the influence of gravity, and somehow create a physiological asymmetry within the cell. This results in differential growth of the upper and lower sides of the cell, and hence upward or downward curvature of the organ concerned, depending on whether it is a root or a shoot. The biochemical mechanisms of this differential growth remain largely unknown, although auxins are required for geotropism in shoots, and also possibly in roots, and there is evidence that calcium ions play a role too. See tropism.

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germ cell

Any cell in the series of cells (the germ line ) that eventually produces gametes, especially the first cell in such a series. In mammals the germ cells are the oogonia (in the ovaries) and the spermatogonia (in the testes). See oogenesis; spermatogenesis.

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germinal epithelium

1 A layer of epithelial cells on the surface of the ovary that are continuous with the mesothelium. These cells do not give rise to the ova (they were formerly thought to do this).

2 The layer of epithelial cells lining the seminiferous tubules of the testis, which gives rise to spermatogonia (see spermatogenesis).
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germination

1 The initial stages in the growth of a seed to form a seedling. The embryonic shoot (plumule) and embryonic root (radicle) emerge and grow upwards and downwards respectively. Food reserves for germination come from endosperm tissue within the seed and/or from the seed leaves (cotyledons). See also epigeal; hypogeal.

2 The first signs of growth of spores and pollen grains.
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germ layers (primary germ layers)

The layers of cells in an animal embryo at the gastrula stage, from which are derived the various organs of the animal's body. There are two or three germ layers: an outer layer (see ectoderm), an inner layer (see endoderm), and in most animal groups a middle layer (see mesoderm). See also development.

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germ plasm

See Weismannism.

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gestation

The period in animals bearing live young (especially mammals) from the fertilization of the egg to birth of the young (parturition). In humans gestation is known as pregnancy and takes about nine months (40 weeks).

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giant chromosome

See polyteny.

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giant fibre

A nerve fibre with a very large diameter, found in many types of invertebrate (e.g. earthworms and squids). Its function is to allow extremely rapid transmission of nervous impulses and hence rapid escape movements in emergencies.

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gibberellic acid (GA_3)

A plant growth substance that is extracted from fungal cultures and is one of the most important commercially available gibberellins (see formula). It was discovered in 1954.

Gibberellic acid

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gibberellin

Any of a group of plant growth substances chemically related to terpenes and occurring naturally in plants and fungi. They promote elongation of stems, e.g. bolting in cabbage plants, and the mobilization of food reserves in germinating seeds, and are influential in inducing flowering and fruit development. Commercially available gibberellins, such as gibberellic acid, are used to manipulate the onset of sexual maturity in various species, e.g. to induce cone-bearing in young conifer trees.

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giga-

Symbol G. A prefix used in the metric system to denote one thousand million times. For example, 10^9 joules = 1 gigajoule (GJ).

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gigantism

See growth hormone.

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gill

1 (in zoology) A respiratory organ used by aquatic animals to obtain oxygen from the surrounding water. A gill consists essentially of a membrane or outgrowth from the body, with a large surface area and a plentiful blood supply, through which diffusion of oxygen and carbon dioxide between the water and blood occurs. Fishes have internal gills , formed as outgrowths from the pharynx wall and contained within gill slits. Water entering the mouth is pumped out through these slits and over the gills. The gills of most aquatic invertebrates and amphibian larvae are external gills , which project from the body so that water passes over them as the animal moves.

2 (in botany) One of the ridges of tissue that radiate from the centre of the underside of the cap of mushrooms. The spores are produced on these gills.
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gill bar

A cartilaginous support for the tissue between the gill slits in lower chordates, such as lancelets.

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gill slit

An opening leading from the pharynx to the exterior in aquatic vertebrates and lancelets. In lancelets they function in filter feeding. In fish they contain the gills and are usually in the form of a series of long slits. They are absent in adult tetrapod vertebrates (except for some amphibians) but their presence in some form in the embryos of all vertebrates is a characteristic of the phylum Chordata.

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gingiva (gum)

The part of the epithelial tissue lining the mouth that covers the jaw bones. It is continuous with the sockets surrounding the roots of the teeth.

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Ginkgophyta

See gymnosperm.

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gizzard

A muscular compartment of the alimentary canal of many animals that is specialized for breaking up food. In birds the gizzard lies between the proventriculus and the duodenum and contains small stones and grit, which assist in breaking up the food when the gizzard contracts. See also gastric mill.

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gland

A group of cells or a single cell in animals or plants that is specialized to secrete a specific substance. In animals there are two types of glands, both of which synthesize their secretions. Endocrine glands discharge their products directly into the blood vessels; exocrine glands secrete through a duct or network of ducts into a body cavity or onto the body surface. Secretory cells are characterized by having droplets (vesicles ) containing their products. See also secretion.
In plants glands are specialized to secrete certain substances produced by the plant. The secretions may be retained within a single cell, secreted into a special cavity or duct, or secreted to the outside. Examples are the water glands (hydathodes) of certain leaves, nectaries (see nectar), and the digestive glands of certain carnivorous plants.

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glenoid cavity

The socket-shaped cavity in the scapula (shoulder blade) that holds the head of the humerus in a ball-and-socket joint.

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glia (glial cells; neuroglia)

Cells of the nervous system that support the neurones. There are four classes of glial cells: astrocytes , oligodendrocytes , ependymal cells , and microglia . Oligodendrocytes form insulating sheaths of myelin round neurones in the central nervous system, preventing impulses from travelling between adjacent neurones. Other functions of glial cells include providing nutrients for neurones and controlling the biochemical composition of the fluid surrounding the neurones.

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global warming

See greenhouse effect.

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globin

See haemoglobin.

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globular protein

See protein.

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globulin

Any of a group of globular proteins that are generally insoluble in water and present in blood, eggs, milk, and as a reserve protein in seeds. Blood serum globulins comprise four types: 1-, 2-, and ß-globulins, which serve as carrier proteins; and -globulins (gamma globulins ), which include the immunoglobulins responsible for immune responses.

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glomerular filtrate

The fluid in the lumen of the Bowman's capsule of the nephron that has been filtered from the capillaries of the glomerulus (see ultrafiltration). The glomerular filtrate has the same composition as the plasma except that it does not contain any of the larger components, such as plasma proteins or cells.

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glomerulus

(pl. glomeruli) A tangled mass of blood capillaries enclosed by the cup-shaped end (Bowman's capsule) of a kidney tubule (see nephron). Fluid is filtered from these capillaries into the Bowman's capsule and down the nephron (see glomerular filtrate).

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glottis

The opening from the pharynx to the trachea (windpipe). In mammals it also serves as the space for the vocal cords. See also epiglottis; larynx.

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glucagon

A hormone, secreted by the (or A) cells of the islets of Langerhans in the pancreas, that increases the concentration of glucose in the blood by stimulating the metabolic breakdown of glycogen. It thus antagonizes the effects of insulin (see antagonism).

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glucan

Any polysaccharide composed only of glucose residues, e.g. starch and glycogen.

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glucocorticoid

See corticosteroid.

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gluconeogenesis

The synthesis of glucose from noncarbohydrate sources, such as fat and protein. This occurs when the glycogen supplies in the liver are exhausted. The pathway is essentially a reversal of glycolysis from pyruvate to glucose and it can utilize many sources, including amino acids, glycerol, and Krebs cycle intermediates. Large-scale protein and fat catabolism normally occurs only in those suffering from starvation or certain endocrine disorders.

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gluconic acid

An optically active hydroxycarboxylic acid, CH_2(OH)(CHOH)4COOH. It is the carboxylic acid corresponding to the aldose sugar glucose, and can be made by the action of certain moulds.

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glucosamine

See amino sugar.

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glucosan

An obsolete name for a glucan.

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glucose (dextrose; grape sugar)

A white crystalline sugar, C_6H_12O_6, occurring widely in nature. Like other monosaccharides, glucose is optically active: most naturally occurring glucose is dextrorotatory. Glucose and its derivatives are crucially important in the energy metabolism of living organisms. It is a major energy source, being transported around the body in blood, lymph, and cerebrospinal fluid to the cells, where energy is released in the process of glycolysis. Glucose is present in the sap of plants, in fruits, and in honey and is also a constituent of many polysaccharides, most notably of starch and cellulose. These yield glucose when broken down, for example by enzymes during digestion.

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glucuronic acid

A compound, OC_6H_9O_6, derived from the oxidation of glucose. It is an important constituent of gums and mucilages. Glucuronic acid can combine with hydroxyl (–OH), carboxyl (–COOH), or amino (–NH_2) groups to form a glucuronide . The addition of a glucuronide group to a molecule (glucuronidation ) generally increases the solubility of a compound; hence glucuronidation plays an important role in the excretion of foreign substances (see phase II metabolism).

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glucuronide

See glucuronic acid.

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glume

Either member of a pair of bracts that are found at the base of a spikelet (see spike) of the grasses. Compare lemma.

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glutamic acid

See amino acid.

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glutamine

See amino acid.

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glutathione

A peptide comprising the amino acids glutamic acid, cysteine, and glycine. It occurs widely in plants, animals, and microorganisms, serving chiefly as an antioxidant. Reduced glutathione reacts with potentially harmful oxidizing agents and is itself oxidized. This is important in ensuring the proper functioning of proteins, haemoglobin, membrane lipids, etc. Glutathione is also involved in amino acid transport across cell membranes.

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gluten

A mixture of two proteins, gliadin and glutenin, occurring in the endosperm of wheat grain. Their amino acid composition varies but glutamic acid (33%) and proline (12%) predominate. The composition of wheat glutens determines the `strength' of the flour and whether or not it is suitable for biscuit or bread making. Sensitivity of the lining of the intestine to gluten occurs in coeliac disease , a condition that must be treated by a gluten-free diet.

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glycan

See polysaccharide.

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glyceraldehyde 3-phosphate (GALP)

A triose phosphate, CHOCH(OH)CH_2OPO_3H_2, that is an intermediate in the Calvin cycle (see also photosynthesis) and glycolysis.

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glycerate 3-phosphate

A phosphorylated three-carbon monosaccharide that is an intermediate in the Calvin cycle of photosynthesis and also in glycolysis. It was formerly known as 3-phosphoglycerate or phosphoglyceric acid (PGA).

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glyceride (acylglycerol)

A fatty-acid ester of glycerol. Esterification can occur at one, two, or all three hydroxyl groups of the glycerol molecule producing monoglycerides , diglycerides , and triglycerides respectively. Triglycerides are the major constituent of fats and oils found in living organisms. Alternatively, one of the hydroxyl groups may be esterified with a phosphate group forming a phosphoglyceride (see phospholipid) or a sugar forming a glycolipid.

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glycerine

See glycerol.

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glycerol (glycerine; propane-1,2,3,-triol)

A trihydric alcohol, HOCH_2CH(OH)CH_2OH. Glycerol is a colourless sweet-tasting viscous liquid, miscible with water but insoluble in ether. It is widely distributed in all living organisms as a constituent of the glycerides, which yield glycerol when hydrolysed.

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glycerophospholipid

See phospholipid.

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glycine

See amino acid.

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glycobiology

The study of carbohydrates and carbohydrate complexes, especially glycoproteins.

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glycocalyx

1 (cell coat) A layer of carbohydrate on the surface of the cell membrane of most eukaryotic cells. It is made up of the oligosaccharide side-chains of the glycolipid and glycoprotein components of the membrane and may include oligosaccharides secreted by the cell. The function of the glycocalyx is not known.

2 The outermost layer of a bacterium, typically consisting of numerous polysaccharides plus various glycoproteins. The glycocalyx varies in thickness and consistency: in some species it forms a flexible slime layer while in others it forms a rigid and relatively impermeable capsule.
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glycogen (animal starch)

A polysaccharide consisting of a highly branched polymer of glucose occurring in animal tissues, especially in liver and muscle cells. It is the major store of carbohydrate energy in animal cells.

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glycogenesis

The conversion of glucose to glycogen, which is stimulated by insulin from the pancreas. Glycogenesis occurs in skeletal muscles and to a lesser extent in the liver. Glucose that is taken up by cells is phosphorylated to glucose 6-phosphate; this is converted successively to glucose 1-phosphate, uridine diphosphate glucose, and finally to glycogen. See also gluconeogenesis. Compare glycogenolysis.

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glycogenolysis

The conversion of glycogen to glucose, which occurs in the liver and is stimulated by glucagon from the pancreas and adrenaline from the adrenal medulla. These hormones activate an enzyme that phosphorylates glucose molecules in the glycogen chain to form glucose 1-phosphate, which is converted to glucose 6-phosphate. This is then converted to glucose by a phosphatase enzyme. In skeletal muscle glycogen is degraded to glucose 6-phosphate, which is then converted into pyruvate and used in ATP production during glycolysis and the Krebs cycle. However, pyruvate can also be converted, in the liver, to glucose; thus muscle glycogen is indirectly a source of blood glucose. Compare glycogenesis.

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glycolipid

Any of a group of sugar-containing lipids, in which the lipid portion of the molecule is usually based on glycerol (see glyceride) or sphingosine and the sugar is typically galactose, glucose, or inositol. Glycolipids are components of biological membranes. In animal plasma membranes they are found in the outer layer of the lipid bilayer; the simplest animal glycolipids are the cerebrosides. Plant glycolipids are glycerides in which the sugar group is most commonly galactose. They are the principal lipid constituents of chloroplasts.

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glycolysis (Embden–Meyerhof pathway)

The series of biochemical reactions in which glucose is broken down to pyruvate with the release of usable energy in the form of ATP (see illustration). One molecule of glucose undergoes two phosphorylation reactions and is then split to form two triose-phosphate molecules. Each of these is converted to pyruvate. The net energy yield is two ATP molecules per glucose molecule. In aerobic respiration pyruvate then enters the Krebs cycle. Alternatively, when oxygen is in short supply or absent, the pyruvate is converted to various products by anaerobic respiration. Other simple sugars, e.g. fructose and galactose, and glycerol (from fats) enter the glycolysis pathway at intermediate stages. Compare gluconeogenesis.

The principal stages of glycolysis

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glycoprotein

A carbohydrate linked covalently to a protein. Formed in the Golgi apparatus in the process of glycosylation, glycoproteins are important components of cell membranes, in which they extend throughout the lipid bilayer. They are also constituents of body fluids, such as mucus, that are involved in lubrication. Many of the hormone receptors on the surfaces of cells have been identified as glycoproteins. Glycoproteins produced by viruses attach themselves to the surface of the host cell, where they act as markers for the receptors of leucocytes. Viral glycoproteins can also act as target molecules and help viruses to detect certain types of host cell; for example, a glycoprotein on the surface of HIV enables the virus to find and infect white blood cells.

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glycosaminoglycan

Any one of a group of polysaccharides that contain amino sugars (such as glucosamine). Formerly known as mucopolysaccharides , they include hyaluronic acid and chondroitin (see cartilage), which provide lubrication in joints and form part of the matrix of cartilage. The three-dimensional structure of these molecules enables them to trap water, which forms a gel and gives glycosaminoglycans their elastic properties.

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glycoside

Any one of a group of compounds consisting of a pyranose sugar residue, such as glucose, linked to a noncarbohydrate residue (R) by a glycosidic bond: the hydroxyl group (–OH) on carbon-1 of the sugar is replaced by –OR. Glycosides are widely distributed in plants; examples are the anthocyanin pigments and the cardiac glycosides , such as digoxin (see digitalis) and ouabain, which are used medicinally for their stimulant effects on the heart.

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glycosidic bond (glycosidic link)

The type of chemical linkage between the monosaccharide units of disaccharides, oligosaccharides, and polysaccharides, which is formed by the removal of a molecule of water (i.e. a condensation reaction). The bond is normally formed between the carbon-1 on one sugar and the carbon-4 on the other (see illustration). An -glycosidic bond is formed when the –OH group on carbon-1 is below the plane of the glucose ring and a ß-glycosidic bond is formed when it is above the plane. Cellulose is formed of glucose molecules linked by 1-4 ß-glycosidic bonds, whereas starch is composed of 1-4 -glycosidic bonds.

Formation of a glycosidic bond

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glycosuria

See insulin.

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glycosylation

The process in which a carbohydrate is joined to another molecule, such as a protein to form a glycoprotein or to a lipid to form a glycolipid. Glycosylation occurs in the rough endoplasmic reticulum and the Golgi apparatus of cells.

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glyoxylate cycle

A metabolic pathway in plants and microorganisms that is a modified form of the Krebs cycle. It utilizes fats as a source of carbon and enables the synthesis of carbohydrate from fatty acids by avoiding the stages of the Krebs cycle in which carbon dioxide is released. It occurs in tissues rich in fats, such as those of germinating seeds; the enzymes involved in the cycle, which have not been found in mammals, are contained in organelles called glyoxysomes . The glyoxylate cycle differs from the Krebs cycle in that it utilizes two molecules of acetyl coenzyme A (rather than one), which are derived from fatty-acid oxidation. Isocitrate is converted to succinate (from which glucose can be synthesized in gluconeogenesis) and glyoxylate (see illustration).

Glyoxylate cycle

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glyoxysome

See glyoxylate cycle.

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glyphosate

N-(phosphonomethyl)glycine: a herbicide, marketed as Roundup and Tumbleweed, that kills a wide range of plants but shows little persistence in soil and has low toxicity to animals. If applied to the leaves it is rapidly translocated to the rest of the plant, and hence can penetrate the roots of even hardy perennials. It works by blocking the synthesis of aromatic amino acids, so that treated plants are unable to manufacture proteins and other key metabolites. Glyphosate inhibits the activity of 5-enolpyruvyl-shikimate-3-phosphate synthase (EPSPS), a key enzyme in the shikimic acid pathway, which occurs only in plants and microorganisms. Certain crops, notably soya bean, have been genetically engineered to give them resistance to glyphosate, by inserting genes for an EPSPS enzyme from Agrobacterium. These `Roundup-ready' crops, which can be sprayed with the herbicide without being affected, are now widely grown in North America and elsewhere.

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Gnathostomata

A subphylum or superclass of chordates consisting of all vertebrates that possess jaws. It contains six extant classes: Chondrichthyes (cartilaginous fishes), Osteichthyes (bony fishes), Amphibia, Reptilia, Aves (birds), and Mammalia. Compare Agnatha.

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Gnetophyta

See gymnosperm.

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gnotobiotic

Designating germ-free conditions, especially those in which experimental animals are inoculated with known strains of microorganisms.

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goblet cell

A goblet-shaped cell, found in the epithelium of the intestine and respiratory system in mammals and in the epidermis of fish, that secretes mucus. Goblet cells have a wide top and constricted base and possess glycoprotein-containing vesicles.

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goitre

Enlargement of the thyroid gland resulting in a swelling of the neck. This may be caused by lack of iodine in the diet (which is required for the production of thyroid hormones), as the gland enlarges to compensate for this deficiency; or by hyperthyroidism.

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Golgi, Camillo

(1843–1926)
Italian cytologist, who experimented with cells and tissues while working as a physician. He later became a professor at Pavia University. He devised a method of staining cells using silver salts, which enabled him to study nerve cells, and identified a type of nerve cell in the brain (later called Golgi cells ) that made connections with many other nerve cells. This led to the establishment, by the Spanish histologist Santiago Ramón y Cajal (1852–1934), of the neurone as the impulse-conducting unit of the nervous system. Golgi is also remembered for his discovery of the cell organelle now called the Golgi apparatus. For his work on the structure of the nervous system he shared the 1906 Nobel Prize for physiology or medicine with Ramón y Cajal.

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Golgi apparatus

An assembly of vesicles and folded membranes within the cytoplasm of eukaryotic cells that stores and transports secretory products (such as enzymes and hormones) and plays a role in formation of a cell wall (when this is present). It is named after its discoverer, Camillo Golgi. See also endoplasmic reticulum.

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gonad

Any of the usually paired organs in animals that produce reproductive cells (gametes). The most important gonads are the male testis, which produces spermatozoa, and the female ovary, which produces ova (egg cells). The gonads also produce hormones that control secondary sexual characteristics.

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gonadotrophin (gonadotrophic hormone)

Any of several hormones, secreted by the mammalian anterior pituitary gland, that stimulate reproductive activity of the testes or ovaries (the gonads). Pituitary gonadotrophins include follicle-stimulating hormone and luteinizing hormone. Chorionic gonadotrophin is a hormone produced by the placenta of higher mammals that maintains the corpus luteum. The presence of large amounts of human chorionic gonadotrophin (hCG) in the urine of women is an indication of pregnancy.

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Gondwanaland

See continental drift.

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gonorrhoea

See sexually transmitted disease.

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G-protein

Any one of a group of proteins that have an important role in relaying signals in mammalian cells. They occur on the inner surface of the plasma membrane and transmit signals from hormone receptors on the outer surface of the cell to adenylate cyclase, which catalyses the formation of the second messenger, cyclic AMP, inside the cell. G-proteins bind to GTP and GDP (see guanosine); the GTP–protein complex is able to activate adenylate cyclase, whereas the GDP–protein complex does not. Normally, deactivation of a G-protein is controlled by the hormone receptor on the cell surface and it also occurs as the GTP of a GTP–protein complex gradually hydrolyses to GDP. The cholera toxin exerts its effects by changing the G-protein in the epithelial cells of the intestine so that it is continually activated, which causes an abnormal increase in cellular adenylate cyclase levels. One consequence of this is that sodium ions are actively pumped into the intestine, causing water to follow by osmosis: the result is diarrhoea and dehydration.

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Graafian follicle (ovarian follicle)

The fluid-filled cavity that surrounds and protects the developing egg cell (oocyte) in the ovary of a mammal. The oocyte is surrounded by granulosa cells, which secrete fluid into the cavity of the follicle. After the release of the ovum the follicle develops into a corpus luteum. It is named after the Dutch anatomist Reinier de Graaf (1641–73).

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grade

A group of organisms that share certain morphological characteristics but are not necessarily related in evolutionary terms. For example, all organisms that possess a coelom can be regarded as a grade. A grade can be contrasted with a clade, which represents phylogenetic lineage.

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graft

An isolated portion of living tissue that is joined to another tissue, either in the same or a different organism, the consequent growth resulting in fusion of the tissues. (The word is also used for the process of joining the tissues.)
Grafting of plant tissues is a horticultural practice used to propagate plants, especially certain bushes and fruit trees, artificially. A shoot or bud of the desired variety (the scion ) is grafted onto a rootstock of either a common or a wild related species (the stock ). The scion retains its desirable characteristics (e.g. flower form or fruit yield) and supplies the stock with food made by photosynthesis. The stock supplies the scion with water and mineral salts and affects only the size and vigour of the scion.
Animal and human grafts are used to replace faulty or damaged parts of the body. An autograft is taken from one part of the body and transferred to another part of the same individual, e.g. a skin graft used for severe burns. A homograft (allograft ) is taken from one individual (the donor ) and implanted in another (the recipient ), the process being known as transplantation , e.g. a heart or kidney transplant. In such cases the graft may be regarded by the body as foreign (a state of incompatibility ): an immune response follows and the graft is rejected (see also histocompatibility).

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graft hybrid

A type of plant chimaera that may be produced when a part of one plant (the scion) is grafted onto another plant of a different genetic constitution (the stock). Shoots growing from the point of union of the graft contain tissues from both the stock and the scion.

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gram

Symbol g. One thousandth of a kilogram. The gram is the fundamental unit of mass in c.g.s. units and was formerly used in such units as the gram-atom , gram-molecule , and gram-equivalent , which have now been replaced by the mole.

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Gram's stain

A staining method used to differentiate bacteria. The bacterial sample is smeared on a microscope slide, stained with a violet dye, treated with acetone-alcohol (a decolourizer), and finally counterstained with a red dye. Gram-positive bacteria retain the first dye, appearing blue-black under the microscope; such bacteria have a thick layer of peptidoglycan in their cell walls. In Gram-negative bacteria, the acetone-alcohol washes out the violet dye and the counterstain is taken up, the cells appearing red. The cell walls of these bacteria have an outer layer of lipoprotein overlying a thin layer of peptidoglycan. The stain is named after the Danish bacteriologist H. C. J. Gram (1853–1938), who first described the technique (since modified) in 1884.

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granulocyte

Any white blood cell (see leucocyte) that contains granular material (secretory vessels ) and lysosomes in its cytoplasm. Neutrophils, basophils, and eosinophils are examples of granulocytes. Compare agranulocyte.

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granulosa cells (granular cells)

Secretory cells in a Graafian follicle that surround the oocyte. They have a function analogous to that of the Sertoli cells in the testis, supplying nutrients to the developing oocyte. After ovulation they form the corpus luteum.

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granum

(pl. grana) A stack of platelike bodies (thylakoids), many of which are found in plant chloroplasts (each chloroplast contains about 50 grana). Grana bear the light-receptive pigment chlorophyll and contain the enzymes responsible for the light reaction of photosynthesis.

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grape sugar

See glucose.

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graptolites

A group of extinct marine colonial animals that were common in the Palaeozoic era. Graptolites are generally regarded as being related to the Cnidaria. They had chitinous outer skeletons in the form of simple or branched stems, the individual polyps occupying minute cups (thecae ) along these stems. Fossils of these skeletons are found in Palaeozoic rocks of all continents; they are particularly abundant in Ordovician and Silurian rock strata, for which they are used as index fossils. At the end of the Silurian many graptolites became extinct but a few groups continued into the early Carboniferous.

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grass-green bacteria

See Cyanobacteria.

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grassland

A major terrestrial biome in which the dominant plants are species of grass; the rainfall is insufficient to support extensive growth of trees, which are also suppressed by grazing animals. Tropical grassland (savanna ), which covers much of Africa south of the Sahara, has widely spaced trees, such as acacias and baobabs, and supports large herds of grazing animals and their predators. Temperate grasslands, such as the steppes of Asia, the prairies of North America, and the pampas of South America, have few trees and are largely used for agriculture.

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gravitropism

See geotropism.

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gray

Symbol Gy. The derived SI unit of absorbed dose of ionizing radiation (see radiation units). It is named after the British radiobiologist L. H. Gray (1905–65).

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grazing

The consumption of vegetation, usually on grassland, by animals, particularly cattle and sheep. Overgrazing can lead to desertification.

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green algae

See Chlorophyta.

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greenhouse effect

An effect occurring in the atmosphere because of the presence of certain gases (greenhouse gases ) that absorb infrared radiation. Light and ultraviolet radiation from the sun are able to penetrate the atmosphere and warm the earth's surface. This energy is re-radiated as infrared radiation, which, because of its longer wavelength, is absorbed by such substances as carbon dioxide. The overall effect is that the average temperature of the earth and its atmosphere is increasing (so-called global warming ). The effect is similar to that occurring in a greenhouse, where light and long-wavelength ultraviolet radiation can pass through the glass into the greenhouse but the infrared radiation is absorbed by the glass and part of it is re-radiated into the greenhouse.
The greenhouse effect is seen as a major environmental hazard. Average increases in temperature could change weather patterns and agricultural output. It might also lead to melting of the polar ice caps and a corresponding rise in sea level. Carbon dioxide, from coal-fired power stations and car exhausts, is the main greenhouse gas. Other contributory pollutants are nitrogen oxides, ozone, methane, and chlorofluorocarbons. Many countries have now agreed targets to limit emissions of greenhouse gases, e.g. by switching to renewable energy sources. See also pollution.

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grey matter

Part of the tissue that makes up the central nervous system of vertebrates. It is brown-grey in colour, consisting largely of nerve cell bodies, synapses, and dendrites. The grey matter is the site of coordination between nerves of the central nervous system. Compare white matter.

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grooming

The actions of an animal of rearranging fur or feathers and cleaning the body surface by biting, scratching, licking, etc., which is important for removing parasites and spreading oils over the body surface. In many mammals, especially primates, grooming between individuals (allogrooming ) has an important role in maintaining social cohesion.

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ground meristem

The meristem in plant shoots and roots, derived from the apical meristem, that gives rise to the cortex and pith (the ground tissues) in stems and the cortex and endodermis in roots.

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ground substance

The matrix of connective tissue, in which various cells and fibres are embedded. The ground substance of cartilage consists of chondrin.

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ground tissues

All the plant tissues formed by the apical meristems except the epidermis and vascular tissue. The principal ground tissues are the cortex, pith, and primary medullary rays, and they consist chiefly of parenchyma. See also collenchyma; sclerenchyma.

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group selection

A mechanism originally proposed to account for the evolution of altruism in social groups of animals. It was suggested by the British ethologist V. C. Wynne-Edwards (1906–97) in 1962, and arose from his observations that individual animals often expose themselves to danger (for instance by warning of predators) or forego reproduction (as with worker bees in a colony) for the greater good of the group as a whole. Hence, groups containing altruistic individuals would have some selective advantage over groups lacking such members. This conflicts with Darwinian orthodoxy, which views natural selection as operating strictly on individuals. Group selection has now been supplanted by the theory of kin selection as an explanation of apparently altruistic acts.

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growth

An increase in the dry weight or volume of an organism through cell division and cell enlargement. Growth may continue throughout the life of the organism, as occurs in woody plants, or it may cease at maturity, as in man and other mammals. See also allometric growth; exponential growth.

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growth factor

Any of various chemicals, particularly polypeptides, that have a variety of important roles in the stimulation of new cell growth and cell maintenance. They bind to the cell surface on receptors. Specific growth factors can cause new cell proliferation (epidermal growth factor , insulin-like growth factors , haemopoietic growth factor – see haemopoietic tissue) and the migration of cells (fibroblast growth factor ) and play a role in wound healing (platelet-derived growth factor ; PDGF ). Some growth factors act in the embryonic stage of development; for example, nerve growth factor stimulates the growth of axons and dendrites from developing sensory and sympathetic neurones. It is thought that some growth factors are involved in the abnormal regulation of growth seen in cancer when produced in excessive amounts.

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growth hormone (GH; somatotrophin)

A hormone, secreted by the mammalian pituitary gland, that stimulates protein synthesis and growth of the long bones in the legs and arms. It also promotes the breakdown and use of fats as an energy source, rather than glucose. Production of growth hormone is greatest during early life. Its secretion is controlled by the opposing actions of two hormones from the hypothalamus: growth hormone releasing hormone (somatoliberin ), which promotes its release; and somatostatin, which inhibits it. Overproduction of human growth hormone (hGH) results in gigantism in childhood and acromegaly in adults; underproduction results in dwarfism. Bovine somatotrophin (BST) has been used to increase milk and meat production in cattle.

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growth ring (annual ring)

Any of the rings that can be seen in a cross-section of a woody stem (e.g. a tree trunk). It represents the xylem formed in one year as a result of fluctuating activity of the vascular cambium. In temperate climates pale soft spring wood , characterized by large xylem vessels, is formed in spring and early summer. Growth slows down in late summer and a darker dense autumn wood with smaller xylem vessels is formed (see illustration). The age of a tree can be determined by counting the rings. Under certain circumstances two or more growth rings may form in one year, giving rise to false annual rings.

Transverse section through a three-year-old woody stem to show the growth rings

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growth substance (phytohormone; plant hormone)

Any of a number of organic chemicals that are synthesized by plants and regulate growth and development. They are usually made in a particular region, such as the shoot tip, and transported to other regions, where they take effect. See abscisic acid; auxin; cytokinin; ethylene; gibberellin.

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guanine

A purine derivative. It is one of the major component bases of nucleotides and the nucleic acids DNA and RNA.

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guano

An accumulation of the droppings of birds, bats, or seals, usually formed by a long-established colony of animals. It is rich in plant nutrients, and some deposits are extracted for use as fertilizer.

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guanosine

A nucleoside consisting of one guanine molecule linked to a D-ribose sugar molecule. The derived nucleotides, guanosine mono-, di-, and triphosphate (GMP, GDP, and GTP, respectively), participate in various metabolic reactions.

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guard cell

See stoma.

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gullet

See oesophagus.

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gum

1 Any of a variety of substances obtained from plants. Typically they are insoluble in organic solvents but form gelatinous or sticky solutions with water. Gum resins are mixtures of gums and natural resins. Gums are produced by the young xylem vessels of some plants (mainly trees) in response to wounding or pruning. The exudate hardens when it reaches the plant surface and thus provides a temporary protective seal while the cells below divide to form a permanent repair. Excessive gum formation is a symptom of some plant diseases. See also mucilage.

2 See gingiva.
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gustatory receptors

See taste bud.

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gut

See alimentary canal.

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guttation

See hydathode.

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gymnosperm

Any plant whose ovules and the seeds into which they develop are borne unprotected, rather than enclosed in ovaries, as are those of the flowering plants (the term gymnosperm means naked seed). In traditional systems of classification such plants were classified as the Gymnospermae, a class of the Spermatophyta, but they are now divided into separate phyla: Coniferophyta (conifers), Cycadophyta (cycads), Ginkgophyta (ginkgo), and Gnetophyta (e.g. Welwitschia).

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gynandromorph

An animal that possesses both male and female characteristics because it is genetically a mosaic, i.e. some of its cells are genetically male and others are female. This phenomenon is found particularly in the insects but also appears in the birds and mammals; it is often due to the loss of an X chromosome in a stem cell of a female (XX), so that all tissues derived from that cell are phenotypically male. Compare intersex.

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gynoecium (gynaecium)

The female sex organs (carpels) of a flower. Compare androecium.

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H

habitat

The place in which an organism lives, which is characterized by its physical features or by the dominant plant types. Freshwater habitats, for example, include streams, ponds, rivers, and lakes. See also microhabitat.

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habituation

1 A simple type of learning consisting of a gradual waning in the response of an animal to a continuous or repeated stimulus that is not associated with reinforcement.

2 The condition of being psychologically, but not physically, dependent on a drug, with a desire to continue its use but not to increase the dosage.
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Hadean

The earliest eon in the history of the earth, from the time of the accretion of planetary material, around 4600 million years ago, to the date of the oldest known rocks – and hence the beginning of the geological record – about 3800 million years ago. The young earth was probably a rocky planet with a hot interior and a moist surface with oceans of liquid water. No evidence of life has been found. Compare Archaean.

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Haeckel, Ernst Heinrich

(1834–1919)
German biologist, who became professor of zoology and comparative anatomy at the University of Jena in 1862. An ardent follower of Darwin, Haeckel was the first to divide animals in protozoan (unicellular) and metazoan (multicellular) forms. However, he had a mistaken belief in the spontaneous combination of the elements to produce elementary life forms. His recapitulation theory, that the stages of embryological development of an individual organism recapitulate the evolution of its species, is no longer accepted.

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haem (heme)

An iron-containing molecule (see formula) that binds with proteins as a cofactor or prosthetic group to form the haemoproteins . These are haemoglobin, myoglobin, and the cytochromes. Essentially, haem comprises a porphyrin with its four nitrogen atoms holding the iron(II) atom as a chelate. This iron can reversibly bind oxygen (as in haemoglobin and myoglobin) or (as in the cytochromes) conduct electrons by conversion between the iron(II) and iron(III) series.

Haem

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haemagglutination

See agglutination.

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haematophagous

Feeding on blood. For example, horseflies and certain leeches are haematophagous.

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haematoxylin

A compound used in its oxidized form (haematein ) as a blue dye in optical microscopy, particularly for staining smears and sections of animal tissue. It stains nuclei blue and is frequently used with eosin as a counterstain for cytoplasm. Haematoxylin requires a mordant, such as iron alum, which links the dye to the tissue. Different types of haematoxylin can be made up depending on the mordant used, the method of oxidation, and the pH. Examples are Delafield's haematoxylin and Ehrlich's haematoxylin .

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haemocoel

The body cavity of arthropods and molluscs, which is filled with blood. The haemocoel is an enlarged blastocoel (see blastula), which greatly reduces the coelom (this is restricted to the cavities of the gonads and excretory organs). The haemocoel can act as a hydrostatic skeleton.

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haemocyanin (hemocyanin)

Any of a group of copper-containing respiratory proteins found in solution in the blood of certain arthropods (e.g. crabs and lobsters) and molluscs. Haemocyanins contain two copper atoms that reversibly bind oxygen, changing between the colourless deoxygenated form (CuI) and the blue oxygenated form (CuII). In some species, haemocyanin molecules form giant polymers with molecular weights of several million.

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haemocytometer

A device used to estimate the concentration of cells in blood samples. Typically a haemocytometer consists of a glass slide with a shallow depression divided into a fine-meshed grid; each of the grid compartments is about 0.05mm square. The slide is placed under a microscope and the number of cells in each square of the grid can be counted, which enables the concentration of blood cells in the sample to be assessed. Haemocytometers are also used in estimating the number of microorganisms present in a sample of water or other fluid.

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haemoerythrin

A red iron-containing respiratory pigment that occurs in the blood of certain worms, especially sipunculids. Its structure is essentially the same as that of haemoglobin except the prosthetic group has a different chemical composition.

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haemoglobin

One of a group of globular proteins occurring widely in animals as oxygen carriers in blood. Vertebrate haemoglobin comprises two pairs of polypeptide chains, known as -chains and ß-chains (forming the globin protein), with each chain folded to provide a binding site for a haem group. Each of the four haem groups binds one oxygen molecule to form oxyhaemoglobin . Dissociation occurs in oxygen-depleted tissues: oxygen is released and haemoglobin is reformed (see Bohr effect; haemoglobinic acid; oxygen dissociation curve). The haem groups also bind other inorganic molecules, including carbon monoxide (to form carboxyhaemoglobin). In vertebrates, haemoglobin is contained in the red blood cells (erythrocytes).

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haemoglobinic acid

A very weak acid formed inside red blood cells when hydrogen ions combine with haemoglobin. The presence of the hydrogen ions, which are produced by the dissociation of carbonic acid (see carbonic anhydrase), encourages oxyhaemoglobin to dissociate into haemoglobin and oxygen (see Bohr effect). The oxygen diffuses into the tissue cells and the haemoglobin acts as a buffer for the excess hydrogen ions, which it takes up to form haemoglobinic acid.

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haemoglobin S

An abnormal form of haemoglobin produced in sickle-cell disease. See polymorphism.

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haemolysis

The breakdown of red blood cells. It may be due to the action of disease-causing microorganisms, poisons, antibodies in mismatched blood transfusions, or certain allergic reactions. It produces anaemia.

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haemophilia

A hereditary sex-linked disease (see sex linkage) in which there is a deficiency or defect of Factor VIII, causing the blood to clot very slowly. There may be prolonged bleeding following injury, and in severe cases spontaneous bleeding into the joints and muscles. The disorder is due to a defective recessive allele of the Factor VIII gene, which is located on the X chromosome. Female carriers of the defective allele are unaffected, whereas all males who inherit a defective allele exhibit the disease.

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haemopoietic tissue

The tissue that gives rise to blood cells in the process of haemopoiesis . The haemopoietic tissue of the embryo and fetal stage of vertebrates is the bone marrow, lymph nodes, yolk sac, liver, spleen, and thymus but after birth haemopoiesis occurs in the red bone marrow (see myeloid tissue). The different types of stem cells in haemopoietic tissue that rise to erythrocytes and leucocytes are all originally derived from haemopoietic stem cells (or haemocytoblasts ). The formation of the different types of blood cell is under the control of haemopoietic growth factors , which include hormones and cytokines. See also erythropoiesis.

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haemostasis

The prevention of blood loss following rupture of blood vessels, which is effected by several physiological processes. Initially, bleeding is restricted by constriction of the damaged vessels, whose endothelial surfaces also stick together. This is followed by the formation of a plug of platelets and then by the various stages of blood clotting.

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hair

1 A multicellular threadlike structure, consisting of many dead keratinized cells, that is produced by the epidermis in mammalian skin. The section of a hair below the skin surface (the root ) is contained within a hair follicle, the base of which produces the hair cells. Hair assists in maintaining body temperature by reducing heat loss from the skin. Bristles and whiskers are specialized types of hair.

2 Any of various threadlike structures on plants, such as a trichome.
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hair follicle

A narrow tubular depression in mammalian skin containing the root of a hair. It is lined with epidermal cells and extends down through the epidermis and dermis to its base in the subcutaneous tissue. The ducts of sebaceous glands empty into hair follicles.

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hallucinogen

A drug or chemical that causes alterations in perception (usually visual), mood, and thought. Common hallucinogenic drugs include lysergic acid diethylamide (LSD) and mescaline. There is no common mechanism of action for this class of compounds although many hallucinogens are structurally similar to neurotransmitters in the central nervous system, such as serotonin and the catecholamines.

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hallux

The innermost digit on the hindlimb of a tetrapod vertebrate. In humans it is the big toe and contains two phalanges. The hallux is absent in some mammals and in many birds it is directed backwards as an adaptation to perching. Compare pollex.

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halophyte

A plant that can tolerate a high concentration of salt in the soil. Such conditions occur in salt marshes and mudflats. Halophytes possess some of the structural modifications of xerophytes; for example, many of them are succulents. In addition, they are physiologically adapted to withstand the high salinity of the soil water: their root cells have a higher than normal concentration of solutes, which enables them to take up water by osmosis from the surrounding soil. Examples of halophytes are mangrove trees (see mangrove swamp), thrift (Armeria), sea lavender (Limonium), and rice grass (Spartina). Compare hydrophyte; mesophyte.

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haploid

Describing a nucleus, cell, or organism with a single set of unpaired chromosomes. The haploid number is designated as n. Reproductive cells, formed as a result of meiosis, are haploid. Fusion of two such cells (see fertilization) restores the normal (diploid) number.

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haplont

An organism that is at the haploid stage of its life cycle. Compare diplont.

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hapten

A foreign molecule that can bind to an antibody but does not evoke an immune response unless combined with a carrier protein. In hypersensitivity reactions haptens bound to endogenous proteins cause the body's immune system to attack both the hapten and the endogenous protein.

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haptonasty

See nastic movements.

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haptotropism

See thigmotropism.

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hardwood

See wood.

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Hardy–Weinberg equilibrium

The balance in the relative numbers of alleles (see allele frequency) that is maintained within a large population over a period of time assuming that: (1) mating is random; (2) there is no natural selection; (3) there is no migration; (4) there is no mutation. In such a stable population, for a gene with two alleles, A (dominant) and a (recessive), if the frequency of A is p and the frequency of a is q, then the frequencies of the three possible genotypes (AA, Aa, and aa) can be expressed by the Hardy–Weinberg equation :
p^2 + 2pq + q^2 = 1,

where p^2 = frequency of AA (homozygous dominant) individuals, 2pq = frequency of Aa (heterozygous) individuals, and q^2 = frequency of aa (homozygous recessive) individuals. The equation can be used to calculate allele frequencies if the numbers of homozygous recessive individuals in the population is known. The equation and the equilibrium are named after British mathematician G. H. Hardy (1877–1947) and German physician W. Weinberg (1862–1937).

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harvesting

1 The processes involved in gathering in ripened crops (see agriculture).

2 The collection of cells from cell cultures or of organs from donors for the purpose of transplantation (see graft).
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Harvey, William

(1578–1657)
English physician, who worked at St Bartholomew's Hospital, London, from 1609 and from 1618 was court physician. He is best known for discovering the circulation of the blood, which he announced in 1628.

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Hatch–Slack pathway

See C4 pathway

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haustorium

(pl. haustoria) A specialized structure of certain parasitic plants and fungi that penetrates the cells of the host plant to absorb nutrients. In parasitic fungi haustoria are formed from enlarged hyphae and in parasitic flowering plants, such as the dodder (Cuscuta), they are outgrowths of the stem.

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Haversian canals

Narrow tubes within compact bone containing blood vessels and nerves. They generally run parallel to the bone surface. Each canal surrounded by a series of rings of bone (lamellae ) is known as a Haversian system . Haversian systems are joined to each other by bone material. They are named after the English anatomist Clopton Havers (1657–1702).

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health physics

The branch of medical physics concerned with the protection of medical, scientific, and industrial workers from the hazards of ionizing radiation and other dangers associated with atomic physics. Establishing the maximum permissible dose of radiation, the disposal of radioactive waste, and the shielding of dangerous equipment are the principal activities in this field.

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hearing

The sense by which sound is detected. In vertebrates the organ of hearing is the ear. In higher vertebrates variation in air pressure caused by sound waves are amplified in the outer and middle ears and transmitted to the inner ear, where sensory cells in the cochlea (see organ of Corti) register the vibrations. The resulting information is transmitted to the brain via the auditory nerve. The ear can distinguish between sounds of different intensity (loudness ) and frequency (pitch ).

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heart

A hollow muscular organ that, by means of regular contractions, pumps blood through the circulatory system (see circulation). The vertebrate heart has a thick wall (see myocardium) composed of a specialized muscle (see cardiac muscle); it is surrounded by the pericardium. Mammals have a four-chambered heart consisting of two atria and two ventricles; the right and left sides are completely separate from each other so there is no mixing of oxygenated and deoxygenated blood (see illustration). Oxygenated blood from the pulmonary veins enters the heart through the left atrium, passes to the left ventricle, and leaves the heart through the aorta. Deoxygenated blood from the venae cavae enters the right atrium and is pumped through the right ventricle to the pulmonary artery, which conveys it to the lungs for oxygenation. The tricuspid and bicuspid valves ensure that there is no backflow of blood. The contractions of the heart are initiated and controlled by the sinoatrial node (see pacemaker); in an average adult human the heart contracts about 70 times per minute. See also cardiac cycle; cardiac output.
The hearts of other vertebrates are similar except in the number of atria and ventricles (there may be one or two) and in the degree of separation of oxygenated and deoxygenated blood. Invertebrates, however, show great variation in the form and functioning of the heart.

Structure of the mammalian heart

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heartwood (duramen)

The wood at the centre of a tree trunk or branch. It consists of dead xylem cells heavily thickened with lignin and provides structural support. Many heartwood cells contain oils, gums, and resins, which darken the wood. Compare sapwood.

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heat-shock protein (HSP)

Any of various proteins that are synthesized by living cells in response to increased temperature. They occur in both eukaryotes and prokaryotes, and function mainly as molecular chaperones, protecting the cell's proteins as they become unfolded due to heating and enabling them to refold correctly. There are several families of heat-shock proteins, named according to their relative molecular mass (in kilodaltons, kDa); the larger ones, including HSP100, HSP90, HSP70, and HSP60, tend to predominate in animal cells. The smaller ones, with molecular masses in the range 17–28 kDa, are more common in plants. Another protein, ubiquitin, is regarded as a heat-shock protein and acts as a marker for proteins destined for degradation. Many of the heat-shock proteins are also induced by other stresses, both natural and unnatural, including reduced oxygen concentration, changes in osmotic potential, ionizing radiation, and toxins.

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heavy-metal pollution

Environmental pollution by metals with a high relative atomic mass, such as lead and mercury. These metals derive from a number of sources, including lead in petrol, industrial effluents, and leaching of metal ions from the soil into lakes and rivers by acid rain. They are easily incorporated into biological molecules and exert their toxic effects by displacing essential metals of a lower binding power in biologically active molecules or by acting as noncompetitive inhibitors of enzymes (see inhibition).

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hecto-

Symbol h. A prefix used in the metric system to denote 100 times. For example, 100 coulombs = 1 hectocoulomb (hC).

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heliotropism

See phototropism.

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helix-turn-helix (helix-loop-helix)

A structural motif characteristic of certain proteins that bind to DNA. It consists of two alpha-helices connected by a short nonhelical segment called a `turn'. One of the helical segments recognizes a specific sequence of nucleotides in the DNA molecule and fits into the groove in the DNA double helix, while the other helix stabilizes the bound configuration. This motif is one of several found in DNA-binding proteins (see also leucine zipper; zinc finger).

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Helmholtz, Hermann Ludwig Ferdinand von

(1821–94)
German physiologist and physicist. In 1850 he measured the speed of a nerve impulse and in 1851 invented the ophthalmoscope. In physics, he discovered the conservation of energy (1847) and introduced the concept of free energy.

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helper T cell

See T cell.

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heme

See haem.

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hemicellulose

A polysaccharide found in the cell walls of plants. The branched chains of this molecule bind to cellulose microfibrils, together with pectins, forming a network of cross-linked fibres.

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Hemichordata

A phylum of soft-bodied marine invertebrates that comprises two classes: the Enteropneusta (acorn worms), which are cylindrical burrowing animals; and the Pterobranchia, which are colonial forms with vase-shaped bodies. The coelom is divided into three regions and the body into three sections: proboscis, collar, and trunk; like chordates, hemichordates possess gill slits but they lack a notochord. Some acorn worms develop via a ciliated larva (tornaria ), which has some similarities to an echinoderm larva.

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hemimetabolous

Describing insect development in which there is incomplete or partial metamorphosis, typically with successive immature stages increasingly resembling the adult. For instance, rudimentary wings are usually apparent, at least in later stages. It is characteristic of the exopterygote insect orders. The immature stages of hemimetabolous species are called nymphs. Compare ametabolous; holometabolous.

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hemiparasite (semiparasite)

1 A parasitic plant that lacks a fully developed root system and forms connections with another plant, from which it obtains some or all of its water and minerals. Such plants have chlorophyll and produce their own food by photosynthesis, and in some cases are capable of limited growth in the absence of the host plant. They tap into the sap-conducting tissue of the host by means of specialized structures called haustoria (see haustorium). Some, such as eyebright (Euphrasia spp.), attach themselves to the roots of their host and appear like normal plants growing in the soil, whereas others grow on the aerial parts of their host. The mistletoes are well-known examples that colonize the branches of trees.

2 A facultative parasite (see parasitism).
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Hemiptera

An order of exopterygote insects comprising the true bugs. Hemipterans typically have oval flattened bodies with two pairs of wings, which are folded back across the abdomen at rest. The forewings are hardened, either at their bases only (in the suborder Heteroptera) or uniformly (in the suborder Homoptera). The mouthparts are modified for piercing and sucking, with long slender stylets forming a double tube. Many bugs feed on plant sap and are serious agricultural pests, including aphids, leaf-hoppers, scale insects, mealy bugs, etc. Others are carnivorous, and the order contains many aquatic species, such as the water boatmen, which have legs adapted for swimming and the exchange of respiratory gases.

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heparin

A glycosaminoglycan (mucopolysaccharide) with anticoagulant properties, occurring in vertebrate tissues, especially the lungs and blood vessels. Heparin salts are administered therapeutically to prevent or dissolve blood clots.

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hepatic

Of or relating to the liver. For example, the hepatic portal vein (see hepatic portal system) and the hepatic artery supply blood to the liver, and the hepatic vein carries blood away from the liver.

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Hepaticae

See Hepatophyta.

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hepatic portal system

The vein (hepatic portal vein ) or veins that transport blood containing the absorbed products of digestion from the intestine directly to the liver.

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hepatocyte

A specialized epithelial cell that is the most abundant type of cell in the liver. Hepatocytes are arranged around a central vein in units called lobules , in close association with branches of the hepatic portal vein, hepatic artery, and bile canaliculi. They are involved with the various functions of the liver, including metabolism, detoxification, and the production of bile. See also Kupffer cells.

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Hepatophyta

A phylum comprising the liverworts – simple plants that lack vascular tissue and possess rudimentary rootlike organs (rhizoids). Liverworts occur in moist situations (including fresh water) and as epiphytes on other plants. Like the mosses (see Bryophyta), liverworts show marked alternation of generations between haploid gamete-bearing forms (gametophytes) and diploid spore-bearing forms (sporophytes), the latter being dependent on the former for nutrients, etc. The plant body (gametophyte) may be a thallus, growing closely pressed to the ground (thallose liverworts, e.g. Pellia), or it may bear many leaflike lobes (leafy liverworts). It gives rise to leafless stalks bearing capsules (sporophytes). Spores formed in the capsules are released and grow to produce new plants. Liverworts were formerly placed in the class Hepaticae, in the phylum Bryophyta, which now contains only the mosses.

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hepatotoxin

Any chemical that has adverse effects on the liver. Alcohol (ethanol) is one of the most common hepatotoxins.

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herb

1 A herbaceous plant, i.e. a seed-bearing plant that does not form hard woody tissue.

2 A plant with medicinal or culinary uses. Culinary herbs are usually plants whose leaves are used for flavouring food.
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herbaceous

Describing a plant that contains little permanent woody tissue. The aerial parts of the plant die back after the growing season. In annuals the whole plant dies; in biennials and herbaceous perennials the plant has organs (e.g. bulbs or corms) that are modified to survive beneath the soil in unfavourable conditions.

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herbicide

See pesticide.

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herbivore

An animal that eats vegetation, especially any of the plant-eating mammals, such as ungulates (cows, horses, etc.). Herbivores are characterized by having teeth adapted for grinding plants and alimentary canals specialized for digesting cellulose (see caecum).

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heredity

The transmission of characteristics from parents to offspring via the chromosomes. The study of heredity (genetics) was first undertaken by Gregor Mendel, who derived a series of laws that govern heredity (see Mendel's laws).

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heritability

A measure of the degree to which the variance of a particular phenotype is caused by genetic factors. It is given by a value between 0 and 1 and effectively measures the extent to which offspring resemble their parents relative to the population mean. Estimates of heritability are important in applied genetics, especially in agriculture and horticulture, because they enable prediction of the response of a population to artificial selection. The higher the heritability value, the greater the response, although heritability declines after several generations of artificial selection due to increasing homozygosity. The term is used in two different ways. Heritability in the narrow sense is the proportion of phenotypic variance due only to the additive genetic effect of all the polygenes controlling a particular trait; hence it measures the proportion that is transmissible to offspring and therefore amenable to selection. Heritability in the broad sense considers other genetic but nontransmissible factors as well, including dominance and epistasis; it is used, for example, in psychology to quantify genetic and environmental influences.

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hermaphrodite (bisexual)

1 An animal, such as the earthworm, that has both male and female reproductive organs.

2 A plant whose flowers contain both stamens and carpels. This is the usual arrangement in most plants. Compare monoecious; dioecious.
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heroin (diacetylmorphine)

A narcotic compound that is a synthetic derivative of morphine (see opiate). The compound is easily absorbed by the brain, due to its lipid-like nature, and is used as a sedative and powerful analgesic. Highly addictive, it is abused by drug users.

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herpesvirus

One of a group of complex DNA-containing viruses causing infections in humans and most other vertebrates that tend to recur. The group includes herpes simplex , the agent of cold sores; herpes varicella/zoster , the virus causing chickenpox and shingles; Epstein–Barr (EB) virus , the causal agent of glandular fever and also implicated in the cancer Burkitt's lymphoma; and the cytomegalovirus.

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hertz

Symbol Hz. The SI unit of frequency equal to one cycle per second. It is named after Heinrich Hertz (1857–94), a German physicist.

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hesperidium

See berry.

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heterochromatin

See chromatin.

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heterochrony

Any change in the relative rates or timing of development of different cell lines in the body. Such changes can lead to significant alterations in form; for example, if a particular organ develops earlier or faster it may be larger. Hence, mutations causing such changes play an important role in evolution. Heterochronic changes affecting the relative rates of development of germ (reproductive) cells and somatic (body) cells are of particular interest. They fall into four categories – acceleration, progenesis, neoteny, and hypermorphosis – depending on whether the change in rate (speeded up or slowed down) affects the somatic tissues or the reproductive tissues (see table). However, they have only two possible outcomes: one is paedomorphosis, in which reproduction occurs in an ancestrally juvenile form; the other is peramorphosis, in which development is extended by the addition of stages to the sequence shown in the ancestral form.

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heterocyst

A specialized cell found in the nitrogen-fixing blue-green bacteria. Heterocysts are enlarged cells with thick cell walls and they lack chlorophyll, giving them a colourless appearance. They are the site of nitrogen fixation, for which they produce the enzyme nitrogenase. The lack of photosynthetic activity (which would produce oxygen), together with the thick cell wall, are thought to maintain the anoxic conditions in heterocysts that are essential for the activity of nitrogenase. Heterocysts are connected by plasmodesmata to surrounding cells, on which they depend for nutrients.

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heterodont

Describing animals that possess teeth of more than one type (i.e. incisors, canine teeth, premolars, and molars), each with a particular function. Most mammals are heterodont. See permanent teeth. Compare homodont.

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heteroduplex DNA

Double-stranded DNA in which the two strands are derived from different DNA molecules. Heteroduplex DNA is formed during genetic recombination (see Holliday intermediate) and can be produced in vitro in DNA hybridization.

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heteroecious

Describing rust fungi (see rusts) that pass different phases of their life cycle on different host plants. Compare autoecious.

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heterogametic sex

The sex that is determined by possession of two dissimilar sex chromosomes (e.g. XY). In humans and many other mammals this is the male sex. The heterogametic sex produces reproductive cells (gametes) of two kinds, half containing an X chromosome and half a Y chromosome. Compare homogametic sex.

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heterogamy

Alternation of generations in which parthenogenesis alternates with bisexual reproduction in the life cycle, as seen in certain aphids.

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heterokaryon

See heterokaryosis.

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heterokaryosis

The presence in the same cell of two or more genetically different nuclei. Heterokaryosis occurs naturally in certain fungi, in which it results from the fusion of the cytoplasm of cells from different strains without the fusion of their nuclei. The cell, and the hypha or mycelium containing it, is known as a heterokaryon ; the most common type of heterokaryon is a dikaryon. Heterokaryosis can also be induced in vitro, to study the interaction between the cellular components from different species (see cell fusion).

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heteromorphosis (hypermetamorphosis)

Development in which there is a radical change in form between successive larval instars. For example, the tiny insects of the order Strepsiptera have an active first-instar larva (called a triungulin ), which is succeeded by several inactive, often legless, grublike larval stages. This change in form reflects quite different functions. The triungulin's task is to seek out a suitable larval host, which is then parasitized by the following inactive larval stages.

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heteroplasmy

The presence within a cell or organism of mitochondria with different genetic constitutions. Generally, an organism inherits its mitochondrial DNA from its mother via the egg cell, with no contribution from the male gamete, and therefore all its mitochondria are genetically identical. However, in some species there is 'leakiness' from the male line. For example, in mice about 1 in 1000 mitochondria originate from the male parent. Exceptionally, in some organisms (such as mussels) mitochondria are apparently inherited equally from both parents. Compare homoplasmy.

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heterosis

See hybrid vigour.

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heterospory

The condition of producing two types of spore, megaspores and microspores. Heterospory occurs in all seed-bearing plants and in some mosses and ferns.

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heterostyly

The condition of flowering plants in which flowers of the same species have styles of different lengths, so that the stigma is positioned below the anthers in some flowers and above them in others. This ensures that pollinating insects are more likely to transfer pollen from the anther of one flower to the stigma of another, thus promoting cross-pollination. Species showing heterostyly include the primrose (see illustration).

Heterostyly in the primrose (Primula vulgaris)

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heterothallic

Describing species of algae and fungi that reproduce sexually by the conjugation of cells, thalli, or mycelia from different strains (mating types), normally referred to as + and – strains. Compare homothallic.

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heterotherm

An organism whose ability to regulate its body temperature is intermediate between an endotherm and an ectotherm. Some small birds and mammals – generally endothermic (`warm-blooded') groups – may reduce their metabolic rate during a particular season or even a certain time of day, allowing their body temperature to fall and entering a state of torpor. In small animals, for example hummingbirds and certain rodents, this avoids the relatively high cost of maintaining a constant body temperature during periods of food scarcity or other adverse conditions. This strategy is sometimes called partial endothermy . At the opposite end of the spectrum, certain animals that are generally regarded as ectothermic (`cold-blooded') have the ability to generate heat internally for limited periods, thus temporarily raising their body temperature to enable activity when the environmental temperature is low. Various insects, including bumblebees, are known to shiver in cold conditions in order to attain the body temperature necessary for flight. This may be termed facultative endothermy .

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heterotrichy

The condition occurring in certain filamentous algae in which the algal body is composed of both prostrate filaments and protruding upright filaments.

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heterotrophic nutrition

A type of nutrition in which energy is derived from the intake and digestion of organic substances, normally plant or animal tissues. The breakdown products of digestion are used to synthesize the organic materials required by the organism. All animals obtain their food this way: they are heterotrophs . See also ingestion. Compare autotrophic nutrition.

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heterozygous

Describing an organism or cell in which the alleles at a given locus on homologous chromosomes are different. The aspect of the feature displayed by the organism will be that determined by the dominant allele. Heterozygous organisms, called heterozygotes , do not breed true. Compare homozygous.

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Hexapoda (Insecta)

A class of the phylum Uniramia comprising about a million known species of arthropods (many more are thought to exist). They are distributed worldwide in nearly all terrestrial habitats. Ranging in length from 0.5 to over 300mm, an insect's body consists of a head, a thorax of three segments and usually bearing three pairs of legs and one or two pairs of wings, and an abdomen of eleven segments. The head possesses a pair of sensory antennae and a pair of large compound eyes, between which are three simple eyes (ocelli). The mouthparts are variously adapted for either chewing or sucking, enabling insects to feed on a wide range of plant and animal material. Insects owe much of their success to having a highly waterproof cuticle (to resist desiccation) and, in most species, wings – outgrowths of the body wall that confer the greater mobility of flight. Breathing occurs through a network of tubes (see trachea).
Most insect species have separate sexes and undergo sexual reproduction. In some, this may alternate with asexual parthenogenesis and in a few, males are unknown and reproduction is entirely asexual. In the wingless insects (subclass Apterygota) metamorphosis is slight or absent. In the winged insects (subclass Pterygota) the newly hatched young grow by undergoing a series of moults. In the more primitive exopterygotes (including the orders Dermaptera, Orthoptera, Dictyoptera, and Hemiptera) the young (called a nymph) resembles the adult. The more advanced endopterygotes (e.g. Coleoptera, Diptera, Lepidoptera, and Hymenoptera) undergo metamorphosis, in which the young (called a larva) is transformed into a quiescent pupa from which the fully formed adult emerges. Insects are of vital importance in many ecosystems and many are of economic significance – as animal or plant pests or disease vectors or beneficially as crop pollinators or producers of silk, honey, etc.
Some authorities regard the Hexapoda as a superclass comprising the classes Protura, Collembola, and Diplura, all of whose members have mouthparts that are enclosed in folds of the head; and Insecta, which have exposed mouthparts.

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hexose

A monosaccharide that has six carbon atoms in its molecules.

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Hfr (high-frequency recombinant)

See sex factor.

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hibernation

A sleeplike state in which some animals pass the winter months as a way of surviving food scarcity and cold weather. Various physiological changes occur, such as lowering of the body temperature and slowing of the pulse rate and other vital processes, and the animal lives on its reserve of body fat. Animals that hibernate include bats, hedgehogs, and many fish, amphibians, and reptiles. See also dormancy. Compare aestivation.

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hierarchy

A type of social organization in which individuals are ranked according to their status or dominance relative to other group members. This affects their behaviour in various ways, e.g. by determining their access to food or to mates. Many vertebrate animals and some invertebrates live in hierarchical social groups.

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high-performance liquid chromatography (HPLC)

A sensitive technique for separating or analysing mixtures, in which the sample is forced through the chromatography column under pressure.

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Hill reaction

The release of oxygen from isolated illuminated chloroplasts when suitable electron acceptors (e.g. potassium ferricyanide) are added to the surrounding water. The reaction was discovered by Robert Hill (1899–1991) in 1939; the electron acceptors substitute for NADP+, the natural acceptor for the light reaction of photosynthesis.

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hilum

A scar on the seed coat of a plant marking the point at which the seed was attached to the fruit wall by the funicle. It is a feature that distinguishes seeds from fruits.

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hindbrain (rhombencephalon)

One of the three sections of the brain of a vertebrate embryo. It develops to form the cerebellum, pons, and medulla oblongata, which control and coordinate fundamental physiological processes (including respiration and circulation of blood). Compare forebrain; midbrain.

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hindgut

1 The posterior part of the alimentary canal of vertebrates, comprising the posterior section of the colon.

2 The posterior section of the alimentary canal of arthropods. See also foregut; midgut.
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hip girdle

See pelvic girdle.

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hippocampus

A part of the vertebrate brain consisting of two ridges, one over each of the two lateral ventricles. It is highly developed in advanced mammals (primates and whales) and its function appears to be related to the expression of responses that generate emotion (such as fear and anger).

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Hirudinea

A class of freshwater and terrestrial annelid worms that comprises the leeches. They have suckers at both anterior and posterior ends but no bristles. Some are blood-sucking parasites of vertebrates and invertebrates but the majority are predators.

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histamine

A substance that is released during allergic reactions, e.g. hay fever. Formed from the amino acid histidine, histamine can occur in various tissues but is concentrated in connective tissue. It causes dilation and increased permeability of small blood vessels, which results in such symptoms as localized swelling, itching, sneezing, and runny eyes and nose. The effects of histamine can be countered by the administration of antihistamine drugs.

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histidine

See amino acid.

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histochemistry

The study of the distribution of the chemical constituents of tissues by means of their chemical reactions. It utilizes such techniques as staining, light and electron microscopy, autoradiography, and chromatography.

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histocompatibility

The degree to which tissue from one organism will be tolerated by the immune system of another organism. For any animal, it is essential that its immune system can distinguish its own tissues from foreign cells or tissues, so that only the latter are attacked. This self-recognition is achieved principally by a set of marker molecules, called histocompatibility proteins (or histocomptibility antigens ), which occur on the surfaces of cells. These proteins (in humans also called human leucocyte antigens , or HLAs ) are encoded in vertebrates by a cluster of genes called the major histocompatibility complex (MHC). Each species has a unique set of MHC proteins, and there is also wide variation within any given species. This explains why in human transplantation it is very difficult to match donor and recipient tissue exactly (see HLA system). MHC proteins also play a vital role in the immune responses of lymphocytes, notably by enabling T cells to identify foreign antigens. The MHC encodes two distinct classes of histocompatibility proteins, both of which are glycoproteins. Class I proteins determine graft rejection and help cytotoxic T cells recognize virus-infected cells. They are found on the surface of most cells in the body. Class II proteins act as receptors for presenting foreign antigen to helper T cells (see antigen-presenting cell). Their distribution is restricted to certain cell types of the immune system, including macrophages, B cells, and activated T cells.

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histocompatibility antigen

See histocompatibility; HLA system.

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histology

The microscopic study of the tissues of living organisms. The study of cells, a specialized branch of histology, is known as cytology.

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histone

Any of a group of water-soluble proteins found in association with the DNA of plant and animal chromosomes. They contain a large proportion of the basic (positively charged) amino acids lysine, arginine, and histidine. They are involved in the condensation and coiling of chromosomes during cell division and have also been implicated in nonspecific suppression of gene activity. Histones do not occur in vertebrate sperm cells (see protamine) or in bacteria, although a very similar protein has been found in the genome of the archaebacterium Thermoplasma.

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HIV (human immunodeficiency virus)

The retrovirus that causes AIDS in humans. It has a specific affinity for the helper T cells of its host. The membrane envelope glycoproteins encasing the virus show great variability in their amino-acid sequences, hence the difficulty of preparing an effective AIDS vaccine. HIV is thought to have originated from chimpanzees in central Africa.

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HLA system (human leucocyte antigen system)

A series of four gene loci (A, B, C, and D) in humans that code for a group of glycoproteins, present on the surface of cell membranes, that act as antigens and are important in determining the acceptance or rejection by the body of a tissue or organ transplant (see graft). These antigens are one group of the so-called histocompatibility proteins (see histocompatibility). Two individuals with identical HLA types are said to be histocompatible . Successful transplantation requires a minimum number of HLA differences between the donor's and recipient's tissues. See also major histocompatibility complex.

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hnRNP

See ribonucleoprotein.

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Hodgkin, Sir Alan

See Eccles, Sir John Carew.

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Hogness box

See TATA box.

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holandric

Describing a trait that is inherited only by the male line. Such traits are determined by genetic loci that occur only on the Y chromosome, and hence are present only in males.

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Holliday intermediate

An intermediate structure in genetic recombination, proposed by Robin Holliday in 1964, in which two double-stranded homologous DNA molecules are joined by means of a reciprocal crossover involving one DNA strand of each molecule. It is formed when a single strand of DNA from each chromosome is broken and joined to the other strand at the point of crossover. The region in which strands from different DNA molecules are paired (a heteroduplex DNA sequence) is extended and two strands of the Holliday intermediate are cleaved. The breaks in the DNA sequences are then repaired to form the recombinant products (see illustration).

Holliday intermediate

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holocarpic

Denoting a fungus in which the entire thallus is differentiated into a reproductive sporangium when mature. Compare eucarpic.

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Holocene (Recent)

The most recent geological epoch of the Quaternary period, comprising roughly the past 10000 years since the end of the Pleistocene up to the present. It follows the final glacial of the Pleistocene and thus is sometimes known as the Postglacial epoch. Some geologists consider the Holocene to be an interglacial phase of the Pleistocene that will be followed by another glacial.

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holocrine secretion

See secretion.

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holoenzyme

A complex comprising an enzyme molecule and its cofactor. Only in this state is an enzyme catalytically active. Compare apoenzyme.

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holometabolous

Describing insect development in which there is complete metamorphosis and the immature stages, called larvae, are markedly different from the adults. Transformation of the larvae into the adult takes place during a resting stage called a pupa. Holometabolous development is characteristic of endopterygote insect orders. Compare ametabolous; hemimetabolous.

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holophytic

Describing organisms that feed like plants, i.e. that are photoautotrophic. See autotrophic nutrition.

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holotype

See type specimen.

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holozoic

Describing organisms that feed by ingesting complex organic matter, which is subsequently digested and absorbed. See heterotrophic nutrition; ingestion.

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homeobox

A nucleotide sequence containing about 180 base pairs, which are identical or very similar in many eukaryotic organisms, that encodes a series of amino acids known as a homeodomain . Present in many eukaryotic regulatory proteins, this domain is an important region involved in the binding of regulatory proteins to the DNA molecule. A homeobox was first identified in the homeotic genes of Drosophila and has since been found in the homeotic genes of many animals (including humans) and in plants.

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homeodomain

See homeobox.

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homeostasis

The regulation by an organism of the chemical composition of its body fluids and other aspects of its internal environment so that physiological processes can proceed at optimum rates. It involves monitoring changes in the external and internal environment by means of receptors and adjusting the composition of the body fluids accordingly; excretion and osmoregulation are important in this process. Examples of homeostatic regulation are the maintenance of the acid–base balance and body temperature (see homoiothermy; poikilothermy).

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homeotic genes

A class of genes that play a central role in controlling the early development and differentiation of embryonic tissues in eukaryotic organisms. They code for proteins that bind to DNA and regulate the expression of a wide range of other genes. This binding capability resides in a structural domain of the protein called a homeodomain, encoded by a nucleotide sequence that is characteristic of homeotic genes (see homeobox). These genes were first identified in Drosophila fruit flies, through the occurrence of mutations that alter the development of entire body segments. Drosophila flies have two major clusters of homeotic genes: the antennapedia complex, which controls development of the head and anterior thoracic segments, and the bithorax complex, which governs the development of posterior segments. For example, one mutation of the bithorax cluster results in the thoracic segment that normally bears the halteres (balancing organs; see Diptera) to be transformed into a segment bearing a pair of wings. In vertebrates there are four clusters of homeotic genes located on separate chromosomes.

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home range

The area in which an animal forages and spends most of its time. The size varies according to the size of the animal and its feeding habits; for example, the home range of African hunting dogs may extend over 4000 km^2, whereas that of a field mouse may be less than 1 ha. Unlike a territory, the home range is not defended against incursion by other members of the same species, although an animal's territory may lie within its home range. Moreover, the ranges of neighbouring individuals or groups often overlap. Within the range there may be a core area, containing the main feeding and drinking areas, within which the animal's activities are focused.

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hominid

Any member of the primate family Hominidae, which includes man and his fossil ancestors (fossil hominids ) in the genus Homo.

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Homo

The genus of primates that includes modern man (H. sapiens, the only living representative) and various extinct species. The oldest Homo fossils are those of H. habilis and H. rudolfensis, which first appeared in Africa 2.3–2.4 million years ago. Both species used simple stone tools. H. habilis appears to have been 1–1.5 m tall and had more manlike features and a larger brain than Australopithecus. H. erectus appeared about 1.9 million years ago. Fossils of H. erectus, which was formerly called Pithecanthropus (ape man), include Java man and Peking man. They are similar to present-day man except that there was a prominent ridge above the eyes and no forehead or chin. They used crude stone tools and fire. H. erectus is thought to have evolved from H. ergaster, a species that may also have given rise to H. heidelbergensis (represented by Heidelberg man and Boxgrove man). These hominids lived around 500000 years ago. H. heidelbergensis is thought to have been the common ancestor of both H. neanderthalensis (Neanderthal man) and H. sapiens. Both species appeared about 100000 years ago. See also Cromagnon man.

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homodont

Describing animals whose teeth are all of the same type. Most vertebrates except mammals are homodont. Compare heterodont.

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homogametic sex

The sex that is determined by possession of two similar sex chromosomes (e.g. XX). In humans and many other mammals this is the female sex. All the reproductive cells (gametes) produced by the homogametic sex have the same kind of sex chromosome (i.e. an X chromosome). Compare heterogametic sex.

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homogamy

The condition in a flower in which the male and female reproductive organs mature at the same time, thereby allowing self-fertilization. Compare dichogamy.

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homoiothermy

The maintenance by an animal of its internal body temperature at a relatively constant value by using metabolic processes to counteract fluctuations in the temperature of the environment. Homoiothermy occurs in birds and mammals, which are described as endotherms. The heat produced by their tissue metabolism and the heat lost to the environment are balanced by various means to keep body temperature constant: 36–38°C in mammals and 38–40°C in birds. The hypothalamus in the brain monitors blood temperature and controls thermoregulation by both nervous and hormonal means. This produces both short-term responses, such as shivering or sweating, and long-term adjustments to metabolism according to seasonal changes in climate (acclimatization). Endotherms generally possess insulating feathers or fur. Their relatively high internal temperature permits fast action of muscles and nerves and enables them to lead highly active lives even in cold climates. However, in certain animals, homoiothermy is abandoned during periods of hibernation. Compare poikilothermy.

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homologous

Describing a character that is shared by a group of species because it is inherited from a common ancestor. Such characters, called homologies , are used in cladistics to determine the evolutionary relationships of species or higher taxa. They are divided into two types: a shared derived homology (see apomorphy) is unique to a particular group and may be used to define a monophyletic group; a shared ancestral homology (see plesiomorphy) is not unique to the group, or may not be exhibited by all descendants of the ancestor in which it arose (see paraphyletic). Even though homologous features share the same evolutionary origin, they may have developed different functions. For example the wings of a bat, the flippers of a dolphin, and the arms of a human are homologous organs, having evolved from the paired pectoral fins of a fish ancestor. Compare analogous.

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homologous chromosomes

Chromosomes having the same structural features. In diploid nuclei, pairs of homologous chromosomes can be identified at the start of meiosis. One member of each pair comes from the female parent and the other from the male. Homologous chromosomes have the same pattern of genes along the chromosome but the nature of the genes may differ (see allele).

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homoplasmy

The presence within a cell or organism of genetically identical mitochondria. This is the common condition for most groups of organisms, although there are some exceptions (see heteroplasmy).

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homoplastic

Denoting similarity resulting from convergent evolution. Compare patristic.

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homothallic

Describing species of algae and fungi that reproduce sexually by the conjugation of cells, thalli, or mycelia from the same strain. Compare heterothallic.

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homozygous

Describing an organism or cell in which the alleles at a given locus on homologous chromosomes are identical (they may be either dominant or recessive). Homozygous organisms, which are called homozygotes , breed true when crossed with genetically identical organisms. Compare heterozygous.

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hopeful monster

A hypothetical new phenotype, or monstrosity, that arises due to mutations that radically alter an individual's developmental pattern. Such an individual, it is claimed, could possess major innovations that equip it for an environment quite different from that of its immediate antecedents. The concept, which was introduced in 1933 by the geneticist R. Goldschmidt, presents a way in which, theoretically, a new group could arise in a single macroevolutionary leap, rather than by the gradual process of natural selection producing many small adaptive changes – the route favoured by orthodox neo-Darwinians. It is regarded by many as implausible and unsupported by fossil evidence.

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horizontal cell

See retina.

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hormogonium

A filament produced by filamentous blue-green bacteria that becomes detached from the parent organism and acts as a means of vegetative propagation.

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hormone

1 A substance that is manufactured and secreted in very small quantities into the bloodstream by an endocrine gland or a specialized nerve cell (see neurohormone) and regulates the growth or functioning of a specific tissue or organ in a distant part of the body. For example, the hormone insulin controls the rate and manner in which glucose is used by the body. Other hormones include the sex hormones, corticosteroids, adrenaline, thyroxine, and growth hormone.

2 A plant growth substance.
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hornworts

See Anthocerophyta.

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horsetails

See Sphenophyta.

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host

1 An organism whose body provides nourishment and shelter for a parasite (see parasitism) or a parasitoid. A definitive (or primary ) host is one in which an animal parasite becomes sexually mature; an intermediate (or secondary ) host is one in which the parasite passes the larval or asexual stages of its life cycle.

2 An organism that lives in close association with an inquiline (see inquilinism).
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HPLC

See high-performance liquid chromatography.

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HSP

See heat-shock protein.

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human chorionic gonadotrophin (hCG)

See gonadotrophin.

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Human Genome Project

A coordinated international project, begun in 1988, to map the entire human genome so that the genes can be isolated and eventually sequenced (see gene sequencing). It involves the production of a gene library. The haploid human genome contains about 3 × 10^9 nucleotide base pairs, making up an unknown number of genes (about 80000 according to most estimates). It is hoped that the project will be completed within the first ten years of the 21st century.

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human growth hormone (hGH)

See growth hormone.

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human immunodeficiency virus

See HIV.

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humerus

The long bone of the upper arm of tetrapod vertebrates. It articulates with the scapula (shoulder blade) at the glenoid cavity and with the ulna and radius (via a condyle) at the elbow.

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humidity

The concentration of water vapour in the atmosphere. The absolute humidity is the mass of water vapour per unit volume of air, usually expressed in kgm–3. A useful measure is the relative humidity , the ratio, expressed as a percentage, of the moisture in the air to the moisture it would contain if it were saturated at the same temperature and pressure.

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humoral

Relating to the blood or other body fluids. For example, humoral immunity is immunity conferred by the antibodies present in the blood, lymph, and tissue fluids (see immunity).

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humus

The dark-coloured amorphous colloidal material that constitutes the organic component of soil. It is formed by the decomposition of plant and animal remains and excrement (see litter) and has a complex and variable chemical composition. Being a colloid, it can hold water and therefore improves the water-retaining properties of soil; it also enhances soil fertility and workability. Acidic humus (mor ) is found in regions of coniferous forest, where the decay is brought about mainly by fungi. Alkaline humus (mull ) is typically found in grassland and deciduous forest: it supports an abundance of microorganisms and small animals (e.g. earthworms).

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Huxley, Sir Andrew

See Eccles, Sir John Carew.

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hyaline cartilage

See cartilage.

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hyaluronic acid

A glycosaminoglycan that is part of the ground substance of connective tissue. Hyaluronic acid binds cells together and helps to lubricate joints. It may play a role in the migration of cells at wounds; this activity ceases when hyaluronidase breaks down hyaluronic acid.

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hyaluronidase

The enzyme that breaks down hyaluronic acid, thereby decreasing its viscosity and increasing the permeability of connective tissue. It is used in medicine to increase the absorption and diffusion of drugs administered by injection or application.

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hybrid

The offspring of a mating in which the parents differ in at least one characteristic. The term is usually used of offspring of widely different parents, e.g. different varieties or species. Hybrids between different animal species are usually sterile, as is the mule (a cross between a horse and a donkey). See also hybrid vigour.

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hybrid dysgenesis

Infertility and an increased incidence of chromosome mutations that is observed in the progeny of laboratory-reared organisms crossed with wild organisms. Hybrid dysgenesis has been best studied in strains of Drosophila fruit flies, when laboratory females are crossed with wild males; it is thought to be caused by transposable elements (see transposon) in the wild-type males, which are activated in eggs from certain strains of laboratory females.

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hybridization

1 The production of one or more hybrid organisms by the mating of genetically different parents.

2 The production of hybrid cells. See cell fusion (somatic cell hybridization).
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3 See DNA hybridization.
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hybridoma

A type of hybrid cell that is produced by the fusion of a tumour cell (a myeloma cell) with a normal antibody-producing B cell (see cell fusion). The resulting hybrid cell line is able to produce large amounts of normal antibody, which is described as monoclonal (see monoclonal antibody) as it results from a cloned cell line.

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hybrid vigour (heterosis)

The increased vigour displayed by the offspring from a cross between genetically different parents. Hybrids from crosses between different crop varieties (F_1 hybrids) are often stronger and produce better yields than the original varieties. Mules, the offspring of mares crossed with donkeys, have greater strength and resistance to disease and a longer lifespan than either parent.

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hybrid zone

A zone that exists between the populations of two related species, whose geographical ranges overlap, which contain hybrids resulting from matings between the two species. The hybrids, which have a reduced fertility, will eventually act as a barrier between further interspecific mating and hence will restrict gene flow between the two species.

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hydathode

A pore found in the epidermis of the leaves of certain plants. Like stomata, hydathodes are surrounded by two crescent-shaped cells but these, unlike guard cells, do not regulate the size of the aperture. Hydathodes are used by the plant to secrete water under conditions in which transpiration is inhibited; for example, when the atmosphere is very humid. This process of water loss is called guttation .

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hydrochloric acid

A strong acid, HCl, that is produced by oxyntic cells in the wall of the stomach and forms a constituent of gastric juice. Hydrochloric acid is required for the conversion of pepsinogen to pepsin in the lumen of the stomach and kills various microorganisms that enter with food.

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hydrocortisone

See cortisol.

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hydrogen acceptor

See hydrogen carrier.

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hydrogen bond

A type of electrostatic interaction between electronegative (fluorine, nitrogen, or oxygen) atoms in one molecule and hydrogen atoms bound to electronegative atoms in another molecule. It is a strong dipole–dipole attraction caused by the electron-withdrawing properties of the electronegative atom. Thus, in the water molecule the oxygen atom attracts the electrons in the O–H bonds. The hydrogen atom has no inner shells of electrons to shield the nucleus, and there is an electrostatic interaction between the hydrogen proton and a lone pair of electrons on an oxygen atom in a neighbouring molecule. Each oxygen atom has two lone pairs and can make hydrogen bonds to two different hydrogen atoms. The strengths of hydrogen bonds are about one tenth of the strengths of normal covalent bonds. Hydrogen bonding does, however, have significant effects on physical properties. Thus it accounts for the unusual properties of water and for its relatively high boiling point. It is also of great importance in living organisms. Hydrogen bonding occurs between bases in the chains of DNA (see base pairing). It also occurs between the C=O and N–H groups in proteins, and is responsible for maintaining the secondary structure.

Hydrogen bonds (shown as dotted lines) between water molecules

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hydrogencarbonate (bicarbonate)

A salt of carbonic acid in which one hydrogen atom has been replaced; it thus contains the hydrogencarbonate ion HCO_3–. See also buffer.

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hydrogen carrier (hydrogen acceptor)

A molecule that accepts hydrogen atoms or ions, becoming reduced in the process (see oxidation–reduction). The electron transport system, whose function is to generate energy in the form of ATP during respiration, involves a series of hydrogen carriers, including NAD and FAD, which pass on the hydrogen (derived from the breakdown of glucose) to the next carrier in the chain.

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hydrolase

Any of a class of enzymes that catalyse the addition of water to, or the removal of water from, a molecule. Hydrolases play an important role in the construction and breakdown of storage materials, such as starch.

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hydrological cycle (water cycle)

The circulation of water between the atmosphere, land, and oceans on the earth. Water evaporates from the oceans and other water bodies on earth to form water vapour in the atmosphere. This may condense to form clouds and be returned to the earth's surface as precipitation (e.g. rainfall, hail, and snow). Some of this precipitation is returned to the atmosphere directly through evaporation or transpiration by plants; some flows off the land surface as overland flow, eventually to be returned to the oceans via rivers; and some infiltrates the ground to flow underground forming groundwater storage.

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hydrophilic

Having an affinity for water.

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hydrophily

A rare form of pollination in which pollen is carried to a flower by water. It occurs by one of two methods. In Canadian pondweed (Elodea canadensis) the male flowers break off and float downstream until they contact the female flowers. In Zostera, a marine species, the filamentous pollen grains are themselves carried in the water. Compare anemophily; entomophily.

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hydrophobic

Lacking affinity for water.

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hydrophyte

Any plant that lives either in very wet soil or completely or partially submerged in water. Structural modifications of hydrophytes include the reduction of mechanical and supporting tissues and vascular tissue, the absence or reduction of a root system, and specialized leaves that may be either floating or finely divided, with little or no cuticle. Examples of hydrophytes are waterlilies and certain pondweeds. Compare halophyte; mesophyte; xerophyte.

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hydroponics

A commercial technique for growing certain crop plants in culture solutions rather than in soil. The roots are immersed in an aerated solution containing the correct proportions of essential mineral salts. The technique is based on various water culture methods used in the laboratory to assess the effects of the absence of certain mineral elements on plant growth.

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hydrosere

The sequence of plant communities (see sere) occurring during the change from shallow open water to forest or bog. The process commences with accumulations of silt, enabling initial colonization by submerged or floating plants, such as water lilies and pondweeds, depending on the rate of flow and nutrient status of the water. As silting increases and organic debris is deposited, reeds, sedges, and similar plants begin to appear, forming a swamp . Organic matter builds up as peat, and conditions progressively become drier, creating a stage called a fen , dominated by herbaceous species, and then a carr , in which shrubs and small trees predominate. Eventually, the substrate is sufficiently stable to support the larger trees of mature forest species. In conditions of high rainfall or low evaporation a different hydrosere may occur. The rate of peat formation may be sufficiently high to create a bog , i.e. a permanently waterlogged raised site that receives all its water and nutrient inputs from rainfall or other precipitation.

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hydrosphere

The water on the surface of the earth. Some 74% of the earth's surface is covered with water, 97% (or some 10^21 kilograms) of which is in the oceans. Icecaps and glaciers contain about 3 × 10^19 kg, rivers about 10^15 kg, lakes and inland seas about 2 × 10^17 kg, and groundwater (down to 4000 metres) about 8 × 10^19 kg. Water in the atmosphere contains only about 10^16 kg.

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hydrostatic skeleton

The system of support found in soft-bodied invertebrates, which relies on the incompressibility of fluids contained within the body cavity. For example, in earthworms the coelomic fluid is under pressure within the coelom and therefore provides support for internal organs.

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hydrotropism

The growth of a plant part in response to water. Roots, for example, grow towards water in the soil. See tropism.

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hydroxonium ion

See acid.

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hydroxyl

Denoting the group –OH in a chemical compound or the ion OH–.

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5-hydroxytryptamine

See serotonin.

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Hydrozoa

See Cnidaria.

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hygroscopic

Describing a substance that can take up water from the atmosphere.

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hymen

A fold of mucous membrane that covers the opening of the vagina at birth. It normally perforates at puberty, to allow the flow of menstrual blood, but if the opening is small it may be ruptured during the first occasion of sexual intercourse.

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Hymenoptera

An order of insects that includes the ants, bees, wasps, ichneumon flies, and sawflies. Hymenopterans generally have a narrow waist between thorax and abdomen. The smaller hindwings are interlocked with the larger forewings by a row of tiny hooks on the leading edges of the hindwings. Some species are wingless. The mouthparts are typically adapted for biting, although some advanced forms (e.g. bees) possess a tubelike proboscis for sucking liquid food, such as nectar. The long slender ovipositor can serve for sawing, piercing, or stinging. Metamorphosis occurs via a pupal stage to the adult form. Parthenogenesis is common in the group.
Ants and some bees and wasps live in colonies, often comprising numerous individuals divided into castes and organized into a coordinated and complex society. The colony of the honeybee (Apis mellifera), for example, consists of workers (sterile females), drones (fertile males), and usually a single fertile female – the queen. The sole concern of the queen is egg laying. She determines the gender of the egg by either withholding or releasing stored sperm. Unfertilized eggs become males; fertilized eggs become females. The workers fulfil a variety of tasks, including nursing the developing larvae, building the wax cells (combs) of the hive, guarding the colony, and foraging for nectar and pollen. The single function of the larger drones is to mate with the young queen on her nuptial flight.

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hyoid arch

The second of seven bony V-shaped arches that support the gills of fish. The dorsal part is specialized as the hyomandibular , which is involved in jaw suspension (see amphistylic jaw suspension; hyostylic jaw suspension). In tetrapods the dorsal part of the hyoid arch has evolved to form the stapes (one of the ear ossicles). The ventral part forms the hyoid bone , which supports the tongue.

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hyomandibular

See hyoid arch.

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hyostylic jaw suspension

A type of jaw suspension seen in most fishes, in which the upper jaw is not directly connected to the cranium. The attachment between cranium and upper jaw is made by a ligament at the front end and by the hyomandibular (see hyoid arch) at the rear end (see illustration). Compare amphistylic jaw suspension; autostylic jaw suspension.

Hyostylic jaw suspension

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hyper-

A prefix denoting over, above, high; e.g. hyperpolarization.

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hyperglycaemia

Excessive amounts of glucose in the bloodstream, which occurs in diabetes mellitus due to underproduction of insulin. Compare hypoglycaemia.

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hypermetamorphosis

See heteromorphosis.

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hypermetropia (hyperopia)

Long-sightedness. A vision defect in which the lens of the eye is unable to accommodate sufficiently to throw the image of near objects onto the retina. It is caused usually by shortness of the eyeball rather than any fault in the lens system. The subject requires spectacles with converging lenses to bring the image from behind the retina back on to its surface (see illustration).

Hypermetropia

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hypermorphosis

A form of heterochrony in which, during the course of evolution, the rate of development is unchanged but the total development time is extended, permitting the addition of new stages to the end of the ancestral development sequence. The morphological outcome is an example of peramorphosis. Hence the development (ontogeny) of such organisms conforms to the theory of recapitulation of their phylogeny.

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hyperparasite

A parasite that lives in or on another parasite. The most common examples are insects that lay their eggs inside or near parasitoid larvae, which are themselves parasitizing the tissues of a host, again usually an insect larva. Hyperparasites, more strictly termed hyperparasitoids , are found mainly among the Hymenoptera (wasps, sawflies, etc.), although some flies and beetles also adopt this strategy.

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hyperplasia

Increase in the size of a tissue or organ due to an increase in the number of its component cells. Compare hypertrophy.

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hyperpolarization

See inhibitory postsynaptic potential.

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hypersensitivity

1 (in immunology) Increased or abnormal sensitivity to compounds, which can elicit a specific immune response accompanied by tissue damage. Hypersensitive reactions include allergies and anaphylaxis. See also hapten.

2 (in botany) A reaction by the tissues of a plant in response to invasion by pathogenic viruses, bacteria, fungi, or other organisms. The reaction varies according to the type of infecting agent, but typically involves the synthesis of enzymes (such as chitinase) that disarm or degrade the pathogen, the production of phytoalexins, which inhibit the growth of pathogens, and strengthening of cell walls in the vicinity of the attack by deposition of lignin and other materials. The final strategy is programmed cell death (apoptosis) around the infection site, which slows the spread of the pathogen to neighbouring healthy tissues. See also elicitor; systemic acquired resistance.
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hypertension

See blood pressure.

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hyperthyroidism

Overactivity of the thyroid gland. This condition causes an increase in basal metabolic rate, with such symptoms as rapid pulse and weight loss. It is sometimes associated with a goitre. Compare hypothyroidism.

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hypertonic solution

A solution that has a lower water potential and a correspondingly higher osmotic pressure than another solution. See osmosis.

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hypertrophy

An increase in the size of a tissue or organ due to an increase in the size of its component cells. Hypertrophy often occurs in response to an increased workload in an organ, which may result from malfunction or disease. Compare hyperplasia.

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hyperventilation

An increase in the amount of air taken into the lungs caused by an increase in the depth or rate of breathing. See also ventilation.

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hypha

(pl. hyphae) A delicate filament in fungi many of which may form either a loose network (mycelium) or a tightly packed interwoven mass of pseudoparenchyma, as in the fruiting body of mushrooms. Hyphae may be branched or unbranched and may or may not possess cross walls. The cell wall consists of either fungal cellulose or a nitrogenous compound called chitin. The cell wall is lined with cytoplasm, which often contains oil globules and glycogen, and there is a central vacuole. The hyphae produce enzymes that in parasitic fungi digest the host tissue, and in saprotrophic fungi digest dead organic matter.

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hypo-

A prefix denoting under, below, low; e.g. hypogyny, hyponasty.

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hypocotyl

The region of the stem beneath the stalks of the seed leaves (cotyledons) and directly above the young root of an embryo plant. It grows rapidly in seedlings showing epigeal germination and lifts the cotyledons above the soil surface. In this region (the transition zone ) the arrangement of vascular bundles in the root changes to that of the stem. Compare epicotyl.

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hypodermis (exodermis)

The outermost layer of cells in the plant cortex, lying immediately below the epidermis. These cells are sometimes modified to give additional structural support or to store food materials or water. After the loss of the piliferous layer of the root the hypodermis takes over the protective functions of the epidermis.

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hypogeal

1 Describing seed germination in which the seed leaves (cotyledons) remain below ground. Examples of hypogeal germination are seen in oak and runner bean. Compare epigeal.

2 Describing fruiting bodies that develop underground, such as truffles and peanuts.
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hypoglycaemia

Below-normal levels of glucose in the blood plasma, causing weakness, dizziness, and sweating. Compare hyperglycaemia.

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hypogyny

A floral arrangement in which the ovary is superior, i.e. it arises from the receptacle above the sepals, petals, and stamens (see illustration). The perianth and stamens are said to be hypogynous with respect to the ovary, as seen in the tulip. Compare epigyny; perigyny.

Hypogyny

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hypolimnion

The lower layer of water in a lake. See thermocline. Compare epilimnion.

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hypophysis

See pituitary gland.

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hypothalamus

Part of the vertebrate brain that is derived from the forebrain and located on the ventral surface below the thalamus and the cerebrum. The hypothalamus regulates a wide variety of physiological processes, including maintenance of body temperature, water balance, sleeping, and feeding, via both the autonomic nervous system (which it controls) and the neuroendocrine system. Its endocrine functions are largely mediated by the pituitary gland. The pituitary responds to releasing hormones produced by the hypothalamus, which thereby indirectly controls hormone production in other glands.

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hypothyroidism

Underactivity of the thyroid gland, which is generally due to a deficiency in thyroid-stimulating hormone. In adults hypothyroidism causes a decrease in basal metabolic rate, with tissue swelling and weight gain (myxoedema ). In children thyroid hormone is required for development of the bones and nervous system, and hypothyroidism can result in cretinism. Compare hyperthyroidism.

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hypotonic solution

A solution that has a higher water potential and a correspondingly lower osmotic pressure than another solution. See osmosis.

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hypoxia

A deficiency of oxygen in body tissues, which can result from living in an oxygen-deficient environment, inadequate inspiration, or deficiency of red blood cells or haemoglobin (required for oxygen transport in the body).

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H zone

The region of a striated muscle fibre that contains only thick (myosin) filaments. The H zone appears as a lighter band in the middle of the dark A band at the centre of a sarcomere.

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I

IAA (indoleacetic acid)

See auxin.

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I band

The region of a striated muscle fibre that contains only thin (actin) filaments. I bands are visible as light bands at either end of a sarcomere.

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ice age

A period in the earth's history during which ice advanced towards the equator and a general lowering of temperatures occurred. The last major ice age, that of the Pleistocene period (sometimes known as the Ice Age ), ended about 10000 years ago. At least four major ice advances (glacials) occurred during the Pleistocene period; these were separated by interglacials during which the ice retreated and temperatures rose. At present it is not known if the earth is between ice ages or is in an interglacial of the Pleistocene Ice Age. It has been established that ice ages also occurred during the Precambrian (over 500 million years ago) and during the Permo-Carboniferous (about 250 million years ago).

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ice-nucleating agent

Any agent that promotes the formation of ice crystals. Such agents include minute solid particles, such as dust or food particles, and large molecules. Certain organisms living in cold climates, such as some insects, molluscs, and nematodes, may manufacture ice-nucleating proteins in order to achieve a slow controlled freezing of their extracellular body water when the temperature falls below freezing. The structure of these proteins encourages the binding of water molecules and prevents sudden and damaging ice formation. However, gut particles can also act as ice-nucleating agents, and hence organisms that cannot tolerate freezing must void their guts at the onset of freezing conditions. See also cryoprotectant.

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ICSH

See luteinizing hormone.

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identical twins (monozygotic twins)

Two individuals that develop from a single fertilized egg cell by its division into two genetically identical parts. Each part eventually gives rise to a separate individual and these twins are therefore identical in every respect. Compare fraternal twins.

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identification key

See key.

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idiogram

See karyogram.

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idiosyncrasy

An abnormal reaction to a drug or other foreign substance shown by an individual, which is usually genetically determined. An individual that shows immunological idiosyncrasy is said to be hypersensitive to a particular substance, agent, etc.

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ileum

The portion of the mammalian small intestine that follows the jejunum and precedes the large intestine. It is a site of digestion and absorption. The internal lining of the ileum bears numerous small outgrowths (see villus), which increase its absorptive surface area.

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ilium

The largest of the three bones that make up each half of the pelvic girdle. The ilium bears a flattened wing of bone that is attached by ligaments to the sacrum (see sacral vertebrae). See also ischium; pubis.

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imaginal disc

A group of undifferentiated cells in an insect larva that develops into a specific adult structure. Imaginal discs are derived from the blastoderm. They do not undergo further development until the pupal stage, when they start to differentiate into adult epidermal structures, such as eyes, antennae, and wings, under the control of the hormone ecdysone.

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imago

The adult sexually mature stage in the life cycle of an insect after metamorphosis.

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imbibition

The uptake of water by substances that do not dissolve in water, so that the process results in swelling of the substance. Imbibition is a property of many biological substances, including cellulose (and other constituents of plant cell walls), starch, and some proteins. It occurs in dry seeds before they germinate and – together with osmosis – is responsible for the uptake of water by growing plant cells.

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immersion objective

An optical microscope objective in which the front surface of the lens is immersed in a liquid on the cover slip of the microscope specimen slide. Cedar-wood oil (for an oil-immersion lens ) or sugar solution is frequently used. It has the same refractive index as the glass of the cover slip, so that the object is effectively immersed in it. The presence of the liquid increases the effective aperture of the objective, thus increasing the resolution.

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immune response

The reaction of the body to foreign or potentially dangerous substances (antigens), particularly disease-producing microorganisms. The response involves the production by specialized white blood cells (B cells) of proteins known as antibodies, which react with the antigens to render them harmless (see immunoglobulin). The antibody–antigen reaction is highly specific. See also anaphylaxis; immunity.

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immunity

The state of relative insusceptibility of an animal to infection by disease-producing organisms or to the harmful effects of their poisons (toxins). Immunity depends on the presence in the blood of antibodies and white blood cells (lymphocytes), which produce an immune response. Inherited (natural or innate) immunity is that with which an individual is born. Acquired immunity is of two types, active and passive. Active immunity arises when the body produces antibodies against an invading foreign substance (antigen), either through infection or immunization; this type of immunity may be humoral , in which B lymphocytes produce free antibodies that circulate in the bloodstream (see B cell), or cell-mediated , caused by the action of T lymphocytes (see T cell). Passive immunity is induced by injection of serum taken from an individual already immune to a particular antigen; it can also be acquired by the transfer of maternal antibodies to offspring via the placenta or breast milk (see colostrum). Active immunity tends to be long-lasting; passive immunity is short-lived. See also autoimmunity.

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immunization

The production of immunity in an individual by artificial means. Active immunization (vaccination ) involves the introduction, either orally or by injection (inoculation ), of specially treated bacteria, viruses, or their toxins to stimulate the production of antibodies (see vaccine). Passive immunization is induced by the injection of preformed antibodies.

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immunoassay

Any of various techniques that measure the amount of a particular substance by virtue of its binding antigenically to a specific antibody. In solid-phase immunoassay the specific antibody is attached to a solid supporting medium, such as a PVC sheet. The sample is added and any test antigens will bind to the antibody. A second antibody, specific for a different site on the antigen, is added. This carries a radioactive or fluorescent label, enabling its concentration, and thus that of the test antigen, to be determined by comparison with known standards. Variations on this technique include ELISA and Western blotting.

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immunoelectrophoresis

An analytical technique used to identify antigens that have been separated by electrophoresis. The gel medium used for the electrophoresis is irrigated with specific antibodies for the required antigens: when an antigen meets its corresponding antibody a precipitate forms, which can be seen and isolated.

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immunofluorescence

A technique used to identify specific proteins in tissue samples by applying antibodies that have been labelled with a fluorescent compound. The labelled antibody combines with the protein under investigation and the resulting complex can be identified under a microscope by its fluorescence. Proteins and enzymes in tissue samples can be identified and located in this way.

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immunogenicity

The degree to which an antigen can produce an immune response.

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immunoglobulin

One of a group of proteins (globulins) in the body that act as antibodies. They are produced by specialized white blood cells called B cells and are present in blood serum and other body fluids. An antibody is typically a Y-shaped structure consisting of four polypeptide chains – two heavy chains and two light chains. Each arm of the `Y' bears an antigen-binding site. There are five classes, with distinct immunological and physical properties. The main immunoglobulin of blood, lymph, and tissue fluid is immunoglobulin G (IgG). It binds to microorganisms, promoting their engulfment by phagocytes (macrophages), and to viruses and bacterial toxins, thereby neutralizing them. It also binds complement, which results in lysis of the target cell, and can cross the placenta to protect the fetus. Immunoglobulin M (IgM) is the first antibody to be produced following immunization or infection. Found in blood and lymph, it is an aggregate of five of the basic Y-shaped units, enabling it to `mop up' microorganisms or other antigens with its ten binding sites. It can also activate complement. Immunoglobulin A (IgA) is found in saliva, tears, breast milk, and mucous secretions, where its role is to neutralize viruses and bacteria as they enter the body. It also occurs in serum. The role of immunoglobulin D (IgD) is uncertain. It is present in serum in very low concentrations, but occurs on the surface of antibody-secreting B cells, whose activity it may regulate. Immunoglobulin E (IgE) also normally has very low concentrations in blood and connective tissues, but it plays a crucial role in allergic reactions. It binds to mast cells, and when triggered by the presence of antigen causes histamine release and hence inflammation and other common allergic symptoms. See also immunity.

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immunological tolerance

See tolerance.

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immunosuppression

The suppression of an immune response. Immunosuppression is necessary following organ transplants in order to prevent the host rejecting the grafted organ (see graft); it is artificially induced by radiation or chemical agents that inhibit cell division of lymphocytes. Immunosuppression occurs naturally in certain diseases, notably AIDS.

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imperfect fungi

See Deuteromycota.

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Imperial units

The British system of units based on the pound and the yard. The former f.p.s. system was used in engineering and was loosely based on Imperial units; for all scientific purposes SI units are now used. Imperial units are also being replaced for general purposes by metric units.

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implant

Any substance, device, or tissue that is inserted into the body. For example, drug implants and heart pacemakers are typically inserted under the skin.

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implantation (nidation)

(in embryology) The embedding of a fertilized mammalian egg into the wall of the uterus (womb) where it will continue its development. After fertilization in the fallopian tube the egg passes into the womb in the form of a ball of cells (blastocyst ). Its outer cells destroy cells of the uterine wall, forming a cavity into which the blastocyst sinks.

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imprinting

1 (in behaviour) A specialized form of learning in which young animals, during a critical period in their early development, learn to recognize and approach some large moving object nearby. In nature this is usually the mother, though simple models or individuals of a different species (including man) may suffice. Imprinting was first described by Konrad Lorenz, working with young ducks and geese. See learning.

2 (in genetics) See molecular imprinting.
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impulse (nerve impulse)

The signal that travels along the length of a nerve fibre and is the means by which information is transmitted through the nervous system. It is marked by the flow of ions across the membrane of the axon caused by changes in the permeability of the membrane, producing a reduction in potential difference that can be detected as the action potential. The strength of the impulse produced in any nerve fibre is constant (see all-or-none response).

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inbreeding

Mating between closely related individuals, the extreme condition being self-fertilization, which occurs in many plants and some primitive animals. A population of inbreeding individuals generally shows less variation than an outbreeding population. Continued inbreeding among a normally outbreeding population leads to inbreeding depression (the opposite of hybrid vigour) and an increased incidence of harmful characteristics. For example, in humans, certain mental and other defects tend to occur more often in families with a history of cousin marriages.

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incisor

A sharp flattened chisel-shaped tooth in mammals that is adapted for biting food and – in rodents – for gnawing. In humans there are normally two pairs of incisors (central and lateral) in each jaw. See permanent teeth.

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inclusion body

Any of various particulate structures formed in plant and animal cells due to viral infection. Inclusion bodies sometimes become embedded in cellular proteins.

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inclusive fitness

The quality that organisms attempt (unconsciously) to maximize as the result of natural selection acting on genes that are influential in controlling their behaviour and physiology. It includes the individual's own reproductive success (usually taken as the number of its offspring that survive to adulthood) and also the effects of the individual's actions on the reproductive success of its relatives, because relatives have a higher probability of sharing some identical genes with the individual than do other members of the population. When interactions between relatives are likely to occur (which happens during the lives of many animals and plants) kin selection will operate.

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incompatibility

1 The condition that exists when foreign grafts or blood transfusions evoke a marked immune response and are rejected.

2 The phenomenon in which pollen from one flower fails to fertilize other flowers on the same plant (self-incompatibility ) or on other genetically similar plants. This genetically determined mechanism prevents self-fertilization (breeding between likes) and promotes cross-fertilization (breeding between individuals with different genetic compositions). See also fertilization; pollination.
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incomplete dominance

The condition that arises when neither allele controlling a characteristic is dominant and the aspect displayed by the organism results from the partial influence of both alleles. For example, a snapdragon plant with alleles for red and for white flowers produces pink flowers. Compare co-dominance.

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incubation

1 The process of maintaining the fertilized eggs of birds and of some reptiles and egg-laying mammals at the optimum temperature for the successful development of the embryos. A period of incubation follows the laying of the eggs and precedes their hatching.

2 The process of maintaining a culture of bacteria or other microorganisms at the optimum temperature for growth of the culture.
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3 The phase in the development of an infectious disease between initial infection and the appearance of the first symptoms.
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incus (anvil)

The middle of the three ear ossicles of the mammalian middle ear.

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indefinite inflorescence

See racemose inflorescence.

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indehiscent

Describing a fruit or fruiting body that does not open to release its seeds or spores when ripe. Instead, release occurs when the fruit wall decays or, if eaten by an animal, is digested. Compare dehiscence.

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independent assortment

The separation of the alleles of one gene into the reproductive cells (gametes) independently of the way in which the alleles of other genes have segregated. By this process all possible combinations of alleles should occur equally frequently in the gametes. In practice this does not happen because alleles situated on the same chromosome tend to be inherited together. However, if the allele pairs Aa and Bb are on different chromosomes, the combinations AB, Ab, aB, and ab will normally be equally likely to occur in the gametes. See meiosis; Mendel's laws.

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index fossil (zone fossil)

An animal fossil of a group that existed continuously during a particular span of geological time and can therefore be used to date the rock in which it is found. Index fossils are found chiefly in sedimentary rocks. They are an essential tool in stratigraphy for comparing the geological ages of sedimentary rock formations. Examples are ammonites and graptolites.

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indicator species

A plant or animal species that is very sensitive to a particular environmental factor, so that its presence (or absence) in an area can provide information about the levels of that factor. For example, some lichens are very sensitive to the concentration of sulphur dioxide (a major pollutant) in the atmosphere. Examination of the lichens present in an area can provide a good indication of the prevailing levels of sulphur dioxide.

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indigenous

Describing a species that occurs naturally in a certain area, as distinct from one introduced by man; native.

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indoleacetic acid (IAA)

See auxin.

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induced-fit model

A proposed mechanism of interaction between an enzyme and a substrate. It postulates that exposure of an enzyme to a substrate causes the active site of the enzyme to change shape in order to allow the enzyme and substrate to bind (see enzyme–substrate complex). This hypothesis is generally preferred to the lock-and-key mechanism.

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inducer

See operon.

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induction

1 (in embryology) The ability of natural stimuli to cause unspecialized embryonic tissue to develop into specialized tissue.

2 (in obstetrics) The initiation of childbirth by artificial means; for example, by injection of the hormone oxytocin.
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indusium

The kidney-shaped covering of the sorus of certain ferns that protects the developing sporangia. It withers when the sorus ripens to expose the sporangia.

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industrial melanism

The increase of melanic (dark) forms of an animal in areas darkened by industrial pollution. The example most often quoted is that of the peppered moth (Biston betularia), melanic forms of which markedly increased in the industrial north of England during the 19th century. Experiments have shown that the dark forms increase in polluted regions because they are less easily seen by birds against a dark background; conversely the paler forms survive better in unpolluted areas. See also directional selection.

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infection

The invasion of any living organism by disease-causing microorganisms (see pathogen), which proceed to establish themselves, multiply, and produce various symptoms in their host. Pathogens may invade via a wound or (in animals) through the mucous membranes lining the alimentary, respiratory, and reproductive tracts, and may be transmitted by an infected individual, a carrier, or an arthropod vector. Symptoms in animals appear after an initial symptomless incubation period and typically consist of localized inflammation, often with pain and fever. Infections are combated by the body's natural defences (see immune response). Treatment with drugs (see antibiotics; antiseptic) is effective against most bacterial, fungal, and protozoan infections; some viral infections respond to antiviral drugs. See also immunization.

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inferior

Describing a structure that is positioned below or lower than another structure in the body. For example, in flowering plants the ovary is described as inferior when it is located below the other organs of the flower (see epigyny). Compare superior.

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inflammation

The defence reaction of tissue to injury, infection, or irritation by chemicals or physical agents. Cells in the affected tissue release a chemical (histamine) that causes localized dilatation of blood vessels so that fluid leaks out and blood flow is increased. This leads to swelling, redness, heat, and often pain. Other chemical mediators of inflammation include kinins and prostaglandins. White blood cells, particularly phagocytes, enter the tissue and an immune response is stimulated. A gradual healing process usually follows.

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inflorescence

A particular arrangement of flowers on a single main stalk of a plant. There are many different types of inflorescence, which are classified into two main groups depending on whether the tip of the flower axis goes on producing new flower buds during growth (see racemose inflorescence) or loses this ability (see cymose inflorescence).

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infraspecific

Occurring within a species. For example, infraspecific variation is the variation occurring between individuals of the same species. Compare interspecific; intraspecific.

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ingestion (feeding)

A method of heterotrophic nutrition in which bulk food is taken into an organism and subsequently digested (see digestion). Ingestion is the principal mechanism of animal nutrition. See also macrophagous; microphagous.

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inguinal

Of, relating to, or situated in the groin. For example, the inguinal canal , which contains the spermatic cord, lies in the abdominal cavity of a male fetus.

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inhalation

See inspiration.

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inheritance

The transmission of particular characteristics from generation to generation by means of the genetic code, which is transferred to offspring in the gametes. See also Mendel's laws.

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inhibition

1 (in biochemistry) (enzyme inhibition) A reduction in the rate of an enzyme-catalysed reaction by substances called inhibitors . Competitive inhibition occurs when the inhibitor molecules resemble the substrate molecules and bind to the active site of the enzyme, so preventing normal enzymatic activity. Competitive inhibition can be reversed by increasing the concentration of the substrate. In noncompetitive inhibition the inhibitor binds to a part of the enzyme or enzyme–substrate complex other than the active site, known as an allosteric site . This deforms the active site so that the enzyme cannot catalyse the reaction. Noncompetitive inhibition cannot be reversed by increasing the concentration of the substrate. The toxic effects of many substances are produced in this way. Inhibition by reaction products (feedback inhibition ) is important in the control of enzyme activity. See also allosteric enzyme.

2 (in physiology) The prevention or reduction of the activity of effectors (such as muscles) by means of certain nerve impulses. Inhibitory activity often provides a balance to stimulation of a process; for example, the impulse to stimulate contraction of a voluntary muscle may be accompanied by an inhibitory impulse to prevent contraction of its antagonist.
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inhibitory postsynaptic potential (IPSP)

The electric potential that is generated in a postsynaptic neurone when an inhibitory neurotransmitter (such as gamma-aminobutyric acid) is released into the synapse and causes a slight increase in the potential difference across the postsynaptic membrane (hyperpolarization ). This makes the neurone less likely to transmit an impulse. Compare excitatory postsynaptic potential.

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initial

One of a group of cells (or, in lower plants, a single cell) that divides to produce the cells of a plant tissue or organ. The cells of the apical meristem, cambium, and cork cambium are initials.

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initiation codon

See start codon.

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initiation factor (IF)

Any of a group of proteins that are required for initiating the translation stage of protein synthesis. Each ribosome is assembled on the messenger RNA (mRNA) chain in two subunits, one smaller than the other. IFs catalyse the binding of an initiator tRNA molecule, which recognizes the start codon of the mRNA, to the smaller subunit of the ribosome. This subunit then binds to the mRNA and the IFs dissociate, allowing the larger ribosomal subunit to bind to the smaller one. IFs also control the rate of translation in some cells.

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initiation site

See promoter.

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innate behaviour

An inherited pattern of behaviour that appears in a similar form in all normally reared individuals of the same sex and species. See instinct.

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innate immunity

See immunity.

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inner ear

The structure in vertebrates, surrounded by the temporal bone of the skull, that contains the organs of balance and hearing. It consists of soft hollow sensory structures (the membranous labyrinth ), containing fluid (endolymph ), surrounded by fluid (perilymph ), and encased in a bony cavity (the bony labyrinth ). It consists of two chambers, the sacculus and utriculus, from which arise the cochlea and semicircular canals respectively.

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innervation

The supply of nerve fibres to and from an organ.

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innominate artery

A short artery that branches from the aorta to divide into the subclavian artery (the main artery to the arm) and the right carotid artery (which supplies blood to the head).

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innominate bone

One of the two bones that form each half of the pelvic girdle in adult vertebrates. This bone is formed by the fusion of the ilium, ischium, and pubis.

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inoculation

1 See vaccine.

2 The placing of a small sample of microorganisms or any other type of cell into a culture medium so that the cells can grow and proliferate.
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inoculum

A small amount of material containing bacteria, viruses, or other microorganisms that is used to start a culture.

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inositol

A cyclic alcohol, C_6H_12O_6, that is a constituent of certain cell phosphoglycerides; the most important isomer is myo-inositol (see formula). It is sometimes classified as a member of the vitamin B complex but it can be synthesized by many animals and it is not regarded as an essential nutrient in humans. Phosphatidyl inositol , a constituent of cell membranes, is a precursor of the intracellular second messenger molecules, inositol 1,4,5-trisphosphate (IP_3) and diacylglycerol ; these are produced in response to the binding of a substance, e.g. serotonin, to receptors on the cell surface. They mediate such cellular events as smooth muscle contraction, adrenaline secretion, and histamine secretion.

Myo-inositol

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inquilinism

An association between members of two different species in which one, the inquiline , lives on or in the other (the host ), or inside the host's home, obtaining shelter and in some instances taking some of the host's food. For example, certain mosquitoes live and breed in the fluid contained in the pitchers of pitcher plants, benefiting from the protection afforded by the pitcher and also making use of nutrients from prey trapped by the plant. The nest of many social insects harbour inquilines, which have evolved ingenious strategies to gain food or avoid being attacked, as evidenced by the staphylinid beetle Atemeles pubicollis. As a larva this lives in ant colonies, adopting the `begging' posture of ant larvae in order to receive food from adult ants.

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Insecta

See Hexapoda.

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insect growth regulator

Any of various substances that interfere with the normal development or growth of insects, some of which are used to control insect pests. The two main categories are distinguished by their modes of action. Juvenile hormone mimics (juvenoids ) disrupt the hormonal control of larval development so that metamorphosis either fails to occur or any adults that do emerge are weak and sterile. Examples include fenoxycarb, methoprene, and pyriproxyfen. In contrast, the chitin synthesis inhibitors , such as diflubenzuron and triflumuron, prevent chitin formation and hence normal replacement of the old cuticle following a moult. Treated insects either fail to moult, or have soft weak cuticles that cannot protect them, and die at or soon after ecdysis. Both types of growth regulator have low toxicity for vertebrates, but can have devastating effects on populations of beneficial insects, such as pollinators and pest predators, when used in the field. Their most common applications are to control insects in food stores and in houses.

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insecticide

See pesticide.

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Insectivora

An order of small, mainly nocturnal, mammals that includes the hedgehogs, moles, and shrews. They have long snouts covered with stiff tactile hairs and their teeth are specialized for seizing and crushing insects and other small prey. The insectivores have changed very little since they evolved in the Cretaceous period, 130 million years ago.

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insectivore

An animal that eats insects, especially a mammal of the order Insectivora (hedgehogs, shrews, etc.).

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insectivorous plant

See carnivorous plant.

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insertion

1 (of muscles) See voluntary muscle.

2 (in genetics) A point mutation in which an extra nucleotide base is added to the DNA sequence. This results in the misreading of the base sequence during the translation stage of protein synthesis.
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insertion sequence

See transposon.

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insight learning

A form of learning in which an animal responds to new situations by adapting experiences gained in other contexts. Insight learning requires an animal to solve problems by viewing a situation as a whole instead of relying wholly on trial-and-error learning. Chimpanzees are capable of insight learning. See learning.

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inspiration (inhalation)

The process by which gas is drawn into the lungs through the trachea (see respiratory movement). In mammals the rib cage is raised by contraction of the external intercostal muscles and the muscles of the diaphragm. These actions enlarge the thorax, so that pressure in the lung cavity is reduced below atmospheric pressure, which causes an influx of air until the pressures are equalized. Compare expiration.

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inspiratory centre

The most important subcentre of the ventilation centre in the brain, which controls the regularity of breathing. The inspiratory centre is situated in the medulla oblongata and is composed of a group of neurones (the dorsal respiratory group ). These neurones receive information from stretch receptors in the bronchial tubes and chemoreceptors in the carotid bodies and generate impulses that pass down the phrenic nerve to the diaphragm, causing it to contract (see inspiration).

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instar

A stage in the larval development of an insect between two moults (ecdyses). There are usually a number of larval instars before metamorphosis. See exopterygote; heteromorphosis.

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instinct

An innate tendency to behave in a particular way, which does not depend critically on particular learning experiences for its development and therefore is seen in a similar form in all normally reared individuals of the same sex and species. Much instinctive behaviour takes the form of fixed action patterns . These are movements that – once started – are performed in a stereotyped way unaffected by external stimuli. For example, a frog's prey-catching tongue flick is performed in the same way whether or not anything is caught. Some complex instinctive behaviour, however, requires some learning by the animal before it is perfected. Birdsong, for example, consists of an innate component that is modified and made more complex by the influence of other birds, the habitat, etc.

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insulin

A protein hormone, secreted by the ß (or B) cells of the islets of Langerhans in the pancreas, that promotes the uptake of glucose by body cells, particularly in the liver and muscles, and thereby controls its concentration in the blood. Insulin was the first protein whose amino-acid sequence was fully determined (in 1955). Underproduction of insulin results in the accumulation of large amounts of glucose in the blood (hyperglycaemia ) and its subsequent excretion in the urine in abnormally high concentrations (glycosuria ). This condition, known as diabetes mellitus , can be treated successfully by insulin injections.

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integrated pest management (IPM)

An approach to controlling insects and other crop pests that combines various physical, chemical, and biological methods in an attempt to reduce reliance on chemical pesticides, and hence minimize pollution and harmful residues in the product. It also helps to counter the threat posed by increasing resistance of many insect pests to conventional insecticides. IPM is now used on certain field crops worldwide, including cotton, rice, alfalfa, and citrus fruits, and with many greenhouse crops. To be effective, IPM requires thorough knowledge of the physiology, ecology and life cycles of not only the pest but also of its natural predators. This enables selection of the most appropriate strategy, drawing from a wide array of control techniques. These include: biological control, which encourages the pest's natural enemies and may involve the introduction of novel predators in certain situations; improving the resistance of crops to pests, either by conventional plant breeding or by genetic engineering (see genetically modified organisms); the use of agricultural practices that lessen the degree of pest damage, such as mixed cropping or timeliness of planting; and the selective use of insecticides or other chemical agents, such as insect growth regulators, so that these have the maximum controlling effect with minimum impact on beneficial insects and the environment.

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integration

(in neurophysiology) The coordination within the brain of separate but related nervous processes. For example, sensory information from the inner ear and the eye are both necessary for the sense of balance. These stimuli must be integrated by the brain not only with each other but also with various motor nerves, which coordinate the muscles that control posture.

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integument

1 The outermost body layer of an animal, characteristically comprising a layer of living cells – the epidermis – together with a superficial protective coat, which may be a secreted hardened cuticle, as in arthropods, or dead keratinized cells, as in vertebrates (see skin).

2 The outer protective covering of a plant ovule. It is perforated by a small pore, the micropyle. Usually two integuments are present in angiosperms and one in gymnosperms. After fertilization the integuments form the testa of the seed.
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intelligence

The coordination of memory, learning, and reasoning in animals. Intelligence has also been defined as the ability of an animal to form associative links between events or objects of which it has had no previous experience (see insight learning). In humans intelligence is generally expressed as an intelligence quotient (IQ): the mental age of the subject (as measured by standard tests) divided by his or her real age × 100.

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intercalary

Occurring between differentiated tissues. For example, intercalary meristem , which is not part of the apical meristem, occurs in the internodes of grasses (between leaf nodes) and enables longitudinal growth of the stem.

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intercellular

Located or occurring between cells. Compare intracellular.

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intercostal muscles

The muscles located between the ribs, surrounding the lungs. Comprising the superficial external intercostal muscles and the deep internal intercostal muscles , they play an essential role in breathing (see expiration; inspiration).

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interfascicular cambium

See cambium.

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interferon (IFN)

Any of a number of proteins (see cytokine) that increase the resistance of a cell to attack by viruses by unmasking genes that synthesize antiviral proteins. In humans, three groups of interferons have been discovered: -interferons from white blood cells; ß-interferons from connective tissue fibroblasts; and -interferons from lymphocytes (see interleukin). Interferons are also produced by lymphocyte killer cells, which attack altered tissue cells, such as cancer cells. This converts other normal lymphocytes to killer cells and effects other changes in the immune system. Interferons produced using genetically engineered bacteria are used for treating some forms of hepatitis, some cancers, and multiple sclerosis.

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intergenic suppressor

A point mutation that negates or lessens the effects of another mutation in the same gene. For example, the effects of a primary frameshift mutation may be suppressed by a second mutation in which the deletion or insertion of a nucleotide occurs.

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interleukin

Any of several cytokines that act specifically as mediators between leucocytes. Interleukin-1 (IL-1) is secreted by antigen-activated macrophages; it induces activated helper T cells to secrete interleukin-2 (IL-2). IL-2 stimulates the production of other cytokines, including B-cell growth factor, B-cell differentiation factor, colony-stimulating factor, and -interferon. Interleukin-3 is involved in regulating mast-cell proliferation, and interleukin-4 induces B cells to proliferate and produce antibodies. At least nine other interleukins are now known to exist.

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intermediate filament

Any of numerous microscopic protein fibres that form part of the cytoskeleton of eukaryote cells. With a diameter of about 10 nm, they are intermediate in size compared to the narrower microfilaments and the wider microtubules – the other main cytoskeletal components. Intermediate filaments are relatively sturdy, forming a three-dimensional mesh within the cell that gives structural support to the nucleus and other cell organelles. Each filament consists of several twisted strands of protein subunits. Intermediate filaments differ in the nature of their protein subunits, often according to the tissue in which the cell is found. For example, those found in protective skin cells consist of keratin subunits, while those of muscle cells consist of desmin subunits.

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internal environment

The conditions that prevail within the body of an organism, particularly with respect to the composition of the tissue fluid. The concept of an internal environment was first proposed by the French physiologist Claude Bernard (1813–78), who stated that maintenance of a constant internal environment was necessary for the survival of an organism in a varying external environment. Selective absorption of materials across cell membranes plays a large part in controlling the internal environment of both animals and plants. Animals in addition can regulate their body fluids by the action of hormones and the nervous system. See homeostasis.

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interneurone

A relatively short neurone, which in vertebrates is confined to the grey matter of the central nervous system, that provides a link between a sensory neurone and a motor neurone in a polysynaptic reflex.

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internode

1 (in botany) The part of a plant stem between two nodes.

2 (in neurology) The myelinated region of a nerve fibre between two nodes of Ranvier. See myelin sheath.
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interoceptor

A receptor that detects stimuli from the internal environment of an organism. Chemoreceptors that detect changes in the levels of oxygen concentration in the blood are examples. Compare exteroceptor.

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interphase

The period following the completion of cell division, when the nucleus is not dividing. During this period changes in both the nucleus and the cytoplasm result in the complete development of the daughter cells. See cell cycle.

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intersex

An organism displaying characteristics that are intermediate between those of the typical male and typical female of its species. For example, a human intersex may have testes that fail to develop, so that although he is technically a man he has the external appearance of a woman. Intersexes may be produced in various ways; for example, by malfunctioning of the sex hormones. See also hermaphrodite. Compare gynandromorph.

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interspecific

Occurring between members of different species. The term is applied, for example, to some types of competition and communication. Compare infraspecific; intraspecific.

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interspecific competition

See competition.

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interstitial cell

A cell that forms part of the connective tissue (the interstitium ) between other tissues and structures, especially any of the cells of the testis that lie between the seminiferous tubules and secrete androgens in response to stimulation by interstitial-cell-stimulating hormone (see luteinizing hormone).

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interstitial-cell-stimulating hormone

See luteinizing hormone.

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interstitial fauna (meiofauna)

Small invertebrate animals that live between individual substrate particles, such as sand grains, in the bed of a sea, lake, or river. See also benthos.

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intervertebral disc

Any of the discs of cartilage that separate the bones of the vertebral column. The intervertebral discs allow the vertebral column a certain degree of flexibility and they also absorb shock.

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intestinal juice

A slightly alkaline liquid containing mucus that is secreted into the lumen of the small intestine from the cells that line the crypts of Lieberkühn. Together with pancreatic juice, the intestinal juice provides an alkaline environment that helps in the absorption of digested food molecules entering the small intestine in chyme from the stomach.

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intestine

The portion of the alimentary canal posterior to the stomach. Its major functions are the final digestion of food matter from the stomach, the absorption of soluble food matter, the absorption of water, and the production of faeces. See large intestine; small intestine.

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intine

See pollen.

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intracellular

(in biology) Located or occurring within cells. Compare intercellular.

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intrafusal

Describing a type of muscle fibre that occurs in the muscle spindles in voluntary muscles. Two to twelve intrafusal fibres enclosed in a capsule are found in each muscle spindle, lying parallel to the main (extrafusal ) fibres. Each intrafusal fibre consists of a noncontractile (equatorial ) region connected to a contractile (polar ) region at each end of the fibre. The equatorial region is connected to stretch receptors.

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intramembranous ossification

See ossification.

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intraspecific

Occurring between members of the same species. The term is applied, for example, to some types of competition and communication. Compare infraspecific; interspecific.

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intraspecific competition

See competition.

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intrinsic factor

See vitamin B complex.

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introgression (introgressive hybridization)

The insertion of the genes of one species into the gene pool of another. This can occur when two species interbreed to produce fertile hybrids. These can then back-cross with individuals of one of the parent species. See also hybrid zone.

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intron (intervening sequence)

A nucleotide sequence in a gene that does not code for the gene product (compare exon). Introns, which occur principally in eukaryotes, are transcribed into messenger RNA but are subsequently removed from the transcript before translation (see gene splicing). The removal of introns is currently regarded as an autocatalytic process – self-splicing – whereby the RNA itself has the properties of an enzyme (see ribozyme). However, messenger RNA produced in the nucleus requires the participation of a spliceosome , a regulatory complex of proteins and RNA. The function of introns is still subject to lively debate. They may simply be sequences of selfish DNA, able to move between different loci within the genome (see transposon) with no benefit to the host. On the other hand, introns may act as `spacers' for exons and facilitate exon shuffling – recombination or rearrangement of exons – which permits rapid evolution of proteins with novel permutations of functional groups. Introns have also been found in certain archaebacteria and blue-green bacteria and in some viruses.

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intussusception

The insertion of new cellulose fibres in the space between existing fibres in the wall of an elongating plant cell. This type of growth results in increasing the surface area of the cell wall. Compare apposition.

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inulin

A polysaccharide, made up from fructose molecules, that is stored as a food reserve in the roots or tubers of many plants, such as the dahlia.

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in utero

Describing any event occurring in the uterus of a mammal during pregnancy.

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inversion

(in genetics) 1 A chromosome mutation caused by reversal of part of a chromosome, so that the genes within that part are in inverse order. Inversion mutations usually occur during crossing over in meiosis.

2 A point mutation caused by the reversal of two or more bases in the DNA sequence within a gene.
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invertebrate

Any animal that lacks a vertebral column (backbone). Invertebrates include all nonchordate animals as well as the more primitive chordates (see Chordata).

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in vitro

Describing biological processes that are made to occur outside the living body, in laboratory apparatus (literally `in glass', i.e. in a test tube). In in vitro fertilization, mature egg cells are removed from the ovary of a woman unable to conceive normally and fertilized externally; the resultant blastocyst is implanted into her uterus. Compare in vivo.

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in vivo

Describing biological processes as they are observed to occur in their natural environment, i.e. within living organisms. Compare in vitro.

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involucre

A protective structure in some flowering plants and bryophytes. In flowering plants it consists of a ring of bracts arising beneath the flower cluster of those species with a capitulum (i.e. members of the dandelion family) or an umbel (i.e. members of the carrot family). In mosses and liverworts the involucre is a projection of tissue from the thallus that arches over the developing archegonium.

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involuntary

(in biology) Not under the control of the will of an individual. Involuntary responses by muscles, glands, etc., occur automatically when required; many such responses, such as gland secretion, heartbeat, and peristalsis, are controlled by the autonomic nervous system and effected by involuntary muscle.

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involuntary muscle (smooth muscle)

Muscle whose activity is not under the control of the will; it is supplied by the autonomic nervous system. Involuntary muscle comprises long spindle-shaped cells without striations. These cells occur singly, in groups, or as sheets in the skin, around hair follicles, and in the digestive tract, respiratory tract, urinogenital tract, and the circulatory system. The cells contract slowly in spontaneous rhythms or when stretched; they may show sustained contraction (tonus) for long periods without fatigue. Compare voluntary muscle.

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involution

1 A decrease in the size of an organ or the body. It may be associated with functional decline, as occurs in the ageing process, or follow enlargement, as when the uterus returns to its normal size after pregnancy.

2 The turning or rolling inwards of cells that occurs during the development of some vertebrate embryos.
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iodine

Symbol I. A dark violet nonmetallic element that is required as a trace element (see essential element) by living organisms; in animals it is concentrated in the thyroid gland as a constituent of thyroid hormones.

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iodopsin

See colour vision.

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ion

An atom or group of atoms that has either lost one or more electrons, making it positively charged (a cation), or gained one or more electrons, making it negatively charged (an anion).

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ion exchange

The exchange of ions of the same charge between a solution (usually aqueous) and a solid in contact with it. The process occurs widely in nature, especially in the absorption and retention of water-soluble fertilizers by soil. For example, if a potassium salt is dissolved in water and applied to soil, potassium ions are absorbed by the soil and sodium and calcium ions are released from it. The soil, in this case, is acting as an ion exchanger. Synthetic ion-exchange resins consist of various copolymers having a cross-linked three-dimensional structure to which ionic groups have been attached. An anionic resin has negative ions built into its structure and therefore exchanges positive ions. A cationic resin has positive ions built in and exchanges negative ions. Ion-exchange resins are used as the stationary phase in ion-exchange chromatography.

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ion-exchange chromatography

See chromatography.

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ionizing radiation

Radiation of sufficiently high energy to cause ionization in the medium through which it passes. It may consist of a stream of high-energy particles (e.g. electrons, protons, alpha-particles) or short-wavelength electromagnetic radiation (ultraviolet, X-rays, gamma-rays). This type of radiation can cause extensive damage to the molecular structure of a substance either as a result of the direct transfer of energy to its atoms or molecules or as a result of the secondary electrons released by ionization. In biological tissue the effect of ionizing radiation can be very serious, usually as a consequence of the ejection of an electron from a water molecule and the oxidizing or reducing effects of the resulting highly reactive species:
2H_2O e– + H_2O + H_2O+

H_2O .OH + .H

H_2O+ + H_2O .OH + H_3O+

where the dot before a radical indicates an unpaired electron and an denotes an excited species.

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ionophore

A relatively small hydrophobic molecule that facilitates the transport of ions across lipid membranes. Most ionophores are produced by microorganisms. There are two types of ionophore: channel formers , which combine to form a channel in the membrane through which ions can flow; and mobile ion carriers , which transport ions across a membrane by forming a complex with the ion. Examples of ionophores include vancomycin and nigericin.

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IPM

See integrated pest management.

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iridium anomaly

The occurrence of unusually high concentrations of the relatively scarce metal iridium at the boundaries of certain geological strata. Two such layers have been discovered, one at the end of the Cretaceous, 65 million years ago, and the second at the end of the Eocene, 34 million years ago. One theory to account for these suggests that on each occasion a huge iridium-containing meteorite may have collided with the earth, producing a cloud of dust that settled out to form an iridium-rich layer. The environmental consequences of such an impact, notably in causing a general warming of the earth by the greenhouse effect, may have led to the extinction of the dinosaurs at the end of the Cretaceous and the extinction of many radiolarians at the end of the Eocene.

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iris

The pigmented ring of muscular tissue, lying between the cornea and the lens, in the eyes of vertebrates and some cephalopod molluscs. It has a central hole (the pupil ) through which light enters the eye and it contains both circular and radial muscles. Reflex contraction of the former occurs in bright light to reduce the diameter of the pupil (see pupillary reflex); contraction of the radial muscles in dim light increases the pupil diameter and therefore the amount of light entering the eye. Colour is determined by the amount of the pigment melanin in the iris. Blue eyes result from relatively little melanin; grey and brown eyes from increasingly larger amounts.

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iron

Symbol Fe. A silvery malleable and ductile metallic element that is the fourth most abundant element in the earth's crust. It is required as a trace element (see essential element) by living organisms. Iron is an important constituent of haemoglobin and the cytochromes, being stored in the liver in the form of ferritin. In animals deficiency of iron results in a form of anaemia.

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irradiation

Exposure to any form of radiation; often exposure to ionizing radiation is implied. See also food preservation.

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irrigation

The provision of water for crops by artificial methods; for example by constructing ditches, pipe systems, and canals. Irrigation can lead to problems when the water leaches trace elements from the soil; selenium, for example, can be toxic to both local fauna and flora. Irrigation can also increase the salinity of the soil, if diverted rivers are used to provide the water. Evaporation of surface water leaves a crust of salt, which can drain down to deeper layers of the soil.

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irritability

See sensitivity.

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ischium

The most posterior of the three bones that make up each half of the pelvic girdle. See also ilium; pubis.

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isidium

(pl. isidia) An outgrowth from the thallus of a lichen, 0.01–0.03 mm in diameter, that includes both algal and fungal cells. Isidia can detach from the main body of the lichen and serve as structures of vegetative propagation. Compare soredium.

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islets of Langerhans

Small groups of cells in the pancreas that function as an endocrine gland. The (or A) cells secrete the hormone glucagon, the ß (or B) cells secrete insulin, and the (or D) cells secrete somatostatin. The islets are named after their discoverer, the German anatomist and microscopist Paul Langerhans (1847–88).

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isoelectric point

The pH of a medium at which a protein carries no net charge and therefore will not migrate in an electric field. Proteins precipitate most readily at their isoelectric points; this property can be utilized to separate mixtures of proteins or amino acids.

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isoenzyme

See isozyme.

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isogamy

Sexual reproduction involving the production and fusion of gametes that are similar in size and structure. It occurs in some protoctists, e.g. certain protozoans and algae. Compare anisogamy.

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isolating mechanism

Any of the biological properties of organisms that prevent interbreeding (and therefore exchange of genetic material) between members of different species that inhabit the same geographical area. These mechanisms include seasonal isolation , in which the breeding seasons of the different populations do not overlap; and behavioural isolation , in which different courtship behaviour in the populations ensures that mating takes place only between members of the same species. Both these are examples of premating mechanisms. Postmating mechanisms include hybrid infertility and inviability.

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isoleucine

See amino acid.

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isomerase

Any of a class of enzymes that catalyse the rearrangement of the atoms within a molecule, thereby converting one isomer into another.

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isomers

Chemical compounds that have the same molecular formulae but different molecular structures or different arrangements of atoms in space. Structural isomers have different molecular structures, i.e. they may be different types of compound or they may simply differ in the position of the functional group in the molecule. Structural isomers generally have different physical and chemical properties. Stereoisomers have the same formula and functional groups, but differ in the arrangement of groups in space. Optical isomers are examples of stereoisomers (see optical activity).

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isoprene

A colourless liquid diene, CH_2:C(CH_3)CH:CH_2. The systematic name is 2-methylbuta-1,3-diene. It is the structural unit in terpenes and natural rubber, and is used in making synthetic rubbers.

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Isoptera

An order of social exopterygote insects that comprises the termites. These mainly tropical insects have a complex system of castes, including wingless workers and soldiers and primary and secondary winged reproductive members. A termite colony is founded by a single reproductive pair, the nest comprising an elaborate system of tunnels in wood or soil. Termites rely on gut microflora for cellulose digestion, causing considerable damage to wooden structures if they invade houses.

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isotonic

Describing solutions that have the same osmotic pressure (see osmosis).

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isotope

One of two or more atoms of the same element that have the same number of protons in their nucleus but different numbers of neutrons. Hydrogen (1 proton, no neutrons), deuterium (1 proton, 1 neutron), and tritium (1 proton, 2 neutrons) are isotopes of hydrogen. Most elements in nature consist of a mixture of isotopes.

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isozyme (isoenzyme)

One of several forms of an enzyme in an individual or population that catalyse the same reaction but differ from each other in such properties as substrate affinity and maximum rates of enzyme–substrate reaction (see Michaelis–Menten curve).

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itaconic acid

A product of the fermentation of the filamentous fungus Aspergillus niger. Itaconic acid is used commercially in the production of adhesives and paints.

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iteroparity

The strategy of reproducing several or many times during a lifetime. Iteroparous organisms include perennial plants and the majority of animals. Organisms that have a specific breeding season and whose breeding population contains individuals of different ages, for example temperate-region trees, are said to show overlapping iteroparity . Organisms that breed repeatedly and do so at any time of year, such as humans, exhibit continuous iteroparity . Compare semelparity.

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J

Jacob–Monod hypothesis

The theory postulated by the French biologists F. Jacob (1920– ) and J. Monod (1910–76) in 1961 to explain the control of gene expression in bacteria (see operon). Jacob and Monod investigated the expression of the gene that codes for the enzyme ß-galactosidase, which breaks down lactose; the operon that regulates lactose metabolism is called the lac operon.

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jasmonate

Any of a group of organic compounds found in plants and thought to play a part in the plant's defensive response against attack by insects, fungi, and other pathogens. Jasmonates, principally jasmonic acid and methyl jasmonate , are synthesized from the unsaturated fatty acid linolenic acid and accumulate in injured parts of the plant. Here they activate genes encoding defensive proteins and promote the rapid build-up of phytoalexins. However, jasmonates are believed to modulate the action of many other genes as well, influencing processes ranging from seed germination to root development. They may represent another class of plant growth substances.

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jaw

The part of the vertebrate skeleton that provides a support for the mouth and holds the teeth. It consists of the upper jaw (maxilla) and the lower jaw (mandible; see also dentary). Members of the Agnatha lack jaws.

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jejunum

The portion of the mammalian small intestine that follows the duodenum and precedes the ileum. The surface area of the lining of the jejunum is greatly increased by numerous small outgrowths (see villus). This facilitates the absorption of digested material, which is the prime function of the jejunum.

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jellyfish

See Cnidaria.

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Jenner, Edward

(1749–1823)
British physician, who is best known for introducing smallpox vaccination to Britain in 1796 (announced two years later), using a vaccine made from cowpox.

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joint

The point of contact between two (or more) bones, together with the tissues that surround it. Joints fall into three classes that differ in the degree of freedom of movement they allow: (1) immovable joints , e.g. the sutures between the bones that form the cranium; (2) slightly movable joints , e.g. the symphyses between the vertebrae of the spinal column; and (3) freely movable or synovial joints , e.g. those that occur between the limb bones. Synovial joints include the ball-and-socket joints (between the limbs and the hip and shoulder girdles), which allow movement in all directions; and the hinge joints (e.g. at the knee and elbow), which allow movement in one plane only (see illustration). A synovial joint is bound by ligaments and lined with synovial membrane.

Types of freely movable joints

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joule

Symbol J. The SI unit of work and energy equal to the work done when the point of application of a force of one newton moves, in the direction of the force, a distance of one metre. 1 joule = 10^7 ergs = 0.2388 calorie. It is named after James Prescott Joule (1818–89).

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jugular vein

A paired vein in the neck of mammals that returns blood from the head to the heart. It joins the subclavian vein at the base of the neck.

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jumping gene

See transposon.

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junk DNA

See selfish DNA.

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Jurassic

The second geological period of the Mesozoic era. It followed the Triassic, which ended about 213 million years ago, and extended until the beginning of the Cretaceous period, about 144 million years ago. It was named in 1829 by A. Brongniart after the Jura Mountains on the borders of France and Switzerland. Jurassic rocks include clays and limestones in which fossil flora and fauna are abundant. Plants included ferns, cycads, ginkgos, rushes, and conifers. Important invertebrates included ammonites (on which the Jurassic is zoned), corals, brachiopods, bivalves, and echinoids. Reptiles dominated the vertebrates and the first flying reptiles – the pterosaurs – appeared. The first primitive bird, Archaeopteryx, also made its appearance.

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juvenile hormone

A hormone secreted by insects from a pair of endocrine glands (corpora allata ) close to the brain. It inhibits metamorphosis and maintains the presence of larval features.

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juvenoid

See insect growth regulator.

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K

Kainozoic

See Cenozoic.

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kairomone

See allelochemical.

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kallidin

See kinin.

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karyogamy

The fusion of nuclei or nuclear material that occurs during sexual reproduction. See fertilization.

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karyogram (idiogram)

A diagram representing the characteristic features of the chromosomes of a species.

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karyokinesis

The division of a cell nucleus. See meiosis; mitosis.

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karyolysis

See karyorrhexis.

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karyoplasm

See nucleoplasm.

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karyorrhexis

A stage of cell death (see necrosis) that involves fragmentation of a cell nucleus. The nucleus breaks down into small dark beads of damaged chromatin. This usually leads to further dissolution of the nucleus (karyolysis ), whereupon the damaged chromatin gradually fades.

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karyotype

The number and structure of the chromosomes in the nucleus of a cell. The karyotype is identical in all the diploid cells of an organism.

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katadromous

Describing the migration of certain fish that spend most of their lives in fresh waters before swimming to deep oceanic waters to breed. For example, the common freshwater eel (Anguilla anguilla), found in European lakes and rivers, travels to the Sargasso Sea in the western Atlantic to breed, from where the young (elvers) may take up to three years to reach their home waters. Compare anadromous.

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katal

Symbol kat. A non-SI unit of enzyme activity defined as the catalytic activity of an enzyme that increases the rate of conversion of a specified chemical reaction by 1 mols–1 under specified assay conditions.

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keel (carina)

The projection of bone from the sternum (breastbone) of a bird or bat, to which the powerful flight muscles are attached. The sterna of flightless birds (e.g. ostrich and emu) lack keels.

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kelp

Any large brown seaweed (see Phaeophyta) or its ash, used as a source of iodine.

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kelvin

Symbol K. The SI unit of thermodynamic temperature equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The magnitude of the kelvin is equal to that of the degree Celsius (centigrade), but a temperature expressed in degrees Celsius is numerically equal to the temperature in kelvins less 273.15 (i.e. °C = K – 273.15). The unit is named after Lord Kelvin (1824–1907).

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keratin

Any of a group of fibrous proteins occurring in hair, feathers, hooves, and horns. Keratins have coiled polypeptide chains that combine to form supercoils of several polypeptides linked by disulphide bonds between adjacent cysteine amino acids. Aggregates of these supercoils form microfibrils, which are embedded in a protein matrix. This produces a strong but elastic structure.

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keratinization (cornification)

The process in which the cytoplasm of the outermost cells of the mammalian epidermis is replaced by keratin. Keratinization occurs in the stratum corneum, feathers, hair, claws, nails, hooves, and horns.

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ketohexose

See monosaccharide.

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ketone

Any one of a group of organic compounds that contain the carbonyl group (>C=O) linked to two hydrocarbon groups. The ketone group is a carbonyl group with two single bonds to other carbon atoms (–CO–). Examples are acetone (propanone), CH_3COCH_3, and methyl ethyl ketone (butanone), CH_3COC_2H_5. See also ketone body.

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ketone body

Any of three compounds, acetoacetic acid (3-oxobutanoic acid, CH_3COCH_2COOH), ß-hydroxybutyric acid (3-hydroxybutanoic acid, CH_3CH(OH)CH_2COOH), and acetone (propanone, CH_3COCH_3), all produced by the liver as a result of the metabolism of body fat deposits. Ketone bodies are normally used as energy sources by peripheral tissues. However, if carbohydrate supply is limited (e.g. during starvation or in diabetics), the blood level of ketone bodies rises and they may be present in urine, giving it a characteristic `pear drops' odour. This condition is called ketosis .

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ketopentose

See monosaccharide.

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ketose

See monosaccharide.

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key (identification key)

A means of identifying a specimen organism by selecting from a series of choices relating to external features. Each stage in the key presents several character descriptions (two in a dichotomous key ); each description is followed by a direction to another stage in the key. Selection of the character that matches the specimen leads to another stage, with another two choices, and so on until the specimen is identified.

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kidney

The main organ of excretion and osmoregulation in vertebrates, through which nitrogenous waste material (usually in the form of urine) and surplus water, ions, etc., are eliminated from the body. In mammals there is a pair of kidneys situated in the abdomen (see illustration). Each has an outer cortex and an inner medulla and is made up of tubular units called nephrons, through which nitrogenous waste is filtered from the blood, with the formation of urine. The nephrons drain into a basin-like cavity in the kidney (the renal pelvis ), which leads to the ureter and bladder.

The kidneys of a mammal

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killer cell

Either of two types of lymphocyte that destroy infected or cancerous host cells. Natural killer cells are distinct from both T cells and B cells in that they act without stimulation by a specific antigen. They attack cancerous and virus-infected cells that lack the normal class I histocompatibility proteins on their surface. This contrasts with the other type of killer cells – cytotoxic T cells – which require the presence on the surface of the target cell of foreign antigen combined with host histocompatibility proteins. Killer cells destroy their target cells by releasing proteins (perforins) that create pores through the target cell membrane, thus causing lysis and cell death.

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kilo-

Symbol k. A prefix used in the metric system to denote 1000 times. For example, 1000 volts = 1 kilovolt (kV).

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kilobase (kb)

A unit used at the molecular level for measuring distances along nucleic acids, chromosomes, or genes, equal to 1000 bases (equivalent to 1000 nucleotides or base pairs). See also bp.

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kilogram

Symbol kg. The SI unit of mass defined as a mass equal to that of the international platinum–iridium prototype kept by the International Bureau of Weights and Measures at Sèvres, near Paris.

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kinase (phosphokinase)

An enzyme that can transfer a phosphate group from a high-energy phosphate, such as ATP, to an organic molecule. Phosphorylation is normally required to activate the molecule, which is often an enzyme. For example, kinases activate the precursors of enzymes secreted in pancreatic juice (see chymotrypsin; trypsin). See also protein kinase.

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kinesis

The movement of a cell or organism in response to a stimulus in which the rate of movement depends on the intensity (rather than the direction) of the stimulus. For example, a woodlouse moves slowly in a damp atmosphere and quickly in a dry one.

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kinetochore

The point at which the microtubules of the spindle attach to the centromere of a chromosome during nuclear division. In higher organisms it consists of protein and RNA arranged in three layers closely apposed to the chromatin. It acts as a motor, pulling the centromere along the attached microtubules towards the spindle pole. This process is thought to involve motor proteins, such as dynein, and disassembly of the microtubule subunits.

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kinetosome (basal body)

See undulipodium.

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kingdom

In most classification systems, the highest category into which organisms are classified. Traditionally two kingdoms were recognized, the Plantae (see plant) and the Animalia (see animal). More recently a number of other kingdoms have been suggested, and most modern classification systems now recognize five kingdoms: Bacteria (or Prokaryotae; see bacteria), Protoctista (including protozoa and algae), Fungi (see fungi), Plantae, and Animalia. See also domain.

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kinin

1 One of a group of peptides, occurring in blood, that are involved in inflammation. Kinins are formed by the splitting of blood plasma globulins (kininogens ) by the enzyme kallikrein at the site of inflammation. Kinins so far identified include bradykinin and kallidin . They cause local increases in the permeability of small blood vessels. Bradykinin is a potent stimulator of pain receptors in the skin.

2 See cytokinin.
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kinomere

See centromere.

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kin selection

Natural selection of genes that tend to cause the individuals bearing them to be altruistic to close relatives. These relatives therefore have a higher probability of bearing identical copies of those same genes than do other members of the population. Thus kin selection for a gene that tends to cause an animal to share food with a close relative will result in the gene being spread through the population because it (unconsciously) benefits itself. The more closely two animals are related, the higher the probability that they share some identical genes and therefore the more closely their interests coincide. Parental care is a special case of kin selection. See inclusive fitness.

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Klinefelter's syndrome

A genetic disorder affecting men in which an individual gains an extra X chromosome, so that the usual karyotype of XY (see sex chromosome) is replaced by one of XXY (see nondisjunction). Symptoms of Klinefelter's syndrome, named after US physician H. F. Klinefelter (1912– ), include underdeveloped testes, infertility, and some female characteristics (such as breast enlargement).

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klinostat

A device used in experiments to test the influence of gravity on the growth movements of plants (see geotropism). It consists of a motor that slowly rotates a drum inside which seedlings are attached. This prevents any single part of the seedlings from receiving uninterrupted gravitational stimulation and results in horizontal growth of the seedlings.

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knee-jerk reflex

See stretch reflex.

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knockout

Describing an organism that has been genetically engineered so that a normal gene is replaced with a defective homologous gene. Experiments with laboratory organisms (especially mice) treated in this way reveal how defects in particular genes can affect the development and life of the organism. Knockout mice are engineered by injecting normal mouse oocytes with defective DNA and selecting transfected embryos that are heterozygous for the defective gene. If adults reared from such embryos are crossed with other similar heterozygotes, 25% of the progeny will be homozygous for the `knocked out' gene, and hence the impact of the defect on such mice can be assessed.

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Kranz anatomy

The arrangement of palisade mesophyll cells in a circle around the vascular bundle of C_4 plants. See bundle sheath cells.

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Krebs, Sir Hans Adolf

(1900–81)
German-born British biochemist, who emigrated to Britain in 1933, working at Sheffield University before moving to Oxford in 1954. Krebs is best known for the Krebs cycle, the basis of which he discovered in 1937. Details were later added by Fritz Lipmann (1899–1986), with whom Krebs shared the 1953 Nobel Prize for physiology or medicine.

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Krebs cycle (citric acid cycle; tricarboxylic acid cycle; TCA cycle)

A cyclical series of biochemical reactions that is fundamental to the metabolism of aerobic organisms, i.e. animals, plants, and many microorganisms (see illustration). The enzymes of the Krebs cycle are located in the mitochondria and are in close association with the components of the electron transport chain. The two-carbon acetyl coenzyme A (acetyl CoA) reacts with the four-carbon oxaloacetate to form the six-carbon citrate. In a series of seven reactions, this is reconverted to oxaloacetate and produces two molecules of carbon dioxide. Most importantly, the cycle generates one molecule of guanosine triphosphate (GTP – equivalent to 1 ATP) and reduces three molecules of the coenzyme NAD to NADH and one molecule of the coenzyme FAD to FADH_2. NADH and FADH_2 are then oxidized by the electron transport chain to generate three and two molecules of ATP respectively. This gives a net yield of 12 molecules of ATP per molecule of acetyl CoA.
Acetyl CoA can be derived from carbohydrates (via glycolysis), fats, or certain amino acids. (Other amino acids may enter the cycle at different stages.) Thus the Krebs cycle is the central `crossroads' in the complex system of metabolic pathways and is involved not only in degradation and energy production but also in the synthesis of biomolecules. It is named after its principal discoverer, Hans Krebs.

Krebs cycle

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K selection

A type of selection that favours organisms with a low rate of reproduction but whose populations expand to the maximum number of individuals that the habitat can support (the carrying capacity of the habitat). K-selected species (or K strategists ) tend to be highly adapted to their environment and are able to compete successfully for food and other resources. They also tend to inhabit stable environments and have relatively long life spans. See survival curve. Compare r selection.

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Kupffer cells

Specialized macrophages that dispose of old blood cells and particulate matter. Kupffer cells, named after Karl Wilhelm von Kupffer (1829–1902), are found in the bloodstream and in the liver, attached to the walls of the sinusoids.

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kwashiorkor

See malnutrition.

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L

labelling

The process of replacing a stable atom in a compound with a radioisotope of the same element to enable its path through a biological or mechanical system to be traced by the radiation it emits. In some cases a different stable isotope is used and the path is detected by means of a mass spectrometer. A compound containing either a radioactive or stable isotope is called a labelled compound and the atom used is a label . If a hydrogen atom in each molecule of the compound has been replaced by a tritium atom, the compound is called a tritiated compound . A radioactive labelled compound will behave chemically and physically in the same way as an otherwise identical stable compound, and its presence can easily be detected. This process of radioactive tracing is widely used in biology and medicine.

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labia

See labium.

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labium

(pl. labia) 1 The lower lip in the mouthparts of an insect, which is used in feeding and is formed by the fusion of a pair of appendages (the second maxillae).

2 Either member of two pairs of fleshy folds that form part of the vulva. The outer and larger pair, the labia majora , are covered by pubic hair and contain adipose tissue; the smaller labia minora lack adipose tissue and pubic hair. Both pairs of labia contain sebaceous glands.
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labrum

The upper lip in the mouthparts of an insect. It is formed from a plate of cuticle hinged to the head above the mouth and is used in feeding.

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labyrinth

The system of cavities and tubes that comprises the inner ear of vertebrates. It consists of a system of membranous structures (membranous labyrinth ) housed in a similar shaped bony cavity (bony labyrinth ).

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lac operon

The operon that regulates lactose metabolism in the bacterium Escherichia coli. Its form was first postulated in 1961 by François Jacob (1920– ) and Jacques Monod (1910–76) to explain control of ß-galactosidase synthesis, and it is used as a model for the structure of all other operons. See Jacob–Monod hypothesis.

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lacrimal gland (lachrymal gland)

The tear gland, present in the eyelids of some vertebrates. The fluid (tears) produced by this gland cleanses and lubricates the exposed surface of the eye; it drains into the nose through the lacrimal duct.

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lactase (ß-galactosidase)

The enzyme that breaks down the milk sugar, lactose, to glucose and galactose by catalysing the hydrolysis of galactose residues.

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lactation

The discharge of milk from the mammary glands. This generally only occurs after birth of the young and is stimulated by the sucking action of the infants. Lactation is under the control of hormones, notably prolactin and oxytocin.

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lacteal

A minute blind-ended lymph vessel that occurs in each villus of the small intestine. Digested fats are absorbed into the lacteals (see chyle) and transported to the bloodstream through the thoracic duct.

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lactic acid (2-hydroxypropanoic acid)

An alpha hydroxy carboxylic acid, CH_3CH(OH)COOH, with a sour taste. Lactic acid is produced from pyruvic acid in active muscle tissue when oxygen is limited (see oxygen debt) and subsequently removed for conversion to glucose by the liver. During strenuous exercise it may build up in the muscles, causing cramplike pains. It is also produced by fermentation in certain bacteria and is characteristic of sour milk.

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lactobacillus

(pl. lactobacilli) Any one of a group of rod-shaped Gram-positive anaerobic bacteria of the genus Lactobacillus, in which lactic acid is an end product of fermentation. Lactobacilli are important in food technology for the production of cheese, yoghurt, and other foods from milk.

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lactogenic hormone

See prolactin.

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lactose (milk sugar)

A sugar comprising one glucose molecule linked to a galactose molecule. Lactose is manufactured by the mammary gland and occurs only in milk. For example, cows' milk contains about 4.7% lactose. It is less sweet than sucrose (cane sugar).

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lacuna

A gap or cavity in the tissues of an organism; for example, the hollow centre of certain plant stems or any of the small cavities in bone in which the bone-forming cells are found.

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laevorotatory

Designating a chemical compound that rotates the plane of plane-polarized light to the left (anticlockwise for someone facing the oncoming radiation). See optical activity.

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laevulose

See fructose.

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lagging strand

See discontinuous replication.

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lag phase

See bacterial growth curve.

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Lamarck, Jean Baptiste Pierre Antoine de Monet, Chevalier de

(1744–1829)
French natural historian. In 1778 he published a flora of France, which included a dichotomous identification key, and later worked on the classification of invertebrates, published in a seven-volume natural history (1815–22). In 1809 he put forward a theory of evolution that has become known as Lamarckism (later rejected in favour of Darwinism).

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Lamarckism

One of the earliest superficially plausible theories of inheritance proposed by Jean-Baptiste de Lamarck in 1809. He suggested that changes in an individual are acquired during its lifetime, chiefly by increased use or disuse of organs in response to “a need that continues to make itself felt”, and that these changes are inherited by its offspring. Thus the long neck and limbs of a giraffe are explained as having evolved by the animal stretching its neck to browse on the foliage of trees. This so-called inheritance of acquired characteristics has never unquestionably been demonstrated to occur and the theory was largely displaced by the genetic theories of Mendel and his successors (see Mendelism). See also Lysenkoism.

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lambda phage

A temperate bacteriophage that infects cells of the bacterium Escherichia coli, where it can either exist as a quiescent prophage (in a state called lysogeny) or undergo replication leading to lysis of the host cell and release of new phage particles. The phage particle consists of an icosahedral head, 64 nm in diameter, and a tail, 150 nm in length. The head contains the double-stranded DNA of the phage genome. Lambda phage has been intensively studied as a model of viral infection and replication and is much used in genetic research and in genetic engineering. Modified lambda phages are used as vectors in gene cloning, especially for packaging relatively large amounts of foreign DNA.

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lamella

(pl. lamellae) 1 (in botany)
a Any of the paired folds of membranes seen between the grana in a plant chloroplast.
b Any of the spore-bearing gills on the underside of the cap of many mushrooms and toadstools. See also middle lamella.

2 (in zoology) Any of various thin layers of membranes, especially any of the thin layers of tissue of which compact bone is formed.
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Lamellibranchia

See Bivalvia.

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lamina

1 The thin and usually flattened blade of a leaf, in which photosynthesis and transpiration occurs. The bulk of the lamina is made up of mesophyll cells interspersed by a network of veins (vascular bundles). The mesophyll is enclosed by a protective epidermis that produces a waxy cuticle.

2 The leaflike part of the thallus of certain algae, notably kelps. See also stipe.
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lampbrush chromosome

A type of chromosome found in the oocytes of animals, notably newts and other amphibians, that is characterized by having numerous loops arising from the long axis of the chromosomal arms. Under the microscope this gives an appearance reminiscent of a cylindrical brush of the type formerly used for cleaning oil lamps. The loops are regions where the chromosome is uncoiled and the DNA exposed for active transcription by an associated matrix of RNA and proteins.

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lancelet

See Cephalochordata.

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large intestine

The portion of the alimentary canal of vertebrates between the small intestine and the anus. It consists of the caecum, colon, and rectum and its principal function is the absorption of water and formation of faeces.

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larva

(pl. larvae) The juvenile stage in the life cycle of most invertebrates, amphibians, and fish, which hatches from the egg, is unlike the adult in form, and is normally incapable of sexual reproduction (see paedogenesis). It develops into the adult by undergoing metamorphosis. Larvae can feed themselves and are otherwise self-supporting. Examples are the tadpoles of frogs, the caterpillars of butterflies, and the ciliated planktonic larvae of many marine animals. Compare nymph.

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larynx

The anterior portion of the trachea (windpipe) of tetrapod vertebrates, which in amphibians, reptiles, and mammals contains the vocal cords. Movement of the cartilage in the walls of the larynx (by means of the laryngeal muscles) alters the tension of the vocal cords. This changes the pitch of the sound emitted by the vocal cords when they vibrate. The final voiced sound is modified by resonance within the oral and nasal cavities.

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lasso cell (colloblast)

A type of cell in comb jellies (phylum Ctenophora) that is similar to the thread cells of cnidarians. Lasso cells are embedded in the tentacles of the organism. Each has two protruding filaments, one straight and one spiralled, with adhesive heads used to catch prey.

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latent learning

A form of learning in which there is apparently no immediate reward for the animal, and what is learnt remains `latent'. The prime example is an animal exploring its surroundings. Learning about the geography of its home area may bring an animal no immediate benefits, but can prove vital in the future when fleeing a predator or searching for food. Many insects learn the details of landmarks near their nest by making orientation flights. This process enables them to locate the nest when returning from distant sites.

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latent period

The short time that elapses between the reception of a stimulus and the start of the response in an irritable tissue. For a contracting muscle the latent period lasts about 0.02 seconds, during which time calcium ions are released from the sarcoplasmic reticulum.

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latent virus

Any virus that remains in its host organism without undergoing replication. Latent viruses, which include herpes simplex virus (the agent of cold sores), may be induced to replicate and cause cell lysis some time after the initial infection, for example when the host's immunity is reduced.

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lateral-line canal

A sense organ in fish consisting of a groove on the head and along each side of the body that contains sensory hairs. These hairs are sensitive to variations in water pressure and are used to detect moving objects, such as predators, and to monitor progress through water during swimming. The canals and the hairs within them collectively form the lateral-line system .

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lateral root

See root.

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latex

A milky fluid of mixed composition found in some herbaceous plants and trees. Its function is not clear but it may assist in protecting wounds (compare gum) and it may be involved in the nutrition of the plant. The latex of some species, notably rubber trees, is collected for commercial purposes.

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Laurasia

See continental drift.

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lauric acid (dodecanoic acid)

A white crystalline fatty acid, CH_3(CH_2)10COOH. Glycerides of the acid are present in natural fats and oils (e.g. coconut and palm-kernel oil).

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Law of Independent Assortment

See Mendel's laws; independent assortment.

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Law of Segregation

See Mendel's laws.

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LD_50

Lethal dose 50, or median lethal dose: the amount of a pharmacological or toxic substance (such as ionizing radiation) that causes death in 50% of a group of experimental animals. For each LD_50 the species and weight of the animal and the route of administration of the substance is specified. LD_50s are used both in toxicology and in the bioassay of therapeutic compounds.

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L-dopa

See dopa.

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leaching

Extraction of the soluble components of a solid mixture by percolating a solvent through it.

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leading strand

See discontinuous replication.

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leaf

A flattened structure that develops from a superficial group of tissues, the leaf buttress, on the side of the stem apex. Each leaf has a lateral bud in its axil. Leaves are arranged in a definite pattern (see phyllotaxis) and usually show limited growth. Each consists of a broad flat lamina (leaf blade) and a leaf base, which attaches the leaf to the stem; a leaf stalk (petiole ) may also be present. The leaves of bryophytes are simple appendages, which are not homologous with the leaves of vascular plants as they develop on the gametophyte generation. Md1506
Leaves show considerable variation in size, shape, arrangement of veins, type of attachment to the stem, and texture. They may be simple or divided into leaflets , i.e. compound (see illustration). Types of leaf include: cotyledons (seed leaves); scale leaves , which lack chlorophyll and develop on rhizomes or protect the inner leaves of a bud; foliage leaves , which are the main organs for photosynthesis and transpiration; and bracts and floral leaves , such as sepals, petals, stamens, and carpels, which are specialized for reproduction.
Leaves may be modified for special purposes. For example the leaf bases of bulbs are swollen with food to survive the winter. In some plants leaves are reduced to spines for protection and their photosynthetic function is carried out by another organ, such as a cladode.

Transverse section through a leaf blade

Simple leaves (images 1 and 2); Compound leaves (images 3 and 4)

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leaf area index (LAI)

The total surface area of the leaves of plants in a given area divided by the area of ground covered by the plants. In an area of dense vegetation, such as a forest, the LAI will be high.

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leaf buttress

See primordium.

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leaf litter

See litter.

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learning

A process by which an animal's response to a particular situation may be permanently altered, usually in a beneficial way, as a result of its experience. Learning allows an animal to respond more flexibly to the situations it encounters: learning abilities in different species vary widely and are adapted to the species' environment. Numerous different categories of learning have been proposed, including habituation, associative learning (through conditioning), trial-and-error learning, insight learning, latent learning, and imprinting.
Learning in animals An animal's survival prospects are greatly improved if the animal alters its behaviour according to its experience. Learning increases its chances of obtaining food, avoiding predators, and adjusting to other often unpredictable changes in its environment. The importance of learning in the development of behaviour was stressed particularly by US experimental psychologists, such as John B. Watson (1878–1958) and B. F. Skinner (1904–90), who studied animals under carefully controlled laboratory conditions. They demonstrated how rats and pigeons could be trained, or ‘conditioned’, by exposing them to stimuli in the form of food rewards or electric shocks. This work was criticized by others, notably the ethologists, who preferred to observe animals in their natural surroundings and who stressed the importance of inborn mechanisms, such as instinct, in behavioural development. A synthesis between these two once-conflicting approaches has now been achieved: learning is regarded as a vital aspect of an animal's development, occurring in response to stimuli in the animal's environment but within constraints set by the animal's genes. Hence young animals are receptive to a wide range of stimuli but are genetically predisposed to respond to those that are most significant, such as those from their mother. Conditioning The classical demonstration of conditioning was undertaken by Ivan Pavlov in the early 1900s. He showed how dogs could learn to associate the ringing of a bell with the presentation of food, and after a while would salivate at the sound of the bell alone. He measured the amount of saliva produced by a dog, and showed that this increased as the animal learnt to associate the sound of the bell with presentation of food. The dog became conditioned to respond to the sound of the bell.
Such learning is widespread among animals. Pavlov's experiment involved positive conditioning, but negative conditioning can also occur. For example, a young bird quickly learns to associate the black-and-orange markings of the cinnabar moth's caterpillars with their unpleasant taste, and to avoid eating such caterpillars in future. Trial-and-error learning This occurs when the spontaneous behaviour of an animal accidentally produces a reward. For example, a hungry cat is placed in a box and required to pull on a string loop to open the door and gain access to food (see illustration). After various scratching and reaching movements, it accidentally pulls the loop and is released from the box. Its behaviour is instrumental in securing a reward. On subsequent occasions, the cat's attention becomes increasingly focused on the loop, until eventually it pulls the loop straightaway on entering the box.

Trial and error learning by a cat
Insight learning Chimpanzees can learn to stack crates or boxes to form a platform or to manipulate poles in order to reach an otherwise inaccessible bunch of bananas. A chimp may apparently solve such a problem suddenly, as if gaining insight after mental consideration of the problem. Such complex learning benefits from previous experience, in this instance by simply ‘playing’ with crates, boxes, or poles.

Insight learning by a chimpanzee
Imprinting This is a form of learning found in young animals, especially young birds, in which they form an attachment to their mother in early life, thereby ensuring that they are taken care of and do not wander off. For example, chicks or ducklings follow the first large moving object that they encounter after hatching. This is normally their mother, but artifically incubated youngsters can become imprinted on a wooden decoy, as illustrated here, or even on a human being – as originally demonstrated in goslings and ducklings by Konrad Lorenz Imprinting occurs during a particularly sensitive period of development: the attachment formed by an animal to an imprinted individual or object lasts well into its adult life.

Imprinting in ducklings

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lecithin (phosphatidylcholine)

A phosphoglyceride (see phospholipid) containing the amino alcohol choline esterified to the phosphate group. It is the most abundant animal phospholipid (being a component of cell membranes) and also occurs in higher plants, but rarely in microorganisms.

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lectin

Any of a group of proteins found in plants, animals, fungi, algae, and bacteria that share the property of binding to specific carbohydrate groups. Hence, lectins derived from plant seeds, such as concanavalin A , can cause cells to clump together by forming cross links between the oligosaccharide groups on cell surfaces. Lectins are widely used for diagnosis and experimental purposes, e.g. to identify mutant cells in cell cultures, to determine blood groups by triggering agglutination of red blood cells, or in mapping the surface of cell membranes. The role of lectins in plants remains unclear. They are especially abundant in seeds, in which they may inhibit the growth of fungi or other pathogens. In legumes it is thought that lectins take part in the recognition of suitable bacterial partners for the plant in establishing root nodule symbioses.

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leeches

See Hirudinea.

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Leewenhoek, Anton van

(1632–1723)
Dutch microscopist, who had little formal education. He is known for accurately grinding small lenses to make simple microscopes, with which he made the first observations of red blood cells, protozoa, and spermatozoa. He communicated regularly with the Royal Society in London, which published many of his findings in its Philosophical Transactions.

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legume (pod)

A dry fruit formed from a single carpel and containing one or more seeds, which are shed when mature. It is the characteristic fruit of the Leguminosae (Fabaceae; pea family). It splits, often explosively, along both sides and the two halves of the fruit move apart to expose the seeds. A special form of the legume is the lomentum.

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Leishman's stain

A neutral stain for blood smears devised by the British surgeon W. B. Leishman (1865–1926). It consists of a mixture of eosin (an acidic stain), and methylene blue (a basic stain) in alcohol and is usually diluted and buffered before use. It stains the different components of blood in a range of shades between red and blue. The similar Wright's stain is favoured by American workers.

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lek

An area of ground divided into territories that are vigorously defended by males for purposes of sexual display and mating during the breeding season. Such a system occurs in various bird species, for example the black grouse and peafowl, and also in some mammals. The most dominant males occupy the smallest territories at the centre of the lek, where they are most likely to attract and copulate with visiting females. The peripheral, and larger, territories are occupied by subordinate males, who have less mating success. Over successive breeding seasons, the younger subordinate males gradually displace older individuals from the most desirable territories and become dominant themselves. The lek territories contain no resources of value to the breeding female, such as food or nesting materials, although males of some species may construct bowers or similar structures used in their display.

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lemma

One of two bracts that protects the floret of grasses. The lemma is situated beneath the second (smaller) bract, known as the palea , which it surrounds. See spike.

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lens

A transparent biconvex structure in the eyes or analogous organs of many animals, responsible for directing light onto light-sensitive cells. In vertebrates it is a flexible structure centred behind the iris and attached by suspensory ligaments to the ciliary body. In terrestrial species its main function is to focus images onto the retina. To focus on near objects, the circular muscles in the ciliary body contract and the lens becomes more convex; contraction of the radial muscles in the ciliary body flattens the lens for focusing on distant objects (see also accommodation).

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lenticel

Any of the raised pores in the stems of woody plants that allow gas exchange between the atmosphere and the internal tissues. The pore is formed by the cork cambium, which, at certain points, produces a loose bulky form of cork that pushes through the outer tissues to create the lenticel.

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Lepidoptera

An order of insects comprising the butterflies and moths, found mainly in tropical regions. Adults possess two pairs of membranous wings, often brightly coloured and usually coupled together. The wings, body, and legs are covered with minute scales. Adult mouthparts are generally modified to form a long proboscis for sucking nectar, fruit juices, etc. Butterflies are typically small-bodied, active during daylight, and rest with their wings folded vertically; moths have larger bodies, are nocturnal, and rest with their wings in various positions. The larvae (caterpillars) have a prominent head and a segmented wormlike body, most segments bearing a pair of legs. They chew leaves and stems, sometimes causing considerable damage to crop plants. The larvae undergo metamorphosis via a pupa (chrysalis) to the adult form. In some groups, the pupa is enclosed in a cocoon of silk derived from silk glands (modified salivary glands); others use leaves, etc. to build a cocoon.

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leptotene

The beginning of the first prophase of meiosis, when the chromatids can be seen and pairing begins.

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lethal allele (lethal gene)

A mutant form of a gene that eventually results in the death of an organism if expressed in the phenotype. Most lethal genes are recessive; for example, sickle-cell anaemia (see polymorphism) results from a recessive lethal gene that causes the production of abnormal and inefficient haemoglobin.

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lethal dose 50

See LD50.

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leucine

See amino acid.

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leucine zipper

A molecular motif, originally discovered in DNA-binding proteins but also found in other proteins, in which a set of four or five consecutive leucine residues is repeated every seven amino acids in the primary sequence. In a helical configuration this produces a line of leucines on one side of the helix. With two such helixes alongside each other, the arrays of leucines can interdigitate like a zip fastener, thus forming a stable link between the two helices.

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leucocyte (white blood cell)

A colourless cell with a nucleus, found in blood and lymph. Leucocytes are formed in lymph nodes and red bone marrow and are capable of amoeboid movement. They can produce antibodies and move through the walls of vessels to migrate to the sites of injuries, where they surround and isolate dead tissue, foreign bodies, and bacteria. There are two major types: those without granules in the cytoplasm, such as lymphocytes and monocytes (see agranulocyte), and those with granular cytoplasm (granulocytes), which include basophils, eosinophils, and neutrophils.

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leucoplast

Any of various organelles in plant cells that contain no pigment and are therefore colourless. Leucoplasts are usually found in tissues not normally exposed to light and frequently contain reserves of starch (in amyloplasts ), protein, or oil. See plastid. Compare chromoplast.

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leukaemia

See cancer.

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leukotrienes (LTs)

A class of compounds synthesized from arachidonic acid in a variety of tissues, including the lungs and leucocytes. There are at least six different types of leukotrienes, all of which play an active role in regulating inflammation. In the immune response, LTs attract neutrophils and eosinophils to sites of infection and they cause vasoconstriction.

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l-form

See optical activity.

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LH

See luteinizing hormone.

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lice

See Mallophaga (bird lice); Siphunculata (sucking lice).

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lichens

A group of organisms that are symbiotic associations (see symbiosis) between a fungus (usually one of the Ascomycota) and a green alga or a blue-green bacterium. The fungal partner (mycobiont ) usually makes up most of the lichen body and the cells of the alga or bacterium (phycobiont ) are distributed within it. The phycobiont photosynthesizes and passes most of its food to the fungus and the fungus protects its partner's cells. The lichen reproduces by means of soredia, isidia, or by fungal spores, which must find a suitable partner on germination. Lichens are slow growing but can live in regions that are too cold or exposed for other plants. They may form a flattened crust or be erect and branching. Many grow as epiphytes, especially on tree trunks. Some species are very sensitive to air pollution and have been used as indicator species. Lichens are classified as fungi, usually being placed in the taxon of the fungal partner; some authorities group them together in the phylum Mycophycophyta.

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life cycle

The complete sequence of events undergone by organisms of a particular species from the fusion of gametes in one generation to the same stage in the following generation. In most animals gametes are formed by meiosis of germ cells in the reproductive organs of the parents. The zygote, formed by the fusion of two gametes, eventually develops into an organism essentially similar to the parents. In plants, however, the products of meiosis are spores, which develop into plants (the gametophyte generation) often very different in form from the spore-forming (sporophyte) generation. The sporophyte generation is restored when gametes, formed by the gametophyte generation, fuse. See alternation of generations.

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ligament

A resilient but flexible band of tissue (chiefly collagen) that holds two or more bones together at a movable joint. Ligaments restrain the movement of bones at a joint and are therefore important in preventing dislocation.

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ligase

Any of a class of enzymes that catalyse the formation of covalent bonds using the energy released by the cleavage of ATP. Ligases are important in the synthesis and repair of many biological molecules, including DNA (see DNA ligase), and are used in genetic engineering to insert foreign DNA into cloning vectors.

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light green

See fast green.

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light reaction (light-dependent reaction)

See photosynthesis.

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lignin

A complex organic polymer that is deposited within the cellulose of plant cell walls during secondary thickening. Lignification makes the walls woody and therefore rigid. See sclerenchyma.

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lignite

See coal.

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ligule

1 A membranous scalelike outgrowth from the leaves of certain flowering plants. Many grasses have a ligule at the base of the leaf blade.

2 A small membranous structure that develops on the upper surface of a young leaf base in certain clubmosses, for example Selaginella. It withers as the plant matures.
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3 A strap-shaped extension from the corolla tube in certain florets of a capitulum. They are termed ligulate (or ray ) florets .
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limb

1 An appendage of a vertebrate animal, such as the leg or arm of a mammal or the wing of a bird. See also pentadactyl limb.

2 The expanded upper part of a sepal, petal, or leaf.
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3 The widened upper section of a gamopetalous corolla.
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limbic system

A group of regions in the brain that is involved in the expression and control of mood and instinct and plays a major role in long-term memory. The limbic system includes the hippocampus and the hypothalamus.

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liming

The application of lime (calcium hydroxide) to soils to increase levels of calcium and decrease acidity.

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limiting factor

Any environmental factor that – by its decrease, increase, absence, or presence – limits the growth, metabolic processes, or distribution of organisms or populations. In a desert ecosystem, for example, low rainfall and high temperature will be factors limiting colonization. When a metabolic process is affected by more than one factor, the law of limiting factors states that its rate is limited by the factor that is nearest its minimum value. For example, photosynthesis is affected by many factors, such as light, temperature, and carbon dioxide concentration, but on a warm sunny day carbon dioxide concentration will be the limiting factor as light and temperature will be at optimum levels.

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limnology

The study of freshwater habitats and ecosystems.

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LINE (long interspersed element)

Any of a class of dispersed moderately repetitive DNA found in eukaryotes, consisting of numerous copies of relatively long (generally 6–8 kb) sequences scattered throughout the genome. LINEs are retrotransposons and can spread by reverse transcription: an RNA transcript is formed, then a DNA copy of this, which subsequently undergoes insertion into the genome. An example found in humans and other mammals is the L1 element . This is 6 kb long, has 50000 copies in the human genome, and contains two large coding regions plus noncoding flanking sequences.

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linear energy transfer (LET)

The energy transferred per unit path length by a moving high-energy charged particle (such as an electron or a proton) to the atoms and molecules along its path. It is of particular importance when the particles pass through living tissue as the LET modifies the effect of a specific dose of radiation. LET is proportional to the square of the charge on the particle and increases as the velocity of the particle decreases.

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linkage

The tendency for two different genes on the same chromosome to remain together during the separation of homologous chromosomes at meiosis. Linkage can be broken by crossing over or by a chromosome mutation, when sections of chromosomes are exchanged and new combinations of genes are produced. See also sex linkage.

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linkage map

See chromosome map.

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Linnaean system

See binomial nomenclature.

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Linnaeus, Carolus

(Carl Linné; 1707–78) Swedish botanist. He travelled round Europe and by 1735 had described more than 100 new species of plants. In 1749 he announced his system of binomial nomenclature, which, with modification, has been used ever since for all organisms.

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linoleic acid

A liquid polyunsaturated fatty acid with two double bonds, CH_3(CH_2)4CH:CHCH_2CH:CH(CH_2)7COOH. Linoleic acid is abundant in many plant fats and oils, e.g. linseed oil, groundnut oil, and soya-bean oil. It is an essential fatty acid.

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linolenic acid

A liquid polyunsaturated fatty acid with three double bonds in its structure: CH_3CH_2CH:CHCH_2CH:CHCH_2CH:CH(CH_2)7COOH. Linolenic acid occurs in certain plant oils, e.g. linseed and soya-bean oil, and in algae. It is one of the essential fatty acids.

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lipase

An enzyme secreted by the pancreas that catalyses the breakdown of fats into fatty acids and glycerol in the small intestine.

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lipid

Any of a diverse group of organic compounds, occurring in living organisms, that are insoluble in water but soluble in organic solvents, such as chloroform, benzene, etc. Lipids are broadly classified into two categories: complex lipids , which are esters of long-chain fatty acids and include the glycerides (which constitute the fats and oils of animals and plants), glycolipids, phospholipids, and waxes; and simple lipids , which do not contain fatty acids and include the steroids and terpenes.
Lipids have a variety of functions in living organisms. Fats and oils are a convenient and concentrated means of storing food energy in plants and animals. Phospholipids and sterols, such as cholesterol, are major components of cell membranes (see lipid bilayer). Waxes provide vital waterproofing for body surfaces. Terpenes include vitamins A, E, and K, and phytol (a component of chlorophyll) and occur in essential oils, such as menthol and camphor. Steroids include the adrenal hormones, sex hormones, and bile acids.
Lipids can combine with proteins to form lipoproteins (e.g. in cell membranes). In bacterial cell walls, lipids may associate with polysaccharides to form lipopolysaccharides .

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lipid bilayer

The arrangement of lipid molecules in biological membranes, which takes the form of a double sheet. Each lipid molecule comprises a hydrophilic `head' (having a high affinity for water) and a hydrophobic `tail' (having a low affinity for water). In the lipid bilayer the molecules are aligned so that their hydrophilic heads face outwards, forming the outer and inner surfaces of the membrane, while the hydrophobic tails face inwards, away from the external aqueous environment.

Structure of a lipid bilayer

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lipoic acid

A vitamin of the vitamin B complex. It is one of the coenzymes involved in the decarboxylation of pyruvate by the enzyme pyruvate dehydrogenase. This reaction has to take place before carbohydrates can enter the Krebs cycle during aerobic respiration. Good sources of lipoic acid include liver and yeast.

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lipolysis

The breakdown of storage lipids in living organisms. Most long-term energy reserves are in the form of triglycerides in fats and oils. When these are needed, e.g. during starvation, lipase enzymes convert the triglycerides into glycerol and the component fatty acids. These are then transported to tissues and oxidized to provide energy.

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lipoprotein

One of a group of compounds consisting of a lipid combined with a protein. Lipoproteins are the main structural materials of the membranes of cells and cell organelles. They also occur in blood and lymph, being the form in which lipids are transported in these media. Cholesterol is transported in the bloodstream mainly in the form of low-density lipoproteins (LDLs) and is removed by means of LDL receptors in cell membranes; the LDLs are bound to the receptors, which are then taken into the cells. Lack of LDL receptors, occurring as a genetic defect in some individuals, is believed to be a cause of high levels of cholesterol in the blood, predisposing to atherosclerosis. Very low-density lipoproteins (VLDLs) are formed in the liver and are the precursors of LDLs, while high-density lipoproteins (HDLs), the smallest of all lipoproteins, are believed to transport cholesterol from tissues to the liver. See also chylomicron.

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liposome

A microscopic spherical membrane-enclosed vesicle or sac (20–30 nm in diameter) made artificially in the laboratory by the addition of an aqueous solution to a phospholipid gel. The membrane resembles a cell membrane and the whole vesicle is similar to a cell organelle. Liposomes can be incorporated into living cells and are used to transport relatively toxic drugs into diseased cells, where they can exert their maximum effects. For example, liposomes containing the drug methotrexate, used in the treatment of cancer, can be injected into the patient's blood. The cancerous organ is heated to a temperature higher than body temperature, so that when the liposome passes through its blood vessels, the membrane melts and the drug is released. Liposomes can also be used as vectors in gene therapy. The study of the behaviour of liposome membranes is used in research into membrane function, particularly to observe the behaviour of membranes during anaesthesia with respect to permeability changes.

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lipotrophin

Either of two peptide hormones produced in the anterior pituitary gland that trigger the mobilization of fat deposits and the transfer of lipid components to the bloodstream. ß-lipotrophin is formed by cleavage of the precursor pro-opiocortin, and is itself cleaved to form -lipotrophin and ß-endorphin.

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Listeria

A genus of rod-shaped aerobic motile Gram-positive bacteria. Only one species, L. monocytogenes, causes disease (listeriosis ). It is resistant to physical and chemical treatments and can occur as a contaminant in certain foods, in faeces, etc. Listeriosis can take various forms, depending on the site of infection: localization in the central nervous system causes meningoencephalitis, while uterine infection can result in abortion or congenital handicap in the fetus.

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litre

Symbol l. A unit of volume in the metric system regarded as a special name for the cubic decimetre. It was formerly defined as the volume of 1 kilogram of pure water at 4°C at standard pressure, which is equivalent to 1.000028dm^3.

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litter

Dead organic matter in the soil that has not yet decomposed. It consists of fallen leaves and other plant remains (leaf litter ), animal excrement, etc. After decomposition by decomposers and detritivores litter becomes humus.

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littoral

Designating or occurring in the marginal shallow-water zone of a sea or lake, especially (in the sea) between high and low tide lines. In this zone enough light penetrates to the bottom to support rooted aquatic plants. Compare profundal; sublittoral.

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liver

A large lobed organ in the abdomen of vertebrates that plays an essential role in many metabolic processes by regulating the composition and concentration of nutrients and toxic materials in the blood. It is made up of units called lobules , each of which is a roughly hexagonal structure consisting largely of hepatocytes arranged around a central vein. The liver receives the products of digestion dissolved in the blood via the hepatic portal vein and its most important functions are to convert excess glucose to the storage product glycogen, which serves as a food reserve; to break down excess amino acids to ammonia, which is converted to urea or uric acid and excreted via the kidneys; and to store and break down fats (see lipolysis). Other functions of the liver are (1) the production of bile; (2) the breakdown (detoxification) of poisonous substances in the blood; (3) the removal of damaged red blood cells; (4) the synthesis of vitamin A and the blood-clotting substances prothrombin and fibrinogen; and (5) the storage of iron (see ferritin).

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liverworts

See Hepatophyta.

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living fossil

Any organism whose closest relatives are extinct and that was once itself thought to be extinct. An example is the coelacanth, a primitive fish that was common in the Devonian era, the first recent living specimen of which was discovered in 1938.

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lizards

See Squamata.

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loam

A fertile soil that is made up of organic matter mixed with clay, sand, and silt. Loams differ in their ratios of clay, sand, and silt, which influences which types of plants they can support.

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lock-and-key mechanism

A mechanism proposed in 1890 by Emil Fischer (1852–1919) to explain binding between the active site of an enzyme and a substrate molecule. The active site was thought to have a fixed structure (the lock), which exactly matched the structure of a specific substrate (the key). Thus the enzyme and substrate interact to form an enzyme–substrate complex. The substrate is converted to products that no longer fit the active site and are therefore released, liberating the enzyme. Observations made by X-ray diffraction studies have shown that the active site of an enzyme is more flexible than the lock-and-key theory would suggest. Compare induced-fit model.

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locomotion

The ability of an organism to move in a particular direction in its environment, which requires a propulsive force acting against a supporting structure. Most animals and many single-celled organisms have powers of locomotion. Some protoctists possess contractile fibres that exert force on the cell membrane to change the shape of the cell; this may be combined with cytoplasmic streaming to bring about locomotion (see amoeboid movement). In many other protoctists and bacteria the propulsive force is provided by the action of undulipodia or flagella. In animals the force required to initiate locomotion is generated by voluntary muscles, which act against a supporting framework provided by a skeleton. See also fins; flight.

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locule (loculus)

A small cavity in a plant or animal body. In plants the locule of the ovary is the cavity containing the ovules and the locules of the anther contain the developing pollen grains.

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locus

(pl. loci) The position of a gene on a chromosome or within a nucleic acid molecule. The two alleles of a gene occupy the same locus on homologous chromosomes.

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lodicule

Either of two small scales situated at the base of the floret of grasses. They may represent the reduced perianth and be involved in the opening of the floret at the time of pollination.

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logarithmic scale

A scale of measurement in which an increase or decrease of one unit represents a tenfold increase or decrease in the quantity measured. Decibels and pH measurements are common examples of logarithmic scales of measurement.

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log phase

See bacterial growth curve.

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lomentum

A type of dry dehiscent fruit formed from a single carpel but divided into one-seeded compartments by constrictions between the seeds. Legumes (e.g. those of Acacia) and siliquas (e.g. those of wild radish) can be divided in this way.

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long-day plant

A plant in which flowering can be induced or enhanced by long days, usually of more than 12 hours of daylight. Examples are spinach and spring barley. See photoperiodism. Compare day-neutral plant; short-day plant.

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long-sightedness

See hypermetropia.

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loop of Henle

The hairpin-shaped section of a kidney tubule situated between the proximal and distal tubules in the nephron. The loop of Henle extends from the cortex into the medulla; it consists of a thin descending limb, which is permeable to water, and a thick ascending limb, which is impermeable to water. Complex movements of ions and water across the walls of the loop enable it to function as a countercurrent multiplier, resulting in the production of concentrated urine in the collecting duct. It is named after German anatomist F. G. J. Henle (1809–85).

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lophophore

An organ characteristic of aquatic invertebrates of the phyla Bryozoa, Phoronida, and Brachiopoda that functions in filter feeding. It consists of a ridge of hollow tentacles being cilia, which waft food particles into the mouth.

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Lorenz, Konrad Zacharias

(1903–89)
Austrian ethologist who studied medicine, becoming a lecturer at Vienna in 1937. Watching the behaviour of birds on his private estate, he made his studies of imprinting. For this work he shared the 1973 Nobel Prize for physiology or medicine with Karl von Frisch and Niko Tinbergen.

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lower critical temperature

The minimum body temperature that can be tolerated by an organism. Below this temperature, the biochemical properties of cell structures, especially membranes, are altered, and reactions are slowed such that the organism cannot maintain its usual bodily functions and death may ensue. Also, at subzero temperatures, there is a risk of the body water turning to ice, with consequent physical disruption of cells. The lower critical temperature varies greatly, depending on the `normal' temperature regime to which the organism is adapted. Hence, plants native to warm regions, e.g. maize and cotton, are much more sensitive to chilling than plants from cooler regions. Many animals have strategies for adjusting to seasonal falls in temperature, often entering a state of torpor in which metabolism is greatly reduced. In this state they may tolerate much lower body temperatures than when fully active during the summer months. Compare upper critical temperature.

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LSD

See lysergic acid diethylamide.

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luciferase

See bioluminescence.

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luciferin

See bioluminescence.

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lumbar vertebrae

The vertebrae in the region of the lower back. They occur below the thoracic vertebrae and above the sacral vertebrae. In mammals they bear processes for the attachment of back muscles.

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lumen

1 The space enclosed by a vessel, duct, or other tubular or saclike organ. The central cavity of blood vessels and of the digestive tract are examples.

2 Symbol lm. The SI unit of luminous flux equal to the flux emitted by a uniform point source of 1 candela in a solid angle of 1 steradian.
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lung

The respiratory organ of air-breathing vertebrates. A pair of lungs is situated in the thorax, within the ribcage. Each consists essentially of a thin moist membrane that is folded to increase its surface area. Exchange of oxygen and carbon dioxide takes place between blood capillaries on one side of the membrane and air on the other. The lung is supplied with air through a bronchus. In mammals and reptiles the membrane of the lung takes the form of numerous sacs (see alveolus) that are connected to the bronchus via bronchioles (see illustration). The lungs themselves contain no muscular tissue and are ventilated by respiratory movements, the mechanisms of which vary with the species.

The lungs and air passages of a mammal (right lung cut open to show internal structure)

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lung book

The respiratory organ of some arachnids. Lung books occur in pairs on the ventral side of the abdomen. They consist of leaflike folds of ectoderm sunk into pockets having slitlike openings at the abdominal surface. Gaseous exchange occurs by diffusion through the ectodermal folds.

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lungfish

See Dipnoi.

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luteal phase

The phase of the oestrous cycle (menstrual cycle in women) that occurs after ovulation. During this phase the corpus luteum secretes oestrogens and progesterone, which prepare the uterine wall to receive a fertilized egg.

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luteinizing hormone (LH; interstitial-cell-stimulating hormone; ICSH)

A hormone, secreted by the anterior pituitary gland in mammals, that stimulates in males the production of sex hormones (androgens) by the interstitial cells of the testes and in females ovulation, progesterone synthesis, and corpus luteum formation.

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luteotrophic hormone

See prolactin.

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lyase

Any of a class of enzymes that catalyse either the cleavage of a double bond and the addition of new groups to a substrate, or the formation of a double bond.

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Lycophyta (Lycopodophyta)

A phylum of tracheophyte plants containing the clubmosses (genus Lycopodium) and related genera (including Selaginella) as well as numerous extinct forms, which reached their peak in the Carboniferous period with giant coal-forming tree species. Lycophytes have roots and their stems are covered with numerous small leaves. Reproduction is by means of spores; the sporangia are usually grouped into cones.

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lymph

The colourless liquid found within the lymphatic system, into which it drains from the spaces between the cells. Lymph (called tissue fluid in the intercellular spaces) resembles blood plasma, consisting mostly of water with dissolved salts and proteins. Fats are found in suspension and their presence varies with food intake. The lymph eventually enters the bloodstream near the heart.

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lymphatic system

The network of vessels that conveys lymph from the tissue fluids to the bloodstream. Tiny lacteals (in the small intestine) and lymph capillaries (in other tissues) drain into larger tubular vessels that converge to form the right lymphatic duct and the thoracic duct, which connect with the venous blood supply to the heart. Associated with the lymphatic vessels at intervals along the system are the lymph nodes. The lymph capillary walls are very permeable, so lymph bathing the body's tissues can drain away molecules that are too large to pass through blood capillary walls. In most vertebrates lymph is pumped by cycles of contraction and relaxation of the lymphatic vessels and also by the action of adjoining muscles. See also lymph heart.

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lymph capillary

See lymphatic system.

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lymph heart

Any of a series of muscular pumping structures that occur in the major lymphatic vessels of certain lower vertebrates and maintain a flow of lymph around the body. Many animals (including all mammals) do not possess lymph hearts; in these the flow of lymph is maintained by a series of muscle contractions and valves (see lymphatic system).

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lymph node

A mass of lymphoid tissue, many of which occur at intervals along the lymphatic system. Lymph in the lymphatic vessels flows through the lymph nodes, which filter out bacteria and other foreign particles, so preventing them from entering the bloodstream and causing infection. The lymph nodes also produce lymphocytes. In humans, major lymph nodes occur in the neck, under the arms, and in the groin.

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lymphocyte

A type of white blood cell (leucocyte) that has a large nucleus and little cytoplasm. Lymphocytes are formed in the lymph nodes and provide about a quarter of all leucocytes. They are important in the body's defence and are responsible for immune reactions as the presence of antigens stimulates them to produce antibodies. There are two populations of lymphocytes: B lymphocytes (see B cell), which produce circulating antibodies and are responsible for humoral immunity; and T lymphocytes (see T cell), which are responsible for cell-mediated immunity. See also killer cell.

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lymphoid tissue

The type of tissue found in the lymph nodes, tonsils, spleen, and thymus. It is responsible for producing lymphocytes and therefore contributes to the body's defence against infection.

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lymphokine

Any soluble factor secreted by lymphocytes that plays a role in cell-mediated immunity by influencing the activity of other cells. Examples include the interleukins, macrophage-activating factor (MAF), and macrophage migration inhibition factor (MIF). See also cytokine.

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lymphoma

See cancer.

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lyophilization

See freeze drying.

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Lysenkoism

The official Soviet science policy governing the work of geneticists in the USSR from about 1940 to 1960. It was named after its chief promoter, the agriculturalist Trofim Lysenko (1898–1976). Lysenkoism dismissed all the advances that had been made in classical genetics, denying the existence of genes, and held that the variability of organisms was produced solely by environmental changes. There was also a return to a belief in the inheritance of acquired characteristics (see Lamarckism). This state of affairs continued, despite overwhelming conflicting evidence from Western scientists, because it provided support for communist theory.

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lysergic acid diethylamide (LSD)

A chemical derivative of lysergic acid that has potent hallucinogenic properties (see hallucinogen). It occurs in the cereal fungus ergot and was first synthesized in 1943. LSD acts as an antagonist at serotonin receptors.

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lysigeny

The localized disruption of plant cells to form a cavity (surrounded by remnants of the broken cells) in which secretions accumulate. Examples are the oil cavities in the leaves of citrus trees. Compare schizogeny.

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lysimeter

An instrument used to measure the loss of water from an area of land covered with vegetation. Water that evaporates from both the soil and from the plants can be determined.

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lysine

See amino acid.

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lysis

The destruction of a living cell. This may be effected by lysosomes or lymphocytes, either as part of the normal metabolic process (as when cells are damaged or worn out) or as a reaction against invading cells (e.g. bacteria). Bacteriophages eventually cause lysis of their host cells.

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lysogeny

The relationship between a temperate phage (see bacteriophage) and a bacterium. A bacterium whose chromosome has been integrated with the DNA of a temperate phage is called a lysogen ; the viral DNA is known as a prophage.

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lysosome

A membrane-bound sac within the cytoplasm of eukaryote cells that contains enzymes responsible for the digestion of material in food vacuoles, the dissolution of foreign particles entering the cell (such as bacteria) and, on the death of the cell, the breakdown of all cell structures.

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lysozyme

An antibacterial enzyme widely distributed in body fluids and secretions, including tears and saliva. It disrupts the polysaccharide components of bacterial cell walls, leaving them susceptible to destruction.

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M

McClintock, Barbara

(1902–92)
US botanist and geneticist, who joined the Cold Spring Harbor Laboratory of the Carnegie Institute. She is best known for her discovery of `jumping genes' (see transposon), which move along a chromosome and exert control over other genes. She carried out her work with maize plants, but such controlling elements were later found in bacteria and other organisms. For this work she was awarded the 1983 Nobel Prize for physiology or medicine.

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macroevolution

Evolution on a relatively large scale, involving, for example, the emergence of entire groups of organisms, such as the flowering plants or the mammals. Compare microevolution.

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macrofauna

The larger animals, collectively, which can be observed without the aid of a microscope (compare microfauna). The macrofauna sometimes includes small soil-dwelling invertebrates, such as annelids and nematodes, but these may be separated into an intermediate category, the mesofauna . Compare meiofauna.

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macromolecule

A very large molecule. Natural and synthetic polymers have macromolecules, as do such substances as haemoglobin. See also colloids.

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macronutrient

A chemical element required by plants in relatively large amounts. Macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulphur, magnesium, calcium, and iron. See also essential element. Compare micronutrient.

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macrophage (histiocyte; clasmatocyte)

A large phagocytic cell (see phagocyte) found in the spleen, lymph nodes, liver, and many other tissues. Macrophages ingest bacteria and cell debris and are responsible for the breakdown of worn-out red blood cells. They develop from monocytes and are characterized by a horseshoe-shaped nucleus and nongranular cytoplasm. Tissues containing a high concentration of macrophages are known collectively as the reticuloendothelial system .

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macrophagous

Describing a method of feeding in heterotrophic organisms in which food is ingested in the form of relatively large chunks. Compare microphagous.

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macrophyll

See megaphyll.

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macula

1 A patch of sensory hair cells in the utriculus and sacculus of the inner ear that provides information about the position of the body in relation to gravity. The hairs of the cells are embedded in an otolith , a gelatinous cap containing particles of calcium carbonate. Movement of the particles in response to gravity pulls the gelatinous mass downwards, which bends the hairs and triggers a nerve impulse to the brain (see illustration).

Macula (when the head is positioned at 90° to the direction of the force of gravity)

2 An area of the retina of the vertebrate eye with increased visual acuity. Maculae occur in some animals that lack foveae and often surround foveae in those animals that possess them.
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3 (macular adherens) See desmosome.
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magnesium

Symbol Mg. A silvery metallic element that is an essential element for living organisms. It functions as a cofactor for various enzymes and is a constituent of chlorophyll in plants. In animals it is involved in the transmission of nerve impulses.

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magnetic resonance imaging (MRI)

See nuclear magnetic resonance.

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magnetoreceptor

A device or organ that detects magnetic fields, particularly the earth's magnetic field. Some form of magnetic sense is found in a wide range of animals, including insects, fishes, amphibians, reptiles, birds, and mammals, but the nature of the magnetoreceptor organs remains elusive. Many species have in their brains small particles of the magnetic material magnetite, which are postulated to transduce the magnetic field into nervous impulses. The animal's photoreceptors may also be involved, and there is evidence to support this in various invertebrate and vertebrate species. One theory is that changes in electron spin resonance caused by the earth's magnetic field elicit nerve impulses. Certain fishes, particularly sharks and rays, use their electroreceptors as magnetoreceptors. This is possible because the animals themselves act as conductors moving through the magnetic field, and so generate electric currents, albeit very weak ones. Seawater is the conductor that completes the circuit and permits detection by the animal's own sense organs. Such a sense is exploited by the animals in navigating through the marine environment.

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major histocompatibility complex (MHC)

A large gene cluster that encodes various components of the immune system, including the histocompatibility proteins and components of the complement system. In humans the MHC is located on chromosome 6 and includes the HLA system. Other vertebrate species have similar MHC regions. Certain MHC genes can have many variant alleles; this produces an enormous diversity of proteins in a population, each individual possessing a unique set.

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malaria

A disease caused by the parasitic protozoan Plasmodium, which requires two hosts, the bloodsucking female Anopheles mosquito and man, in order to complete its complex life cycle. Symptoms of fever and anaemia in humans are caused by invasion and destruction of the red blood cells during an asexual phase of the life cycle. See Apicomplexa.

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male

1 Denoting the gamete (sex cell) that, during sexual reproduction, fuses with a female gamete in the process of fertilization. Male gametes are generally smaller than the female gametes and are usually motile (see spermatozoon).

2 (Denoting) an individual whose reproductive organs produce only male gametes. Compare hermaphrodite.
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malic acid (2-hydroxybutanedioic acid)

A crystalline solid, HOOCCH(OH)CH_2COOH. L-malic acid occurs in living organisms as an intermediate metabolite in the Krebs cycle and also (in certain plants) in photosynthesis. It is found especially in the juice of unripe fruits, e.g. green apples.

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malignant

Describing a mutant cell or group of cells that proliferates at a faster rate than normal cells and has the capacity to spread to other sites in the body. See cancer.

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malleus (hammer)

The first of the three ear ossicles of the mammalian middle ear.

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Mallophaga

An order of secondarily wingless insects comprising the bird lice. Bird lice are minute with dorsoventrally flattened ovoid bodies, reduced eyes, and biting mouthparts. They are ectoparasites of birds, feeding on particles of dead skin, feather fragments, and sometimes blood. The eggs hatch to form nymphs resembling the adults.

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malnutrition

The condition arising due to the lack of one or more of the nutrients that are required in the diet to maintain health. Malnutrition can result from a reduced intake of nutrients (undernourishment ), an inability to use absorbed nutrients, failure to meet a required increase in nutrient intake, or nutrient losses. There are three stages in the process of malnutrition: first, the carbohydrate stores in the body are depleted; secondly, the fat reserves are metabolized (see fatty-acid oxidation); and finally, proteins are broken down to provide energy. Death may result after protein levels have been reduced to half their normal value. Kwashiorkor is a type of malnutrition that develops when the diet lacks proteins and hence essential amino acids. Malnutrition due to reduced absorption of nutrients in the intestine can develop with a cereal-based diet, due to sensitivity of the intestinal lining to gluten, a protein found in cereals. See also mineral deficiency.

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Malpighian body (Malpighian corpuscle)

The part of a nephron in the kidney that consists of its cup-shaped end together with the glomerulus that it encloses. It is named after its discoverer, the Italian anatomist M. Malpighi (1628–94).

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Malpighian layer (stratum germinativum)

The innermost layer of the epidermis of mammalian skin, separated from the underlying dermis by a fibrous basement membrane. It is only in this layer of the epidermis that active cell division (mitosis) occurs. As the cells produced by these divisions age and mature, they migrate upwards through the layers of the epidermis to replace the cells being continuously worn away at the surface.

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Malpighian tubule

Any of the organs that are involved in the excretion of nitrogenous waste in insects, arachnids, and centipedes. They lie in the haemocoel. In insects Malpighian tubules open into the intestine; they selectively extract from the blood uric acid, which – together with water and salts – is deposited into the hindgut and excreted in the faeces. Some reabsorption of water and salts can occur in the tubule itself but most reabsorption occurs in the hindgut. See also cryptonephridial system.

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malt

The product of the hydrolysis of starch by ß-amylase that occurs during the germination of barley in brewing. See also maltose.

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maltase (-glucosidase)

A membrane-bound enzyme in the small intestine that hydrolyses the disaccharide maltose into glucose.

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maltose (malt sugar)

A sugar consisting of two linked glucose molecules that results from the action of the enzyme amylase on starch. Maltose occurs in high concentrations in germinating seeds; malt, used in the manufacture of beer and malt whisky, is produced by allowing barley seeds to germinate and then slowly drying them.

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malt sugar

See maltose.

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Mammalia

A class of vertebrates containing some 4250 species. Mammals are warm-blooded animals (see homoiothermy), typically having sweat glands whose secretion cools the skin and an insulating body covering of hair. All mammals have mammary glands, which secrete milk to nourish the young. Mammalian teeth are differentiated into incisors, canines, premolars, and molars and the middle ear contains three sound-conducting ear ossicles. The four-chambered heart enables complete separation of oxygenated and deoxygenated blood and a muscular diaphragm takes part in breathing movements, both of which ensure that the tissues are well supplied with oxygen. This, together with well-developed sense organs and brain, have enabled mammals to pursue an active life and to colonize a wide variety of habitats.
Mammals evolved from carnivorous reptiles in the Triassic period about 225 million years ago. There are two subclasses: the primitive egg-laying Prototheria (monotremes) and the Theria, which includes all other mammals and consists of the infraclasses Metatheria (marsupials) and Eutheria (placental mammals).

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mammary glands

The milk-producing organs (possibly modified sweat glands) of female mammals, which provide food for the young (see milk; colostrum). Their number (2 to 20) and position (on the chest or abdomen) vary according to the species. In most mammals the gland openings project as a nipple or teat . Nipples have a number of milk-duct openings; teats have one duct leading from a storage cavity.

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mandible

1 One of a pair of horny mouthparts in insects, crustaceans, centipedes, and millipedes. The mandibles lie in front of the weaker maxillae and their lateral movements assist in biting and crushing the food.

2 The lower jaw of vertebrates.
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3 Either of the two parts of a bird's beak.
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Mandibulata

See Uniramia.

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manganese

Symbol Mn. A grey brittle metallic element that is a trace element (see essential element) for living organisms. It functions as a cofactor for several enzymes.

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mangrove swamp

A region of vegetation, found along tropical coasts, in which mangrove trees (Rhizophora species) predominate. The waterlogged soil is highly saline, and – like other halophytes – mangroves are adapted to withstand these conditions; they also possess aerial roots (pneumatophores ) through which gaseous exchange occurs, to counteract effects of the badly aerated soil.

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mannan

See mannose.

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mannitol

A polyhydric alcohol, CH_2OH(CHOH)4CH_2OH, derived from mannose or fructose. It is the main soluble sugar in fungi and an important carbohydrate reserve in brown algae. Mannitol is used as a sweetener in certain foodstuffs.

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mannose

A monosaccharide, C_6H_12O_6, stereoisomeric with glucose, that occurs naturally only in polymerized forms called mannans . These are found in plants, fungi, and bacteria, serving as food energy stores.

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manometer

A device for measuring pressure differences, usually by the difference in height of two liquid columns. The simplest type is the U-tube manometer, which consists of a glass tube bent into the shape of a U. If a pressure to be measured is fed to one side of the U-tube and the other is open to the atmosphere, the difference in level of the liquid in the two limbs gives a measure of the unknown pressure.

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mantle

The fold of skin covering the dorsal surface of the body of molluscs, which extends into lateral flaps that protect the gills in the mantle cavity (the space between the body and mantle). The outer surface of the mantle secretes the shell (in species that have shells).

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marker gene

1 A gene used to identify a particular bacterial colony or bacteriophage plaque. Such genes are incorporated into cloning vectors to enable the isolation and replication of colonies containing a desired vector. Typically, marker genes confer resistance to specific antibiotics or produce colour changes.

2 (or genetic marker) A gene or short sequence of DNA that acts as a tag for another, closely linked, gene. Such markers are used in mapping the order of genes along chromosomes and in following the inheritance of particular genes: genes closely linked to the marker will generally be inherited with it. Markers must be readily identifiable in the phenotype, for instance by controlling an easily observable feature (such as eye colour). Restriction fragment length polymorphisms (RFLPs) are now widely used as markers, for example in tracking the inheritance of genes associated with diseases (see gene tracking).
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marsupials

See Metatheria.

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masquerade

The resemblance of an organism to some inanimate object in its environment so that it remains effectively 'hidden' from predators. Many insects have evolved particular shapes and colours so they resemble leaves, twigs, or other features of their natural surroundings, making it difficult for predators to detect them visually. For example, some larvae look like bird droppings, certain butterflies have very leaflike wings, while stick insects fully live up to their name. See also camouflage.

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mass flow (pressure flow)

A hypothesis to explain the movement of sugars in the phloem tissue of plants. At a source (site of production) sugars are actively secreted from phloem companion cells into the sieve elements, causing water to follow by osmosis. The pressure of water in the tubes (the hydrostatic pressure) causes it to move along the tubes to a sink (site of utilization), where the reverse process occurs. Here sugars are actively transported from the sieve elements into the companion cells and then into the surrounding tissues, establishing a concentration gradient from source to sink. Although different solutes can be transported in the phloem in different directions at the same time, it is argued that the mass flow hypothesis can still apply provided transport occurs in different sieve elements.

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mass spectroscopy

A technique used to determine relative atomic masses and the relative abundance of isotopes and for chemical analysis, as in the identification of metabolites, drugs, and other molecules isolated from biological samples. In a mass spectrometer a sample (usually gaseous) is ionized and the positive ions produced are accelerated into a high-vacuum region containing electric and magnetic fields. These fields deflect and focus the ions onto a detector. The fields can be varied in a controlled way so that ions of different types can impinge on the detector. A mass spectrum is thus obtained consisting of a series of peaks of variable intensity to which mass/charge (m/e) values can be assigned. For organic molecules, the mass spectrum consists of a series of peaks, one corresponding to the parent ion and the others to fragment ions produced by the ionization process. Different molecules can be identified by their characteristic pattern of lines. Analysis of mixtures can be done by gas chromatography–mass spectroscopy (see gas–liquid chromatography).

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mast cells

Large amoeboid cells that are found in vertebrate connective tissue. They may be connected with the deposition of fat. In certain situations they produce histamine and serotonin, responsible for allergy symptoms. They also produce heparin, an anticoagulant, which may prevent fibrin from forming clots in the intercellular spaces.

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mastication

The process of chewing food, which involves movements of the jaws and teeth. Mastication breaks up the food into small particles, which provides a greater surface area for digestion and enables the formation of a bolus, which is small enough to pass through the oesophagus.

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mastoid process

An outgrowth from the temporal bone of the skull containing air cavities that communicate with the cavity of the middle ear. In man it is a route through which infection may spread from the middle ear.

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mating

See sexual intercourse.

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mating season

See breeding season.

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mating type

The equivalent in microorganisms, fungi, and algae of the male and female individuals of higher organisms. Mating types are morphologically the same and therefore difficult to distinguish. For this reason different mating types, of the same species, are designated as + and –. The mating types are distinct strains that are compatible in sexual reproduction; they are able to give rise to a zygote, which can develop into a new individual.

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matrix

(in histology) The component of tissues (e.g. bone and cartilage) in which the cells of the tissue are embedded.

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maturation

1 The process of becoming fully developed, especially the final phase in the development of germ cells, which renders the egg or sperm capable of fertilization.

2 Changes in the neuromuscular system as an animal develops that improve coordination regardless of experience.
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maturation-promoting factor (M-phase-promoting factor; MPF)

A kinase that induces mitosis in eukaryotic cells by promoting the transition from the G_2 stage to the M stage in the cell cycle. MPF catalyses the phosphorylation of proteins that may in turn cause the breakdown of the nuclear membrane. It may also phosphorylate certain histone proteins, possibly leading to the condensation of chromosomes before the M phase.

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maturity

1 The stage in a life cycle that is reached when a developing organism has taken on the appearance of the adult form and is capable of reproduction.

2 The stage reached in the formation of gametes (gametogenesis) following meiotic division of precursor cells and their development into fully functional gametes.
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maxicell

A bacterial cell that has been irradiated with ultraviolet light to intentionally damage the cell's own DNA so that plasmid genes, which are relatively spared, can be preferentially expressed.

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maxilla

(pl. maxillae) 1 One of a pair of mouthparts in insects, crustaceans, centipedes, and millipedes. They lie behind the mandibles and their lateral movements assist in feeding. Crustaceans have two pairs of maxillae but in insects the second pair are fused together forming the labium.

2 One of a pair of large tooth-bearing bones in the upper jaw of vertebrates. In mammals they carry all the upper teeth except the incisors.
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maxilliped

A paired appendage in crustaceans that contributes to the mouthparts. There may be up to three pairs of maxillipeds, which are located behind the mandibles and are used to move food to the mouth.

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maxillula

A paired appendage that contributes to the mouthparts in certain crustaceans. Lying between the mandibles and the maxillae, they may bear hairs or spines designed to move food particles deeper into the mouth.

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maximum permissible dose

See dose.

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M band (M line)

See sarcomere.

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mean

The average value of a set of n numbers, i.e. the sum of the numbers divided by n.

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meatus

A small canal or passage in the body. An example is the external auditory meatus of the outer ear in mammals, which connects the exterior opening to the eardrum.

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mechanoreceptor

A receptor that responds to such mechanical stimuli as touch, sound, and pressure. The skin is rich in mechanoreceptors.

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meconium

The first faecal material passed by a newborn mammal, which is usually dark green in colour.

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Medawar, Sir Peter Brian

(1915–87)
British immunologist. Born in Brazil and educated at Oxford, he held posts in zoology there and at Birmgingham and London. He turned to medical biology and studied rejection in tissue grafts, experimenting with mouse embryos and demonstrating the phenomenon of acquired immunological tolerance. For this work he shared the 1960 Nobel Prize for physiology or medicine with Sir Macfarlane Burnet.

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median

The middle number or value in a series of numbers or values.

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median eye (pineal eye)

An eyelike structure, with a lens and retina, found on the top of the head of some lizards, Sphenodon, and the Cyclostomata (lampreys) as well as in many fossil vertebrates. It corresponds to the pineal gland of other vertebrates and is thought to act as a photoreceptor, detecting changes in light intensity and modifying the physiology and behaviour of the animal.

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median lethal dose

See LD50.

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mediastinum

1 A membrane in the midline of the thorax of mammals that separates the lungs.

2 The space between the two lungs, which is occupied by the heart and oesophagus.
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medulla

1 (in zoology) The central tissue of various organs, including the adrenal glands (adrenal medulla ) and kidneys (renal medulla ).

2 (in botany) See pith.
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medulla oblongata

Part of the vertebrate brainstem, derived from the hindbrain, that is continuous with the spinal cord. Its function is to regulate the reflex responses controlling respiration, heart beat, blood pressure, and other involuntary processes. It gives rise to many of the cranial nerves.

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medullary ray (ray)

Any of the vertical plates of parenchyma cells running radially through the cylinder of vascular tissue in the stems and roots of plants. Each may be one to many cells in width. Primary medullary rays occur in young plants and in those not showing secondary thickening; they pass from the cortex through to the pith. Secondary medullary rays are produced by the vascular cambium and terminate in xylem and phloem tissues. Medullary rays store and transport food materials.

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medullated nerve fibre

A nerve fibre that is characterized by a myelin sheath, which insulates the axon.

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medusa

The free-swimming stage in the life cycle of the Cnidaria. Medusae are umbrella-shaped, with tentacles round the edge and the mouth in the centre underneath. They swim by pulsations of the body and reproduce sexually. In the Hydrozoa (e.g. Hydra) they alternate in the life cycle with polyps, from which they are produced by budding. In the Scyphozoa, which includes all the common jellyfish, the medusa is the dominant form and the polyp is reduced or absent.

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mega-

1 A prefix denoting large size; e.g. meganucleus, megasporangium.

2 Symbol M. A prefix used in the metric system to denote one million times. For example, 10^6 volts = 1 megavolt (MV).
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meganucleus

See nucleus.

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megaphyll

A type of foliage leaf in ferns and seed plants that has branched or parallel vascular bundles running through the lamina. The megaphylls of ferns are large pinnate leaves called fronds . A megaphyll was formerly called a macrophyll . Compare microphyll.

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megasporangium

See sporangium.

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megaspore

See megaspore mother cell; sporophyll.

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megaspore mother cell (megasporocyte)

A diploid cell in plants that divides by meiosis to give rise to four haploid megaspores (see sporophyll). In flowering plants the megaspore mother cell (or embryo mother cell ) is situated in the ovule. One of the megaspores it produces develops into the embryo sac; the others abort.

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megasporocyte

See megaspore mother cell.

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megasporophyll

See sporophyll.

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meiofauna

1 (meiobenthic fauna) Animals inhabiting the bottom of a river, lake, or sea that are just visible to the naked eye; for example, small polychaetes, bivalves, and nematodes. They have dimensions in the range 0.1 to 1 mm, intermediate between microfauna and macrofauna.

2 See interstitial fauna.
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meiosis (reduction division)

A type of nuclear division that gives rise to four reproductive cells (gametes) each with half the chromosome number of the parent cell. Two consecutive divisions occur (see illustration). In the first, homologous chromosomes become paired and may exchange genetic material (see crossing over) before moving away from each other into separate daughter nuclei. This is the actual reduction division because each of the two nuclei so formed contains only half of the original chromosomes. The daughter nuclei then divide by mitosis and four haploid cells are produced. See also prophase; metaphase; anaphase; telophase.

The stages of meiosis in a cell containing two pairs of homologous chromosomes

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Meissner's corpuscles

Receptors for touch that are found in the papillary (upper) region of the dermis of the skin. Named after German anatomist Georg Meissner (1829–1905), each consists of a mass of dendrites encapsulated by connective tissue. The corpuscles are found in abundance in the palms of the hands, fingertips, and soles of the feet as well as in the nipples and tip of the penis.

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melanin

Any of a group of polymers, derived from the amino acid tyrosine, that cause pigmentation of eyes, skin, and hair in vertebrates. Melanins are produced by specialized epidermal cells called melanophores (or melanocytes ); their dispersion in these cells is controlled by melanocyte-stimulating hormone and melatonin. Certain invertebrates, fungi, and microorganisms also produce melanin pigments. The `ink' of the octopus and squid is a notable example. Hereditary albinism is caused by the absence of the enzyme tyrosinase, which is necessary for melanin production.

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melanism

Black coloration of the body caused by overproduction of the pigment melanin, often as a reaction to the environment. There are several species of melanic moths in industrially polluted areas (see industrial melanism) and the panther is a melanic form of leopard.

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melanocyte-stimulating hormone (MSH)

A hormone, secreted in the anterior region of the pituitary gland, that stimulates the concentration of melanin granules in the chromatophores of the skin of lower vertebrates, such as amphibians. The role of MSH in humans is not clearly understood.

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melanophore

See chromatophore.

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melatonin

A hormone derived from serotonin and secreted by the pineal gland and retinas of vertebrates. Melatonin secretion by the pineal is linked to the dark–light cycle of the organism's environment, being greatest at night and lowest by day. The hormone is involved in regulating certain diurnal and seasonal changes in the body, such as the reproductive cycle in seasonally breeding animals. Melatonin also controls pigmentation changes; it triggers aggregation of the pigment melanin into melanophores in the skin, causing the skin to turn pale.

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membrane

A sheetlike tissue that covers, connects, or lines biological cells and their organelles, organs, and other structures. Membranes consist of polar lipids (predominantly the phospholipids lecithin and cephalin) and protein: the lipids form a double layer (see lipid bilayer) with protein molecules suspended within it. Membranes are permeable to water and fat-soluble substances but impermeable to polar substances (e.g. sugars). Movement of ions across a membrane occurs by carrier-mediated processes, including active transport and facilitated diffusion. See also cell membrane; osmosis.

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membrane bone (dermal bone)

Bone formed directly in connective tissue, i.e. by intramembranous ossification, rather than by replacing cartilage (compare cartilage bone). Some face bones, skull bones, and part of the clavicle are membrane bones. Small areas of membrane become jelly-like and attract calcium salts. Bone-forming cells break down these areas forming a bone lattice, which eventually fills in.

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membranous labyrinth

The soft tubular sensory structures that form the inner ear of vertebrates and are housed within the bony labyrinth.

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memory

The means by which information is stored in the brain. The exact mechanism of processing and storing information is not known but is thought to involve the construction of circuits of neurones, which are strengthened by repeated use. Memory is essential to the processes of learning and recognition of individuals and objects.

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memory cell

See B cell.

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Mendel, Johann Gregor

(1822–84)
Austrian geneticist, who from 1843 lived as a monk in Brün (now Brno, in the Czech Republic). His fame rests on the plant-breeding experiments he began in 1856, which eventually produced the rules of inheritance summarized in Mendel's laws. His work was ignored during his lifetime and only rediscovered in 1900 by Hugo de Vries and others.

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Mendelism

The theory of heredity that forms the basis of classical genetics, proposed by Gregor Mendel in 1866 and formulated in two laws (see Mendel's laws; particulate inheritance). Mendel suggested that individual characteristics were determined by inherited `factors', and when improved microscopes revealed details of cell structure the behaviour of Mendel's factors could be related to the behaviour of chromosomes during meiosis.

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Mendel's laws

Two laws summarizing Gregor Mendel's theory of inheritance (see also Mendelism). The Law of Segregation states that each hereditary characteristic is controlled by two `factors' (now called alleles), which segregate (separate) and pass into separate germ (reproductive) cells. The Law of Independent Assortment states that pairs of `factors' segregate independently of each other when germ cells are formed (see also independent assortment). These laws are the foundation of genetics.

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meninges

The three membranes that surround the brain and spinal cord of vertebrates: the pia mater, the arachnoid membrane, and the outer dura mater. The pia and arachnoid are separated by the subarachnoid space , which contains cerebrospinal fluid.

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meniscus

See surface tension.

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menopause

The time in a woman's life when ovulation and menstruation cease (see menstrual cycle). It normally occurs between the ages of 45 and 55. The effects of the gonadotrophic hormones, follicle-stimulating hormone and luteinizing hormone, in the ovaries decrease so that the follicles do not develop properly. There is a change in the balance of the hormones oestrogen and progesterone, secreted by the ovaries, which may be associated with certain physical symptoms, such as weight gain and `hot flushes', and there may also be mood changes. These symptoms can be treated by long-term hormone replacement therapy with oestrogens and progestogens.

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menstrual cycle (sexual cycle)

The approximately monthly cycle of events associated with ovulation that replaces the oestrous cycle in most primates (including humans). The lining of the uterus becomes progressively thicker with more blood vessels in preparation for the implantation of a fertilized egg cell (blastocyst). Ovulation occurs during the middle of the cycle (the fertile period). If fertilization does not occur the uterine lining breaks down and is discharged from the body (menstruation ); the discharge is known as a `period'. In women the fertile period is 11–15 days after the end of the last menstruation.

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menstruation

See menstrual cycle.

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mericarp

See schizocarp.

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meristem

A plant tissue consisting of actively dividing cells that give rise to cells that differentiate into new tissues of the plant. The most important meristems are those occurring at the tip of the shoot and root (see apical meristem) and the lateral meristems in the older parts of the plant (see cambium; cork cambium).

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meristoderm

The outer meristematic layer of the thallus of certain brown algae (Phaeophyta). The cells of the meristoderm divide to increase the width of the thallus.

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merocrine secretion

See secretion.

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meromictic lake

See dimictic lake.

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Merostomata

See Chelicerata.

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merozygote

A bacterial cell that contains more than the haploid number of chromosomes but less than the full diploid number. Merozygotes arise when the genetic material from one bacterial cell is only partially transferred into another cell during conjugation, transduction, or transformation.

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mesencephalon

See midbrain.

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mesenteric artery

The artery that delivers blood to the intestines.

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mesentery

A thin sheet of tissue, bounded on each side by peritoneum, that supports the gut and other organs in the body cavities of animals. Vertebrates have a well-developed dorsal mesentery that anchors the stomach and intestine and contains blood vessels and nerves supplying the gut. The reproductive organs and their ducts are also supported by mesenteries.

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mesocarp

See pericarp.

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mesoderm

The layer of cells in the gastrula that lies between the ectoderm and endoderm. It develops into the muscles, circulatory system, and sex organs and in vertebrates also into the excretory system and skeleton. See also germ layers.

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mesofauna

See macrofauna.

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mesoglea

The gelatinous noncellular layer between the endoderm and ectoderm in the body wall of coelenterates. It may be thin, as in Hydra, or tough and fibrous, as in the larger jellyfish and sea anemones. It often contains cells that have migrated from the two body layers but these do not form tissues and organs and the mesoglea is not homologous with the mesoderm of triploblastic animals.

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mesopelagic zone

See euphotic zone.

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mesophilic

Describing an organism that lives and grows optimally at moderate temperatures, typically between 10 and 40°C. The vast majority of organisms are mesophiles, occupying all the major biomes of temperate and tropical climates. Compare psychrophilic; thermophilic.

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mesophyll

The internal tissue of a leaf blade (lamina), consisting of parenchyma cells. There are two distinct forms. Palisade mesophyll lies just beneath the upper epidermis and consists of cells elongated at right angles to the leaf surface. They contain a large number of chloroplasts and their principal function is photosynthesis. Spongy mesophyll occupies most of the remainder of the lamina. It consists of spherical loosely arranged cells containing fewer chloroplasts than the palisade mesophyll. Between these cells are air spaces leading to the stomata.

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mesophyte

Any plant adapted to grow in soil that is well supplied with water and mineral salts. Such plants wilt easily when exposed to drought conditions as they are not adapted to conserve water. The majority of flowering plants are mesophytes. Compare halophyte; hydrophyte; xerophyte.

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mesothelium

A single layer of thin platelike cells covering the surface of the inside of the abdominal cavity and thorax and surrounding the heart, forming part of the peritoneum and pleura (see serous membrane). It is derived from the mesoderm. Compare endothelium; epithelium.

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mesotrophic

Describing a body of water, such as a lake, that is intermediate between a eutrophic lake and an oligotrophic lake in the amount of nutrients contained within it.

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Mesozoic

The geological era that extended from the end of the Palaeozoic era, about 248 million years ago, to the beginning of the Cenozoic era, about 65 million years ago. It comprises the Triassic, Jurassic, and Cretaceous periods. The Mesozoic era is often known as the Age of Reptiles as these animals, which included the dinosaurs, pterosaurs, and ichthyosaurs, became the dominant lifeform; most became extinct before the end of the era.

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messenger RNA

See RNA.

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metabolic pathway

See metabolism.

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metabolic rate

A measure of the energy used by an animal in a given time period. The metabolic rate of an animal is affected by several interacting factors, including temperature and the level of activity. The metabolic rate of a resting animal is known as the basal metabolic rate (BMR).

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metabolic waste

The waste products, collectively, of metabolism.

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metabolism

The sum of the chemical reactions that occur within living organisms. The various compounds that take part in or are formed by these reactions are called metabolites . In animals many metabolites are obtained by the digestion of food, whereas in plants only the basic starting materials (carbon dioxide, water, and minerals) are externally derived. The synthesis (anabolism) and breakdown (catabolism) of most compounds occurs by a number of reaction steps, the reaction sequence being termed a metabolic pathway . Some pathways (e.g. glycolysis) are linear; others (e.g. the Krebs cycle) are cyclic. The changes at each step in a pathway are usually small and are promoted by efficient biological catalysts – the enzymes. In this way the amounts of energy required or released at any given stage are minimal, which helps in maintaining a constant internal environment. Various feedback mechanisms exist to govern metabolic rates.

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metabolite

See metabolism.

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metacarpal

One of the bones in the metacarpus.

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metacarpus

The hand (or corresponding part of the forelimb) in terrestrial vertebrates, consisting of a number of rod-shaped bones (metacarpals ) that articulate with the bones of the wrist (see carpus) and those of the fingers (see phalanges). The number of metacarpals varies between species: in the basic pentadactyl limb there are five, but this number is reduced in many species.

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metacentric

See centromere.

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metafemale

A fruit fly (Drosophila sp.) with three instead of the normal two X chromosomes. In such flies the disproportion of sex chromosomes to autosomes leads to impaired development and often failure to emerge from the pupae. Compare metamale.

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metamale

A fruit fly (Drosophila sp.) with three (instead of the normal two) copies of each autosome. The disproportion of autosomes to the normal complement of one X chromosome (plus one Y chromosome, which plays little or no part in sex determination) impairs development, and such flies are typically weak and sterile. Compare metafemale.

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metameric segmentation (metamerism; segmentation)

The division of an animal's body (except at the head region – see cephalization) into a number of compartments (segments or metameres ) each containing the same organs. Metameric segmentation is most strongly marked in annelid worms (e.g. earthworms), in which the muscles, blood vessels, nerves, etc. are repeated in each segment. In these animals the segmentation is obvious both externally and internally. It also occurs internally in arthropods and in the embryonic development of all vertebrates, in which it is confined to parts of the muscular, skeletal, and nervous systems and does not show externally.

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metamorphosis

The rapid transformation from the larval to the adult form that occurs in the life cycle of many invertebrates and amphibians. Examples are the changes from a tadpole to an adult frog and from a pupa to an adult insect. Metamorphosis often involves considerable destruction of larval tissues by lysosomes, and in both insects and amphibians it is controlled by hormones.

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metaphase

The stage of cell division during which the membrane around the nucleus breaks down, the spindle forms, and centromeres attach the chromosomes to the equator of the spindle. In the first metaphase of meiosis pairs of chromosomes (bivalents) are attached, while in mitosis and the second metaphase of meiosis, individual chromosomes are attached.

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metaphloem

The part of the primary phloem that develops in a plant after the stem or other part has finished elongating. Compare protophloem.

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metaplasia

The transformation of a tissue into a different type. This is an abnormal process; for example, metaplasia of the epithelium of the bronchi may be an early sign of cancer.

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metastasis

See cancer.

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metatarsal

One of the bones in the metatarsus.

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metatarsus

The foot (or corresponding part of the hindlimb) in terrestrial vertebrates, consisting of a number of rod-shaped bones (metatarsals ) that articulate with the bones of the ankle (see tarsus) and those of the toes (see phalanges). The number of metatarsals varies between species: in the basic pentadactyl limb there are five, but this number is reduced in some species.

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Metatheria

An infraclass of mammals containing the marsupials. The female bears an abdominal pouch (marsupium ) into which the newly born young, which are in a very immature state, move to complete their development. They obtain nourishment from the mother's mammary teats. Modern marsupials are restricted to Australasia (where they include the kangaroos, koala bears, phalangers, and bandicoots) and America (the opossums). Marsupials evolved during the late Cretaceous period, 80 million years ago. In Australia, where the marsupials have been isolated for millions of years, they show the greatest diversity of form, having undergone adaptive radiation to many of the niches occupied by placental mammals elsewhere. Compare Eutheria; Prototheria.

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metaxylem

The part of the primary xylem that develops in a plant after the stem or other part has finished elongating (compare protoxylem). The walls of the metaxylem are more extensively lignified than those of the protoxylem.

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Metazoa (Eumetazoa)

A subkingdom comprising all multicellular animals. It excludes the Porifera (sponges), which are placed in a separate subkingdom, Parazoa.

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methanogen

Any of various archaebacteria (see Archaea) that produce methane; they include such genera as Methanobacillus and Methanothrix. Methanogens are obligate anaerobes found in oxygen-deficient environments, such as marshes, swamps, sludge (formed during sewage treatment), and the digestive systems of ruminants. Mostly they obtain their energy by reducing carbon dioxide and oxidizing hydrogen, with the production of methane:
CO_2 + 4H_2 CH_4 + 2H_2O

Formate, methanol, or acetate may also be used as substrates by certain types. Methanogenic bacteria are important in the production of biogas.

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methionine

See amino acid.

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methylene blue

A blue dye used in optical microscopy to stain nuclei of animal tissues. It is also suitable as a vital stain and a bacterial stain.

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metre

Symbol m. The SI unit of length that is equal to 39.37 inches. It is formally defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second. This definition, adopted by the General Conference on Weights and Measures in October, 1983, replaced the 1967 definition based on the krypton lamp, i.e. 1650763.73 wavelengths in a vacuum of the radiation corresponding to the transition between the levels 2p^10 and 5d^5 of the nuclide krypton-86. This definition (in 1958) replaced the older definition of a metre based on a platinum-iridium bar of standard length.

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MHC

See major histocompatibility complex.

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micelle

An aggregate of molecules in a colloid. For example, phospholipids in aqueous solution form micelles – small clusters of molecules in which the nonpolar hydrocarbon groups are in the centre and the hydrophilic polar groups are on the outside. The products of fat digestion are dispersed into micelles by the action of bile salts, which facilitates their absorption in the small intestine.

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Michaelis–Menten curve

A graph that shows the relationship between the concentration of a substrate and the rate of the corresponding enzyme-controlled reaction. Named after L. Michaelis (1875–1949) and M. L. Menten (1879–1960), the curve only applies to enzyme reactions involving a single substrate. The graph can be used to calculate the Michaelis constant (Km), which is the concentration of a substrate required in order for an enzyme to act at half of its maximum velocity (Vmax). The Michaelis constant is a measure of the affinity of an enzyme for a substrate. A low value corresponds to a high affinity, and vice versa. See also enzyme kinetics.

Michaelis–Menten curve

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micro-

1 A prefix denoting very small size; e.g. microgamete, micronucleus.

2 Symbol µ. A prefix used in the metric system to denote one millionth. For example, 10–6 metre = 1 micrometre (µm).
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microbe

See microorganism.

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microbiology

The scientific study of microorganisms (e.g. bacteria, viruses, and fungi). Originally this was directed towards their effects (e.g. in causing disease and decay), but during the 20th century the emphasis has shifted to their physiology, biochemistry, and genetics. Microbes are now recognized as important vehicles for the study of biochemical and genetic processes common to all living organisms, and their rapid growth enables their laboratory culture in large numbers for studies in genetics.

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microclimate

The local climate of a small area or of a particular habitat, which is different from the macroclimate or the larger surrounding geographical area.

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microdissection (micromanipulation)

A technique used for the dissection of living cells under the high power of an optical microscope. It utilizes minute mechanically manipulated instruments, such as needles, scalpels, and pipettes. For example, the instruments may be used to remove a single nucleus from one cell and to implant it in another.

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microevolution

Evolution on a relatively small scale, involving the emergence of new species, or of new groups below the species level, such as races and subspecies. Compare macroevolution.

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microfauna

1 Animals that cannot be seen with the naked eye. They are normally observed with the aid of a microscope. Compare macrofauna.

2 The animals that live in a particular microhabitat.
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microfibril

A microscopic fibre. Plant cell walls contain microfibrils, about 5nm in diameter, each consisting of some 50–60 parallel cellulose chains that are associated together to form a rod or a flat ribbon. Cellulose microfibrils are arranged in layers at right angles to each other.

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microfilament

Any of numerous microscopic protein fibres, typically 3–6 nm in diameter, that form one of the main components of the cytoskeleton of eukaryotic cells. Each microfilament consists of two helically twisted strands, each comprising a chain of globular subunits of the protein actin. Bundles of microfilaments often occur just beneath the cell surface, typically oriented parallel to the long axis of the cell, and some are anchored to the plasma membrane. With the aid of the motor protein myosin the microfilaments can slide relative to each other, causing contractile movements, as in muscle cells, or other changes in cell shape, such as those occurring in amoeboid movement. They are also involved in the transport of materials within the cell, and in the peripheral flow of cytoplasm and cell organelles known as cytoplasmic streaming. Hence microfilaments play a crucial role in the growth of cell extensions, for example the pollen tube that develops from a germinating pollen grain. Compare microtubule. See also intermediate filament.

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microflora

1 Plants and algae that cannot be seen with the naked eye. They are normally observed with the aid of a microscope.

2 The plants and algae that live in a particular microhabitat.
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microfossil

A fossil that is so small that it can only be studied under a microscope. Microfossils include bacteria, diatoms, and protozoa and parts of organisms, such as plant pollen and skeletal fragments. Microfossils are important in the correlation of rocks where only small samples are available. The study of microfossils, particularly pollen, is known as palynology.

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microhabitat

The local habitat of a particular organism or microorganism. There are normally a number of different microhabitats within a large habitat (macrohabitat ), each with its distinct set of environmental conditions. For example, in a stream macrohabitat there will exist different microhabitats, depending on oxygen content, pH, speed of water flow, and other factors in localized areas of the stream.

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micromanipulation

See microdissection.

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micronucleus

See nucleus.

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micronutrient

A chemical element required by plants in relatively small quantities. Micronutrients are typically found in cofactors and coenzymes. They include copper, zinc, molybdenum, manganese, cobalt, and boron. See essential element. Compare macronutrient.

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microorganism (microbe)

Any organism that can be observed only with the aid of a microscope. Microorganisms include bacteria, viruses, protoctists (including certain algae), and fungi. See microbiology.

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microphagous

Describing the method of feeding of those heterotrophic organisms that take in their food in the form of tiny particles. Filter feeding and ciliary feeding are examples of this type of feeding. Compare macrophagous.

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microphyll

A type of foliage leaf in clubmosses and horsetails that has a single unbranched midrib. Such leaves are generally no more than a few millimetres long. Compare megaphyll.

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micropropagation

The in vitro propagation of plants by cloning. Typically, this involves culturing excised meristematic tissue on a special medium that encourages axillary bud development. The new shoots are then separated and cultured, and the cycle is repeated until finally the shoots are transferred to a medium that promotes root development, to produce plantlets. Micropropagation is used in agriculture, horticulture, and forestry as special genotypes can be bred and maintained, the process is rapid, and plants can be kept disease-free. See also clone.

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micropyle

1 A small opening in the surface of a plant ovule through which the pollen tube passes prior to fertilization. It results from the incomplete covering of the nucellus by the integuments. It remains as an opening in the testa of most seeds through which water is absorbed.

2 A small pore in some animal cells or tissues; for example, in insect eggs (see chorion).
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microsatellite DNA

A form of highly repetitive DNA consisting of large numbers of very short base sequences scattered throughout the eukaryote genome. Each consists of a core motif, varying in size from a few to 60 bp, which may be repeated tens of thousands of times. Because these motifs occur with great variability among individuals of any population, they can be used as a means of creating a DNA fingerprint (see DNA fingerprinting). Microsatellite DNA is named by analogy with satellite DNA, although it does not form visible satellite peaks on centrifugation.

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microscope

A device for forming a magnified image of a small object. The simple microscope consists of a biconvex magnifying glass or an equivalent system of lenses, either hand-held or in a simple frame. The compound microscope uses two lenses or systems of lenses, the second magnifying the real image formed by the first (see illustration). The lenses are usually mounted at the opposite ends of a tube that has mechanical controls to move it in relation to the object. An optical condenser and mirror, often with a separate light source, provide illumination of the object. The widely used binocular microscope consists of two separate instruments fastened together so that one eye looks through one while the other eye looks through the other. This gives stereoscopic vision. See Chronology. See also electron microscope; phase-contrast microscope; ultraviolet microscope.

Compound microscope

MICROSCOPY

c.1590

Dutch spectacle-makers Hans and Zacharias Janssen invent the compound microscope.

1610

German astronomer Johannes Kepler (1571–1630) invents the modern compound microscope.

1675

Anton van Leeuwenhoek invents the simple microscope.

1826

British biologist Dames Smith (d. 1870) constructs a microscope with much reduced chromatic and spherical aberrations.

1827

Italian scientist Giovanni Amici (1786–1863) invents the reflecting achromatic microscope.

1861

British chemist Joseph Reade (1801–70) invents the kettledrum microscope condenser.

1912

British microscopist Joseph Barnard (1870–1949) invents the ultramicroscope.

1932

Dutch physicist Frits Zernike (1888–1966) invents the phase-contrast microscope.

1936

German-born US physicist Erwin Mueller (1911–77) invents the field-emission microscope.

1938

German engineer Ernst Ruska (1906–88) develops the electron microscope.

1940

Canadian scientist James Hillier (1915– ) makes a practical electron microscope.

1951

Erwin Mueller invents the field-ionization microscope.

1978

US scientists of the Hughes Research Laboratory invent the scanning ion microscope.

1981

Swiss physicists Gerd Binning (1947– ) and Heinrich Rohrer (1933– ) invent the scanning tunnelling microscope.

1985

Gerd Binning invents the atomic force microscope.

1987

James van House and Arthur Rich invent the positron microscope.

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microsome

A fragment of endoplasmic reticulum formed when cells or tissues are disrupted. Microsomes can be isolated by centrifugation and are commonly used to investigate the functional properties of endoplasmic reticulum, such as enzymic activity and protein synthesis.

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microsporangium

See sporangium.

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microspore

See microspore mother cell; sporophyll.

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microspore mother cell (microsporocyte)

A diploid cell in plants that divides by meiosis to give rise to four haploid microspores (see sporophyll). In flowering plants microspore mother cells are formed within the pollen sacs of the anthers by mitosis; the microspores they produce develop into pollen grains.

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microsporocyte

See microspore mother cell.

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microsporophyll

See sporophyll.

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microtome

A machine used to cut thin sections (3–5µm thick) of plant or animal tissue for microscopical observation. There are various designs of microtome, each basically consisting of a steel knife, a block for supporting the specimen, and a device for moving the specimen towards the knife. The specimen is usually supported by being embedded in wax; if a freezing microtome is used, the specimen is frozen. An ultramicrotome is used to cut much thinner sections (20–100nm thick) for electron microscopy. The biological material is embedded in plastic or resin, sectioned with a glass or diamond knife, and the cut sections are allowed to float on the surface of water in an adjacent water bath.

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microtubule

A microscopic tubular structure, with an external diameter of 24 nm and of variable length, found in a wide range of eukaryotic cells. Microtubules are composed of numerous subunits of the globular protein tubulin and occur singly or in pairs, triplets, or bundles. Microtubules help cells to maintain their shape (see cytoskeleton); they also occur in cilia and eukaryotic flagella (see undulipodium) and the centrioles and form the spindle during nuclear division. A further role is in the intracellular transport of materials and movement of organelles. Compare microfilament; intermediate filament.

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microvillus

One of a number of minute finger-like projections on the free surfaces of epithelial cells. Microvilli are covered with cell membrane and their cytoplasm is continuous with the main cell cytoplasm. Their purpose is probably to increase the absorptive or secretory surface area of the cell, and they are abundant on the villi of the intestine, where they form a brush border.

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micturition

The discharge of urine from the bladder. It is brought about by reflex contraction of the detrusor muscle after voluntary relaxation of the sphincter muscle at the junction of the bladder with the urethra.

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midbrain (mesencephalon)

One of the three sections of the brain of a vertebrate embryo. Unlike the forebrain and the hindbrain, the midbrain does not undergo further subdivision to form additional zones. In mammals it becomes part of the brainstem, but in amphibians, reptiles, and birds the roof of the midbrain becomes enlarged as the tectum , a dominant centre for integration, and may include a pair of optic lobes .

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middle ear (tympanic cavity)

The air-filled cavity within the skull of vertebrates that lies between the outer ear and the inner ear. It is linked to the pharynx (and therefore to outside air) via the Eustachian tube and in mammals contains the three ear ossicles, which transmit auditory vibrations from the outer ear (via the tympanum) to the inner ear (via the oval window).

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middle lamella

A thin layer of material, consisting mainly of pectins, that binds together the walls of adjacent plant cells. See also cell plate.

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midgut

1 The middle section of the alimentary canal of vertebrates, which is concerned with digestion and absorption. It comprises most of the small intestine.

2 The middle section of the alimentary canal of arthropods. See also foregut; hindgut.
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migration

The seasonal movement of complete populations of animals to a more favourable environment. It is usually a response to lower temperatures resulting in a reduced food supply, and is often triggered by a change in day length (see photoperiodism). Migration is common in mammals (e.g. porpoises), fish (e.g. eels and salmon), and some insects but is most marked in birds. The Arctic tern, for example, migrates annually from its breeding ground in the Arctic circle to the Antarctic – a distance of some 17600 km. Migrating animals possess considerable powers of orientation; birds seem to possess a compass sense, using the sun, pole stars, and (in cloud) the earth's magnetic lines of force as reference points (see navigation).

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milk

The fluid secreted by the mammary glands of mammals. It provides a balanced and highly nutritious food for offspring. Cows' milk comprises about 87% water, 3.6% lipids (triglycerides, phospholipids, cholesterol, etc.), 3.3% protein (largely casein), 4.7% lactose (milk sugar), and, in much smaller amounts, vitamins (especially vitamin A and many B vitamins) and minerals (notably calcium, phosphorus, sodium, potassium, magnesium, and chlorine). Composition varies among species; human milk contains less protein and more lactose.

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milk sugar

See lactose.

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milk teeth

See deciduous teeth.

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milli-

Symbol m. A prefix used in the metric system to denote one thousandth. For example, 0.001 metre = 1 millimetre (mm).

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millipedes

See Diplopoda.

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Millon's reagent

A solution of mercuric nitrate and nitrous acid used to test for proteins. The sample is added to the reagent and heated for two minutes at 95°C; the formation of a red precipitate indicates the presence of protein in the sample. The reagent is named after French chemist Auguste Millon (1812–67).

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mimicry

The resemblance of one animal to another, which has evolved as a means of protection. In Batesian mimicry , named after British naturalist Henry Bates (1825–92), the markings of certain harmless insects closely resemble the warning coloration of another insect (the model ). Predators that have learnt to avoid the model will also avoid good mimics of it. This phenomenon is often found among butterflies. Müllerian mimicry, named after German zoologist J. F. T. Müller (1821–97), involves the mutual resemblance of a group of animals, all harmful, such as the wasp, bee, and hornet, so that a predator, having experienced one, will subsequently avoid them all.

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mineral deficiency

Lack of any essential mineral nutrient, such as nitrogen, phosphorus, or potassium, in living organisms, which can result in mineral deficiency diseases. For example, lack of calcium causes poor bone development in humans and stunted growth in plants; lack of nitrogen can cause the disease kwashiorkor, due to a deficiency in protein intake (see malnutrition); in plants it results in stunted growth and chlorosis. Lack of phosphorus can cause reduced growth in plants. A deficiency of trace elements (see essential element) also leads to diseases; for example, a deficiency of iron can cause anaemia in humans and chlorosis in plants.

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mineralocorticoid

See corticosteroid.

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mineral salts

Inorganic salts that need to be ingested or absorbed by living organisms for healthy growth and maintenance. They comprise the salts of the trace elements in animals (see essential element) and the micronutrients of plants.

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minicell

1 A small roughly spherical cell produced by the abnormal division of a rod-shaped bacterium that contains no chromosomal DNA and is unable to grow or divide. It is used experimentally to express nucleic acids and proteins encoded by DNA introduced to the cell, e.g. in the form of a cloning vector.

2 (microcell) An experimentally produced body consisting of a nucleus surrounded by a thin layer of cytoplasm and a plasma membrane and containing variable numbers of chromosomes. It is used as a vector for transferring one or several chromosomes into a normal cell following cell fusion.
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Miocene

The fourth epoch of the Tertiary period, stretching from the end of the Oligocene, about 24 million years ago, to the start of the Pliocene, roughly 5 million years ago. It saw the radiation of several modern mammal groups, including the ruminants (deer, cattle, and antelopes), certain rodents (beavers, porcupines, and cavies) and the apes. Cooling of the climate during the Oligocene resulted in a continuing shift to deciduous hardwood species, such as oak and maple, at the expense of conifers during the Miocene.

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miracidium

The first larval stage of trematode worms (flukes), which hatches from eggs excreted in the faeces of the primary host. Its leaflike body is covered with cilia, enabling the larva to swim towards a secondary host, in which it continues its development.

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mismatch repair

A form of DNA repair that replaces mismatched bases in newly replicated DNA in living cells. For example, if a guanine (G) on the template strand pairs with thymine (T) instead of the usual cytosine (C), the resulting abnormality in the DNA duplex is detected by the mismatch repair system. A length of strand containing the mismatched T is cut out and the gap is repaired with a new length containing the correct base (C), using the parental strand as template. The mismatch repair system closely follows the Y-shaped fork where the parental DNA unwinds and replication takes place, so enabling immediate correction of replication errors. It accounts for the vast majority of DNA repairs.

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missense mutation

A type of point mutation that converts one codon to another, specifying a different amino acid. This results in incorporation of the wrong amino acid in the polypeptide chain during translation and, consequently, a potentially faulty protein.

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mites

See Acarina.

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mitochondrial DNA (mtDNA)

A circular ring of DNA found in mitochondria. In mammals mtDNA makes up less than 1% of the total cellular DNA, but in plants the amount is variable. It codes for ribosomal and transfer RNA but only some mitochondrial proteins (up to 30 proteins in animals), the nuclear DNA being required for encoding most of these. Human mtDNA codes for 13 proteins and some RNA. Mitochondrial DNA is generally inherited via the female line only, although there are some exceptions to this (see heteroplasmy). See also mitochondrial Eve.

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mitochondrial Eve

The hypothetical female claimed by some biologists to be the ancestor of all humankind. Analysis of mitochondrial DNA (mtDNA) from groups of people throughout the world suggests that mitochondrial Eve lived 100000–200000 years ago, probably in Africa (hence she is also known as `African Eve'). Mitochondrial DNA is particularly useful for investigating recent genetic history as it mutates quickly (ten times more rapidly than nuclear DNA) and in humans is inherited solely through the female line (therefore it does not undergo recombination by crossing over). The uniformity of the different samples of mtDNA indicates that modern humans evolved relatively recently from a single region in Africa. This view has been reinforced by studies of Y chromosomes from different groups around the world, which are transmitted only through the male line.

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mitochondrion

(pl. mitochondria) A structure within the cytoplasm of eukaryotic cells that carries out aerobic respiration: it is the site of the Krebs cycle and electron transport chain, and therefore the cell's energy production. Mitochondria vary greatly in shape, size, and number but are typically oval or sausage-shaped and bounded by two membranes, the inner one being folded into finger-like projections (cristae ); they contain their own DNA (see mitochondrial DNA). They are most numerous in cells with a high level of metabolic activity.

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mitosis

A type of nuclear division that results in two daughter cells each having a nucleus containing the same number and kind of chromosomes as the mother cell. The changes during divisions are clearly visible with a light microscope. Each chromosome divides lengthwise into two chromatids, which separate and form the chromosomes of the resulting daughter nuclei. The process is divided into four stages, prophase, metaphase, anaphase, and telophase, which merge into each other (see illustration). Mitotic divisions ensure that all the cells of an individual are genetically identical to each other and to the original fertilized egg. See also cell cycle.

The stages of mitosis in a cell containing two pairs of homologous chromosomes

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mitral valve

See bicuspid valve.

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mixed function oxidase

An enzyme complex found in animals that oxidizes toxic compounds to render them more susceptible to metabolism and excretion. Such complexes are induced in a wide range of species following exposure to toxic organic substances, such as alkaloids, phenolics, terpenoids, and quinones. The enzymes are localized in the liver in vertebrates and in similar tissues, such as the hepatopancreas, in invertebrates. They catalyse the introduction to the toxic molecule of single oxygen atoms in the form of hydroxyl groups, which requires energy. High levels of these enzymes occur in plant-eating insects, in which they detoxify the natural toxins of the plants. Such enzymes may also render insects more resistant to insecticides.

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mixed function oxygenase (mono-oxygenase)

An enzyme that introduces an oxygen atom into a substrate. Mixed function oxidases are essential components of many important metabolic pathways. An example of such an enzyme is cytochrome oxidase.

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mmHg

A unit of pressure equal to that exerted under standard gravity by a height of one millimetre of mercury, or 133.322 pascals.

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modern synthesis

See neo-Darwinism.

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modifier gene

A gene that influences the expression of another gene. For example, one gene controls whether eye colour is blue or brown but other (modifier) genes can also influence the colour by affecting the amount or distribution of pigment in the iris.

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molality

See concentration.

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molar

1 A broad ridged tooth in the adult dentition of mammals (see permanent teeth), found at the back of the jaws behind the premolars. There are two or more molars on each side of both jaws; their surfaces are raised into ridges or cusps for grinding food during chewing. In humans the third (and most posterior) molar does not appear until young adulthood: these molars are known as wisdom teeth .

2 Denoting that an extensive physical property is being expressed per amount of substance, usually per mole. For example, the molar heat capacity of a compound is the heat capacity of that compound per unit amount of substance, i.e. it is usually expressed in J K–1 mol–1.
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molarity

See concentration.

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mole

Symbol mol. The SI unit of amount of substance. It is equal to the amount of substance that contains as many elementary units as there are atoms in 0.012kg of carbon–12. The elementary units may be atoms, molecules, ions, radicals, electrons, etc., and must be specified. 1 mole of a compound has a mass equal to its relative molecular mass expressed in grams.

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molecular biology

The study of the structure and function of large molecules associated with living organisms, in particular proteins and the nucleic acids DNA and RNA. Molecular genetics is a specialized branch, concerned with the analysis of genes (see gene sequencing).

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molecular chaperone

Any of a group of proteins in living cells that assist newly synthesized or denatured proteins to fold into their functional three-dimensional structures. The chaperones bind to the protein and prevent improper interactions within the polypeptide chain, so that it assumes the correct folded orientation. This process requires energy in the form of ATP. One class of chaperones, called chaperonins , occur in E. coli, chloroplasts, and mitochondria. They are heat-shock proteins, manufactured in response to raised temperature, and are presumed to help protect the cell from damage by refolding proteins that have been partially denatured by heat.

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molecular clock

The concept that during evolution the number of substitutions in the nucleotides of nucleic acids (DNA or RNA), and hence in the proteins encoded by the nucleic acids, is proportional to time. Hence, by comparing the DNA or proteins of species that diverged a known length of time ago (e.g. determined from fossil evidence), it is possible to calculate the average substitution rate, thereby calibrating the `molecular clock'. Comparative studies of different proteins in various groups of organisms tend to show that the average number of amino-acid substitutions per site per year is typically around 10–9. These results indicate a fairly constant rate of molecular evolution in comparable sequences of macromolecules in different organisms.

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molecular imprinting

The phenomenon in which there is differential expression of a gene depending on which parent it is inherited from. Genes are now thought to carry either a female or a male `imprint', which influences their expression in offspring. For example, the development of Huntington's disease in humans is delayed until midlife if the gene is inherited from the father (as occurs in the majority of cases), whereas inheritance from the mother causes the onset of symptoms early in childhood. Various other genetic disorders show a similar pattern of differential expression according to the source of the defective gene. One theory is that the genes are chemically `marked' in the germ cells with a characteristic male or female pattern of methylation, and that this methylation pattern influences their expression in the somatic cells of the offspring.

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molecular systematics (biochemical taxonomy)

The use of amino-acid or nucleotide-sequence data in determining the evolutionary relationships of different organisms. Essentially it involves comparing the sequences of functionally homologous molecules from each organism being studied, and determining the number of differences between them. The greater the number of differences, the more distantly related the organisms are likely to be. Moreover, since the number of nucleotide substitutions, and hence substitutions of corresponding amino acids, is generally proportional to time, some indication of the time scale involved can be obtained (see molecular clock). This information has proved particularly useful where there are gaps in the fossil record and can be combined with other evidence from morphology, physiology, and embryology to produce more accurate phylogenetic trees. In microbiology molecular systematics has transformed bacterial phylogeny, in particular prompting the view that there are two quite distinct lineages of bacteria, the archaebacteria and eubacteria.

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Molisch's test

See alpha-naphthol test.

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Mollusca

A phylum of soft-bodied invertebrates characterized by an unsegmented body differentiated into a head , a ventral muscular foot used in locomotion, and a dorsal visceral hump covered by a fold of skin – the mantle – which secretes a protective shell in many species. Respiration is by means of gills (ctenidia) or a lunglike organ and the feeding organ is a radula. Molluscs occur in marine, freshwater, and terrestrial habitats and there are six classes, including the Gastropoda (snails, slugs, limpets, etc.), Bivalvia (bivalves, e.g. mussels, oysters), and Cephalopoda (squids and octopuses).

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molybdenum

Symbol Mo. A silvery hard metallic element that is a trace element required by living organisms. See essential element.

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Monera

See prokaryote.

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monoamine oxidase (MAO)

An enzyme that breaks down monoamines (e.g. adrenaline and noradrenaline) in the body by oxidation. Drugs that inhibit this enzyme are used to treat forms of depression.

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monochasium

See cymose inflorescence.

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monocistronic

Describing a type of messenger RNA that can encode only one polypeptide per RNA molecule. In eukaryotic cells virtually all messenger RNAs are monocistronic. Compare polycistronic.

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monoclonal antibody

A specific antibody produced by one of numerous identical cells derived from a single parent cell. (The population of these cells comprises a clone and each cell is said to be monoclonal .) The parent cell is obtained by the fusion of a normal antibody-producing cell (a lymphocyte) with a cell derived from a malignant tumour of lymphoid tissue of a mouse (see cell fusion). The resulting hybridoma cell then multiplies rapidly and yields large amounts of antibody. Monoclonal antibodies are used to identify a particular antigen within a mixture and can therefore be used for identifying blood groups; they also enable the production of highly specific, and therefore effective, vaccines. Above all, they have transformed medical and biological diagnostics by ushering in a huge range of cheap and convenient kits for identifying and quantifying biological materials (see immunoassay).

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Monocotyledonae

One of the two classes of flowering plants (see Anthophyta), distinguished by having one seed leaf (cotyledon) within the seed. The monocotyledons generally have parallel leaf veins, scattered vascular bundles within the stems, and flower parts in threes or multiples of three. Monocotyledon species include some crop plants (e.g. cereals, onions, fodder grasses), ornamentals (e.g. tulips, orchids, lilies), and a very limited number of trees (e.g. the palms). Compare Dicotyledonae.

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monoculture

See agriculture.

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monocyte

The largest form of white blood cell (leucocyte) in vertebrates. It has a kidney-shaped nucleus and is actively phagocytic, ingesting bacteria and cell debris (see phagocyte).

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monoecious

Describing plant species that have separate male and female flowers on the same plant. Examples of monoecious plants are maize and birch. Compare dioecious.

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monoglyceride

See glyceride.

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monohybrid cross

A genetic cross between parents that differ in the alleles they possess for one particular gene, one parent having two dominant alleles and the other two recessives. All the offspring (called monohybrids ) have one dominant and one recessive allele for that gene (i.e. they are hybrid at that one locus). Crossing between these offspring yields a characteristic 3:1 (monohybrid) ratio in the following generation of dominant:recessive phenotypes (see illustration). Compare dihybrid cross.

A monohybrid cross: the inheritance of stem lengths in garden peas

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monokine

See cytokine.

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monolayer culture

A type of culture in which cells are grown in a single layer on a flask or Petri dish containing the culture medium. Compare suspension culture.

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monomer

A molecule (or compound) that consists of a single unit and can join with others in forming a dimer, trimer, or polymer.

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mononuclear phagocyte system

See reticuloendothelial system.

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monophyletic

In systematics, describing a group of organisms that contains all the descendants of a particular single common ancestor. In cladistics such a grouping is called a clade and is the only type of group regarded as valid when constructing classification schemes. Hence the monophyletic grouping Theria contains the marsupial and placental mammals, together with their extinct Mesozoic relatives, all of which share an immediate common ancestor not shared by the more distantly related egg-laying mammals (comprising the Prototheria). Similarly, birds and crocodiles are the living representatives of a monophyletic group, Archosauria, and are more closely related to each other than to other living reptilian descendants. Consequently the grouping `reptiles', used in many modern classification systems, is not monophyletic but paraphyletic, since it excludes the birds (and mammals). Compare polyphyletic.

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monophyodont

Describing a type of dentition that consists of a single set of teeth that last for the entire lifespan of an animal. Compare diphyodont; polyphyodont.

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monopodium

The primary axis of growth in such plants as pine trees. It consists of a single main stem that continues to grow from the tip and gives rise to lateral branches. Compare sympodium.

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monosaccharide (simple sugar)

A carbohydrate that cannot be split into smaller units by the action of dilute acids. Monosaccharides are classified according to the number of carbon atoms they possess: trioses have three carbon atoms; tetroses , four; pentoses , five; hexoses , six; etc. Each of these is further divided into aldoses and ketoses , depending on whether the molecule contains an aldehyde group (–CHO) or a ketone group (–CO–). For example glucose, having six carbon atoms and an aldehyde group, is an aldohexose whereas fructose is a ketohexose . These aldehyde and ketone groups confer reducing properties on monosaccharides: they can be oxidized to yield sugar acids. They also react with phosphoric acid to produce phosphate esters (e.g. in ATP), which are important in cell metabolism. Monosaccharides can exist as either straight-chain or ring-shaped molecules (see illustration). They also exhibit optical activity, giving rise to both dextrorotatory and laevorotatory forms.

Monosaccharides

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monosomy

See aneuploid.

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monosynaptic reflex

A simple reflex that involves transmission of information from a sensory neurone to the appropriate motor neurone across a single synapse in the spinal cord. The knee-jerk reflex action is an example of a monosynaptic reflex (see stretch reflex). Compare polysynaptic reflex.

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monotremes

See Prototheria.

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monozygotic twins

See identical twins.

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Morgan, Thomas Hunt

(1866–1945)
US geneticist, who held professorships at Bryn Mawr College (1891–1904), Columbia University (1904–28), and the California Institute of Technology (1928–45). He established that chromosomes were the carriers of Mendel's `factors' of inheritance (genes). Working with fruit flies (Drosophila), he demonstrated the phenomenon of linkage and modified Mendel's law of independent assortment by stating that this could only apply to genes located on different chromosomes. He showed that linkage could be broken by crossing over and went on to produce the first chromosome maps. For his work Morgan was awarded the 1933 Nobel Prize for physiology or medicine.

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morphine

An alkaloid present in opium (see opiate). It is an analgesic and narcotic, used medically for the relief of severe pain.

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morphogen

A substance that determines the developmental fate of part of an embryo. During development, different morphogens are produced within the embryo or by cells of the surrounding maternal tissue. They diffuse through the embryonic tissue, each one establishing its own concentration gradient, and together they form a chemical pattern on which embryonic development is based. The complex interplay of these morphogen gradients regulates the activity of genes in different regions of the embryo and ultimately brings about the differentiation of the tissues and organs appropriate to the different regions of the embryo, i.e. morphogenesis. In the development of the fruit fly Drosophila, for example, the anterior and posterior ends of the egg are `signposted' by proteins encoded respectively by bicoid and nanos genes in maternal follicle cells. The messenger RNAs from these genes accumulate at opposite ends of the egg cell (oocyte), and their products subsequently diffuse into the fertilized egg, thereby establishing its polarity at the outset of development. These represent the beginning of a cascade of morphogens that influence gene activity in an increasingly precise manner.

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morphogenesis

The development, through growth and differentiation, of form and structure in an organism.

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morphology

The study of the form and structure of organisms, especially their external form. Compare anatomy.

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mortality

See death rate.

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mosaic

1 An organism made up of cells that have different genotypes but have developed from the same zygote. See also chimaera; gynandromorph.

2 A viral disease in plants that causes yellow patches to develop on the leaves, giving these a variegated appearance (see variegation). An example is tobacco mosaic, caused by tobacco mosaic virus.
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mosses

See Bryophyta.

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moths

See Lepidoptera.

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motivation

The internal conditions responsible for temporary reversible changes in the responsiveness of an animal to external stimulation. Thus an animal that has been deprived of food will accept less palatable food than one that has not been deprived: the difference is attributed to a change in feeding motivation. Changes in responsiveness due to maturation, learning, or injury are not usually readily reversible and are therefore not considered to be due to changes in motivation. Early attempts to describe motivation in terms of a number of separate drives (e.g. food drive, sex drive) have not found general favour, partly because drives interact with one another; for example, water deprivation often affects an animal's willingness to feed.

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motor neurone

A neurone that transmits nerve impulses from the central nervous system to an effector organ (such as a muscle or gland) and thereby initiates a physiological response (e.g. muscle contraction).

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moulting

1 The seasonal loss of hair, fur, or feathers that occurs in mammals and birds.

2 The periodic loss of the integument of arthropods and reptiles. See ecdysis.
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mouth

The opening of the alimentary canal, which in most animals is used for the ingestion of food. It leads to the buccal cavity (mouth cavity).

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mouth cavity

See buccal cavity.

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mouthparts

Modified paired appendages on the head segments of arthropods, used for feeding. A typical insect has a labium (lower lip), one pair each of mandibles and maxillae, and a labrum (upper lip), although in many the mouthparts are modified to form piercing stylets or a sucking proboscis. Crustaceans, centipedes, and millipedes have one pair of mandibles and two pairs of maxillae used for cutting and holding the food. Crustaceans also have several pairs of maxillipeds (see also maxillula). Arachnids have chelicerae and pedipalps.

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mRNA

See RNA.

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MSH

See melanocyte-stimulating hormone.

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mtDNA

See mitochondrial DNA.

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mucilage

A gumlike substance frequently present in the cell walls of aquatic plants and in the seed coats of certain other species. Mucilages are hard when dry and slimy when wet. Like gums they probably have a general protective function or serve to anchor the plant. See also glycocalyx.

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mucin

See mucus.

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mucopolysaccharide

See glycosaminoglycan.

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mucosa

A mucous membrane, especially that forming the lining of the wall of the mammalian stomach and intestine. In these organs the mucosa includes an outer muscular layer, the muscularis mucosae, which lies adjacent to the submucosa.

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mucous membrane (mucosa)

A layer of tissue comprising an epithelium supported on connective tissue. Within the epithelium are goblet cells, which secrete mucus onto the surface, and the epithelium often bears cilia. Mucous membranes line body cavities communicating with the exterior, including the alimentary and respiratory tracts. Compare serous membrane.

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mucus

The slimy substance secreted by goblet cells onto the surface of a mucous membrane to protect and lubricate it and to trap bacteria, dust particles, etc. Mucus consists of water, the glycoprotein mucin , cells, and salts.

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Müllerian mimicry

See mimicry.

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multicellular

Describing tissues, organs, or organisms that are composed of a number of cells. Compare unicellular.

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multienzyme system

A complex of enzymes within a cell that form a reaction sequence of a biochemical pathway so that the product of the first enzyme reaction is transferred directly to the next enzyme and immediately undergoes a second reaction, and so on. The rate of an enzyme reaction often depends on the concentration of the enzyme and the substrate, both being required in relatively high amounts. Multienzyme systems, such as those involved in RNA and protein synthesis, help maintain a high rate of cellular metabolism since the intermediate products are transferred directly to the next enzyme and are therefore not required in large concentrations.

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multifactorial inheritance

See polygenic inheritance.

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multiple alleles

Three or more alternative forms of a gene (alleles) that can occupy the same locus. However, only two of the alleles can be present in a single organism. For example, the ABO system of blood groups is controlled by three alleles, only two of which are present in an individual.

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multipolar neurone

A neurone that has one axon and several dendrons extending from its cell body in different directions. Most vertebrate motor neurones and interneurones are multipolar neurones. Compare bipolar neurone; unipolar neurone.

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Musci

See Bryophyta.

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muscle

A tissue consisting of sheets or bundles of cells (muscle fibres ) that are capable of contraction, so producing movement or tension in the body. There are three types of muscle. Voluntary muscle produces voluntary movement (e.g. at joints); involuntary muscle mainly effects the movements of hollow organs (e.g. intestine and bladder); and cardiac muscle occurs only in the heart.

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muscle spindle

A stretch receptor in vertebrate muscle. Muscle spindles run parallel to normal muscle fibres; each consists of a capsule containing small striated muscle fibres (intrafusal fibres). Muscle spindles are responsible for the adjustment of muscle tone and play an important role in the subconscious maintenance of posture and movement. See also stretch reflex.

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muscularis mucosae

A thin layer of smooth muscle that forms the outermost part of the mucosa of the mammalian stomach and intestine.

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mutagen

An agent that causes an increase in the number of mutants (see mutation) in a population. Mutagens operate either by causing changes in the DNA of the genes, so interfering with the coding system, or by causing chromosome damage. Various chemicals (e.g. colchicine) and forms of radiation (e.g. X-rays) have been identified as mutagens.

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mutant

A gene or an organism that has undergone a heritable change, especially one with visible effects (i.e. the change in genotype is associated with a change in phenotype). See mutation.

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mutation

A sudden random change in the genetic material of a cell that may cause it and all cells derived from it to differ in appearance or behaviour from the normal type. An organism affected by a mutation (especially one with visible effects) is described as a mutant . Somatic mutations affect the nonreproductive cells and are therefore restricted to the tissues of a single organism but germ-line mutations , which occur in the reproductive cells or their precursors, may be transmitted to the organism's descendants and cause abnormal development.
Mutations occur naturally at a low rate but this may be increased by radiation and by some chemicals (see mutagen). Most are point (or gene) mutations, which consist of invisible changes in the DNA of the chromosomes, but some (the chromosome mutations) affect the appearance or the number of the chromosomes. An example of a chromosome mutation is that giving rise to Down's syndrome.
The majority of mutations are harmful, but a very small proportion may increase an organism's fitness; these spread through the population over successive generations by natural selection. Mutation is therefore essential for evolution, being the ultimate source of genetic variation.

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mutation frequency

The average frequency with which a particular mutation occurs in a population. This frequency can be increased by radiation or by exposure to chemicals, such as mustard gas or hydrogen peroxide.

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mutualism

An interaction between two species in which both species benefit. (The term symbiosis is often used synonymously with mutualism.) A well-known example of mutualism is the association between termites and the specialized protozoans that inhabit their guts. The protozoans, unlike the termites, are able to digest the cellulose of the wood that the termites eat and release sugars that the termites absorb. The termites benefit by being able to use wood as a foodstuff, while the protozoans are supplied with food and a suitable environment. See also mycorrhiza.

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mycelium

(pl. mycelia) A network of hyphae that forms the body of a fungus. It consists of feeding hyphae together with reproductive hyphae, which produce sporangia and gametangia.

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mycobiont

The fungal component of a lichen.

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mycology

The scientific study of fungi.

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Mycophycophyta

See lichens.

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mycoplasmas

A group of eubacteria that lack a rigid cell wall and are among the smallest of all living cells (diameter 0.1 µm–0.8 µm). They are either saprotrophic or parasitic and are found on animal mucous and synovial membranes, in insects, and in plants (in which they seem to inhabit sieve tubes). They cause a variety of diseases, including pleuropneumonia in cattle – hence they were formerly known as pleuropneumonia-like organisms (PPLO). Due to their small size and flexible cell wall they can pass through a 0.2-µm-diameter filter and they represent a major contaminant of biotechnological products, such as monoclonal antibodies and vaccines, and of other cell cultures, in which they may exist symbiotically with the cells. Eight genera have been described (including Mycoplasma) with over 120 species. Some authorities place them in the phylum Aphragmabacteria.

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mycorrhiza

The mutually beneficial association (see mutualism) formed between fungi and the roots of plants. This is a very common form of mutualism; the absorption of mineral ions by the plant roots is enhanced by the presence of the fungus, which benefits by obtaining soluble organic nutrients from the root cells. Ectotrophic mycorrhizas form a network of hyphae around the root and grow into the air spaces between the cells of the root. The hyphae of endotrophic mycorrhizas enter the cortical cells of the host roots.

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Mycota

In older classification systems, a kingdom comprising the fungi.

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myelin

A phospholipid produced by the Schwann cells of the nervous system. Myelin forms an insulating layer around the nerve fibres (see myelin sheath).

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myelin sheath (medullary sheath)

The layer of fatty material that surrounds and electrically insulates the axons of most vertebrate and some invertebrate neurones. The myelin sheath enables a more rapid transmission of nerve impulses (at speeds up to 120 ms–1). It consists of layers of membrane derived from Schwann cells. The sheath is interrupted at intervals along the axon by nodes of Ranvier ; myelinated sections of axon are called internodes .

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myeloid tissue

Tissue within red bone marrow that produces the blood cells. It is found around the blood vessels and contains various cells that are precursors of the blood cells. See haemopoietic tissue.

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myeloma

See cancer.

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myocardium

The muscular wall of the heart. The thickness of the myocardium varies, reflecting the magnitude of the pressure generated in the heart during contraction: the myocardium is thickest around the left ventricle, which does most of the work in the heart. The inner surface of the myocardium is lined by a layer of endothelium (the endocardium ), which is continuous with the endothelium lining the blood vessels.

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myocyte

A contractile cell, especially a muscle cell.

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myofibril

See voluntary muscle.

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myogenic

Originating in or produced by muscle cells. The contractions of cardiac muscle fibres are described as myogenic, since they are produced spontaneously, without requiring stimulation from nerve cells (see pacemaker).

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myoglobin

A globular protein occurring widely in muscle tissue as an oxygen carrier. It comprises a single polypeptide chain and a haem group, which reversibly binds a molecule of oxygen. This is only relinquished at relatively low external oxygen concentrations, e.g. during strenuous exercise when muscle oxygen demand outpaces supply from the blood. Myoglobin thus acts as an emergency oxygen store.

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myometrium

The thick layer of smooth muscle that makes up the bulk of the uterus. The myometrium is lined by the endometrium.

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myopia

Short-sightedness. It results from the lens of the eye refracting the parallel rays of light entering it to a focus in front of the retina generally because of an abnormally long eyeball. The condition is corrected by using diverging spectacle lenses to move the image back to the retina.

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myosin

A contractile protein that interacts with actin to bring about contraction of muscle or cell movement. The type of myosin molecule found in muscle fibres consists of a tail, by which it aggregates with other myosin molecules to form so-called `thick filaments'; and a globular head, which has sites for the attachment of actin and ATP molecules. See sarcomere; sliding filament theory.

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myotome

One of a series of segmented muscle blocks found in fishes and lancelets. Myotomes are arranged in pairs on either side of the body that work antagonistically (see antagonism) against the backbone (or notochord), providing a means of locomotion by causing the tail to sweep from side to side.

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Myriapoda

In some classifications, a subphylum of arthropods of the phylum Uniramia that comprises the classes Chilopoda (centipedes), Diplopoda (millipedes), Pauropoda (pauropods), and Symphyla (symphilids). In other classifications the Myriapoda is a class containing only the centipedes and millipedes.

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myrmecochory

The collection and dispersal of plant seeds by ants. A variety of plant species possess hard seeds that are inedible to ants but are nevertheless gathered by them and taken to the ants' nest. The ants perform this service because the seeds are equipped with special food bodies (elaiosomes ). These are variously shaped appendages derived from ovarian tissue and containing proteins, lipids, and carbohydrates. In the nest the ants detach the elaiosomes and feed them to their larvae, discarding the seeds either within or near the nest. The ant benefits by receiving food, while the seed dispersal may benefit the plant in several ways: for example, protection of the seeds inside the ants' nest; reduction of competition from its own seedlings; or removal of the seeds to a suitable germination site.

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Myxomycota

See slime moulds.

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myxovirus

One of a group of RNA-containing viruses associated with various diseases of man and other vertebrates. Orthomyxoviruses produce diseases of the respiratory tract, e.g. influenza; paramyxoviruses include the causal agents of mumps, measles, and fowl pest.

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N

n

See haploid.

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NAD (nicotinamide adenine dinucleotide)

A coenzyme, derived from the B vitamin nicotinic acid, that participates in many biological dehydrogenation reactions (see formula). NAD is characteristically loosely bound to the enzymes concerned. It normally carries a positive charge and can accept one hydrogen atom and two electrons to become the reduced form, NADH . NADH is generated during the oxidation of food, especially by the reactions of the Krebs cycle. It then gives up its two electrons (and single proton) to the electron transport chain, thereby reverting to NAD+ and generating three molecules of ATP per molecule of NADH.
NADP (nicotinamide adenine dinucleotide phosphate ) differs from NAD only in possessing an additional phosphate group. It functions in the same way as NAD although anabolic reactions (see anabolism) generally use NADPH (reduced NADP) as a hydrogen donor rather than NADH. Enzymes tend to be specific for either NAD or NADP as coenzyme.

NAD

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nano-

Symbol n. A prefix used in the metric system to denote 10–9. For example, 10–9 metre = 1 nanometre (nm).

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narcotic

Any drug that induces stupor and relieves pain, especially morphine and other opiates. Such narcotics are addictive and cause dependence, and their medical use is strictly controlled.

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nares (nostrils)

The paired openings of the nasal cavity in vertebrates. All vertebrates have external nares , which open to the exterior; in some species these are situated on a nose. Internal nares (or choanae ) are present only in air-breathing vertebrates (including lungfish) and open into the mouth cavity. In mammals they open posteriorly, beyond the secondary palate.

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nasal cavity

The cavity in the head of a vertebrate that is lined by a membrane rich in sensitive olfactory receptors (see olfaction). It is connected to the exterior by external nostrils and (in air-breathing vertebrates) to the respiratory system by internal nares.

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nastic movements

Movements of plant organs in response to external stimuli that are independent of the direction of the stimuli. Examples are the opening of crocus and tulip flowers in response to a rise in temperature (thermonasty ), the opening of evening primrose flowers at night (photonasty ), and the folding up and drooping of leaves of the sensitive plant (Mimosa pudica) when lightly touched (haptonasty ). Compare tropism. See also nyctinasty.

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natality

See birth rate.

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natural group

A group of organisms of any taxonomic rank that are believed to be descended from a common ancestor (see monophyletic). For example, man and the apes are often regarded as a natural group, descended from fossil ancestors, the dryopithecines (or their near relatives). In an ideal natural classification all taxa should be natural groups. See also cladistics.

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natural history

1 The study of living organisms in their natural habitats.

2 The study of all natural phenomena.
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naturalized

See alien.

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natural order

In the classification of plants, the former name for a family.

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natural selection

The process that, according to Darwinism, brings about the evolution of new species of animals and plants. Darwin noted that the size of any population tends to remain constant despite the fact that more offspring are produced than are needed to maintain it. He also saw that variations existed between individuals of the population and concluded that disease, competition, and other forces acting on the population eliminated those individuals less well adapted to their environment. The survivors would pass on any heritable advantageous characteristics (i.e. characteristics with survival value) to their offspring and in time the composition of the population would change in adaptation to a changing environment. Over a long period of time this process could give rise to organisms so different from the original population that new species are formed. See also adaptive radiation. Compare punctuated equilibrium.

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nature and nurture

The combined effects of inherited factors (nature) and environmental factors (nurture) on the development of an organism. The genetic potential of an organism will only be realized under appropriate environmental conditions. See also phenotype.

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nauplius

The free-swimming larva of many marine and freshwater crustaceans. It has an unsegmented body with a single eye at the front (the nauplius eye ), mandibles, antennae, and three pairs of limbs.

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navigation

The complex process that enables animals to travel along a particular course in order to reach a specific destination. Navigation is an important aspect of behaviour in many animals, particularly those, such as birds, fish, and some insects, that undergo migrations. Landmarks, such as coastlines and mountain ranges, are important reference points for navigation but many animals can navigate successfully without the aid of these, by using the sun, stars, magnetic fields, odours, and polarized light. For example, birds use the sun and stars as landmarks and are sensitive to the earth's magnetic fields, while salmon can identify the unique odour of their home river.

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Neanderthal man

A form of fossil man that appeared in Europe and western Asia about 100000 years ago. He was thought to be a subspecies of Homo sapiens but is now generally regarded as a distinct species, H. neanderthalensis. The fossil remains indicate that Neanderthals were fairly short and had low brows but that the brain size was the same as, or larger than, modern man's. They were nomadic cave dwellers who buried their dead. Neanderthal man became extinct about 35000 years ago. The name is derived from the site in the Neander valley, Germany, where fossils were found in 1856.

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Nearctic region

See faunal region.

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near point

The nearest point at which the human eye can focus an object. As the lens becomes harder with age, the extent to which accommodation can bring a near object into focus decreases. Therefore with advancing age the near point recedes – a condition known as presbyopia.

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necrosis

The death of cells, which can be caused by a variety of chemicals and toxic substances. It often involves denaturation of proteins and may be preceded by a change in the appearance of cells and their contents (such as swelling of the mitochondria and karyorrhexis) and the appearance of lysosomes. Compare apoptosis.

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nectar

A sugary liquid produced in plants by nectaries , regions of secretory cells on the receptacle or other parts of a flower. It attracts pollinating insects or other animals.

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nectary

See nectar.

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negative feedback

See feedback.

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nekton

Pelagic organisms that actively swim through the water. Examples are fish, jellyfish, turtles, and whales. Compare benthos; neuston; plankton.

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nematoblast

See thread cell.

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nematocyst

See thread cell.

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Nematoda

A phylum of pseudocoelomate invertebrates comprising the roundworms. They are characterized by a smooth narrow cylindrical unsegmented body tapered at both ends. They shed their tough outer cuticle four times during life to allow growth. The microscopic free-living forms are found in all parts of the world, where they play an important role in the destruction and recycling of organic matter. The many parasitic nematodes are much larger; they include the filaria (Wuchereria) and Guinea worm (Dracunculus), which cause serious diseases in man.

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neo-Darwinism (modern synthesis)

The current theory of the process of evolution, formulated between about 1920 and 1950, that combines evidence from classical genetics with the Darwinian theory of evolution by natural selection (see Darwinism). It makes use of modern knowledge of genes and chromosomes to explain the source of the genetic variation upon which selection works. This aspect was unexplained by traditional Darwinism.

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neo-Lamarckism

Any of the comparatively modern theories of evolution based on Lamarck's theory of the inheritance of acquired characteristics (see Lamarckism). These include the unfounded dogma of Lysenkoism and controversial experiments on the inheritance of acquired immunological tolerance in mice.

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Neolithic

The New Stone Age, beginning in the Middle East approximately 9000 BC and lasting until 6000 BC, during which man first developed agriculture. Grinding and polishing of stone tools was also practised.

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neopallium (neocortex)

An area of the cerebral cortex of the vertebrate brain. The neopallium is most highly developed in mammals, in which it forms a surface layer covering most of the forebrain. The neopallium is the major centre for the coordination of sensory and motor information.

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neoplasm (tumour)

Any new abnormal growth of cells, forming either a harmless (benign) tumour or a malignant one (see cancer).

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neoteny

The retention of the juvenile body form, or particular features of it, in a mature animal. For example, the axolotl, a salamander, retains the gills of the larva in the adult. Neoteny is thought to have been an important mechanism in the evolution of certain groups, such as man, who is believed to have developed from the juvenile form of apes. See also heterochrony.

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Neotropical region

See faunal region.

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nephridiopore

See nephridium.

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nephridium

(pl. nephridia) The excretory organ of many invertebrates, consisting of a simple or branched tube, formed by the ingrowth of ectoderm, with cilia at the inner end. Excretory products diffuse into the nephridium and are wafted to the exterior by ciliary action. The most primitive type is known as a protonephridium and consists of a system of flame cells; it occurs in platyhelminths and rotifers. The metanephridium , which occurs in the earthworm and some other annelids, opens into the coelom by combining with a coelomoduct; the internal opening is known as the nephrostome and the external opening is the nephridiopore . Metanephridia superficially resemble kidney tubules.

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nephron

The excretory unit of the vertebrate kidney (see illustration). Many constituents of the blood are filtered from the glomerulus into the Bowman's capsule at one end of the nephron. The glomerular filtrate passes along the length of the nephron and some of its water, plus some salts, glucose, and amino acids, are reabsorbed into the surrounding blood capillaries (see proximal convoluted tubule; loop of Henle; distal convoluted tubule). More water is reabsorbed in the collecting duct, and the resulting concentrated solution of nitrogen-containing waste matter (urea in most mammals) plus inorganic salts drains from the collecting ducts of the nephrons and is discharged as urine into the ureter.

Structure of a single nephron

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nephrostome

See nephridium.

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nephrotoxin

Any toxic substance that targets the kidneys. Common examples of nephrotoxins are mercury salts and certain herbicides, such as Paraquat.

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neritic zone

The region of the sea over the continental shelf, which is less than 200 metres deep (approximately the maximum depth for organisms carrying out photosynthesis). Compare oceanic zone.

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nerve

A strand of tissue comprising many nerve fibres plus supporting tissues (see glia), enclosed in a connective-tissue sheath. Nerves connect the central nervous system with the organs and tissues of the body. A nerve may carry only motor nerve fibres (motor nerve ) or only sensory fibres (sensory nerve ) or it may be mixed and carry both types (mixed nerve ). Although the nerve fibres are in close proximity within the nerve, their physiological responses are independent of each other.

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nerve cell

See neurone.

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nerve cord

A large bundle of nerve fibres, running down the longitudinal axis of the body, that forms an important part of the central nervous system. Most invertebrates have a pair of solid nerve cords, situated ventrally and bearing segmentally arranged ganglia. All animals of the phylum Chordata have a dorsal hollow nerve cord; in vertebrates this is the spinal cord.

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nerve fibre

The axon of a neurone together with the tissues associated with it (such as a myelin sheath). The length and diameter of nerve fibres are very variable, even within the same organism. See also giant fibre.

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nerve growth factor (NGF)

1 A polypeptide produced by neurones and their supporting tissues (e.g. astrocytes and Schwann cells) that stimulates the growth of neurones. It is also produced in various other tissues, having a stimulatory effect on B lymphocytes (see B cell) and triggering histamine release by mast cells.

2 Any of various related polypeptides that promote the growth and maintenance of neurones, including neurotrophins .
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nerve impulse

See impulse.

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nerve net

A network of nerve cells connected with each other by synapses or fusion. The nervous system of certain invertebrates (e.g. coelenterates and echinoderms) consists exclusively of a nerve net in the body wall.

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nervous system

The system of cells and tissues in multicellular animals by which information is conveyed between sensory cells and organs and effectors (such as muscles and glands). It consists of the central nervous system (in vertebrates the brain and spinal cord; in invertebrates the nerve cord and ganglia) and the peripheral nervous system. Its function is to receive, transmit, and interpret information and then to formulate appropriate responses for the effector organs. It also serves to coordinate responses that require more than one physiological process. Nervous tissue consists of neurones, which convey the information in the form of impulses, and supporting tissue.

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net primary productivity

See productivity.

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neural plate

A strip of ectoderm, above the notochord, that lies along the central axis of the early embryos (see gastrula) of chordates and develops into the central nervous system. The neural plate folds and forms the neural tube; this process is called neurulation . During the stage when the neural tube is forming the embryo is known as a neurula .

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neural tube

A hollow tube of tissue in the early embryo of vertebrates that subsequently develops into the brain and spinal cord. It forms by folding of the ectodermal neural plate and has a central canal running through it. Sometimes the folds of the neural plate fail to close properly, resulting in neural tube defects (such as spina bifida) in the fetus.

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neurilemma cell

See Schwann cell.

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neuroendocrine system

Any of the systems of dual control of certain activities in the body of some higher animals by nervous and hormonal stimulation. For example, the posterior pituitary gland and the medulla of the adrenal gland receive direct nervous stimulation to secrete their hormones, whereas the anterior pituitary gland is stimulated by releasing hormones from the hypothalamus.

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neurofibril

Any of the fibres in the cytoplasm of a nerve axon. Neurofibrils include neurofilaments and neurotubules , microtubules that play a role in the transport of proteins and other substances within the cytoplasm.

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neurofilament

A type of intermediate filament found in the axons of nerve cells. Neurofilaments serve as elements of the cytoskeleton supporting the axon cytoplasm.

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neurohormone

Any hormone that is produced not by an endocrine gland but by a specialized nerve cell and is secreted from nerve endings into the bloodstream or directly to the tissue or organ whose growth or function it controls (see neurosecretion). Examples of neurohormones are noradrenaline, antidiuretic hormone, and hormones associated with metamorphosis and moulting in insects (see ecdysone; juvenile hormone).

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neurohypophysis

See pituitary gland.

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neuromuscular junction

The point where a muscle fibre comes into contact with a motor neurone carrying nerve impulses from the central nervous system. The impulses travel from the neurone to the muscle fibre by means of a neurotransmitter, in a similar way to the transmission of impulses across a synapse between two neurones. The neurotransmitter is released from vesicles at the end of the motor neurone into a small gap (the cleft), where it diffuses to the end plate of the muscle fibre and depolarizes the membrane. When depolarization has reached a certain threshold an action potential is triggered in the muscle fibre.

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neurone (neuron; nerve cell)

An elongated branched cell that is the fundamental unit of the nervous system, being specialized for the conduction of impulses. A neurone consists of a cell body, containing the nucleus and Nissl granules; dendrites, which receive incoming impulses and pass them towards the cell body; and an axon, which conducts impulses away from the cell body, sometimes over long distances. Impulses are passed from one neurone to the next via synapses. Sensory neurones transmit information from receptors to the central nervous system. Motor neurones conduct information from the central nervous system to effectors (e.g. muscles). (See illustration.) See also bipolar neurone; multipolar neurone; unipolar neurone.

Sensory neurone

Motor neurone

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neurone theory

The hypothesis, now accepted, that the nervous system consists of nerve cells (neurones), which are functionally linked at synapses but are physically separate. This theory has superseded the idea that the cytoplasm of the cells of the nervous system is continuous.

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neurosecretion

The secretion of neurohormones by neurosecretory cells , which possess characteristics of both nerve cells and endocrine cells. They are found, for example, in the hypothalamus, where they receive nerve impulses from other parts of the brain but transmit these signals to the pituitary gland by neurohormones that are released into the blood.

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neurotoxin

A chemical that either physically damages a nerve or reduces or alters the function of the nerve. An example of a neurotoxin is 6-hydroxydopamine, which damages the nerve terminals of the sympathetic neurones. This compound is structurally similar to dopamine and noradrenaline and can enter the nerve terminal by existing transport systems.

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neurotransmitter (transmitter)

A chemical that mediates the transmission of a nerve impulse across a synapse or a neuromuscular junction. Examples are adrenaline, noradrenaline, dopamine, and serotonin (in adrenergic nerves), acetylcholine (in cholinergic nerves), and gamma-aminobutyric acid. The neurotransmitter is released at the synaptic knob at the tip of the axon into the synaptic cleft. It diffuses across to the opposite membrane (the postsynaptic membrane), where it stimulates receptors and initiates the propagation of a nerve impulse in the next neurone. At a neuromuscular junction, the neurotransmitter transmits impulses to the muscle-fibre membrane.

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neurotrophin

See nerve growth factor.

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neurotubule

See neurofibril.

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neurula

See neural plate.

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neuston

The organisms that colonize the surface of an aquatic habitat. They are most abundant in freshwater habitats and include pond skaters, certain types of water beetles, and floating plants. Compare benthos; nekton; plankton.

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neuter

An organism that does not possess either male or female reproductive organs. Cultivated ornamental flowers that have neither pistils nor stamens are called neuters.

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neutral

Describing a compound or solution that is neither acidic nor basic. A neutral solution is one that contains equal numbers of both protonated and deprotonated forms of the solvent.

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neutral theory of molecular evolution

The theory, originally proposed in the late 1960s by Motoo Kimura (1924–94) and others, that most evolutionary changes at the molecular level are due to the random process of genetic drift acting on mutations, rather than natural selection. Its proponents, while recognizing the importance of selection in determining functionally significant traits, hold that the great majority of the differences in macromolecular structures observed between individuals in a population are of no adaptive significance and have no impact on the reproductive success of the individual in which they arise. Hence, frequencies of the corresponding mutant alleles are governed by purely random events. This contrasts with the orthodox neo-Darwinian view that nearly all evolutionary changes have adaptive value for the organism and arise through natural selection. For example, many enzymes exhibit polymorphism with regard to their amino acid sequence, giving rise to morphological variants that are detectable by electrophoresis. However, these variants may apparently perform equally well, and `neutralists' would argue that evolution consists essentially of random shuffling between them. The `selectionists' retort that such variants are likely to have subtle differences in function and are susceptible to selective forces, such as minor environmental changes. The degree to which the neutral theory applies to polymorphisms in proteins and nucleic acids is still a matter of controversy and debate.

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neutrophil

A type of white blood cell (leucocyte) that has a lobed nucleus and granular cytoplasm (see granulocyte). Neutrophils engulf bacteria (see phagocytosis) and release various substances, such as lysozyme and oxidizing agents.

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newton

Symbol N. The SI unit of force, being the force required to give a mass of one kilogram an acceleration of 1 ms–2. It is named after Sir Isaac Newton (1642–1727).

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nexus

See gap junction.

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niacin

See nicotinic acid.

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niche

See ecological niche.

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nicotinamide

See nicotinic acid.

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nicotinamide adenine dinucleotide

See NAD.

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nicotinamide adenine dinucleotide phosphate (NADP)

See NAD.

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nicotine

A colourless poisonous alkaloid present in tobacco. It is used as an insecticide.

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nicotinic acid (niacin)

A vitamin of the vitamin B complex. It can be manufactured by plants and animals from the amino acid tryptophan. The amide derivative, nicotinamide , is a component of the coenzymes NAD and NADP. These take part in many metabolic reactions as hydrogen acceptors. Deficiency of nicotinic acid causes the disease pellagra in humans. Apart from tryptophan-rich protein, good sources are liver and groundnut and sunflower meals.

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nictitating membrane

A clear membrane forming a third eyelid in amphibians, reptiles, birds, and some mammals (but not man). It can be drawn across the cornea independently of the other eyelids, thus clearing the eye surface and giving added protection without interrupting the continuity of vision.

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nidation

See implantation.

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nidicolous

See altricial species.

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nidifugous

See precocial species.

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ninhydrin

A compound that reacts with amino acids to give a blue colour. Ninhydrin is commonly used in chromatography to analyse the amino-acid content of proteins.

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nipple

See mammary glands.

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Nissl granules (Nissl bodies)

Particles seen within the cell bodies of neurones, consisting of granular endoplasmic reticulum and polyribosomes. They are rich in RNA and stain strongly with basic dyes. They are named after F. Nissl (1860–1919), the German neurologist who discovered them.

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nitrate

A salt or ester of nitric acid. The salts contain the ion NO_3–.

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nitric oxide (nitrogen monoxide)

A gaseous mediator in mammals and other vertebrates, especially in the cardiovascular and nervous systems. It is produced in tissues from molecular oxygen and the amino acid arginine by the enzyme nitric oxide synthase and diffuses to neighbouring cells, where it stimulates formation of the intracellular messenger cyclic GMP. The effects of nitric oxide include dilation of blood vessels and inhibition of platelet aggregation and adhesion. It may also act as a neurotransmitter in some tissues, and influence neuronal development. Certain cells of the immune system also produce nitric oxide, which is converted to the cytotoxic peroxynitrite anion (–O–O–N=O). This has nonspecific activity against tumour cells and pathogens, including protozoan and metazoan parasites.

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nitrification

A chemical process in which nitrogen (mostly in the form of ammonia) in plant and animal wastes and dead remains is oxidized at first to nitrites and then to nitrates. These reactions are effected mainly by the nitrifying bacteria Nitrosomonas and Nitrobacter respectively. Unlike ammonia, nitrates are readily taken up by plant roots; nitrification is therefore a crucial part of the nitrogen cycle. Nitrogen-containing compounds are often applied to soils deficient in this element, as fertilizer. Compare denitrification.

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nitrifying bacteria

See nitrification.

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nitrite

A salt or ester of nitrous acid. The salts contain the ion NO_2–.

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nitrogen

Symbol N. A colourless gaseous element that occurs in air (as dinitrogen, N_2, about 78% by volume) and is an essential constituent of proteins and nucleic acids in living organisms (see nitrogen cycle).

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nitrogenase

An important enzyme complex that is present in those microorganisms that are capable of fixing atmospheric nitrogen (see nitrogen fixation). Nitrogenase catalyses the conversion of atmospheric nitrogen into ammonia, which can then be used to synthesize nitrites, nitrates, or amino acids. The two main enzymes within the nitrogenase complex are dinitrogenase reductase and dinitrogenase .

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nitrogen cycle

One of the major cycles of chemical elements in the environment (see biogeochemical cycle). Nitrates in the soil are taken up by plant roots and may then pass along food chains into animals. Decomposing bacteria convert nitrogen-containing compounds (especially ammonia) in plant and animal wastes and dead remains back into nitrates, which are released into the soil and can again be taken up by plants (see nitrification). Though nitrogen is essential to all forms of life, the huge amount present in the atmosphere is not directly available to most organisms (compare carbon cycle). It can, however, be assimilated by some specialized bacteria (see nitrogen fixation) and is thus made available to other organisms indirectly. Lightning flashes also make some nitrogen available to plants by causing the combination of atmospheric nitrogen and oxygen to form oxides of nitrogen, which enter the soil and form nitrates. Some nitrogen is returned from the soil to the atmosphere by denitrifying bacteria (see denitrification). See illustration.

The nitrogen cycle

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nitrogen fixation

A chemical process in which atmospheric nitrogen is assimilated into organic compounds in living organisms and hence into the nitrogen cycle. The ability to fix nitrogen, by means of nitrogenase enzymes, is limited to certain bacteria (e.g. Azotobacter, Anabaena). Some bacteria (e.g. Rhizobium, Bradyrhizobium) are able to fix nitrogen in association with cells in the roots of leguminous plants, such as peas and beans, in which they form characteristic root nodules (see bacteroid); cultivation of legumes is therefore one way of increasing soil nitrogen. Certain nonleguminous plants are also hosts to nitrogen-fixing bacteria. For example, alder trees develop root nodules containing Frankia, a streptomycete-like organism. Various chemical processes are used to fix atmospheric nitrogen in the manufacture of fertilizers. These include the Birkeland–Eyde process, the cyanamide process, and the Haber process.

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nitrogenous base

A basic compound containing nitrogen. The term is used especially of organic ring compounds, such as adenine, guanine, cytosine, and thymine, which are constituents of nucleic acids.

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nitrogenous waste

Any metabolic waste product that contains nitrogen. Urea and uric acid are the most common nitrogenous waste products in terrestrial animals; freshwater fish excrete ammonia and marine fish excrete both urea and trimethylamine oxide.

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nitrogen oxides

Oxides of nitrogen (NOx), such as nitrogen monoxide (NO) and dinitrogen oxide (N_2O), many of which are pollutants contributing to acid rain. Nitrogen oxides are expelled in the emissions from car exhausts, aircraft, and factories. See also air pollution.

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nitrosamines

A group of carcinogenic compounds with the general formula RR'NNO, where R and R' are side groups with a variety of possible structures. Nitrosamines, which are a component of cigarette smoke, cause cancer in a number of organs, particularly in the liver, kidneys, and lungs. An example of a nitrosamine is dimethylnitrosamine, which has two methyl side groups (CH_3–).

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NMR

See nuclear magnetic resonance.

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node

1 (in botany) The part of a plant stem from which one or more leaves arise. The nodes at the stem apex are very close together and remain so in species of monocotyledons that form bulbs. In older regions of the stem they are separated by areas of stem called internodes .

2 (in anatomy) A natural thickening or bulge in an organ or part of the body. Examples are the sinoatrial node that controls the heartbeat (see pacemaker) and the lymph nodes.
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node of Ranvier

See myelin sheath.

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nodule

(in botany) See root nodule.

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nomad

(in cytology) A cell that migrates or wanders from its site of formation. Certain types of phagocytes are nomads.

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noncompetitive inhibition

See inhibition.

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noncyclic phosphorylation (noncyclic photophosphorylation)

See photophosphorylation.

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nondisjunction

The phenomenon that occurs when a pair of homologous chromosomes do not separate in meiosis but migrate to the same pole of the cell, resulting in an uneven number of chromosomes being present in the daughter cells. Klinefelter's syndrome results from nondisjunction of the sex chromosomes.

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nonreducing sugar

A sugar that cannot donate electrons to other molecules and therefore cannot act as a reducing agent. Sucrose is the most common nonreducing sugar. The linkage between the glucose and fructose units in sucrose, which involves aldehyde and ketone groups, is responsible for the inability of sucrose to act as a reducing sugar.

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nonrenewable energy sources

Sources of energy that use up the earth's finite mineral resources; these include fossil fuels. Concern about the exhaustion of nonrenewable energy sources, together with the fact that burning fossil fuels contributes to air pollution and the greenhouse effect, is leading to increased use or investigation of renewable energy resources , which are not exhaustible. These include the sun (for solar heating and solar cells), wind power (for aerogenerators) and water (for hydroelectric generators).

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nonsense mutation

A type of point mutation that converts a codon normally specifying an amino acid to one of the stop codons, thus signalling termination of translation and causing synthesis of the polypeptide chain to cease prematurely.

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noradrenaline (norepinephrine)

A hormone produced by the adrenal glands and also secreted from nerve endings in the sympathetic nervous system as a chemical transmitter of nerve impulses (see neurotransmitter). Many of its general actions are similar to those of adrenaline, but it is more concerned with maintaining normal body activity than with preparing the body for emergencies.

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norepinephrine

See noradrenaline.

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Northern blotting

See Southern blotting.

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nose

The protuberance on the face of some vertebrates that contains the nostrils (see nares) and part of the nasal cavity. It therefore forms part of the olfactory system (see olfaction) and the external opening of the respiratory system.

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nostrils

See nares.

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notochord

An elastic skeletal rod lying lengthwise beneath the nerve cord and above the alimentary canal in the embryos or adults of all chordate animals (see Chordata). Its function is to strengthen and support the body and act as a protagonist for the muscles. It is found in both adult and larval lancelets but in adult vertebrates it is largely replaced by the vertebral column.

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nucellus

The tissue that makes up the greater part of the ovule of seed plants. It contains the embryo sac and nutritive tissue. It is enclosed by the integuments except for a small gap, the micropyle. In certain flowering plants it may persist after fertilization and provide nutrients for the embryo.

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nuclear–cytoplasmic ratio

A measure of the size of a cell nucleus in relation to the cytoplasm. The nuclear–cytoplasmic ratio is often used as an index in the comparison of cells from normal and abnormal tissues. For example, cultured cancer cells show an increase in the nuclear–cytoplasmic ratio.

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nuclear envelope

The double membrane that separates the nucleoplasm (see nucleus) of a cell from the cytoplasm. The membranes consist of lipid bilayers that are separated by a perinuclear space (or compartment ). The outer membrane is continuous with the rough endoplasmic reticulum and is structurally and functionally distinct from the inner membrane. The envelope is perforated at intervals by nuclear pores , which provide a channel for the selective transfer of water-soluble molecules between the nucleus and the cytoplasm. Each nuclear pore is surrounded by a disc-shaped structure (nuclear pore complex ) consisting of an octagonal arrangement of eight protein granules.

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nuclear magnetic resonance (NMR)

The absorption of electromagnetic radiation (radio waves) by certain atomic nuclei placed in a strong and stable magnetic field. This results in a change of orientation of the nuclei, which respond to the magnetic field like miniature bar magnets. The main application of NMR is in a form of spectroscopy (NMR spectroscopy ) used for chemical and biochemical analysis and structure determination. There are two methods of NMR spectroscopy. In continuous wave (CW) NMR, the sample is subjected to a strong magnetic field, which can be varied in a controlled way. As the field changes, absorption of radiation occurs at certain points; this produces oscillations in the field, which can be detected. Fourier transform (FT) NMR uses a fixed magnetic field and the sample is subjected to a high-intensity pulse of radiation covering a range of frequencies. The signal produced is analysed mathematically to give the NMR spectrum. The 1H nucleus is the one commonly studied; other biochemically useful nuclei are 31P, 13C, 14N, and 19F, although these have lower natural abundance than hydrogen and produce weaker signals. The spectrum produced is characteristic of the molecule absorbing the radiation. In medicine, magnetic resonance imaging (MRI) has been developed, in which images of soft tissue are produced. This technique is useful for locating tumours and tissue abnormalities.

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nuclear pore

See nuclear envelope.

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nuclease

Any enzyme that breaks down nucleic acids to nucleotides. Nucleases are found in the small intestine. See DNAase; endonuclease; exonuclease.

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nucleic acid

A complex organic compound in living cells that consists of a chain of nucleotides. There are two types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

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nucleic acid hybridization

The association, in vitro, of two complementary nucleic-acid strands to form a hybrid double strand. See DNA hybridization.

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nucleoid (nuclear region)

The part of a cell of a bacterium (i.e. a prokaryotic cell) that contains the genetic material DNA and therefore controls the activity of the cell. It corresponds to the nucleus of the more advanced eukaryotic cells but is not bounded by a membrane.

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nucleolar organizer

A segment of a chromosome of a eukaryotic cell containing genes that encode ribosomal RNA. In the nondividing cell it is associated with the nucleolus in the assembly of ribosomes. The nucleolar organizer consists of numerous tandem repeats of a single gene; each repeat is transcribed simultaneously to form multiple copies of a precursor RNA molecule containing all the larger ribosomal RNA subunits.

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nucleolus

A small dense round body within the nucleus of a nondividing eukaryotic cell that is the site of ribosome assembly. It forms around the nucleolar organizer, which encodes most of the segments of ribosomal RNA. Ribosomal proteins migrate to the nucleolus from their assembly sites in the cytoplasm and are packaged into ribonucleoproteins, which then return to the cytoplasm where they become mature ribosome particles.

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nucleoplasm (karyoplasm)

The material contained within the nucleus of a cell. The nucleoplasm is bound by the nuclear envelope, which separates it from the cytoplasm.

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nucleoprotein

Any compound present in cells of living organisms that consists of a nucleic acid (DNA or RNA) combined with a protein. Chromosomes consist of nucleoprotein (DNA and proteins, mostly histones), as do ribosomes (RNA and protein). See also ribonucleoprotein.

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nucleoside

An organic compound consisting of a nitrogen-containing purine or pyrimidine base linked to a sugar (ribose or deoxyribose). An example is adenosine. Compare nucleotide.

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nucleosome

The fundamental unit of chromatin, the material of which eukaryotic chromosomes are made. It consists of a core of histone proteins around which are coiled about 160 base pairs of DNA. The nucleosomes themselves are thought to coil in a helical fashion, and in the fully condensed chromosome this coil is believed to undergo a further level of coiling.

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nucleotidase

An enzyme that catalyses the breakdown of nucleotides. It is present in the epithelial cells of the small intestine and plays an important role in the digestion of proteins.

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nucleotide

An organic compound consisting of a nitrogen-containing purine or pyrimidine base linked to a sugar (ribose or deoxyribose) and a phosphate group. DNA and RNA are made up of long chains of nucleotides (i.e. polynucleotides ). Compare nucleoside.

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nucleus

The large body embedded in the cytoplasm of all eukaryote cells that contains the genetic material DNA. The nucleus functions as the control centre of the cell. It is bounded by a double membrane (the nuclear envelope) and contains a viscous sap (nucleoplasm ). When the cell is not dividing a nucleolus is present in the nucleus and the genetic material is dispersed in the sap as chromatin; in dividing cells the chromatin is organized into chromosomes and the nucleolus disappears. In certain protozoans there are two nuclei per cell, a macronucleus (or meganucleus ) concerned with vegetative functions and a micronucleus with sexual reproduction.

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numerical taxonomy

See taxonomy.

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nut

A dry single-seeded fruit that develops from more than one carpel and does not shed its seed when ripe. The fruit wall is woody or leathery. Many nuts are enclosed in a hard or membranous cup-shaped structure, the cupule. The term nut is often loosely used of any hard fruit. For example, the walnut and coconut are in fact drupes and the Brazil nut is a seed.

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nutation

The spiral movement of a plant organ during growth, also known as circumnutation . It is seen in climbing plants and helps the plant find a suitable support to twine around. Examples are the coiling movements of the shoot tips of runner beans and of the tendrils of sweet peas.

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nutrient

Any substance that is required for the nourishment of an organism, providing a source of energy or structural components. In animals nutrients form part of the diet and include the major nutrients , i.e. carbohydrates, proteins (see also essential amino acid), and lipids (see also essential fatty acids), as well as vitamins and certain minerals (see essential element). Plant nutrients, derived from carbon dioxide in the atmosphere and water (containing minerals) absorbed from the soil by the roots, are divided into macronutrients and micronutrients.

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nutrition

The process by which organisms obtain energy (in the form of food) for growth, maintenance, and repair. There are two main types of nutrition: heterotrophic nutrition, employed by animals, fungi, and certain bacteria; and autotrophic nutrition, found in most plants and bacteria.

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nyctinasty (sleep movements)

Nastic movements of plant organs in response to the changes in light and temperature that occur between day and night (and vice versa). Examples are the opening and closing of many flowers and the folding together of the leaflets of clover and other plants at night.

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nymph

The juvenile stage of exopterygote insects, especially terrestrial species, such as grasshoppers, cockroaches, and earwigs; the juveniles of aquatic species (e.g. dragonflies, mayflies, stoneflies) are sometimes called naiads or larvae . The nymph resembles the adult except that the wings and reproductive organs are undeveloped. There is no pupal stage, the nymph developing directly into the adult. Compare larva.

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O

objective

The lens or system of lenses nearest to the object being examined through an optical instrument.

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obligate anaerobes

Organisms that cannot use free oxygen for respiration. Oxygen may inhibit the growth of obligate anaerobes or kill them. See anaerobic respiration.

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occipital condyle

A single or paired bony knob that protrudes from the occipital bone of the skull and articulates with the first cervical vertebra (the atlas). In humans there is a pair of occipital condyles, one on each side of the foramen magnum. Occipital condyles are absent in most fish, which cannot move their heads.

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oceanic zone

The region of the open sea beyond the edge of the continental shelf, where the depth is greater than 200 metres. Compare neritic zone.

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ocellus

A simple eye occurring in insects and other invertebrates. It typically consists of light-sensitive cells and a single cuticular lens.

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ocular

See eyepiece.

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Odonata

An order of exopterygote insects containing the dragonflies and damselflies, most of which occur in tropical regions. Adult dragonflies have a pair of prominent compound eyes, a compact thorax bearing two pairs of delicate membranous wings, and a long slender abdomen. They are strong fliers and prey on other insects, either in flight or at rest. The eggs are laid near or in water, and the newly hatched nymphs (naiads) are aquatic and resemble the adults, with rudimentary wings. They breathe through gills and feed on small aquatic animals. The nymph leaves the water for its final moult into the terrestrial adult.

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odontoblast

A cell that is responsible for producing the dentine of vertebrate teeth. Odontoblasts are found around the lining of the pulp cavity and have processes that extend into the dentine.

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oedema

The accumulation of tissue fluid in the tissues of the body, causing swelling of the affected part. Localized oedema occurs during inflammation. Generalized oedema can result from a variety of pathological conditions, including kwashiorkor (see malnutrition) and heart or kidney failure; it can also occur as a side-effect of certain drugs and as a reaction to toxic chemicals.

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oesophagus (gullet)

The section of the alimentary canal that lies between the pharynx and the stomach. It is a muscular tube whose function is to transfer food to the stomach by means of wavelike contractions (peristalsis) along its length.

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oestrogen

One of a group of female sex hormones, produced principally by the ovaries, that promote the onset of secondary sexual characteristics (such as breast enlargement and development in women) and control the oestrous cycle (menstrual cycle in humans). Oestradiol is the most important. Oestrogens are secreted at particularly high levels during ovulation, stimulating the uterus to prepare for pregnancy. They are used in oral contraceptives (with progestogens) and as treatment for various disorders of the female reproductive organs. Small amounts of oestrogens are produced by the adrenal glands and testes.

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oestrous cycle (sexual cycle)

The cycle of reproductive activity shown by most sexually mature nonpregnant female mammals except most primates (compare menstrual cycle). There are four phases:
(1) pro-oestrus (follicular phase ) – Graafian follicles develop in the ovary and secrete oestrogens;
(2) oestrus (heat ) – ovulation normally occurs, the female is ready to mate and becomes sexually attractive to the male;
(3) metoestrus (luteal phase ) – corpus luteum develops from ruptured follicle;
(4) dioestrus – progesterone secreted by corpus luteum prepares uterus for implantation.

The length of the cycle depends on the species: larger mammals typically have a single annual cycle with a well-defined breeding season (they are described as monoestrous ). The males have a similar cycle of sexual activity. Other species may have many cycles per year (i.e. they are polyoestrous ) and the male may be sexually active all the time.

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oestrus (heat)

See oestrous cycle.

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offset

See runner.

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offspring (progeny)

New individual organisms that result from the process of sexual or asexual reproduction. See also F1; F2.

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oil

Any of various viscous liquids that are generally immiscible with water. Natural plant and animal oils are either volatile mixtures of terpenes and simple esters (e.g. essential oils) or are glycerides of fatty acids.

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oil-immersion lens

See immersion objective.

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Okazaki fragment

See discontinuous replication.

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oleaginous

Producing or containing oil or lipids. Oleaginous microorganisms, which normally contain 20–25% oil, are of interest in biotechnology as alternative sources of conventional oils or as possible sources for novel oils. The majority of the oils produced by oleaginous eukaryotic microorganisms are similar to plant oils. One possibility under consideration is the production of oils and fats from waste material, to be used in animal feed.

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olecranon process

A bony process on the ulna in the forelimb of vertebrates that projects beyond the joint between the humerus and the ulna.

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oleic acid

An unsaturated fatty acid with one double bond, CH_3(CH_2)7CH:CH(CH_2)7COOH. Oleic acid is one of the most abundant constituent fatty acids of animal and plant fats, occurring in butterfat, lard, tallow, groundnut oil, soya-bean oil, etc. Its systematic chemical name is cis-octadec-9-enoic acid .

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oleosome

See spherosome.

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olfaction

The sense of smell or the process of detecting smells. This is achieved by receptors in olfactory organs (such as the nose) that are sensitive to air- or water-borne chemicals. Stimulation of these receptors results in the transmission of information to the brain via the olfactory nerve .

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olfactory lobe

Either member of a pair of lobes in the forebrain, at the anterior end of the cerebrum. They contain the endings of the olfactory nerves (the first pair of cranial nerves) and are concerned with the sense of smell, being prominent in the dogfish and other animals that depend on this sense.

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Oligocene

The third geological epoch of the Tertiary period. It began about 38 million years ago, following the Eocene epoch, and extended for about 13.5 million years to the beginning of the Miocene epoch. The epoch was characterized by the continued rise of mammals; the first pigs, rhinoceroses, and tapirs made their appearance.

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Oligochaeta

A class of hermaphrodite annelid worms that bear only a few bristles (chaetae). Oligochaetes are very abundant in freshwater and terrestrial habitats. The most familiar members of the class are the earthworm (Lumbricus) and the freshwater bloodworm (Tubifex).

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oligonucleotide

A short polymer of nucleotides.

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oligotrophic

Describing a body of water (e.g. a lake) with a poor supply of nutrients and a low rate of formation of organic matter by photosynthesis. Compare dystrophic; eutrophic; mesotrophic.

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omasum

The third of four chambers that form the stomach of ruminants. See Ruminantia.

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ommatidium

(pl. ommatidia) See compound eye.

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omnivore

An animal that eats both animal and vegetable matter. Pigs, for example, are omnivorous. Compare carnivore; herbivore.

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oncogene

A gene that is capable of transforming a normal cell into a cancerous cell. Oncogenes result from the mutation of normal genes (proto-oncogenes ); they occur in cancer cells and oncogenic viruses. Viral oncogenes are derived from normal host genes that have become incorporated into the viral genome and subsequently undergo mutation.

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oncogenic

Describing a chemical, organism, or environmental factor that causes the development of cancer. Some viruses are oncogenic to vertebrates, notably the retroviruses (including the Rous sarcoma virus of chickens), and some are suspected of being oncogenic (e.g. some of the adenoviruses and papovaviruses). Many of these viruses contain genes (known as oncogenes) that are responsible for the transformation of a normal host cell into a cancerous cell. See also growth factor.

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one gene–one polypeptide hypothesis

The theory that each gene is responsible for the synthesis of a single polypeptide. It was originally stated as the one gene–one enzyme hypothesis by the US geneticist George Beadle in 1945 but later modified when it was realized that genes also encoded nonenzyme proteins and individual polypeptide chains. It is now known that some genes code for various types of RNA involved in protein synthesis.

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ontogeny

The developmental course of an organism from the fertilized egg through to maturity. It has been suggested that “ontogeny recapitulates phylogeny”, i.e. the stages of development, especially of the embryo, reflect the evolutionary history of the organism. This idea is now discredited. See recapitulation.

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oocyte

See oogenesis.

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oogamy

Sexual reproduction involving the formation and subsequent fusion of a large, usually stationary, female gamete and a small motile male gamete. The female gamete may contain nourishment for the development of the embryo, which is often retained and protected by the parent organism.

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oogenesis

The production and growth of the ova (egg cells) in the animal ovary. Special cells (oogonia ) within the ovary divide repeatedly by mitosis to produce large numbers of prospective egg cells (primary oocytes ). When mature, these undergo meiosis, which halves the number of chromosomes. During the first meiotic division a polar body and a secondary oocyte are produced. At the second meiotic division the secondary oocyte produces an ovum and a second polar body. Oocytes may be present in the ovaries at birth and may represent the total number of eggs to be produced.

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oogonium

(pl. oogonia) 1 The female sex organ (gametangium) of algae and fungi.

2 Any of the immature sex cells in the animal ovary that give rise to oocytes by mitotic divisions (see oogenesis).
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Oomycota

A phylum of the Protoctista that includes the water moulds, downy mildews, and potato blight (Phytophthora), formerly classified as a class of fungi (Oomycetes). They are coenocytic and the cell wall is made of cellulose. Oomycotes are either saprotrophic or parasitic; they feed by extending hypha-like threads into the food source or host's body. Asexual reproduction is by means of flagellated zoospores, which are released from a sporangium. Sexual reproduction involves the fusion of an antheridium and an oogonium and results in the production of a zygote, which can develop a wall of chitin and become a resistant oospore.

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oosphere (ovum; egg cell)

The nonmotile female gamete in plants and some algae. In angiosperms (flowering plants) it is a cell in the embryo sac of the ovule. In other plants it is situated in an archegonium. In algae, such as Fucus, the oosphere is protected by an oogonium until it is shed into the water prior to fertilization. Many oospheres store food in the form of starch or oil droplets.

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oospore

A zygote that is produced as a result of oogamy in certain algae and fungi. It contains food reserves, develops a protective outer covering, and enters a resting phase before germination. Compare zygospore.

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open circulation

See circulation.

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open reading frame

See reading frame.

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operant conditioning

See conditioning.

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operculum

1 (in zoology) A lid or flap of skin covering an aperture, such as the gill slit cover of fish and larval amphibians and the horny calcareous operculum secreted by many gastropod molluscs, which closes the opening of the shell when the animal is inside.

2 (in botany) The cone-shaped lid of the capsule of mosses, which is forcibly detached to release the spores.
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operon

A functionally integrated genetic unit for the control of gene expression in bacteria, as proposed in the Jacob–Monod hypothesis. It comprises structural genes , coding for protein, and adjacent loci controlling their expression – an operator site and a promoter site . Transcription of the structural genes is prevented by binding of a repressor molecule to the operator site. Another molecule, the inducer , can bind to the repressor molecule, preventing it from binding to the operator and thus allowing the promoter site to bind the enzyme RNA polymerase, thereby initiating transcription. The repressor molecule is encoded by a regulator gene , which may be close to or distant from the operon. Some operons also have an attenuator site preceding the first structural gene, where transcription may either stall or proceed according to the amount of end-product in the cell. See also lac operon.

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opiate

One of a group of drugs derived from opium , an extract of the poppy plant Papaver somniferum that depresses brain function (a narcotic action). Opiates include morphine and its synthetic derivatives, such as heroin and codeine. They are used in medicine chiefly to relieve pain, but the use of morphine and heroin is strictly controlled since they can cause drug dependence and tolerance.

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opioid

Any one of a group of substances that produce pharmacological and physiological effects similar to those of morphine. Opioids are not necessarily structurally similar to morphine, although a subgroup of opioids, the opiates, are morphine-derived compounds.

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opisthosoma

The posterior section of the body (see tagma) of arachnids and other arthropods of the phylum Chelicerata, which consists of those body segments that do not bear legs. Compare prosoma.

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opportunistic

Describing a species that can quickly exploit new resources as they arise, for example by rapidly colonizing a new environment. Such species characteristically exhibit r selection.

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opsin

The lipoprotein component of rhodopsin, the light-sensitive pigment that occurs in the rod cells of the retina.

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opsonin

See opsonization.

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opsonization

The process in which certain antibodies in the blood (known as opsonins ) bind to the surface of an invading microorganism, which render it more susceptible to phagocytosis. See also complement.

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optical activity

The ability of certain substances to rotate the plane of plane-polarized light as it passes through a crystal, liquid, or solution. It occurs when the molecules of the substance are asymmetric, so that they can exist in two different structural forms each being a mirror image of the other. The two forms are optical isomers or enantiomers (see illustration). The existence of such forms is also known as enantiomorphism (the mirror images being enantiomorphs ). One form will rotate the light in one direction and the other will rotate it by an equal amount in the other. The two possible forms are described as dextrorotatory or laevorotatory according to the direction of rotation, and the prefixes (+)- and (–)- are used, respectively, to designate the isomer, as in (+)-tartaric and (–)-tartaric acids. (The prefixes d- and l- are now obsolete.) An equimolar mixture of the two forms is not optically active. It is called a racemic mixture (or racemate ) and designated by (±)-. In addition, certain molecules can have a meso form in which one part of the molecule is a mirror image of the other. Such molecules are not optically active.
Molecules that show optical activity have no plane of symmetry. The commonest case of this is in organic compounds in which a carbon atom is linked to four different groups. An atom of this type is said to be a chiral centre . Many naturally occurring compounds show optical isomerism and usually only one isomer occurs naturally. For instance, glucose is found in the dextrorotatory form. The other isomer, (–)-glucose, can be synthesized in the laboratory, but cannot be synthesized by living organisms.

Isomers of tartaric acid

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optical fibre

A glass fibre through which light can be transmitted with very little leakage through the sidewalls. In the step-index fibre a pure glass core, with a diameter between 6 and 250 micrometres, is surrounded by a glass or plastic cladding of lower refractive index. The cladding is usually between 10 and 150 micrometres thick. The interface between core and cladding acts as a cylindrical mirror at which total internal reflection of the transmitted light takes place. This structure enables a beam of light to travel through many kilometres of fibre. In the graded-index fibre , each layer of glass, from the fibre axis to its outer wall, has a slightly lower refractive index than the layer inside it. This arrangement also prevents light from escaping through the fibre walls by a combination of refraction and total internal reflection, and can be made to give the same transit time for rays at different angles.
Fibre-optic systems use optical fibres to transmit information, in the form of coded pulses or fragmented images (using bundles of fibres), from a source to a receiver. They are used, for example, in medical instruments (endoscopes or fibrescopes ) to examine internal body cavities, such as the stomach and bladder.

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optical isomers

See optical activity.

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optical microscope

See microscope.

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optic chiasma

The point where some of the fibres from both optic nerves cross over to the opposite side, forming an X-shaped structure. Sensory information from the right side of both the right and left eye is transmitted to the right visual cortex in the brain (via the right lateral geniculate nucleus ), while information from the left side of each eye is carried to the left visual cortex. Thus each half of the brain receives sensory information from both eyes.

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optic lobes

See midbrain.

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optic nerve

The second cranial nerve: a paired sensory nerve that runs from each eye to the brain. It is responsible for conveying visual stimuli received by the rods and cones in the retina to the brain for interpretation. See also optic chiasma.

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optic vesicle

The outgrowth from the forebrain of a vertebrate embryo that develops into the retina of the eye. When the optic vesicle comes into contact with the ectoderm covering the head of the embryo, these ectodermal cells become thicker: the ectoderm in this region invaginates and eventually becomes detached from the adjacent ectodermal cells to form the lens of the eye.

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optimal foraging theory

A theory, first formulated in 1966 by R. H. MacArthur and E. R. Pianka, stating that natural selection favours animals whose behavioural strategies maximize their net energy intake per unit time spent foraging. Such time includes both searching for prey and handling (i.e. killing and eating) it. The theory was originally devised in an attempt to explain why, out of the wide range of foods available, animals often restrict themselves to a few preferred types. The prediction is that an animal strikes a balance between two contrasting strategies: spending a long time (i.e. using more energy) searching for highly `profitable' food items, or devoting minimal time (i.e. using less energy) to more common but less profitable food items. Various factors can cause animals to deviate from optimal foraging. For example, the risk of predation may force the animal to select less profitable food items in a relatively safe location, rather than opting for the energetically most efficient feeding strategy.

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oral cavity

See buccal cavity.

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oral contraceptive

Any hormonal preparation taken in the form of a pill to prevent conception (see birth control). The most common form is the combined pill, which contains an oestrogen and a progestogen. Both act to suppress ovulation, while the progestogen additionally causes changes in the viscosity of cervical mucus and alters the lining of the womb, both of which decrease the chances of fertilization should ovulation occur. The so-called `minipill' contains only a progestogen. The so-called `morning-after pill' contains high doses of oestrogen and progestogen, and can be taken for up to 72 hours after intercourse to prevent possible pregnancy.

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oral groove

A ciliated channel found in certain protozoa and aquatic invertebrates down which food is directed into the mouth.

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orbit

(in anatomy) Either of the two sockets in the skull of vertebrates that house the eyeballs.

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order

(in taxonomy) A category used in the classification of organisms that consists of one or several similar or closely related families. Similar orders form a class. Order names typically end in -ales in botany, e.g. Rosales (roses and orchard fruits), and in -a in zoology, e.g. Carnivora (flesh eaters).

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Ordovician

The second geological period of the Palaeozoic era, following the Cambrian and preceding the Silurian periods. It began about 505 million years ago and lasted for about 67 million years. The period was named by the British geologist Charles Lapworth (1842–1920) in 1879. Graptolites, in deep-water deposits, are the dominant fossils. Other fossils include trilobites, brachiopods, ectoprocts, gastropods, bivalves, echinoids, crinoids, nautiloid cephalopods, and the first corals.

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organ

Any distinct part of an organism that is specialized to perform one or a number of functions. Examples are ears, eyes, lungs, and kidneys (in animals) and leaves, roots, and flowers (in plants). A given organ will contain many different tissues.

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organ culture

The culture of complete living organs (explants ) of animals and plants outside the body in a suitable culture medium. Animal organs must be small enough to allow the nutrients in the culture medium to penetrate all the cells. Whole plant roots and even root systems can be kept alive in such conditions for a considerable period of time. See also explantation.

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organelle

A minute structure within a eukaryotic cell that has a particular function. Examples of organelles are the nucleus, mitochondria, and lysosomes.

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organic evolution

The process by which changes in the genetic composition of populations of organisms occur in response to environmental changes. See adaptation; evolution. Compare biochemical evolution.

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organism

An individual living system, such as an animal, plant, or microorganism, that is capable of reproduction, growth, and maintenance.

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organizer

An area of an animal embryo that causes adjacent areas of the embryo to develop in a certain way. The primary organizer (blastopore lip or archenteron roof) causes the gastrula to develop as a complete organism.

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organ of Corti

The sensory part of the cochlea in the inner ear, which responds to sound. This organ, named after Italian anatomist A. G. G. Corti (1822–88), projects into the cochlear duct and consists of two membranes that run parallel to each other: sensory hair cells, rooted in the basilar membrane , are in contact with the overlying tectorial membrane (see illustration). During the transmission of sound waves the basilar membrane vibrates, causing the sensory hairs to flex against the tectorial membrane; this results in the production of impulses, which are transmitted to the brain via the auditory nerve.

Organ of Corti (vertical section through one turn of the cochlea)

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organogenesis

The formation of organs during embryonic development. In animals this begins following the rearrangement of the cells at gastrulation, when the three germ layers are fully formed in their correct positions. Dividing cells of the gastrula begin to differentiate and the rudimentary organs and organ systems begin to form. See differentiation; ectoderm; endoderm; mesoderm.

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orgasm

The climax of sexual excitement in humans, which – in males – coincides with ejaculation. A sense of physiological and emotional release is accompanied by a feeling of extreme pleasure.

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Oriental region

See faunal region.

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origin of life

The process by which living organisms developed from inanimate matter, which is generally thought to have occurred on earth between 3500 and 4000 million years ago. It is supposed that the primordial atmosphere was like a chemical soup containing all the basic constituents of organic matter: ammonia, methane, hydrogen, and water vapour. These underwent a process of chemical evolution using energy from the sun and electric storms to combine into ever more complex molecules, such as amino acids, proteins, and vitamins. Eventually self-replicating nucleic acids, the basis of all life, could have developed. The very first organisms may have consisted of such molecules bounded by a simple membrane.

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ornithine (Orn)

An amino acid, H_2N(CH_2)3CH(NH_2)COOH, that is not a constituent of proteins but is important in living organisms as an intermediate in the reactions of the urea cycle and in arginine synthesis.

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ornithine cycle

See urea cycle.

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orthogenesis

An early theory of the nature of evolutionary change, which proposed that organisms evolve along particular paths predetermined by some factor in their genetic make-up. More recent understanding of selection pressure and other external forces that can be shown experimentally to affect the survival of organisms has proved the improbability of the theory.

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Orthoptera

A large order of exopterygote insects containing the grasshoppers, locusts, crickets, and – in some classification systems – the cockroaches (see Dictyoptera). They are characterized by enlarged hind legs modified for jumping and biting mouthparts and produce sounds by stridulation. The crickets and long-horned grasshoppers (e.g. Gryllus, Tettigonia) have long threadlike antennae and stridulate by rubbing together modified veins on their forewings. The hearing organs are on the front legs. The short-horned grasshoppers and locusts (e.g. Chorthippus, Locusta) have short antennae and stridulate by rubbing pegs on the hind leg against a hardened vein on the forewing. The hearing organs are on the abdomen.

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orthotropism

The tendency for a tropism (growth response of a plant) to be orientated directly in line with the stimulus concerned. An example is the vertical growth of main stems and roots in response to gravity (orthogeotropism ). Compare plagiotropism.

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osculum

(pl. oscula) 1 The mouthlike aperture in the body wall of a sponge (see Porifera) through which water leaves the body cavity.

2 Any of the suckers on the head (scolex) of a tapeworm, by which it attaches itself to the gut wall of its host.
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osmium tetroxide (osmium(IV) oxide)

A yellow solid, OsO_4, made by heating osmium in air. It is used as a fixative in electron microscopy.

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osmolyte

Any compound that protects cells from desiccation by maintaining a high intracellular osmolality (osmotic concentration). Osmolytes are active in the process of osmoregulation and are of particular use in the kidneys, where cells are exposed to highly concentrated fluids. Compounds that are known osmolytes include polyols, amines (e.g. trimethylamine), certain amino acids, and urea.

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osmoreceptor

A receptor situated in the hypothalamus of the brain that responds to an increase in the concentration of the extracellular fluid. This results in the release of antidiuretic hormone (ADH) and the subsequent conservation of water, thereby maintaining the homeostasis of the body fluids.

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osmoregulation

The control of the water content and the concentration of salts in the body of an animal or protoctist. In freshwater species osmoregulation must counteract the tendency for water to pass into the animal by osmosis. Various methods have been developed to eliminate the excess, such as contractile vacuoles in protozoans and kidneys with well-developed glomeruli in freshwater fish. Marine vertebrates have the opposite problem: they prevent excessive water loss and enhance the excretion of salts by having kidneys with few glomeruli and short tubules (see also chloride secretory cell). In terrestrial vertebrates the dangers of desiccation are reduced by the presence of long convoluted renal tubules, which increase the reabsorption of water and salts.

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osmosis

The net movement of water molecules from a region where their concentration is high to a region where their concentration is low through a partially permeable membrane. The distribution of water in living organisms is dependent to a large extent on osmosis, water entering the cells through their partially permeable plasma membranes. The pressure required to stop the flow of pure water into a solution across a partially permeable membrane is a characteristic of the solution, and is called the osmotic pressure . Thus water will move from a region of low osmotic pressure to a region of high osmotic pressure. In terms of water potential, water moves from an area of high (less negative) water potential to an area of low (more negative) water potential. Both water potential and osmotic pressure can be used to explain osmosis but it is now recommended that only water potential be used in plant studies (see also plasmolysis; turgor). Animals have evolved various means to counteract the effects of osmosis (see osmoregulation); in animals solutions are still described in terms of osmotic pressure (see hypertonic solution; hypotonic solution; isotonic).

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osmotic pressure

See osmosis.

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osmotroph

Any heterotrophic organism that obtains its nutrients by absorbing organic matter in solution from its surroundings. Compare phagotroph.

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ossicle

Any small bony or chitinous structure found in various skeletal parts of animals. The term ossicle usually refers to any of the chain of bones in the mammalian ear (see ear ossicles).

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ossification

The process of bone formation. It is brought about by the action of special cells called osteoblasts, which deposit layers of bone in connective tissue. In intramembranous ossification bones are formed directly in connective tissue (see membrane bone); in endochondral ossification the bones are formed by the replacement of cartilage (see cartilage bone).

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Osteichthyes

The class of vertebrates comprising the bony fishes – marine and freshwater fish with a bony skeleton. All have gills covered with a bony operculum, and a layer of thin overlapping bony scales covers the entire body surface. Bony fish have a swim bladder, which acts as a hydrostatic organ enabling the animal to remain suspended in the water at any depth. In some fish this bladder acts as a lung. See also Dipnoi; Teleostei. Compare Chondrichthyes.

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osteoblast

Any of the cells, found in bone, that secrete collagen and other substances that form the matrix of bone (see osteoid). Osteoblasts are derived from osteoprogenitor cells in the bone marrow; they eventually become osteocytes. See also ossification.

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osteoclast

Any of the cells, found on the surface of bone, that are involved in the breakdown of the bone matrix to enable the further development and restructuring of bone during growth and repair. See also parathyroid hormone; periosteum.

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osteocyte

Any of the cells, found in bone, that perform the cellular activities, such as respiration and exchange of materials with the blood, that are required for maintenance of the bone tissue. They are derived from osteoblasts.

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osteoid

A soft material, consisting mainly of collagen, that is secreted by osteoblasts and constitutes the uncalcified matrix of bone. Osteoid is converted into hard bone matrix when it combines with calcium phosphate (hydroxyapatite) deposited from the blood (see osteonectin).

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osteonectin

A protein in bone that binds to collagen (see osteoid) and is involved in the formation of growth sites for calcium phosphate crystals, which are required for forming the hard bone matrix.

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ostiole

A pore in the fruiting body of certain fungi and algae, through which either spores or gametes are released. An ostiole occurs, for example, in the perithecium of ascomycete fungi (see ascocarp) and in the conceptacle of brown algae.

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ostium

(pl. ostia) 1 Any one of the pores in the body wall of a sponge (see Porifera) through which water enters the body cavity.

2 Any one of the apertures in the arthropod heart through which blood enters from the haemocoel.
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otolith

A gelatinous mass containing a high concentration of particles of calcium carbonate, which forms part of the macula of the inner ear.

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outbreeding

Mating between unrelated or distantly related individuals of a species. Outbreeding populations usually show more variation than inbreeding ones and have a greater potential for adapting to environmental changes. Outbreeding increases the number of heterozygous individuals, so disadvantageous recessive characteristics tend to be masked by dominant alleles.

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outer ear (external ear)

The part of the ear external to the tympanum (eardrum). It is present in mammals, birds, and some reptiles and consists of a tube (the external auditory meatus ) that directs sound waves onto the tympanum. In mammals it may include an external pinna, which extends beyond the skull.

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outgroup

A species or higher taxon used in systematics for comparison with a group of closely related species or taxa in order to assess whether particular characters shared by the group members are derived (see apomorphy) or ancestral (see plesiomorphy). For the outgroup comparison to be valid, the outgroup must be outside the phylogenetic group under consideration but not too distantly related, otherwise the comparison yields no useful information. For example, the reptiles, birds, and mammals are all amniotes known to comprise a monophyletic group, the Amniota. Suppose that the evolutionary relations between several representatives of this group are being investigated, using reproductive physiology as one of a set of homologies (see homologous). A dog and a kangaroo both bear live young (viviparity), whereas a sparrow, a turtle, and a crocodile all lay eggs (oviparity). Which is the ancestral trait and which is the derived trait? Outgroup comparison with frogs or fishes reveals that these anamniote fellow vertebrates exhibit oviparity: this is therefore inferred as the ancestral trait for the amniotes as a whole, and hence viviparity is a derived trait for mammals.

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oval window (fenestra ovalis)

A membrane-covered opening between the middle ear and the inner ear (see ear), situated above the round window. Vibrations of the tympanum are transferred across the middle ear by the ear ossicles and transmitted to the inner ear by the oval window, which is connected to the third ear ossicle (stapes). See also vestibular canal.

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ovarian follicle

See Graafian follicle.

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ovary

1 The reproductive organ in female animals in which eggs (ova) are produced. In most vertebrates there are two ovaries (in some fish the ovaries fuse together to form a single structure and in birds the left ovary only is functional). As well as eggs, they produce steroid hormones (see oestrogen; progesterone). In mammals each ovary is situated close to the opening of a fallopian tube; it contains numerous follicles in which the eggs develop and from which they are released in a regular cycle. See also Graafian follicle; menstrual cycle; oogenesis; ovulation; reproductive system.

2 The hollow base of the carpel of a flower, containing one or more ovules. After fertilization, the ovary wall develops into the fruit enclosing the seeds. In some species, the carpels are fused together to form a complex ovary.
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overpopulation

The situation that arises when rapid growth of a population, usually a human population, results in numbers that cannot be supported by the available resources, such as space and food. This occurs when the birth rate exceeds the death rate, or when immigration exceeds emigration, or when a combination of these factors exists. See population growth.

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oviduct

The tube that conveys an animal egg cell from the ovary to other parts of the reproductive system or to the outside. Eggs are passed along the oviduct by the action of muscles and cilia. See fallopian tube.

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oviparity

Reproduction in which fertilized eggs are laid or spawned by the mother and hatch outside her body. It occurs in most animals except marsupial and placental mammals. Compare ovoviviparity; viviparity.

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ovipositor

An organ at the hind end of the abdomen of female insects through which eggs are laid. It consists of a pair of modified appendages and is often long and piercing, so that eggs can be laid in otherwise inaccessible places. The sting of bees and wasps is a modified ovipositor.

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ovoviviparity

Reproduction in which fertilized eggs develop and hatch in the oviduct of the mother. It occurs in many invertebrates and in some fish and reptiles (e.g. the viper). Compare oviparity; viviparity.

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ovulation

The release of an egg cell from the ovary, which in mammals is stimulated by luteinizing hormone. The developing egg cell within its follicle migrates to the ovary surface; when mature, it is released from the follicle (which breaks open) into the body cavity, from where it passes into the oviduct. See also menstrual cycle.

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ovule

The part of the female reproductive organs of seed plants that consists of the nucellus, embryo sac, and integuments. The ovules of gymnosperms are situated on ovuliferous scales of the female cones while those of angiosperms are enclosed in the carpel. After fertilization, the ovule becomes the seed.

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ovuliferous scale

One of a group of large woody specialized leaves that form the female cone of conifers and related trees. It bears the ovules, which develop into seeds.

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ovum (egg cell) (pl. ova)

1 (in zoology) The mature reproductive cell (see gamete) of female animals, which is produced by the ovary (see oogenesis). It is spherical, has a nucleus, is covered with an egg membrane, and is not mobile.

2 (in botany) The oosphere of plants.
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oxalic acid (ethanedioic acid)

A crystalline solid, (COOH)2, that is slightly soluble in water. Oxalic acid is strongly acidic and very poisonous. It occurs in certain plants, e.g. sorrel and the leaf blades of rhubarb.

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oxaloacetic acid

A compound, HO_2CCH_2COCO_2H, that plays an integral role in the Krebs cycle. The anion, oxaloacetate, reacts with the acetyl group from acetyl coenzyme A to form citrate.

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oxidase

Any enzyme that catalyses oxidation–reduction reactions that involve the transfer of electrons to molecular oxygen.

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oxidation

See oxidation–reduction.

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oxidation–reduction (redox)

Originally, oxidation was simply regarded as a chemical reaction with oxygen. The reverse process – loss of oxygen – was called reduction . Reaction with hydrogen also came to be regarded as reduction. Later, a more general idea of oxidation and reduction was developed in which oxidation was loss of electrons and reduction was gain of electrons. This wider definition covered the original one and also applies to reactions that do not involve oxygen. However, it applies only to reactions in which electron transfer occurs – i.e. to reactions involving ions. It can be extended to reactions between covalent compounds by using the concept of oxidation number (or state ). This is a measure of the electron control that an atom has in a compound compared to the atom in the pure element. An oxidation number consists of two parts:
(1) its sign, which indicates whether the control has increased (negative) or decreased (positive);
(2) its value, which gives the number of electrons over which control has changed.

The change of electron control may be complete (in ionic compounds) or partial (in covalent compounds). Oxidation is a reaction involving an increase in oxidation number and reduction involves a decrease. Thus in
2H_2 + O_2 2H_2O

the hydrogen in water is +1 and the oxygen –2. The hydrogen is oxidized and the oxygen is reduced. Compounds that tend to undergo reduction readily are oxidizing agents ; those that undergo oxidation are reducing agents .

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oxidative burst

The production of various highly reactive oxygen derivatives by certain cells, notably macrophages of the vertebrate immune system. These toxic compounds are used to kill bacteria engulfed by the macrophage, or secreted to attack larger parasites outside the macrophage. The enzyme NADPH oxidase catalyses the formation of superoxide anions (O_2–), which then form hydrogen peroxide (H_2O_2), singlet oxygen, and hydroxyl radicals (OH·). Another phagocytic enzyme, myeloperoxidase, catalyses a reaction between chloride ions and hydrogen peroxide to yield hypochlorous acid (HOCl), which reacts with further hydrogen peroxide to yield more singlet oxygen. Nitric oxide, and its peroxynitrite derivatives, are also produced. The net effect of this toxic cocktail is to disable the target cell by oxidizing crucial components of its cellular apparatus.

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oxidative deamination

A reaction involved in the catabolism of amino acids that assists their excretion from the body. An example of an oxidative deamination is the conversion of glutamate to -ketoglutarate, a reaction catalysed by the enzyme glutamate dehydrogenase. See deamination.

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oxidative decarboxylation

The reaction in the Krebs cycle in which oxygen, derived from two water molecules, is used to oxidize two carbon atoms to two molecules of carbon dioxide. The two carbon atoms result from the decarboxylation reactions that occur during the Krebs cycle as the six-carbon compound citrate is converted to the four-carbon compound oxaloacetate.

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oxidative phosphorylation

A reaction occurring during the final stages of aerobic respiration, in which ATP is formed from ADP and phosphate coupled to electron transport in the electron transport chain. The reaction occurs in the mitochondria (see chemiosmotic theory). See also phosphorylation.

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oxidoreductase

Any of a class of enzymes that catalyse oxidation–reduction reactions, i.e. they are involved in the transfer of hydrogen or electrons between molecules. They include the oxidases and dehydrogenases.

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oxygen

Symbol O. A colourless odourless gaseous element. It is the most abundant element in the earth's crust (49.2% by weight) and is present in the atmosphere (28% by volume), mainly as dioxygen (O_2), with much smaller amounts of ozone (O_3). Atmospheric dioxygen is of vital importance for all organisms that carry out aerobic respiration.

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oxygen cycle

The cycling of oxygen between the biotic and abiotic components of the environment (see biogeochemical cycle). The oxygen cycle is closely linked to the carbon cycle and the water cycle (see hydrological cycle). In the process of respiration oxygen is taken in by living organisms and released into the atmosphere, combined with carbon, in the form of carbon dioxide. Carbon dioxide enters the carbon cycle or is taken up by plants for photosynthesis. During photosynthesis oxygen is evolved by the chemical splitting of water and returned to the atmosphere. In the upper atmosphere, ozone is formed from oxygen and dissociates to release oxygen (see also ozone layer).

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oxygen debt

The physiological state that exists in a normally aerobic animal when insufficient oxygen is available for metabolic requirements (e.g. during a period of strenuous physical activity). To meet the body's increased demand for energy, pyruvate is converted anaerobically (i.e. in the absence of oxygen) to lactic acid, which requires oxygen for its breakdown and accumulates in the tissues. When oxygen is available again lactic acid is oxidized in the liver, thus repaying the debt.

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oxygen dissociation curve

The S-shaped curve produced when the percentage saturation of haemoglobin with oxygen (i.e. the percentage of binding sites of haemoglobin that are occupied by oxygen molecules) is plotted against the partial pressure of oxygen (pO_2), which is a measure of the oxygen concentration in the surrounding medium. The steep rise of the curve indicates the high affinity of haemoglobin for oxygen: a small increase in pO_2 results in a relatively sharp increase in the percentage saturation of haemoglobin with oxygen. Therefore in the lungs, where the pO_2 is high, the blood is rapidly saturated with oxygen. Conversely, a small drop in pO_2 results in a large drop in percentage saturation of haemoglobin. Thus in tissues that utilize oxygen at a high rate, where the pO_2 is low, oxygen readily dissociates from haemoglobin and is released for use by the tissues. See also Bohr effect.

Oxygen dissociation curve

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oxygen-evolving complex (OEC)

A small complex of proteins and manganese ions associated with the reaction centre of photosystem II (PSII) that splits (oxidizes) water molecules to yield oxygen during photosynthesis. Essentially, this oxidation provides the low-energy electrons from water that are excited by PSII using the light energy trapped by chlorophyll pigments. The resultant high-energy electrons can then flow through the electron transport chain. PSII draws electrons singly from a cluster of four manganese ions held by each OEC. When this cluster has accumulated four positive charges, the OEC oxidizes two molecules of water, producing one molecule of oxygen and four protons. The OEC is located on the lumen side of the thylakoid membrane in the chloroplast and requires chloride ions to function. See photosystems I and II.

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oxygen sag curve

The curve obtained when the concentration of dissolved oxygen in a river into which sewage or some other pollutant has been discharged is plotted against the distance downstream from the sewage outlet (see graph). Samples of water are taken at areas upstream and downstream from the sewage outlet. The presence of sewage reduces the oxygen content of the water and increases the biochemical oxygen demand. This is due to the action of saprotrophic organisms that decompose the organic matter in the sewage and in the process use up the available oxygen.

Oxygen sag curve

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oxyhaemoglobin

See haemoglobin.

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oxyntic cell (parietal cell)

Any of the cells in the wall of the stomach that produce hydrochloric acid, which forms part of the gastric juice. The oxyntic cells also produce intrinsic factor, which is involved in the absorption of vitamin B_12 in the small intestine (see vitamin B complex).

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oxytocin

A hormone, produced by birds and mammals, that in mammals causes both contraction of smooth muscle in the uterus during birth and expulsion of milk from the mammary glands during suckling. Like antidiuretic hormone, oxytocin is produced in the neurosecretory cells of the hypothalamus (see neurosecretion) but is stored and secreted by the posterior pituitary gland.

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ozonation

The formation of ozone (O_3) in the earth's atmosphere. In the stratosphere, about 20–50km above the surface of the earth, oxygen molecules (O_2) dissociate into their constituent atoms under the influence of ultraviolet radiation of short wavelength (below about 240nm). These atoms combine with oxygen molecules to form ozone (see ozone layer). Ozone is also formed in the lower atmosphere from nitrogen oxides and other pollutants by photochemical reactions (see photochemical smog).

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ozone hole

See ozone layer.

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ozone layer (ozonosphere)

A layer of the earth's atmosphere in which most of the atmosphere's ozone is concentrated. It occurs 15–50 km above the earth's surface and is virtually synonymous with the stratosphere. In this layer most of the sun's ultraviolet radiation is absorbed by the ozone molecules, causing a rise in the temperature of the stratosphere and preventing vertical mixing so that the stratosphere forms a stable layer. By absorbing most of the solar ultraviolet radiation the ozone layer protects living organisms on earth. The fact that the ozone layer is thinnest at the equator is believed to account for the high equatorial incidence of skin cancer as a result of exposure to unabsorbed solar ultraviolet radiation. In the 1980s it was found that depletion of the ozone layer was occurring over both the poles, creating ozone holes . This is thought to have been caused by a series of complex photochemical reactions involving nitrogen oxides produced from aircraft and, more seriously, chlorofluorocarbons (CFCs) and halons. CFCs rise to the stratosphere, where they react with ultraviolet light to release chlorine atoms; these atoms, which are highly reactive, catalyse the destruction of ozone. See also air pollution.

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P

P (parental generation)

The individuals that are selected to begin a breeding experiment, crosses between which yield the F1 generation. Only pure-breeding (homozygous) individuals are selected for the P generation.

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p53

See tumour-suppressor gene.

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pacemaker

1 (or sinoatrial node) A small mass of specialized muscle cells in the mammalian heart, found in the wall of the right atrium near the opening for the vena cava. The cells initiate and maintain the heart beat: by their rhythmic and spontaneous contractions they stimulate contraction of the atria (see also atrioventricular node). The cells themselves are controlled by the autonomic nervous system, which determines the heart rate. Similar pacemakers occur in the hearts of other vertebrates.

2 An electronic or nuclear battery-charged device that can be implanted surgically into the chest to produce and maintain the heart beat. These devices are used when the heart's own pacemaker is defective or diseased.
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pachytene

The period in the first prophase of meiosis when paired homologous chromosomes are fully contracted and twisted around each other.

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Pacinian corpuscle (lamellated corpuscle)

Any of the sensory receptors in the subcutaneous layer of the skin that are sensitive to pressure. Each consists of a nerve ending surrounded by an oval capsule made of connective tissue. The receptors are named after Italian anatomist Filipo Pacini (1812–83).

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paedogenesis

Reproduction by an animal that is still in the larval or pre-adult form. Paedogenesis is a form of neoteny and is particularly marked in the axolotl, a larval form of the salamander, which retains its larval features owing to a thyroid deficiency but can breed, producing individuals like itself. If the thyroid hormone thyroxine is given, metamorphosis occurs.

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paedomorphosis

The evolutionary process in which larval or juvenile features of an ancestral organism are displaced to the adult forms of its descendants. It can arise by neoteny or progenesis. Paedomorphosis is thought to have occurred in the evolution of higher chordates from free-swimming larval tunicates, in which metamorphosis was eventually lost and sexual development accelerated until larval forms were capable of breeding.

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PAGE (polyacrylamide gel electrophoresis)

A type of electrophoresis used to determine the size and composition of proteins. Proteins are placed on a matrix of polyacrylamide gel and an electric field is applied. The protein molecules migrate towards the positive pole, the smaller molecules moving at a faster rate through the pores of the gel. The proteins are then detected by applying a stain, such as coomassie blue.

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pairing (synapsis)

The close association between homologous chromosomes that develops during the first prophase of meiosis. The two chromosomes move together and an exact pairing of corresponding points along their lengths occurs as they lie side by side. The resulting structure is called a bivalent .

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Palaearctic region

See faunal region.

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palaeobotany

The branch of palaeontology concerned with the study of plants through geological time, as revealed by their fossil remains (see also palynology). It overlaps with other aspects of plant study, including anatomy, ecology, evolution, and taxonomy.

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Palaeocene

The earliest geological epoch of the Tertiary period. It began about 65 million years ago, following the Cretaceous period, and extended for about 11.1 million years to the beginning of the Eocene (the Palaeocene is sometimes included in the Eocene). It was named by the palaeobotanist W. P. Schimper in 1874. A major floral and faunal discontinuity occurred between the end of the Cretaceous and the beginning of the Palaeocene: following the extinction of many reptiles the mammals became abundant on land. By the end of the epoch primates and rodents had evolved.

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palaeoclimatology

The study of climates of earlier geological periods. This is based largely on the study of sediments that were laid down during these periods and of fossils. The changes in the positions of the continents as a result of continental drift and plate tectonics complicate the study.

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palaeoecology

The study of the relationships of fossil organisms to each other and to their environments. It involves the study both of the fossils and of the surrounding rocks in which they are found. Trace fossils may provide information on the behaviour of the organism.

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Palaeolithic

The Old Stone Age, lasting in Europe from about 2.5 million to 9000 years ago, during which man used primitive stone tools made by chipping stones and flints.

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palaeontology

The study of extinct organisms, including their structure, environment, evolution, and distribution, as revealed by their fossil remains. Palaeontological work also makes important contributions to geology in revealing stratigraphic relationships between rock strata and determining the physical appearance and climate of the earth during past geological ages. See also palaeobotany; palaeoecology; palaeozoology.

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Palaeozoic

The first era of Phanerozoic time. It follows the Precambrian and is subdivided into the Lower Palaeozoic, comprising the Cambrian, Ordovician, and Silurian periods, and the Upper Palaeozoic, comprising the Devonian, Carboniferous, and Permian periods, It extended from about 590 million years ago to about 248 million years ago, when it was succeeded by the Mesozoic era.

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palaeozoology

The branch of palaeontology concerned with the study of animals throughout geological time, as revealed by their fossil remains.

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palate

The roof of the mouth cavity of vertebrates, which separates the buccal and nasal cavities. In mammals it is divided into two zones, the bony hard palate and the soft palate , and completely separates the buccal cavity from the air passage to enable simultaneous eating and breathing.

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palea

See lemma.

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palindromic

Describing a section of double-stranded DNA in which the sequence of bases on one strand is inverted and repeated on the other. Thus the following sequence is palindromic:
—ACTTGCAAGT—

—TGAACGTTCA—

Palindromic sequences are common in DNA and are the sites at which the DNA is cleaved by restriction enzymes.

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palisade (palisade mesophyll)

See mesophyll.

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pallium

See cerebral cortex.

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palmitic acid (hexadecanoic acid)

A saturated fatty acid, CH_3(CH_2)14COOH. Glycerides of palmitic acid occur widely in plant and animal oils and fats.

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palp

An elongated sensory organ, usually near the mouth, in many invertebrates. Examples are the tactile head appendages of polychaete worms, the ciliated flap of tissue that produces feeding currents in bivalve molluscs, the distal part of the mandibles of crustaceans, and the olfactory parts of the first and second maxillae of some insects.

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palynology (micropalaeontology)

The study of fossil pollen and spores (pollen analysis ) and various other microfossils, such as coccoliths and dinoflagellates. Palynology is used in stratigraphy, palaeoclimatology, and archaeology. Pollen and spores are very resistant to decay and therefore their fossils are found in sedimentary rocks. They may be extracted by various methods, including boiling with potassium hydroxide solution, washing with strong oxidizing mixtures, and centrifuging repeatedly. Spores and pollen are classified according to shape, form of aperture, and both internal and external details of the exine (outer coat). They indicate the nature of the dominant flora, and therefore the climate and conditions of the period in which they lived.

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pancreas

A gland in vertebrates lying between the duodenum and the spleen. Under the influence of the hormone secretin it secretes pancreatic juice containing digestive enzymes or their precursors (mainly trypsin, chymotrypsin, amylase, and lipase) into the duodenum via the pancreatic duct (see acinus). It also contains groups of cells – the islets of Langerhans – that function as an endocrine gland, producing the hormones insulin and glucagon, which regulate blood sugar levels.

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pancreatin

A mixture of digestive enzymes that have been extracted from the pancreas.

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pancreozymin

See cholecystokinin.

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Paneth cells

See crypts of Lieberkühn.

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panicle

A type of flowering shoot common in the grass family. The primary axis bears groups of racemes and is itself racemose, as the youngest groups of flowers are at the top (e.g. oat). The term may be used loosely for any form of branched racemose inflorescence; for example, the horse chestnut is a raceme of cymes. Both these arrangements are seen in the family Polygonaceae (docks and sorrels).

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pantothenic acid

A vitamin of the vitamin B complex. It is a constituent of coenzyme A, which performs a crucial role in the oxidation of fats, carbohydrates, and certain amino acids. Deficiency rarely occurs because the vitamin occurs in many foods, especially cereal grains, peas, egg yolk, liver, and yeast.

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papain

A protein-digesting enzyme (see protease) occurring in the fruit of the West Indian papaya tree (Carica papaya). It is used as a digestant and in the manufacture of meat tenderizers.

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paper chromatography

A technique for analysing mixtures by chromatography, in which the stationary phase is absorbent paper. A spot of the mixture to be investigated is placed near one edge of the paper and the sheet is suspended vertically in a solvent, which rises through the paper by capillary action carrying the components with it. The components move at different rates, partly because they absorb to different extents on the cellulose and partly because of partition between the solvent and the moisture in the paper. The paper is removed and dried, and the different components form a line of spots along the paper. Colourless substances are detected by using ultraviolet radiation or by spraying with a substance that reacts to give a coloured spot (e.g. ninhydrin gives a blue coloration with amino acids). The components can be identified by the distance they move in a given time.

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papilla

Any cone-shaped protuberance projecting from the surface of an organ or organism. Papillae occur, for example, on the tongue, in the kidneys, and, in plants, on the surface of many petals.

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papovavirus

One of a group of DNA-containing viruses that produce tumours in their hosts. Papillomaviruses produce nonmalignant tumours (such as warts) in all vertebrates; polyomaviruses produce malignant tumours in only certain classes of vertebrates (not including man).

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pappus

A group of modified sepals, often in the form of a ring of silky hairs. For example, when the fruit of the dandelion matures a pappus of hairs persists at the top of a thin stalk forming a parachute-like structure, which serves to disperse the fruit.

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parallel evolution

The development of related organisms along similar evolutionary paths due to strong selection pressures acting on all of them in the same way. It is debatable if the phenomenon really exists: many argue that all evolution is ultimately convergent or divergent (see adaptive radiation).

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paramorph

A variant form within a species for which a more specific description does not exist because its taxonomic status cannot be determined.

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parapatric speciation

A type of speciation in which there is free exchange of genes between two populations of organisms living in directly adjacent but environmentally different habitats. Although individuals from the two populations can interbreed, the offspring will not be very successful in either habitat, therefore natural selection tends to promote any mechanisms that decrease the number of crosses between such populations.

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paraphyletic

In systematics, describing a group of organisms that excludes one or more descendants of a particular single common ancestor. For example, the taxonomic group `reptiles' used in evolutionary systematics is paraphyletic because it excludes birds and mammals, which share the same common ancestor as the reptiles. In cladistics such groups are regarded as invalid when constructing classification schemes, since cladists allow only monophyletic groups, or clades, as a basis for taxonomic groupings. However, in evolutionary systematics paraphyletic groups, or evolutionary grades , are sometimes permitted in order to reflect biological similarities. Thus `reptiles' is used as a group because its members are closely related to each other and they share certain essential characteristics, such as ectothermy (`cold-bloodedness'), that differ markedly from their fellow clade members, the birds and mammals. Compare polyphyletic.

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paraphysis

(pl. paraphyses) Any of the sterile filaments of cells that are found around the sex organs of mosses (Bryophyta), brown algae (Phaeophyta), and certain fungi (the Ascomycota and Basidiomycota).

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parapodium

(pl. parapodia) See chaeta; Polychaeta.

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Paraquat

Trade name for an organic herbicide used to control broadleaved weeds and grasses (see viologen dyes). It is poisonous to humans, having toxic effects on the liver, lungs, and kidneys if ingested. Paraquat is not easily broken down and can persist in the environment adsorbed to soil particles. See also pesticide.

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parasexual cycle

A type of reproduction in fungi that results in genetic recombination without the formation and fusion of gametes. It occurs in heterokaryotic mycelia, in which two haploid nuclei fuse and then divide by mitosis, during which a form of crossing over occurs. During subsequent divisions chromosomes are progressively lost from the resultant diploid nucleus so that the haploid condition is restored. The asexual spores produced during the cycle will be genetically different from the original parent mycelium as a result of the crossing over that has occurred.

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parasitism

An association in which one organism (the parasite ) lives on (ectoparasitism ) or in (endoparasitism ) the body of another (the host), from which it obtains its nutrients. Some parasites inflict comparatively little damage on their host, but many cause characteristic diseases (these are, however, never immediately fatal, as killing the host would destroy the parasite's source of food; compare parasitoid). Parasites are usually highly specialized for their way of life, which may involve one host or several (if the life cycle requires it). They typically produce vast numbers of eggs, very few of which survive to find their way to another suitable host. Obligate parasites can only survive and reproduce as parasites; facultative parasites can also live as saprotrophs. The parasites of man include fleas and lice (which are ectoparasites), various bacteria, protozoans, and fungi (endoparasites causing characteristic diseases), and tapeworms (e.g. Taenia solium, which lives in the gut). See also hemiparasite.

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parasitoid

An animal that lives in or on another animal (the host ), which it consumes and eventually kills. Hence parasitoids may be regarded as intermediate between a true parasite (see parasitism) and a predator. Many wasps and other hymenopterans, and also some flies, are parasitoids for part of their life cycle, laying their eggs commonly in or on the larvae of other insects. The hatched parasitoids then use the tissues of their host for food during development into free-living adults. Idiobont parasitoids paralyse or kill the host at the time of egg-laying. However, more common are kainobiont parasitoids , which allow the host to remain alive for extended periods. In some cases the parasitoid can prolong the development of the host larva and prevent it metamorphosing. See also hyperparasite.

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parasympathetic nervous system

Part of the autonomic nervous system. Its nerve endings release acetylcholine as a neurotransmitter and its actions tend to antagonize those of the sympathetic nervous system. For example, the parasympathetic nervous system increases salivary gland secretion, decreases heart rate, promotes digestion (by increasing peristalsis), and dilates blood vessels, while the sympathetic nervous system has opposite effects.

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parathyroid glands

Two pairs of endocrine glands situated behind, or embedded within, the thyroid gland in higher vertebrates. They produce parathyroid hormone, which controls the amount of calcium in the blood. See also C cell.

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parathyroid hormone (parathyrin)

A hormone secreted by the parathyroid gland in response to low levels of calcium in the blood. It acts to maintain normal blood levels of calcium by (1) increasing the number of osteoclasts, which break down the bone matrix and release calcium into the blood; (2) increasing the reabsorption of calcium and magnesium ions in the kidney tubules, so that their concentration is maintained in the blood; (3) converting vitamin D to its active form, which increases calcium absorption in the intestine. Parathyroid hormone acts in opposition to calcitonin.

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Parazoa

See Metazoa.

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parenchyma

1 A plant tissue consisting of roughly spherical relatively undifferentiated cells, frequently with air spaces between them. The cortex and pith are composed of parenchyma cells (see ground tissues).

2 Loose connective tissue formed of large cells. Its function is to pack the spaces between organs in some simple acoelomate animals, such as flatworms (Platyhelminthes).
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parent

1 Either male or female partner that together produce offspring in the process of sexual reproduction. See also P.

2 Denoting an organism or cell that gives rise to new organisms or cells, as by asexual reproduction or cell division.
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parental care

Any behaviour pattern in which a parent invests time or energy in feeding and protecting its offspring. Parental care is a form of altruism since this type of behaviour involves increasing the fitness of the offspring at the expense of the parents. The degree of parental care differs widely. For example, most species of fish show little or no parental care while humans and many other mammals care for their offspring until they reach adolescence.

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parietal

Of or relating to the wall of a body cavity or other structure.

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parietal cell

See oxyntic cell.

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parthenocarpy

The formation of fruit without prior fertilization of the flower by pollen. The resulting fruits are seedless and therefore do not contribute to the reproduction of the plant; examples are bananas and pineapples. Plant growth substances may have a role in this phenomenon, which can be induced by auxins in the commercial production of tomatoes and other fruits.

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parthenogenesis

The development of an organism from an unfertilized egg. This occurs sporadically in many plants (e.g. dandelions and hawkweeds) and in a few animals, but in some species it is the main and sometimes only method of reproduction. For example, in some species of aphid, males are absent or very rare. The eggs formed by the females contain the full (diploid) number of chromosomes and are genetically identical. Variation is consequently very limited in species that reproduce parthenogenetically. See arrhenotoky; thelytoky.

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partially permeable membrane

A membrane that is permeable to the small molecules of water and certain solutes but does not allow the passage of large solute molecules. This term is preferred to semipermeable membrane when describing membranes in living organisms. See osmosis.

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particulate inheritance

The transmission from parent to offspring of separate units that determine characteristics. Gregor Mendel observed that recessive characteristics, absent in the offspring of a cross in which only one parent possessed them, reappeared repeatedly in the progeny of subsequent crosses. This led him to formulate his theory of inherited `factors' (now called alleles) that retain their identity through succeeding generations (see Mendel's laws). Compare blending inheritance.

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parturition

The act of giving birth to young at the end of the gestation period. It is triggered by the secretion of ACTH by the fetal pituitary gland, which causes increased secretion of corticosteroid hormones by the fetal adrenal glands, which in turn act on the womb and lead to the muscular contractions of labour.

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pascal

The SI unit of pressure equal to one newton per square metre.

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passive immunity

See immunity.

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passive transport

See diffusion.

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Pasteur, Louis

(1822–95)
French chemist and microbiologist, who held appointments in Strasbourg (1849–54) and Lille (1854–57), before returning to Paris to the Ecole Normale and the Sorbonne. From 1888 to his death he was director of the Pasteur Institute. In 1848 he discovered optical activity, in 1860 relating it to molecular structure. In 1856 he began work on fermentation, and by 1862 was able to disprove the existence of spontaneous generation. He introduced pasteurization (originally for wine) in 1863. He went on to study disease and developed vaccines against cholera (1880), anthrax (1882), and rabies (1885).

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pasteurization

The treatment of milk to destroy disease-causing bacteria, such as those of tuberculosis, typhoid, and brucellosis. Milk is heated to 65°C for 30 minutes or to 72°C for 15 minutes followed by rapid cooling to below 10°C. The method was devised by the French microbiologist Louis Pasteur (1822–95). See also food preservation.

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patch clamp technique

A method for studying the activation of voltage-dependent transmembrane channels in neurones. A small piece of membrane containing only a few channels is isolated and microelectrodes are sealed to both sides of the membrane.

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patella (kneecap)

A small rounded movable bone that is situated in a tendon in front of the knee joint in most mammals (including humans). The function of the patella is to protect the knee.

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pathogen

Any disease-causing microorganism. Pathogens include viruses and many bacteria, fungi, and protozoans. See infection.

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pathogenesis-related proteins

See systemic acquired resistance.

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pathology

The study of the changes in organs and tissues that are caused by or give rise to disease. This involves the examination of tissue samples, X-ray photographs, or other evidence taken from living patients or from cadavers. Clinical pathology applies these findings to clinical cases, particularly in the development of diagnostic tests and treatments. In experimental pathology , disease processes are studied using experimental animals, cell cultures, or other means.

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patristic

Denoting similarity between organisms resulting from common ancestry. Compare homoplastic.

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pattern recognition

A branch of statistics used to classify data based upon several variables. This type of statistical analysis is particularly useful in interpreting biological data that consist of multiple observations on large numbers of individuals.

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Pavlov, Ivan Petrovich

(1849–1936)
Russian physiologist, who became professor of physiology in St Petersburg in 1886. While working on the physiology of digestion he discovered that the mere sight of food stimulates the production of digestive juices. For this work he was awarded the 1904 Nobel Prize for physiology or medicine. Pavlov went on to demonstrate operant conditioning in dogs and other animals. See also learning.

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PCR

See polymerase chain reaction.

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PDGF

See growth factor.

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peat

A mass of dark-brown or black fibrous plant debris produced by the partial disintegration of vegetation in wet places (see hydrosere). It may accumulate in depressions. When subjected to burial and hence pressure and heat it may be converted to coal. Peat is used to improve soil and as a fuel, especially in Ireland and Sweden.

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peck order

See dominant.

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pecten

Any of various comblike structures in animals. The pecten in the eyes of birds consists essentially of a network of blood vessels attached to the optic nerve and projecting into the vitreous humour. Its function is uncertain, but it may be involved in supplying the retina with nutrients and oxygen. A simple form of this structure is found in the eyes of reptiles.

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pectic substances

A group of polysaccharides made up primarily of sugar acids. They are important constituents of plant cell walls and the middle lamella between adjacent cell walls. Normally they are present in an insoluble form, but in ripening fruits and in tissues affected by certain diseases they change into a soluble form, which is evidenced by softening of the tissues.

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pectin

A type of pectic substance. It is used in making jam as it forms a gel with sucrose.

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pectoral fins

See fins.

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pectoral girdle (shoulder girdle)

The bony or cartilaginous structure in vertebrates to which the anterior limbs (pectoral fins, forelegs, or arms) are attached. In mammals it consists of two dorsal scapulae (shoulder blades) attached to the backbone and two ventral clavicles (collar bones) attached to the sternum (breastbone).

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pedicel

The stalk attaching a flower to the main floral axis (see peduncle). Some flowers, described as sessile , do not have a pedicel and arise directly from the peduncle.

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pedipalp

Either member of the second pair of appendages in arachnids, immediately posterior to the chelicerae. In species with large chelicerae the pedipalps are typically adapted for walking or sensory functions; otherwise they are variously adapted for defence, killing prey, or digging.

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pedology

The science of the study of soils, including their origin and characteristics and their utilization.

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peduncle

1 The main stalk of a plant that bears the flowers, which may be solitary or grouped in an inflorescence. Compare pedicel.

2 See Brachiopoda.
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Pekilo process

A process developed in Finland for the production of Pekilo protein , a single-cell protein used as an animal feed. The fungus Paecilomyces varioti uses a substrate of sulphite waste liquor or wood hydrolysates for fermentation.

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pelagic

Describing organisms that swim or drift in a sea or a lake, as distinct from those that live on the bottom (see benthos). Pelagic organisms are divided into plankton and nekton.

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Pelecypoda

See Bivalvia.

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pellagra

A disease resulting from a deficiency of nicotinic acid, which is characterized by dermatitis and mental disorder.

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pellicle

The thin outer covering, composed of protein, that protects and maintains the shape of certain unicellular organisms, e.g. Euglena. It is transparent and in ciliated organisms, e.g. Paramecium, contains small pores through which the cilia emerge.

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pelvic fins

See fins.

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pelvic girdle (pelvis; hip girdle)

The bony or cartilaginous structure in vertebrates to which the posterior limbs (pelvic fins or legs) are attached. The pelvic girdle articulates dorsally with the backbone; it is made up of two halves, each produced by the fusion of the ilium, ischium, and pubis.

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pelvis

1 See pelvic girdle.

2 The lower part of the abdomen in the region of the pelvic girdle.
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3 A conical chamber in the kidney into which urine drains from the kidney tubules before passing to the ureter.
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penicillin

An antibiotic derived from the mould Penicillium notatum; specifically it is known as penicillin G and belongs to a class of similar substances called penicillins. They are all active against a wide variety of bacteria, producing their effects by disrupting synthesis of the bacterial cell wall, and are used to treat a variety of infections caused by these bacteria.

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penis

The male reproductive organ of mammals (and also of some birds and reptiles) used to introduce sperm into the female reproductive tract to ensure internal fertilization. It contains a duct (the urethra) through which the sperms pass. The penis becomes erect during precopulatory activity, either by filling with blood or haemolymph or by the action of muscles, and can be inserted into the vagina (or cloaca). In mammals the urine also leaves the body through the penis.

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pentadactyl limb

A limb with five digits, characteristic of tetrapod vertebrates (amphibians, reptiles, birds, and mammals). It evolved from the paired fins of primitive fish as an adaptation to locomotion on land and is not found in modern fish. The limb has three parts (see illustration): the upper arm or thigh containing one long bone, the forearm or shank containing two long bones, and the hand or foot, which contains a number of small bones. This basic design is modified in many species, according to the function of the limb, particularly by the loss of fusion of the terminal bones.

A basic pentadactyl forelimb, as exemplified by the human arm

The modified pentadactyl forelimb of various vertebrates

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pentose

A sugar that has five carbon atoms per molecule. See monosaccharide.

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pentose phosphate pathway (pentose shunt)

A series of biochemical reactions that results in the conversion of glucose 6-phosphate to ribose 5-phosphate and generates NADPH, which provides reducing power for other metabolic reactions, such as synthesis of fatty acids. Ribose 5-phosphate and its derivatives are components of such molecules as ATP, coenzyme A, NAD, FAD, DNA, and RNA. In plants the pentose phosphate pathway also plays a role in the synthesis of sugars from carbon dioxide. In animals the pathway occurs at various sites, including the liver and adipose tissue.

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PEP

See phosphoenolpyruvate.

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pepo

See berry.

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pepsin

An enzyme that catalyses the breakdown of proteins to polypeptides in the vertebrate stomach. It is secreted as an inactive precursor, pepsinogen.

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pepsinogen

The inactive precursor of the enzyme pepsin. Pepsinogen is secreted by the lining of the vertebrate stomach into the lumen, where it is converted to pepsin by hydrochloric acid and also by the action of pepsin itself.

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peptidase

See endopeptidase; exopeptidase; protease.

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peptide

Any of a group of organic compounds comprising two or more amino acids linked by peptide bonds . These bonds are formed by the reaction between adjacent carboxyl (–COOH) and amino (–NH_2) groups with the elimination of water (see illustration). Dipeptides contain two amino acids, tripeptides three, and so on. Polypeptides contain more than ten and usually 100–300. Naturally occurring oligopeptides (of less than ten amino acids) include the tripeptide glutathione and the pituitary hormones antidiuretic hormone and oxytocin, which are octapeptides. Peptides also result from protein breakdown, e.g. during digestion.

Formation of a peptide bond

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peptide mapping (peptide fingerprinting)

The technique of forming two-dimensional patterns of peptides (on paper or gel) by partial hydrolysis of a protein followed by electrophoresis and chromatography. The peptide pattern (or fingerprint ) produced is characteristic for a particular protein and the technique can be used to separate a mixture of peptides.

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peptidoglycan

A macromolecule that is a component of the cell wall of eubacteria; it is not found in eukaryotes. Consisting of chains of amino sugars (N-acetylglucosamine and N-acetylmuramic acid) linked to a tripeptide (of alanine, glutamic acid, and lysine or diaminopimelic acid), it confers strength and shape to the cell wall. Archaebacteria possess a similar polysaccharide, pseudopeptidoglycan , with N-acetyltalosaminuronic acid instead of N-acetylmuramic acid.

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peramorphosis

Any outcome arising from evolutionary changes in developmental rates (see heterochrony) that involves the addition of new stages to the end of the ancestral development sequence. Peramorphic forms may arise either by acceleration or by hypermorphosis and conform to the theory of recapitulation. In contrast, paedomorphic forms (see paedomorphosis), in which a new stage is added within the developmental sequence, do not exhibit recapitulation.

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perception

The interpretation of sensory information using both the raw data detected by the senses and previous experience. Compare sensation.

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perennation

The survival of biennial or perennial plants from one year to the next by vegetative means. In biennials and herbaceous perennials the aerial parts of the plant die down and the plants survive by means of underground storage roots (e.g. carrot), rhizomes (e.g. couch grass, Solomon's seal), tubers (e.g. dahlia), bulbs (e.g. daffodil, snowdrop), or corms (e.g. crocus, gladiolus). These perennating organs are also frequently responsible for vegetative propagation. Woody perennials survive the winter by reducing their metabolic activity (e.g. by leaf loss in deciduous trees and shrubs).

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perennial

A plant that lives for a number of years. Woody perennials (trees and shrubs) have a permanent aerial form, which continues to grow year after year. Herbaceous (i.e. nonwoody) perennials have aerial shoots that die down each autumn and are replaced in spring by new shoots from an underground structure (see perennation). Lupin and rhubarb are examples of herbaceous perennials. Compare annual; biennial; ephemeral.

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perfusion techniques

Methods of maintaining a live organ in isolation by circulating fluid containing essential nutrients and oxygen through and around the organ. Maintaining the circulation through an organ allows the delivery of nutrients to the tissues and the removal from them of toxins and waste products. Perfusion techniques are particularly useful in studying the metabolism of drugs in isolation in an intact organ, such as the liver or kidney.

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perianth

The part of a flower situated outside the stamens and carpels. In dicotyledons it consists of two distinct whorls, the outer of sepals (see calyx) and the inner of petals (see corolla). In monocotyledons the two whorls are similar and often brightly coloured. In wind-pollinated flowers both whorls may be reduced or absent. In many horticultural varieties the number of perianth parts is multiplied, but the resulting `double' flowers are often sterile.

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pericardial cavity

The cavity in vertebrates that contains the heart and is bounded by a membrane (the pericardium). It is part of the coelom.

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pericardium (pericardial membrane)

The membrane that encloses the pericardial cavity, containing the vertebrate heart. The pericardium holds the heart in position while allowing it to relax and contract. It consists of two main parts: a tough outer fibrous layer (fibrous pericardium ) and the more delicate serous pericardium , which consists of a double layer of serous membrane, the inner layer being in close contact with the heart.

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pericarp (fruit wall)

The part of a fruit that develops from the ovary wall of a flower. The type of fruit that develops depends on whether the pericarp becomes dry and hard or soft and fleshy. The pericarp can be made up of three layers. The outer skin (epicarp or exocarp ) may be tough and hard; the middle layer (mesocarp ) may be succulent as in peach, hard as in almond, or fibrous as in coconut; and the inner layer (endocarp ) may be hard and stony as in many drupes, membranous as in citrus fruits, or indistinguishable from the mesocarp, as in many berries.

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perichondrium

A dense layer of fibrous connective tissue that covers the surface of cartilage.

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periclinal

(in botany) Parallel to the surface of an organ or part. In periclinal cell division the plane of division is parallel to the surface of the plant body. Compare anticlinal.

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pericycle

A plant tissue comprising the outermost layer of the root vascular tissue, lying immediately beneath the endodermis. Lateral roots originate from the pericycle.

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periderm

See cork cambium.

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perigyny

A floral arrangement in which the ovary is situated in a cup-shaped or flattened receptacle, from the margin of which the perianth and stamens arise (see illustration). The perianth and stamens are said to be perigynous with respect to the ovary, as seen in cherry and plum flowers. Compare epigyny; hypogyny.

Perigyny

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perilymph

The fluid of the inner ear that fills the space between the bony labyrinth and the membranous labyrinth. See also cochlea. Compare endolymph.

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perinuclear space (perinuclear compartment)

See nuclear envelope.

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period

See menstrual cycle.

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periodontal membrane

The membrane of connective tissue that surrounds the root of a tooth and anchors it to its socket in the jawbone. Fibres of the periodontal membrane pass into the cement covering the root, which provides a firm attachment.

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periosteum

The outer membrane that surrounds a bone. It contains connective tissue, capillaries, nerves, and a number of types of cell, including osteoclasts. The periosteum plays an important role in bone repair and growth.

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peripheral nervous system

All parts of the nervous system excluding the central nervous system. It consists of all the cranial and spinal nerves and their branches, which link the receptors and effectors with the central nervous system. See also autonomic nervous system.

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periphyton

The organisms, collectively, that live attached to the stems and leaves of freshwater plants.

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periplasm (periplasmic space)

The zone between the cytoplasmic membrane and the outer membrane in Gram-negative bacteria. It contains a thin layer of peptidoglycan, has a gel-like consistency, and contains various types of proteins concerned with the cell's metabolism.

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Perissodactyla

An order of mammals having hoofed feet with an odd number of toes. They are all herbivores and include the tapirs, rhinoceros, and horse. The teeth are large and specialized for grinding. Cellulose digestion occurs in the caecum and large intestine. Fossils of the Eocene epoch, 60 million years ago, show that these animals were at that time already distinct from the cloven-hoofed Artiodactyla.

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peristalsis

Waves of involuntary muscular contraction and relaxation that pass along the alimentary canal, forcing food contents along. It is brought about by contraction of the circular muscles of the gut wall in sequence.

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peristome

1 A ring of toothlike structures around the opening of a moss capsule. The teeth tend to bend and twist in dry weather, so opening the mouth of the capsule and allowing the spores to escape. In wet weather they close over the opening of the capsule.

2 The area around the mouth in many invertebrates and some protoctists. It sometimes assists in food collecting. Examples are the spirally ciliated groove around the mouth of some ciliate protozoans and the first segment of the earthworm.
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peritoneum

The thin layer of tissue (see serous membrane) that lines the abdominal cavity of vertebrates and covers the abdominal organs. See also mesentery.

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permanent teeth

The second and final set of teeth that mammals produce after shedding the deciduous teeth. An adult human normally has 32 permanent teeth, consisting of incisors, canines, molars, and premolars (see illustration). These usually appear between the ages of approximately 6 and 21 years. See also dental formula; diphyodont.

Permanent teeth of an adult human

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Permian

The last geological period in the Palaeozoic era. It extended from the end of the Carboniferous period, about 286 million years ago, to the beginning of the Mesozoic era, about 248 million years ago. It was named by the British geologist Roderick Murchison (1792–1871) in 1841 after the Perm province in Russia. In some areas continental conditions prevailed, which continued into the following period, the Triassic. These conditions resulted in the deposition of the New Red Sandstone. During the period a number of animal groups became extinct, including the trilobites, tabulate and rugose corals, and blastoids. Amphibians and reptiles continued to be the dominant land animals and gymnosperms replaced ferns, clubmosses, and horsetails as the dominant plants.

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pernicious anaemia

See vitamin B complex.

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peroxisome

An organelle, found in the cytoplasm of virtually all plant and animal cells, that contains several enzymes involved in oxidation processes. Peroxisomes are bound by a single membrane; the enzymes they contain include urate oxidase and catalase. Peroxisomes are active in the process of detoxification, being particularly abundant in liver parenchyma cells and renal proximal convoluted tubule cells.

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persistent

Describing a pesticide or other pollutant that is not readily broken down and can persist for long periods, causing damage in the environment. For example, the herbicides Paraquat and DDT can persist in the soil for many years after their application.

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pest

Any of various organisms, such as fungi, insects, rodents, and plants, that harm crops or livestock or otherwise interfere with the wellbeing of human beings. Weeds are plant pests that grow where they are not wanted – often on cultivated land, where they compete with crop plants for space, light, nutrients, etc. Pests are controlled by the use of pesticides and biological control methods.

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pesticide

Any chemical compound used to kill pests that destroy agricultural production or are in some way harmful to humans. Pesticides include herbicides (such as 2,4-D and Paraquat), which kill unwanted plants or weeds; insecticides (such as pyrethrum), which kill insect pests; fungicides , which kill fungi; and rodenticides (such as warfarin), which kill rodents. The problems associated with pesticides are that they are very often nonspecific and may therefore be toxic to organisms that are not pests; they may also be nonbiodegradable, so that they persist in the environment and may accumulate in living organisms (see bioaccumulation). Organophosphorus insecticides, such as malathion and parathion, are biodegradable but can also damage the respiratory and nervous systems in humans as well as killing useful insects, such as bees. They act by inhibiting the action of the enzyme cholinesterase. Organochlorine insecticides, such as dieldrin, aldrin, and DDT, are very persistent and not easily biodegradable.

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peta-

Symbol P. A prefix used in the metric system to denote one thousand million million times. For example, 10^15 metres = 1 petametre (Pm).

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petal

One of the parts of the flower that make up the corolla. Petals of insect-pollinated plants are usually brightly coloured and often scented. Those of wind-pollinated plants are usually reduced or absent. Petals are considered to be modified leaves but their structure is simpler. Epidermal hairs may be present and the cuticle is often covered by lines or dots known as honey guides , which direct insects to the nectar.

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petiole

The stalk that attaches a leaf blade to the stem. Leaves without petioles are described as sessile .

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Petri dish

A shallow circular flat-bottomed dish made of glass or plastic and having a fitting lid. It is used in laboratories chiefly for culturing bacteria and other microorganisms. It was invented by the German bacteriologist J. R. Petri (1852–1921).

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petrification

See fossil.

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pH

See pH scale.

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PHA

See phytohaemagglutinin.

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Phaeophyta (brown algae)

A phylum of algae in which the green chlorophyll pigments are usually masked by the brown pigment fucoxanthin. Brown algae are usually marine (being abundant in cold water) and many species, such as the wracks (Fucus), inhabit intertidal zones. They vary in size from small branched filaments to ribbon-like bodies (known as kelps) many metres long.

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phage

See bacteriophage.

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phagemid

See expression vector.

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phagocyte

A cell that is able to engulf and break down foreign particles, cell debris, and disease-producing microorganisms (see phagocytosis). Some protoctists and certain mammalian cells (e.g. macrophages and monocytes) are phagocytes. Phagocytes are important elements in the natural defence mechanism of most animals.

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phagocytosis

The process by which foreign particles invading the body or minute food particles are engulfed and broken down by certain animal cells (known as phagocytes). The cell membrane of the phagocyte invaginates to capture the particle and then closes around it to form a sac or vacuole. The vacuole coalesces with a lysosome, which contains enzymes that break down the particle. See endocytosis. Compare pinocytosis.

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phagotroph (macroconsumer)

Any heterotrophic organism that feeds by ingesting organisms or organic particles, which are digested within its body. Compare osmotroph.

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phalanges

The bones that make up the digits of the hand or foot in vertebrates. They articulate with the metacarpals of the hand or with the metatarsals of the foot. In the basic pentadactyl limb there are two phalanges for the first digit (the thumb or big toe in humans) and three for each of the others.

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Phanerozoic

The most recent eon of geological time, represented by rock strata containing clearly recognizable fossils. It comprises the Palaeozoic, Mesozoic, and Cenozoic eras and has extended for about 570 million years from the beginning of the Cambrian period. Compare Proterozoic.

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pharate

`Cloaked': describing a larva or adult when inside the cuticle of the previous developmental stage. For example, a newly metamorphosed adult insect may remain `cloaked' for some hours or even days within the pupal case until it receives the appropriate environmental cue that triggers emergence.

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pharmacodynamics

The study of the action of drugs on the body. Compare pharmacokinetics.

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pharmacokinetics

The movement of foreign substances, particularly drugs, throughout the body of an animal. Processes that influence the pharmacokinetics of a compound include uptake, distribution throughout the body tissues, the length of time the compound remains in the body, and its rate of clearance (e.g. by metabolism or excretion).

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pharmacology

The study of the properties of drugs and their effects on living organisms. Clinical pharmacology is concerned with the effects of drugs in treating disease.

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pharynx

1 The cavity in vertebrates between the mouth and the oesophagus and windpipe (trachea), which serves for the passage of both food and respiratory gases. The presence of food in the pharynx stimulates swallowing (see deglutition). In fish and aquatic amphibians the pharynx is perforated by gill slits.

2 The corresponding region in invertebrates.
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phase-contrast microscope

A type of microscope that is widely used for examining cells and tissues. It makes visible the changes in phase that occur when nonuniformly transparent specimens are illuminated. In passing through an object the light is slowed down and becomes out of phase with the original light. With transparent specimens having some structure diffraction occurs, causing a larger phase change in light outside the central maximum of the pattern. The phase-contrast microscope provides a means of combining this light with that of the central maximum by means of an annular diaphram and a phase-contrast plate , which produces a matching phase change in the light of the central maximum only. This gives greater contrast to the final image, due to constructive interference between the two sets of light waves. This is bright contrast ; in dark contrast a different phase-contrast plate is used to make the same structure appear dark, by destructive interference of the same waves.

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phase I metabolism

The first stage in the conversion of a foreign compound, such as a drug or toxin, into a form that can be eliminated by the body. Common reactions during this phase are oxidation, reduction, and hydrolysis; the resulting metabolites are chemically more reactive than the parent compound, enabling them to undergo the reactions of the second stage (see phase II metabolism).

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phase II metabolism

The second stage in adapting foreign compounds for elimination from the body (compare phase I metabolism). Phase II metabolism involves the addition of chemical groups (e.g. glycine or acetate), which usually makes the compound less toxic to body tissues and easier to excrete.

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phellem

See cork.

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phelloderm

See cork cambium.

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phellogen

See cork cambium.

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phenetic

Describing a system of classification of organisms based on similarities and differences in as many observable characteristics as possible. A phenetic system does not aim to reflect evolutionary descent, although it may well do so. Compare phylogenetic.

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phenocopy

A phenotype that is not genetically determined but mimics one that is. This occurs most commonly when environmental influences alone, such as diet, evoke a developmental trait that has a very similar genetic counterpart. For example, dietary deficiency of vitamin D causes the bone disease rickets, and this form cannot be readily distinguished from genetically determined (nondietary) forms of the disease due to malabsorption or excessive loss of bone minerals.

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phenology

The study of the ways in which the timing and other aspects of periodic events, such as flowering in plants and breeding and migration in animals, are affected by climate and other environmental factors.

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phenolphthalein

A dye used as an acid-base indicator. It is colourless below pH 8 and red above pH 9.6. It is used in titrations involving weak acids and strong bases. It is also used as a laxative.

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phenotype

The observable characteristics of an organism. These are determined by its genes (see genotype), the dominance relationships between the alleles, and by the interaction of the genes with the environment.

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phenylalanine

See amino acid.

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phenylketonuria

A genetic disorder in which there is disordered metabolism of the amino acid phenylalanine, leading to severe mental retardation of affected children. The disease is caused by the absence or deficiency of the enzyme phenylalanine hydroxylase, which results in the accumulation of phenylalanine in all body fluids. There are also high levels of the ketone phenylpyruvate in the urine, hence the name of the disease. The disease occurs in individuals who are homozygous for the defective recessive allele; both parents of such individuals are thus heterozygous carriers of the allele. The advent of gene probes has greatly aided accurate diagnosis, of both phenylketonurics and carriers.

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pheophytin

The first electron acceptor in the light reaction of photosynthesis. It accepts an electron from the excited form of photosystem II (see photosystems I and II), and passes it on via another acceptor, QA, to plastoquinone. Pheophytin is a form of chlorophyll a in which the magnesium ion is replaced by two hydrogen ions. It participates in the crucial step of converting light energy to chemical energy.

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pheromone (ectohormone)

A chemical substance (see semiochemical) emitted by an organism into the environment as a specific signal to another organism, usually of the same species. Pheromones play an important role in the social behaviour of certain animals, especially insects and mammals. They are used to attract mates, to mark trails, and to promote social cohesion and coordination in colonies (see queen substance). Pheromones are usually highly volatile organic acids or alcohols and can be effective at minute concentrations.

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phloem (bast)

A tissue that conducts food materials in vascular plants from regions where they are produced (notably the leaves) to regions, such as growing points, where they are needed. It consists of hollow tubes (sieve tubes) that run parallel to the long axis of the plant organ and are formed from elongated cells (sieve elements) joined end to end and closely associated with companion cells. The end walls of these cells are broken down to a greater or lesser extent to allow passage of materials. In young plants and in newly formed tissues of mature plants primary phloem is formed by the activity of the apical meristem (see protophloem; metaphloem). In most plants secondary phloem is later differentiated by the vascular cambium and this replaces the earlier formed phloem in older regions. See also mass flow. Compare xylem.

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phloem protein

See P-protein.

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phloroglucinol

A red dye (usually acidified with hydrochloric acid) that stains lignin in plant cells red.

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phoresy

A method of dispersal in which an animal attaches itself to an animal of another species in order to be transported to a new site, where it releases itself, with little or no harm to the carrier. This method is adopted by various animals searching for new food sources, or by parasites when seeking a new host. For example, when their host dies, certain bird lice may attach themselves to blood-sucking flies in their effort to find a new avian host, and some insect parasitoids, which lay their eggs inside the eggs of other insects, attach themselves to adults of the host species to ensure immediate access to the eggs when the adult host lays them. Eggs may also be transferred in this manner. The Neotropical human bot fly, Dermatobia hominis, `captures' a suitable carrier, such as a blood-sucking mosquito, to which it attaches 30 or so of its eggs. When the carrier insect reaches a human or bovine host, the bot fly eggs quickly hatch and the larvae release themselves from the carrier and invade the hair follicles of their host, where they develop.

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phosphagen

A compound found in animal tissues that provides a reserve of chemical energy in the form of high-energy phosphate bonds. The most common phosphagens are creatine phosphate, occurring in vertebrate muscle and nerves, and arginine phosphate, found in most invertebrates. During tissue activity (e.g. in muscle contraction) phosphagens give up their phosphate groups, thereby generating ATP from ADP. The phosphagens are then reformed when ATP is available.

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phosphatase

An enzyme that catalyses the removal of a phosphate group from an organic compound. See alkaline phosphatase.

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phosphatide

See phospholipid.

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phosphatidylcholine

See lecithin.

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phosphodiester bond

The covalent bond that links a phosphate group and a sugar group, by means of an oxygen bridge, in the sugar–phosphate backbone of a nucleic acid molecule. See DNA.

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phosphoenolpyruvate (PEP)

A three-carbon compound that is the substrate for carbon dioxide fixation during photosynthesis in C_4 plants (see C4 pathway). PEP is also an intermediate in glycolysis (being the immediate precursor of pyruvate) and the glyoxylate cycle.

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phosphoglyceric acid (PGA; 3-phosphoglycerate)

See glycerate 3-phosphate.

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phosphokinase

See kinase.

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phospholipid (phosphatide)

One of a group of lipids having both a phosphate group and one or more fatty acids. Glycerophospholipids (or phosphoglycerides ) are based on glycerol; the three hydroxyl groups are esterified with two fatty acids and a phosphate group, which may itself be bound to one of a variety of simple organic groups (e.g. in lecithin (phosphatidylcholine) it is choline; see formula). Sphingolipids are based on the alcohol sphingosine and contain only one fatty acid linked to an amino group. With their hydrophilic polar phosphate groups and long hydrophobic hydrocarbon `tails', phospholipids readily form membrane-like structures in water. They are a major component of cell membranes (see lipid bilayer).

Structure of the phospholipid lecithin (phosphatidylcholine), a phosphoglyceride

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phosphorus

Symbol P. A nonmetallic element that is a major essential element for living organisms. It is an important constituent of tissues (especially bones and teeth) and of cells, being required for the formation of nucleic acids and energy-carrying molecules (e.g. ATP) and also involved in various metabolic reactions.

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phosphorus cycle

The cycling of phosphorus between the biotic and abiotic components of the environment (see biogeochemical cycle). Inorganic phosphates (PO_43–, HPO_42–, or H_2PO_4–) are absorbed by plants from the soil and bodies of water and eventually pass into animals through food chains. Within living organisms phosphates are built up into nucleic acids and other organic molecules. When plants and animals die, phosphates are released and returned to the abiotic environment through the action of bacteria. On a geological time scale, phosphates in aquatic environments eventually become incorporated into and form part of rocks; through a gradual process of erosion, these phosphates are returned to the soil, seas, rivers, and lakes. Phosphorus-containing rocks are mined for the manufacture of fertilizers, which provide an additional supply of inorganic phosphate to the abiotic environment.

The phosphorus cycle

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phosphorylase

See phosphorylation.

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phosphorylation

The introduction of a phosphate group into a biomolecule in a reaction that is normally controlled by a phosphorylase enzyme. Phosphate is able to combine easily with inert organic compounds, making them chemically active. The first stage in many biochemical reactions is phosphorylation. The conversion of AMP and ADP to ATP occurs by phosphorylation reactions in two main metabolic pathways, oxidative phosphorylation and photophosphorylation. The formation of nucleotides also involves a phosphorylation reaction. The activity of many enzymes is controlled by phosphorylation: certain enzymes are activated when they are phosphorylated (see kinase), while others are deactivated. Phosphorylation of these enzymes is under the control of hormones and other messengers.

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photic zone

See euphotic zone.

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photoautotroph

An autotrophic organism, such as a green plant or a photosynthetic bacterium, that synthesizes its organic materials from inorganic components using energy derived from the sun (solar energy) in the process of photosynthesis. See autotrophic nutrition; phototroph.

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photoblastic

Describing a seed whose germination is influenced by light. Seeds that are stimulated to germinate by light are described as positively photoblastic ; seeds whose germination is inhibited by light are said to be negatively photoblastic . The response to light is apparently mediated by a phytochrome.

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photochemical smog

A noxious smog produced by the reaction of nitrogen oxides with hydrocarbons in the presence of ultraviolet light from the sun. The reaction is very complex and one of the products is ozone. See also air pollution.

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photoheterotroph

Any organism that uses energy derived from the sun to manufacture organic compounds from organic precursors in photosynthesis. For example, under certain conditions some purple sulphur bacteria use organic acids (rather than hydrogen sulphide) as a source of hydrogen. See phototroph.

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photolysis

A chemical reaction produced by exposure to light or ultraviolet radiation. The photolysis of water, using energy from sunlight absorbed by chlorophyll, produces gaseous oxygen, electrons, and hydrogen ions and is a key reaction in photosynthesis. See photophosphorylation; photosystems I and II.

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photomicrography

The use of photography to obtain a permanent record (a photomicrograph ) of the image of an object as viewed through a microscope.

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photomorphogenesis

The development of plants under the influence of light. All the processes crucial to the growth and development of plants are triggered by light, including seed germination, stem elongation, chloroplast formation, and flowering. These light responses are mediated by various types of light-sensitive molecules, principally phytochrome. It is thought that phytochrome brings about these responses by regulating gene transcription, although the precise mechanisms involved are unknown.

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photonasty

See nastic movements.

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photoperiod

See photoperiodism.

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photoperiodism

The response of an organism to changes in day length (photoperiod ). Many plant responses are controlled by day length, the most notable being flowering in many species (see florigen; day-neutral plant; long-day plant; short-day plant). In plants the internal biological clock and the pigment phytochrome are both thought to be involved in the regulation of photoperiodic responses (see also dark period). Activities in animals that are determined by photoperiod include breeding, migration, and other seasonal events. See also melatonin.

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photophore

A gland or organ that is specialized for the production of light (see bioluminescence). Photophores are a common feature of invertebrates and fish living in the deep sea, often being arranged in lines or other patterns over the body surface to produce a characteristic display of light. They contain the light-producing chemicals or symbiotic bacteria responsible for the bioluminescence. The cells of many different tissues can be modified to form a photophore, including mucous glands in various fish and even the suckers of some deep-sea octopuses.

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photophosphorylation

The formation of ATP from ADP and inorganic phosphate using light energy in photosynthesis (compare oxidative phosphorylation). There are two pathways, noncyclic and cyclic photophosphorylation, which occur in the thylakoid membranes of the chloroplasts. In noncyclic photophosphorylation electrons derived from the photolysis of water are raised to higher energy levels in photosystems I and II and pass along an electron transport chain of carrier molecules (see ferredoxin; plastocyanin; plastoquinone) to NADP reductase. This enzyme transfers electrons to NADP+ to make NADPH, which provides reducing power for the dark reaction of photosynthesis. In cyclic photophosphorylation the electrons from photosystem I that are raised to a higher energy level are recycled through the electron carrier system back to photosystem I. Both pathways of electron flow cause H+ ions to be pumped by a group of cytochromes, the cytochrome b_6–f complex , across the thylakoid membrane. This creates a proton gradient that drives the phosphorylation of ADP to ATP by the enzyme ATP synthetase (see chemiosmotic theory).

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photopic vision

The type of vision that occurs when the cones in the eye are the principal receptors, i.e. when the level of illumination is high. Colours can be identified with photopic vision. Compare scotopic vision.

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photoprotection

Protection of a plant's photosynthetic apparatus from the harmful effects of light. During periods of peak light intensity plants are able to utilize less than half the incoming energy. The surplus energy poses the risk of photooxidation, and the formation of highly reactive superoxide radicals that can destroy the cell's chlorophyll and many other cellular components. Much of the excess energy is trapped and dissipated as heat by carotenoids of the xanthophyll cycle. Also, chloroplasts contain the enzyme superoxide dismutase, which scavenges superoxide radicals.

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photoreactivation

See DNA photolyase.

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photoreceptor

A sensory cell or group of cells that reacts to the presence of light. It usually contains a pigment that undergoes a chemical change when light is absorbed, thus stimulating a nerve. See eye.

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photorespiration

A metabolic pathway that occurs in plants in the presence of light, in which ribulose bisphosphate carboxylase/oxygenase (rubisco), the enzyme involved in carbon dioxide fixation with ribulose bisphosphate, accepts oxygen, in place of carbon dioxide, resulting in the formation of a two-carbon compound, glycolate, which is subsequently metabolized to carbon dioxide. Unlike respiration, there is no production of ATP. In C_3 plants (see C3 pathway) photorespiration has the effect of reducing the rate of photosynthesis, as atmospheric oxygen can combine with rubisco. In C_4 plants (see C4 pathway) the effect of photorespiration is negligible as the affinity of phosphoenolpyruvate carboxylase for carbon dioxide is extremely high. As oxygen is a competitive inhibitor of rubisco, photorespiration will increase as oxygen concentration increases or as carbon dioxide concentration decreases.

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photosynthesis

The chemical process by which green plants and other phototrophs synthesize organic compounds from carbon dioxide and water in the presence of sunlight. In plants and most algae it occurs in the chloroplasts and there are two principal reactions. In the light-dependent (or light ) reaction , which requires the presence of light, energy from sunlight is absorbed by photosynthetic pigments (chiefly the green pigment chlorophyll) and used to bring about the photolysis of water:
H_2O 2H+ + 2e– + ½O_2

The electrons released by this reaction pass along a series of electron carrier molecules; as they do so they lose their energy, which is used to convert ADP to ATP in the process of photophosphorylation. The electrons and protons produced by the photolysis of water are used to reduce NADP:
2H+ + 2e– + NADP+ NADPH + H+

The ATP and NADPH produced during the light reaction provide energy and reducing power, respectively, for the ensuing light-independent (or dark ) reaction , which can take place in light or darkness. During this reaction carbon dioxide is reduced to carbohydrate in a metabolic pathway known as the Calvin cycle. Photosynthesis can be summarized by the equation:
CO_2 + 2H_2O [CH_2O] + H_2O + O_2

Since virtually all other forms of life are directly or indirectly dependent on plants for food, photosynthesis is the basis for all life on earth. Furthermore virtually all the atmospheric oxygen has originated from oxygen released during photosynthesis.

Photosynthesis

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photosynthetic carbon reduction cycle

See Calvin cycle.

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photosynthetic pigments

The pigments responsible for the capture of light energy during the light reaction of photosynthesis. In plants, algae, and cyanobacteria the green pigment chlorophyll a is the primary pigment , absorbing blue and red light (see photosystems I and II). The carotenoids and various other pigments are accessory pigments, absorbing light energy and passing this on to the chlorophyll a molecules.

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photosystems I and II

The two systems of photosynthetic pigments in the thylakoid membranes of chloroplasts that are involved in the light reaction of photosynthesis. Each photosystem contains about 300 chlorophyll molecules that trap light energy, which is then passed to a reaction centre , comprising a chlorophyll a molecule, in each photosystem. In photosystem II a chlorophyll a molecule, known as P680, utilizes light of wavelength 680nm; in photosystem I the chlorophyll a molecule, known as P700, absorbs light at a wavelength of 700nm. Light energy is used in each reaction centre to raise electrons to higher energy levels to enable them to be taken up by electron acceptors. This causes P680 and P700 to become positively charged, or oxidized. The chlorophyll in photosystem II replaces its lost electrons with ones supplied by an associated complex of proteins called the oxygen-evolving complex, which is responsible for the photolysis of water:
2H_2O 4H+ + 4e– + O_2

The oxygen produced is given off as a gas, and the H+ ions, together with the electrons from photosystem I, reduce NADP+ (see photophosphorylation).

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phototaxis

The movement of a cell (e.g. a gamete) or a unicellular organism in response to light. For example, certain algae (e.g. Chlamydomonas) can perceive light by means of a sensitive eyespot and move to regions of higher light concentration to enhance photosynthesis. See taxis.

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phototroph

Any organism that uses energy derived from the sun to manufacture organic compounds by photosynthesis. Most phototrophs are photoautotrophs; a few bacteria are photoheterotrophs.

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phototropism (heliotropism)

The growth of plant organs in response to light. Aerial shoots usually grow towards light, while some aerial roots grow away from light. The phototropic response is thought to be controlled by the differential distribution of the plant growth substance auxin in the illuminated part, which in turn causes differential growth of the shoot or root. See tropism.

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phrenic nerve

The nerve that forms a network in the diaphragm and is involved in the control of ventilation. See inspiratory centre.

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pH scale

A logarithmic scale for expressing the acidity or alkalinity of a solution. To a first approximation, the pH of a solution can be defined as –log_10c, where c is the concentration of hydrogen ions in moles per cubic decimetre. A neutral solution at 25°C has a hydrogen-ion concentration of 10–7 moldm–3, so the pH is 7. A pH below 7 indicates an acid solution; one above 7 indicates an alkaline solution. pH stands for `potential of hydrogen'. The scale was introduced by S. P. Sørensen (1868–1939) in 1909.

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phycobiliprotein

Any one of a class of pigments in the blue-green bacteria, red algae, and photosynthetic cryptophytes that act as accessory pigments in photosynthesis, being attached to the thylakoid membranes. Each comprises a protein bound to a phycobilin , a coloured tetrapyrrole prosthetic group (see porphyrin). The main phycobiliproteins are phycocyanin and phycoerythrin.

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phycobiont

The algal or bacterial component of a lichen.

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phycocyanin

An accessory photosynthetic pigment occurring mainly in the blue-green bacteria and red algae. It is a phycobiliprotein, in which the pigmented prosthetic group is phycocyanobilin, which gives phycocyanin its blue colour.

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phycoerythrin

An accessory photosynthetic pigment occurring mainly in the red algae and blue-green bacteria. It is a phycobiliprotein, in which the pigmented prosthetic group is phycoerythrobilin, which gives phycoerythrin its red colour.

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phycomycetes

In older classification schemes, all primitive fungi, many of which are found in water (e.g. the water moulds, which may be parasitic on fish) or in damp areas. Many are unicellular but those that form mycelia generally have hyphae lacking cross walls, which distinguishes them from the Ascomycota and Basidiomycota. They include the Zygomycota.

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phyletic

See phylogenetic.

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phyllotaxis (phyllotaxy)

The arrangement of leaves on a plant stem. The leaves may be inserted in whorls or pairs at each node or singly up the stem. When arranged in pairs the two leaves arise on opposite sides of the stem and are usually at right angles to the leaf pairs above and below them. Single leaves may be inserted alternately or in a spiral pattern up the stem. Phyllotaxis generally results in the minimum of shading of leaves by those above them.

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phylogenetic (phyletic)

Describing a system of classification of organisms that aims to show their evolutionary history. Compare phenetic.

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phylogeny

The evolutionary history of an organism or group of related organisms. Compare ontogeny.

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phylum

(pl. phyla) A category used in the classification of organisms that consists of one or several similar or closely related classes. Examples of phyla are the Rhodophyta, Ascomycota, Bryophyta, Chelicerata, and Chordata. Phyla are grouped into kingdoms. In traditional plant classification schemes, the division was used instead of the phylum.

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physiological saline

A liquid medium in which animal tissues may be kept alive for a few hours during experiments without pathological changes or distortion of the cells taking place. Such fluids are salt solutions that are isotonic with and have the same pH as the body fluids of the animal. A well-known example is Ringer's solution , formulated by the British physiologist S. Ringer (1835–1910), which is a mixture of sodium chloride, calcium chloride, sodium bicarbonate, and potassium chloride solutions.

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physiological specialization

The occurrence within a species of several forms that are identical in appearance but differ in physiology: these are termed physiological races . For example, many pathogenic fungi develop new physiological races in response to the strong selection pressure exerted when disease-resistant crop varieties are sown over large areas.

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physiology

The branch of biology concerned with the vital functions of plants and animals, such as nutrition, respiration, reproduction, and excretion.

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physisorption

See adsorption.

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phyto-

Prefix denoting plants. For example, phytopathology is the study of plant diseases.

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phytoalexin

An antimicrobial substance that is produced by a plant in response to infection by fungi or bacteria and helps to defend the plant by inhibiting the growth of invading microbes. Phytoalexins vary in their chemical nature; for example, in legumes they are mainly isoflavonoids, whereas in the potato family (Solanaceae) they are predominantly terpenes. Substances, such as small polysaccharides and proteins, that are produced by the invading pathogen act as elicitors, triggering the manufacture of phytoalexins in the tissues surrounding the infection site. The production of phytoalexins is one component of a broader defensive reaction, known as hypersensitivity.

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phytochelatin

Any of a group of sulphur-rich peptides found in plants exposed to high concentrations of heavy metals (e.g. cadmium, zinc, lead, copper, mercury) and thought to play a role in detoxification. They are unusual peptides, with the general formula (-glutamic acid-cysteine)n-glycine (compare glutathione), where n is 2–11, and bind the metal ions in the cell cytoplasm. They may serve to shuttle the toxic metal ions to the cell vacuole, where they are sequestered by organic acids.

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phytochrome

A protein-based plant pigment present in small quantities in many plant organs. It exists in two interconvertible forms: a physiologically active form, which forms when the plant is illuminated with red light or normal daylight; and an inactive form, formed when the plant is exposed to far-red light or darkness. The active form regulates many plant processes, such as seed germination and the initiation of flowering. See also photomorphogenesis; photoperiodism.

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phytoecdysone

See ecdysone.

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phytogeography

See plant geography.

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phytohaemagglutinin (PHA)

Any of various plant-derived compounds that induce changes in lymphocytes normally associated with antigen challenge. These changes include cell enlargement, increased RNA and DNA synthesis, and, finally, cell division. Response to PHAs is used to test for competence of cell-mediated immunity, for example in patients suffering chronic virus infections.

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phytohormone

See growth substance.

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phytoplankton

The photosynthesizing plankton, consisting chiefly of microscopic algae, such as diatoms, and dinoflagellates. Near the surface of the sea there may be many millions of such organisms per cubic metre. Members of the phytoplankton are of great importance as they form the basis of food for all other forms of aquatic life, being the primary producers. Compare zooplankton.

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phytotelm

A small body of water found on a plant. Examples include the contents of a pitcher plant or the small pool within a tree hole. Although often transitory, phytotelms may contain abundant life and represent well-demarcated animal communities that are very amenable to ecological study.

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pia mater

The innermost of the three membranes (meninges) that surround the brain and spinal cord of vertebrates. The pia mater lies immediately adjacent to the central nervous system, and the choroid plexus, which secretes cerebrospinal fluid, is an extension of it.

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pico-

Symbol p. A prefix used in the metric system to denote 10–12. For example, 10–12 farad = 1 picofarad (pF).

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picornavirus

One of a group of small RNA-containing viruses (pico = small; hence pico-RNA-virus) commonly present in the alimentary and respiratory tracts of vertebrates. They cause mild infections of these tracts but the group also includes the polioviruses, which attack the central nervous system causing poliomyelitis; and the causal agent of foot and mouth disease in cattle, sheep, and pigs.

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pie chart

A diagram in which percentages are shown as sectors of a circle. If x percent of the prey of a carnivore comprises species X, y percent species Y, and z percent species Z, a pie chart would show three sectors having central angles 3.6x°, 3.6y°, and 3.6z°.

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pigment

A compound that gives colour to a tissue. Pigments perform a variety of functions. For example, haemoglobin in vertebrate erythrocytes gives blood its characteristic red colour and enables the transport of oxygen throughout the body (see respiratory pigment). Other biological pigments include chlorophyll, a photosynthetic pigment in plants that is responsible for their green coloration; and melanin, a brown pigment in animals that provides protection from ultraviolet light and can be used in camouflaging colorations.

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pileus

The umbrella-shaped cap of certain fungi, such as mushrooms. Spores are produced from gills or pores on the lower surface.

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piliferous layer

The part of the root epidermis that bears root hairs. It extends over a region about 4–10 mm behind the root tip. Beyond this the piliferous layer is sloughed off to reveal the hypodermis.

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pilomotor

Describing nerve fibres or the muscles innervated by them that can cause the position of the body hair to be altered. The arrector pili is a pilomotor muscle.

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Piltdown man

Fossil remains, purported to have been found by Charles Dawson (1864–1916) at Piltdown, Sussex, in 1912, that were named Eoanthropus dawsoni and described as a representative of the true ancestors of modern man. The skull resembled that of a man but the jaw was apelike. In 1953 dating techniques showed the specimen to be a fraud.

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pineal eye

See median eye.

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pineal gland

An outgrowth of the forebrain. In man its functions are obscure, but in other vertebrates it acts as an endocrine gland, secreting the hormone melatonin.

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pinna (auricle)

The visible part of the outer ear, present in some mammals. It is made of cartilage and its function is to channel sound waves into the external auditory meatus. In some species the pinna is movable and aids in detecting the direction from which a sound originates.

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pinocytosis

The process by which a living cell engulfs a minute droplet of liquid. It involves a technique similar to phagocytosis. See endocytosis.

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pipette

A graduated tube used for transferring measured volumes of liquid.

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Pisces

In some classifications, a superclass of the Gnathostomata (jawed chordates) comprising the fishes (compare Tetrapoda). The whole body of a fish is covered with a tough, usually scaly, skin (see scales), which extends over the eye and contains pigments and sometimes slime glands. The circulatory system is a single circuit with blood passing through two sets of capillaries, one at the gills and the other in the body tissues. Three semicircular canals are present in the inner ear. The sense of smell is particularly well developed and pressure waves are detected by the lateral-line system (see lateral-line canal). Fossils of fish date back to the Ordovician period, 505–438 million years ago. There are two classes of modern fish: Chondrichthyes (cartilaginous fishes) and Osteichthyes (bony fishes).

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pistil

The female part of a flower, consisting either of a single carpel (simple pistil ) or a group of carpels (compound pistil ).

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pit

1 A depression or cavity in the secondary wall of a plant cell that facilitates the movement of substances between adjacent cells. The equivalent structures in primary cell walls are called primary pit fields , areas where plasmodesmata are concentrated and pit development usually occurs. A pit comprises a pit membrane , consisting of the middle lamella plus the primary wall; and a pit cavity , the depression in the secondary wall. Pits usually occur in pairs (called pit pairs ) on either side of the middle lamella between two adjacent cells.

2 (coated pit) See endosome.
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pitfall trap

A simple trap for small invertebrate animals consisting of a tin that is placed in the ground with its rim at ground level. The trap, which contains some kind of bait, can be covered by a tile suspended above ground level by stones so that rain does not enter the tin.

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pith

1 (or medulla) The cylinder of parenchyma tissue found in the centre of plant stems to the inside of the vascular tissue. It is light in weight and has been put to various commercial uses, notably the manufacture of pith helmets.

2 (not in scientific usage) The white tissue below the rind of many citrus fruits.
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3 To destroy the central nervous system of an animal, especially a laboratory animal such as a frog, by severing its spinal cord.
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Pithecanthropus

See Homo.

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pituitary gland (pituitary body; hypophysis)

A pea-sized endocrine gland attached by a thin stalk to the hypothalamus at the base of the brain. It consists of two lobes: the anterior and the posterior. The anterior pituitary (or adenohypophysis ) secretes such hormones as growth hormone, the gonadotrophins, prolactin, thyroid-stimulating hormone, and ACTH. Because these hormones regulate the growth and activity of several other endocrine glands, the anterior pituitary is often referred to as the master endocrine gland . Activity of the anterior pituitary itself is regulated by specific releasing factors produced by the hypothalamus (see also neuroendocrine system). The posterior pituitary (or neurohypophysis ) secretes the hormones oxytocin and antidiuretic hormone.

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placenta

1 The organ in mammals and other viviparous animals by means of which the embryo is attached to the wall of the uterus. It is composed of embyronic and maternal tissues: extensions of the chorion and allantois grow into the uterine wall so that materials (e.g. oxygen, nutrients) can pass between the blood of the embryo and its mother (there is, however, no direct connection between the maternal and embryonic blood). The placenta is eventually expelled as part of the afterbirth.

2 A ridge of tissue on the ovary wall of flowering plants to which the ovules are attached. The arrangement of ovules on the placenta (placentation ) is variable, depending on the number of carpels and whether they are free (see apocarpy) or fused (see syncarpy).
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Placentalia

See Eutheria.

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placoid scale (denticle)

See scales.

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plagiotropism

The tendency for a tropism (growth response of a plant) to be orientated at an angle to the line of action of the stimulus concerned. For example, the growth of lateral branches and lateral roots is at an oblique angle to the stimulus of gravity (plagiogeotropism ). Compare orthotropism.

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planarians

See Turbellaria.

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plankton

Minute pelagic organisms that drift or float passively with the current in a sea or lake. Plankton includes many microscopic organisms, such as algae, protozoans, various animal larvae, and some worms. It forms an important food source for many other members of the aquatic community and is divided into zooplankton and phytoplankton. Compare benthos; nekton; neuston.

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plant

Any living organism of the kingdom Plantae. Plants are distinguished from other organisms by their life cycles, in which haploid male and female organisms develop from spores produced by meiosis in the adult diploid organisms and produce gametes by mitosis; fertilization results in a diploid embryo that undergoes its early development in the haploid female. Most plants manufacture carbohydrates by photosynthesis, in which simple inorganic substances are built up into organic compounds. The radiant energy needed for this process is absorbed by chlorophyll, a complex pigment not found in animals. Plants also differ from animals in the possession of cell walls (usually composed of cellulose). Plants are immobile, as there is no necessity to search for food, and they respond slowly to external stimuli. For a classification of the plant kingdom, see see plant kingdom.

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plant geography (phytogeography)

The study of the distribution of world vegetation, with particular emphasis on the influence of the environmental factors that determine this distribution.

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plant kingdom, simplified classification of

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plantigrade

Describing the gait of many mammals, including man, in which the whole lower surface of the foot is on the ground. Compare digitigrade; unguligrade.

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planula

The ciliated free-swimming larva of many cnidarians, consisting of a solid mass of cells. It eventually settles on a suitable surface and develops into a polyp.

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plaque

1 A thin layer of organic material covering all or part of the exposed surface of a tooth. It contains dissolved food (mostly sugar) and bacteria. The bacteria in plaque metabolize the sugar and produce acid, which eats into the surface of the enamel of the tooth and eventually causes tooth decay (dental caries).

2 A clear area in a bacterial culture grown on an agar plate due to lysis of the bacteria by a bacteriophage.
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plasma

See blood plasma.

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plasma cells

Antibody-producing cells found in the epithelium of the lungs and gut and also in bone-forming tissue. They develop in the lymph nodes, spleen, and bone marrow when antigens stimulate lymphocytes to form the precursor cells that give rise to them (see B cell).

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plasmagel

The specialized outer gel-like cytoplasm of living cells (such as Amoeba) that move by extruding part of the cell (known as a pseudopodium) in the direction of motion. A reversible conversion of plasmagel to the more fluid plasmasol is involved in the continuous flow forward of cytoplasm necessary for forming a pseudopodium (see amoeboid movement. See also cytoplasmic streaming.

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plasmalemma

See cell membrane.

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plasma membrane

See cell membrane.

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plasma protein

Any of the protein constituents of blood plasma. They comprise albumins, globulins, and fibrinogen.

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plasmasol

The specialized inner sol-like cytoplasm of living cells that move by producing pseudopodia. Compare plasmagel.

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plasmid

A structure in bacterial cells consisting of DNA that can exist and replicate independently of the chromosome. Plasmids provide genetic instructions for certain cell activities (e.g. resistance to antibiotic drugs). They can be transferred from cell to cell in a bacterial colony (see sex factor). Plasmids are widely used as vectors to produce recombinant DNA for gene cloning.

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plasmin (fibrinase; fibrinolysin)

An enzyme, present in blood plasma, that breaks down a blood clot by destroying the fibrin threads of the clot and by inactivating factors involved in blood clotting, such as prothrombin and the clotting factors. This occurs during fibrinolysis. Plasmin is derived from an inactive precursor, plasminogen.

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plasminogen

The inactive precursor of the enzyme plasmin. Plasminogen is incorporated into blood clots and is converted to plasmin during fibrinolysis.

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plasmodesmata

(sing. plasmodesma) Fine cytoplasmic strands that connect the protoplasts of adjacent plant cells by passing through their cell walls. Plasmodesmata are cylindrical in shape (about 20–40 nm in diameter) and are lined by the plasma membrane of the two adjacent cells. The endoplasmic reticula of the two adjacent cells are connected by a narrower structure, the desmotubule , which runs through the centre of a plasmodesma. Plasmodesmata tend to occur in groups, forming distinct areas called primary pit fields (see pit).

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plasmodium

(pl. plasmodia) A mass of cytoplasm containing many nuclei that comprises a stage in the life cycle of a plasmodial slime mould. The plasmodium is the vegetative phase of the slime mould, often seen as a slimy layer on decaying wood; it is capable of slow amoeboid movement and feeds by engulfing particles of decaying organic matter.

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plasmogamy

Fusion of the cytoplasm of two (or more) cells. It precedes union of the nuclei in fertilization and it occurs in heterokaryosis.

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plasmolysis

The loss of water by osmosis from a plant cell to the extent that the cytoplasm shrinks away from the cell wall. This happens when the cell is placed in a solution that has a higher solute concentration than that of the cell sap, i.e. it has a lower (more negative) water potential, since water always moves from an area of high water potential to an area of low water potential. Compare turgor.

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plastid

An organelle within a plant cell, often occurring in large numbers. Apart from the nucleus, plastids are the largest solid inclusions in a plant cell. For convenience they are classified into those containing pigments (chromoplasts) and those that are colourless (leucoplasts), although changes from one to the other frequently occur. Plastids develop from proplastids , colourless bodies found in meristematic and immature cells; they also arise by division of existing plastids. See also chloroplast.

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plastocyanin

A blue copper-containing protein that is found in chloroplasts and acts as an electron carrier molecule in the light reaction of photosynthesis (see photophosphorylation). Plastocyanin consists of amino acid groups in association with a copper molecule which gives this compound a blue colour.

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plastome

All the genetic material that is contained in a cell's cytoplasmic organelles, such as mitochondria or chloroplasts.

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plastoquinone

A quinone, found in chloroplasts, that functions as one of the carrier molecules of the electron transport chain in the light reaction of photosynthesis. See photophosphorylation.

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plastron

See carapace.

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plate count

An estimate of the number of viable cells in a culture. A plate count of a bacterial culture is made by inoculating the culture plate with a dilute solution of the microorganisms and counting the number of cells that appear in the resultant culture.

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platelet (thrombocyte)

A minute disc-shaped cell fragment in mammalian blood. Platelets are formed as fragments of larger cells (megakaryocytes ) found in red bone marrow; they have no nucleus. They play an important role in blood clotting and release serotonin and other chemicals, which cause a chain of events leading to the formation of a plug at the site of the damage, thus preventing further blood loss. There are about 250000 platelets per cubic millimetre of blood.

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platelet-derived growth factor

See growth factor.

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plate tectonics

The theory that the surface of the earth is made of lithospheric plates, which have moved throughout geological time resulting in the present-day positions of the continents. The theory explains the locations of mountain building as well as earthquakes and volcanoes. The rigid lithospheric plates consist of continental and oceanic crust together with the upper mantle, which lie above the weaker plastic asthenosphere. These plates move relative to each other across the earth. Six major plates (Eurasian, American, African, Pacific, Indian, and Antarctic) are recognized, together with a number of smaller ones. The plate margins coincide with zones of seismic and volcanic activity.
A constructive (or divergent ) plate margin occurs when two plates move away from each other. It is marked by a mid-oceanic ridge where basaltic material wells up from the mantle to form new oceanic crust, in a process known as sea-floor spreading . The production of new crust at constructive plate margins is compensated for by the destruction of material along a destructive (or convergent ) plate margin. Along these margins, which are also known as subduction zones and marked by an oceanic trench, one plate (usually oceanic) is forced to plunge down beneath the other (which may be continental or oceanic). The crust becomes partially melted and rises to form a chain of volcanoes in the upper plate parallel to the trench. When two continental plates collide the compression results in the formation of mountain chains. A third type of plate margin – the transform plate margin – occurs where two plates are slipping past each other.

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Platyhelminthes

A phylum of acoelomate invertebrates comprising the flatworms, characterized by a flattened unsegmented body. The simple nervous system shows some concentration of cells at the head end. The mouth leads to a simple branched gut without an anus. Flatworms are hermaphrodite but self-fertilization is unusual. Many species are parasitic. The phylum contains the classes Turbellaria (planarians), Trematoda (flukes), and Cestoda (tapeworms).

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pleiomorphism

The existence of distinctly different forms during the life cycle of an individual, e.g. the caterpillar, pupa, and winged adult of a butterfly.

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pleiotropic

Describing an allele that has more than one effect in an organism. For example, the allele that causes the erythrocytes to have a distorted form in sickle-cell anaemia also causes these blood cells to rupture easily, thereby inducing anaemia.

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Pleistocene

The earlier epoch of the Quaternary period (compare Holocene). It extended from the end of the Pliocene, about 2 million years ago, to the beginning of the Holocene, about 10000 years ago. The Pleistocene is often known as the Ice Age as it was characterized by a series of glacials, in which ice margins advanced towards the equator, separated by interglacials when the ice retreated. See also ice age.

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plesiomorphy (ancestral trait)

An evolutionary trait that is homologous within a particular group of organisms but is not unique to members of that group (compare apomorphy) and therefore cannot be used as a diagnostic or defining character for the group. For example, vertebrae are found in zebras, cheetahs, and orang-utans, but the common ancestor in which this trait first evolved is so distant that the trait is shared by many other animals. Therefore, possession of vertebrae sheds no light on the phylogenetic relations of these three species.

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pleura (pleural membrane)

The double membrane that lines the thoracic cavity and covers the exterior surface of the lungs. It is a serous membrane forming a closed sac, with a small space (the pleural cavity ) between the two layers.

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plexus

A compact branching network of nerves or blood vessels, such as the brachial plexus – a network of spinal nerves that supply branches to the forelimbs in vertebrates. See also choroid plexus.

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Pliocene

The fifth and final epoch of the Tertiary period. Preceded by the Miocene and followed by the Pleistocene, it began about 5 million years ago and lasted for about 3 million years. Mammals similar to modern forms existed during the epoch and the australopithecines (see Australopithecus), early forerunners of man, appeared.

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plumule

1 (in zoology) A down feather.

2 (in botany) The part of a plant embryo that develops into the shoot system. It consists of the stem apex and first leaves. In seedlings showing epigeal germination the plumule grows above the soil surface together with the cotyledons; in seeds showing hypogeal germination, the plumule alone emerges. Compare radicle.
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pneumatophore

1 An aerial negatively geotropic root that acts as an organ of gaseous exchange. Pneumatophores are produced by mangroves and other plants that grow in waterlogged oxygen-deficient soils.

2 The gas-filled float of certain colonial cnidarians of the class Hydrozoa, such as Physalis (Portuguese man-of-war).
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pod

See legume.

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podocyte

A type of epithelial cell found in Bowman's capsule. Podocytes possess major footlike processes, each supporting a series of minor processes. These minor processes are interwoven with those from other podocytes to form a number of slits, through which filtration can occur. These slits (approximately 0.1 µm wide) permit the passage of all plasma constituents but act as a barrier to blood cells.

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poikilothermy

The passive variation in the internal body temperature of an animal, which depends on the temperature of the environment. All animals except birds and mammals exhibit poikilothermy and are described as ectotherms. Although unable to maintain a constant body temperature, they can respond to compensate for very low or very high temperatures. For example, the tissue composition (especially cell osmotic pressure) can change to regulate the blood flow to peripheral tissues (and thus increase heat loss or heat absorption), and the animals can actively seek sun or shade. Seasonal changes in metabolism are usually under hormonal control. In particularly hot climates, ectotherms may undergo aestivation to escape the heat. Compare homoiothermy.

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point mutation (gene mutation)

A change in the nucleotide sequence of the DNA within a gene; a gene in which such a change has occurred is known as a mutant gene or allele (see mutation). The DNA sequence can be altered in several ways; for example by insertion, substitution, deletion, and inversion. Point mutations result in a misreading of the genetic code during the translation phase of protein synthesis and usually change the order of amino acids making up a protein, which may or may not affect the function of that protein. See also intergenic suppressor; missense mutation. Compare chromosome mutation.

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polar body

See oogenesis.

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polarity

1 The property of a cell, tissue, or organism of being structurally and/or functionally different at opposite ends of its long axis. For example, plants consist of roots, which grow in the direction of the force of gravity, and stems, which grow away from the gravitational force (see geotropism).

2 The property of molecules of having an uneven distribution of electrons, so that one part has a positive charge and the other a negative charge. Such polar molecules include water.
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polar molecule

See polarity.

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polar nuclei

Two haploid nuclei in the centre of the embryo sac of flowering plants. These nuclei fuse with a male gamete nucleus to form a triploid endosperm nucleus, which subsequently divides to form the endosperm. See also double fertilization.

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pollen

The mass of grains containing the male gametes of seed plants, which are produced in large numbers in the pollen sacs. The pollen grains of insect-pollinated plants may be spiny or pitted and are usually larger than those of wind-pollinated plants, which are usually smooth and light. The pollen grain represents the male gametophyte generation; it contains two male nuclei: a generative nucleus and a tube nucleus. The wall of the mature pollen grain consists of the tough outer wall (exine ) and the more delicate narrower intine . The latter gives rise to the pollen tube. See also pollination.

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pollen analysis

See palynology.

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pollen sac

The structure in seed plants in which pollen is produced. In angiosperms there are usually four pollen sacs in each anther; they contain the microspore mother cells. In gymnosperms variable numbers of pollen sacs are borne on the microsporophylls that make up the male cone.

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pollen tube

An outgrowth of a pollen grain, which transports the male gametes to the ovule. It will only grow if the pollen grain is compatible with the female tissue. In angiosperms, the pollen grain is deposited on the stigma and the pollen tube grows down through the style and into the ovule. In some conifers, e.g. Pinus (pines), the pollen tube penetrates the nucellus but does not develop further until the following year, when the female part of the plant is mature. See also embryo sac; fertilization; tube nucleus.

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pollex

The innermost digit on the forelimb of a tetrapod vertebrate. It contains two phalanges (see pentadactyl limb) and in humans and higher primates it is the thumb, which is opposable (i.e. capable of facing and touching the other digits) and gives the hand greater manipulating ability. In some mammals a pollex is absent. Compare hallux.

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pollination

The transfer of pollen from an anther (the male reproductive organ) to a stigma (the receptive part of the female reproductive organ), either of the same flower (self-pollination ) or of a different flower of the same species (cross-pollination ). Cross-pollination involves the action of a pollinating agent to effect transfer of the pollen (see anemophily; entomophily; hydrophily). See also fertilization; incompatibility.

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pollutant

Any substance, produced and released into the environment as a result of human activities, that has damaging effects on living organisms. Pollutants may be toxic substances (e.g. pesticides) or natural constituents of the atmosphere (e.g. carbon dioxide) that are present in excessive amounts. See pollution.

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pollution

An undesirable change in the physical, chemical, or biological characteristics of the natural environment, brought about by man's activities. It may be harmful to human or nonhuman life. Pollution may affect the soil, rivers, seas, or the atmosphere (see air pollution). There are two main classes of pollutants: those that are biodegradable (e.g. sewage), i.e. can be rendered harmless by natural processes and need therefore cause no permanent harm if adequately dispersed or treated; and those that are nonbiodegradable (e.g. heavy metals (such as lead) in industrial effluents (see heavy-metal pollution) and DDT and other chlorinated hydrocarbons used as pesticides), which eventually accumulate in the environment and may be concentrated in food chains. Other forms of pollution in the environment include noise (e.g. from jet aircraft, traffic, and industrial processes) and thermal pollution (e.g. the release of excessive waste heat into lakes or rivers causing harm to wildlife). Recent pollution problems include the disposal of radioactive waste; acid rain; photochemical smog; increasing levels of human waste; high levels of carbon dioxide and other greenhouse gases in the atmosphere (see greenhouse effect); damage to the ozone layer by nitrogen oxides, chlorofluorocarbons (CFCs), and halons; and pollution of inland waters by agricultural fertilizers and sewage effluent, causing eutrophication (see eutrophic). Attempts to contain or prevent pollution include strict regulations concerning factory emissions, the use of smokeless fuels, the banning of certain pesticides, the increasing use of lead-free petrol, restrictions on the use of chlorofluorocarbons, and the introduction, in some countries, of catalytic converters to cut pollutants in car exhausts.

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polyacrylamide gel electrophoresis

See PAGE.

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Polychaeta

A class of annelid worms in which each body segment has a pair of flattened fleshy lobes (parapodia ) bearing numerous bristles (chaetae). All polychaetes are aquatic and most of them are marine. They include the fanworms (Sabella), which construct tubes of sand, etc., in which they live; and the lugworms (Arenicola) and ragworms (Nereis), which burrow in sand or mud.

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polycistronic

Describing a type of messenger RNA that can encode more than one polypeptide separately within the same RNA molecule. Bacterial messenger RNA is generally polycistronic. Compare monocistronic.

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polyembryony

1 The formation of more than one embryo in a plant seed. Often one embryo develops from the fertilized egg cell, while the others have formed asexually from other tissues in the ovule.

2 The formation of more than one embryo from a single animal zygote. Identical twins are produced in this way.
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polygene

Any of a group of genes influencing a quantitative characteristic, e.g. height in humans. See polygenic characters; polygenic inheritance.

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polygenic characters

Phenotypic characters that are controlled by polygenes. Polygenic characters display continuous variation, each polygene having only a small influence on the particular character in question. See also polygenic inheritance.

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polygenic inheritance (multifactorial inheritance; quantitative inheritance)

The determination of a particular characteristic, e.g. height or skin colour, by many genes (polygenes), each having a small effect individually. Characteristics controlled in this way show continuous variation.

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polymer

A substance having large molecules consisting of repeated units (the monomers). There are a number of natural polymers, such as polysaccharides.

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polymerase

Any enzyme that catalyses the elongation of a polymeric molecule. RNA polymerases (types I to III) catalyse the synthesis of RNA using as a template either an existing DNA strand (DNA-dependent RNA polymerase ) or an RNA strand. Type I is responsible for the synthesis of ribosomal RNA, type II for messenger RNA synthesis (see transcription), and type III for transfer RNA synthesis. DNA polymerases catalyse the elongation of a new DNA strand during DNA replication, using an existing DNA strand as template. RNA-directed DNA polymerase is more usually known as reverse transcriptase.

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polymerase chain reaction (PCR)

A technique used to replicate a fragment of DNA so as to produce many copies of a particular DNA sequence. PCR is commonly employed as an alternative to gene cloning as a means of amplifying genetic material for gene sequencing, and is also used to measure gene expression. The technique has also proved invaluable in forensic science, enabling amplification of minute traces of genetic material for DNA fingerprinting. The two strands of the DNA are separated by heating and short sequences of a single DNA strand (primers ) are added, together with a supply of free nucleotides and DNA polymerase obtained from a bacterium that can withstand extreme heat. In a series of heating and cooling cycles, the DNA sequence doubles with each cycle and is thus rapidly amplified.

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polymorphism

The existence of three or more distinctly different forms within a plant or animal species. An example is the caste system of social insects, in which there are the workers, drones, and queens. This is an environmental polymorphism (see polyphenism), i.e. the differences are caused by environmental rather than genetic factors, in this case by the larvae receiving different types of food. There are also heritable or genetic polymorphisms , in which differences are the result of inherited characteristics. Two forms of genetic polymorphism exist: transient polymorphism , in which a particular form is in the process of spreading through a population, causing the relative proportion of phenotypes to alter; and balanced polymorphism , in which two or more forms coexist in a stable ratio within a population, each form possessing both advantageous and disadvantageous characteristics. An example of balanced polymorphism is the occurrence of sickle-cell anaemia , a genetic disease that principally affects Black populations of central Africa and is characterized by an abnormal form of the blood pigment haemoglobin (haemoglobin S ) and sickle-shaped red blood cells. Three different types of individual occur in such populations: those who have two genes (AA) for normal haemoglobin and therefore do not suffer from the disease; those with one normal and one abnormal gene (AS), who suffer from a relatively mild form; and those with two abnormal genes (SS), who suffer a chronic and eventually fatal form of anaemia. Normally such a harmful gene would have been eliminated from the population by the process of natural selection, but it is maintained in this case because people with the mild form of the disease are resistant to a severe form of malaria endemic in central Africa.
See also restriction fragment length polymorphism.

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polynucleotide

See nucleotide.

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polyol (polyhydric alcohol)

Any compound that contains more than one alcohol group. The term is usually restricted to the sugar alcohols, which include such biologically important molecules as myo-inositol glycerol, sorbitol (common in the fruits of many plants), and ribitol (found in bacterial cell walls). Certain polyols act as osmolytes in the kidney cells or function as antifreeze molecules.

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polyp

The sedentary stage in the life cycle of the Cnidaria, consisting of a cylindrical body fixed at one end to a firm base and having a mouth surrounded by a ring of tentacles at the other. Some polyps (e.g. Hydra) are single; others (e.g. the corals and Obelia) form colonies. Polyps typically reproduce asexually by budding to form either new polyps or medusae. The latter reproduce sexually giving rise to new polyps. Sea anemones are solitary polyps that reproduce sexually to form new polyps.

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polypeptide

A peptide comprising ten or more amino acids. Polypeptides that constitute proteins usually contain 100–300 amino acids. Shorter ones include certain antibiotics, e.g. gramicidin, and some hormones, e.g. ACTH, which has 39 amino acids. The properties of a polypeptide are determined by the type and sequence of its constituent amino acids.

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polypetalous

Describing a flower in which the corolla consists of separate petals. Compare gamopetalous.

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polyphenism (environmental polymorphism)

The occurrence within a species of distinct morphological types (morphs ) in response to environmental cues. The morphs, which may overlap in space and/or time, are the result of particular signals in the animals' environment, such as nutrition, temperature, or daylength, that interact with the animals' genes and `switch' development of the young from one `program' to another. The resultant morphs are better adapted to their environmental conditions or to fulfil a certain role in a social group. Polyphenism occurring in successive generations exposed to seasonal changes in the environment is termed cyclomorphosis. Compare polymorphism.

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polyphyletic

Describing a group of organisms that contains the descendants of two or more different ancestors, while excluding other descendants of a single ultimate common ancestor. Such a group may be constructed on the basis of certain shared traits, which may have evolved convergently (see convergent evolution), but it does not necessarily reflect any close evolutionary relationship, and is therefore rejected as a basis for phylogenetic classification. For example, mammals and birds share the trait of endothermy (`warm-bloodedness'), but the grouping `endotherms' is polyphyletic because its members have quite different immediate ancestors and the defining trait evolved independently in each member. Compare monophyletic; paraphyletic.

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polyphyodont

Describing a type of dentition in which the teeth are continuously shed and replaced during the lifetime of the animal. Sharks and frogs have a polyphyodont dentition. Compare diphyodont; monophyodont.

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polyploid

Describing a nucleus that contains more than two sets of chromosomes (see diploid) or a cell or organism containing such nuclei. For example, triploid plants have three sets of chromosomes and tetraploid plants have four. Polyploidy is far more common in plants than in animals; many crops, in particular, are polyploid (bread wheat, for example, is hexaploid , i.e. 6n). It can be induced chemically with colchicine. See also allopolyploid; autopolyploid.

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polyribosome (polysome)

An aggregate of ribosomes in association with a single messenger RNA molecule during the translation process of protein synthesis. In eukaryotes, polyribosomes are attached to the surface of the rough endoplasmic reticulum and the outer membrane of the nucleus; in bacteria they are found free in the cytoplasm.

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polysaccharide (glycan)

Any of a group of carbohydrates comprising long chains of monosaccharide (simple sugar) molecules. Homopolysaccharides consist of only one type of monosaccharide; heteropolysaccharides contain two or more different types. Polysaccharides may have molecular weights of up to several million and are often highly branched. Some important examples are starch, glycogen, and cellulose.

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polysepalous

Describing a flower that possesses a calyx consisting of separate sepals. Compare gamosepalous.

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polysome

See polyribosome.

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polyspermy

The entry of several sperms into the egg during fertilization although only one sperm nucleus actually fuses with the egg nucleus. Polyspermy occurs in animals with yolky eggs (e.g. birds).

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polysynaptic reflex

A reflex action that involves an electrical impulse being transferred from a sensory neurone to a motor neurone via at least one connecting neurone (interneurone ) in the spinal cord. For example, stimulation of pain receptors in the skin initiates a withdrawal reflex , which involves several synapses with several motor neurones and results in the removal of the organism or part from the stimulus.

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polyteny

The condition of a chromosome, nucleus, or cell in which the DNA has repeatedly replicated, without subsequently separating (see endomitosis). This results in a cablelike giant chromosome , consisting of up to 1000 parallel chromatids and often showing conspicuous transverse banding. Polyteny has been observed in the cells of certain insects of the order Diptera, notably in the salivary glands of Drosophila. Swellings (called puffs ) have been observed in giant chromosomes at specific stages in the development of these insects. Puffs are associated with messenger RNA and are sites of gene transcription.

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polythetic

Describing a type of classification in which membership of a taxon is based on its constituent organisms sharing a large number of characteristics.

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polytopic

Describing a taxon that has arisen and exists in more than one geographical region.

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polytypic

Describing a species that exists in a variety of forms or subspecies that inhabit different geographical regions.

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pome

A type of fruit characteristic of apples and pears. The flesh of the fruit develops from the receptacle of the flower, which completely encloses the fused carpels. After fertilization the carpels form the `core' of the fruit, which contains the seeds. See also pseudocarp.

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pons (pons Varolii)

A thick tract of nerve fibres in the brain that links the medulla oblongata to the midbrain. Its function is to relay impulses between different parts of the brain. The pons is named after its discoverer, the Italian anatomist C. Varoli (1543–75).

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population

(in ecology) 1 A group of individuals of the same species within a community. The nature of a population is determined by such factors as density, sex ratio, birth and death rates, emigration, and immigration.

2 The total number of individuals of a given species or other class of organisms in a defined area, e.g. the population of rodents in Britain.
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population dynamics

The study of the fluctuations that occur in the numbers of individuals in animal and plant populations and the factors controlling these fluctuations. An important distinction is maintained between those factors that are dependent on population density and have a stabilizing effect (e.g. food supply) and those that are independent of population density (e.g. catastrophes, such as flooding).

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population genetics

The study of the distribution of inherited variation among a group of organisms of the same species. The potential for change depends on the sum total of alleles that are available to the organisms (the gene pool ), and estimates of changes in allele frequency in a population give an indication of its response to a changing condition.

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population growth

The increase in a population that occurs when the birth rate is higher than the death rate, or when immigration exceeds emigration, or when a combination of these factors is present. A growth curve, obtained by plotting population size against time, is typically S-shaped (sigmoid) or J-shaped (see graph). A sigmoid growth curve shows an initial phase of exponential growth. The curve levels off when the environment has reached its carrying capacity , i.e. when food, space, and other conditions can support a given number of individuals without an increase in population numbers. A J-shaped growth curve shows an initial phase of exponential growth that ceases abruptly, with a sudden decrease in population numbers. This decrease may be caused by a number of factors, such as the end of the life cycle of the prey or any other factor contributing to environmental resistance that may suddenly take effect. See also bacterial growth curve.

Population growth curves

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Porifera

The phylum of marine and freshwater invertebrates that comprise the sponges, which live permanently attached to rocks or other surfaces. The body of a sponge is hollow and consists basically of an aggregation of cells between which there is little nervous coordination. The body is supported by an internal skeleton of spicules of chalk, silica, or fibrous protein (bath sponges have protein skeletons). Undulipodium-bearing (flagellated) cells (choanocytes ) cause water to flow in through openings (ostia ) in the body wall and out through openings (oscula ) at the top; food particles are filtered from the water by the choanocytes.

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porin

Any of a class of proteins that form water-filled channels across cell membranes. In the outer membrane of Gram-negative bacteria, most porins consist of three identical subunits that associate and extend fully through the membrane to form channels about 1 nm in diameter. These permit the entry and exit of hydrophilic low-molecular-weight substances; some contain a specific binding site and allow the passage only of certain substances. The pores can be opened and closed, which has significance for antibiotic resistance in pathogenic bacteria. Similar porin proteins occur in the mitochondrial outer membrane of eukaryotic cells, and they are now known to exist in eukaryotic plasma membranes, where they may be responsible for controllable movements of water in or out of the cell associated with osmosis.

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porphyrin

Any of a group of organic pigments characterized by the possession of a cyclic group of four linked nitrogen-containing rings (a tetrapyrrole nucleus), the nitrogen atoms of which are often coordinated to metal ions. Porphyrins differ in the nature of their side-chain groups. They include the chlorophylls, which contain magnesium; and haem, which contains iron and forms the prosthetic group of haemoglobin, myoglobin, and the cytochromes.

Generalized structure of a porphyrin

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portal vein (portal circulation; portal system)

Any vein that collects blood from one network of capillaries and transports it directly to a second capillary network in another region of the body, without returning to the heart. See hepatic portal system.

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positive feedback

See feedback.

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postcaval vein

See vena cava.

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posterior

1 Designating the part of an animal that is to the rear, i.e. that follows when the animal is moving. In man and bipedal animals (e.g. kangaroos) the posterior surface is equivalent to the dorsal surface.

2 Designating the side of a flower or axillary bud that faces towards the flower stalk or main stem, respectively. Compare anterior.
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postreplicative repair (recombinational repair)

A form of DNA repair that fills large gaps in a new DNA strand resulting from failure of replication. In E. coli, in which this repair process is best understood, thymine dimers in a damaged template strand cause the DNA polymerase to skip several hundred bases, leaving a large gap in the newly synthesized daughter strand and the template strand unpaired in this region. The repair is performed primarily by the action of an enzyme, the RecA protein (encoded by the recA locus), which fills in the gap by appropriating a complementary segment of DNA from the other, undamaged, newly synthesized duplex (double-stranded) DNA. The enzyme coats the unpaired template strand region and insinuates it into the other DNA duplex. Here it pairs with the complementary region of its original sister strand (the other template strand), displacing the newly formed strand. The two ends of this paired region are nicked by an endonuclease enzyme, hence freeing the original damaged region with its complementary `filler' strand. The strand from which the `filler' has been taken is then itself filled in by DNA polymerase I. Although the thymine dimer remains in the original damaged strand, it can be excised at a later stage. Most importantly, the integrity of the DNA is maintained. Many of the enzymes and processes of postreplicative repair are also involved in recombination.

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postsynaptic membrane

The membrane at the end of a neurone that receives an impulse at a synapse.

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potassium

Symbol K. A soft silvery metallic element that is an essential element for living organisms. The potassium ion, K+, is the most abundant cation in plant tissues, being absorbed through the roots and being used in such processes as protein synthesis. In animals the passage of potassium and sodium ions across the nerve-cell membrane is responsible for the changes of electrical potential that accompany the transmission of impulses.

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potassium–argon dating

A dating technique for certain rocks that depends on the decay of the radioisotope potassium–40 to argon–40, a process with a half-life of about 1.27 × 10^10 years. It assumes that all the argon–40 formed in the potassium-bearing mineral accumulates within it and that all the argon present is formed by the decay of potassium–40. The mass of argon–40 and potassium–40 in the sample is estimated and the sample is then dated from the equation:
40Ar = 0.1102 40K(et – 1),

where is the decay constant and t is the time in years since the mineral cooled to about 300°C, when the 40Ar became trapped in the crystal lattice. The method is effective for micas, feldspar, and some other minerals.

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potentiation

The synergistic interaction of two substances to produce an effect that is greater than the effect of either one of the substances in isolation. See synergism.

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potometer

An apparatus used to measure the rate of water loss from a shoot (see transpiration) under natural or artificial conditions.

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poxvirus

One of a group of DNA-containing viruses, often enclosed in an outer membrane, that typically produce skin lesions in vertebrates. They include those viruses causing smallpox (variola), cowpox (vaccinia), and myxomatosis. Some poxviruses produce tumours.

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P-protein (phloem protein)

A protein found in large amounts in the sap-conducting sieve elements of phloem tissue in plants. It takes various forms in the mature sieve element, depending on plant species, ranging from a network of filaments to discrete crystalline bodies. One general property is its ability to form a gel, and it may function as a 'puncture repair' substance, forming a plug at any site of damage in the sieve element, thus preventing loss of food materials being translocated by the phloem. In an intact functioning sieve element, the P-protein is mainly found lining the interior wall.

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preadaptation

An anatomical structure, physiological process, or behaviour pattern in an organism that is by chance highly suited to a new habitat to which the organism migrates or that improves the chance of the organism surviving following a change in environmental conditions. For example, the lungs that developed in certain fish were probably originally a buoyancy aid before these fish began to adapt to a new environment on dry land. See also exaptation.

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Precambrian

Describing the time from the formation of the earth, nearly 5 billion years ago, to the start of the Cambrian period, around 570 million years ago. The term `Precambrian' is no longer used for a specific geological time interval, but remains as a general adjective. Precambrian time is now divided into three eons: Hadean, Archaean, and Proterozoic, the latter extending to the start of the present (Phanerozoic) eon. Fossils are rare, although stromatolites indicate that there were flourishing populations of cyanobacteria and other bacteria. However, subsequent metamorphism of Precambrian rocks makes correlation of rocks and events extremely difficult. The largest areas of exposed Precambrian rocks are the shield areas, such as the Canadian (Laurentian) Shield and the Baltic Shield.

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precaval vein

See vena cava.

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precipitin

Any antibody that combines with its specific soluble antigen to form a precipitate. The term is sometimes applied to the precipitate itself. See also agglutination.

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precocial species

Any species of bird in which hatching occurs at a relatively late stage of development. The young of such species are capable of locomotion and leave the nest soon after hatching; they are described as nidifugous . Compare altricial species.

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predation

An interaction between two populations of animals in which one (the predator ) hunts, captures, and kills the other (the prey ) for food. Predator–prey relationships form important links in many food chains. They are also important in regulating population sizes of both predator and prey, especially when the predator relies on a single prey species. The term predation is also used, more loosely, for any feeding relationship in which an organism feeds on any other living organism (animal or plant).

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predator

An animal that obtains its food by predation. All predators are carnivores, although not all carnivores are predators.

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pregnancy

See gestation.

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premolar

A broad ridged tooth in mammals that is situated behind the canine teeth (when present) and in front of the molars. Premolars are adapted for grinding and chewing food and are present in both the deciduous and permanent teeth.

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presbyopia

A loss of accommodation that normally develops in human eyes over the age of 45–50 years. Vision of distant objects remains unchanged but accommodation of the eye to near objects is reduced as a result of loss of elasticity in the lens of the eye. The defect is corrected by reading glasses using weak converging lenses.

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pressure flow

See mass flow.

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pressure potential

Symbol p. The component of water potential due to the hydrostatic pressure that is exerted on water in a cell. In turgid plant cells it usually has a positive value as the entry of water causes the protoplast to push against the cell wall (see turgor). In xylem cells there is a negative pressure potential, or tension, as a result of transpiration.

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presumptive

Describing embryonic tissue that is not yet determined but which will eventually develop into a certain kind of tissue by virtue of its position in the embryo.

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presynaptic membrane

The membrane of a neurone that releases neurotransmitter into the synaptic cleft between nerve cells (see synapse).

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prey

An animal that is a source of food for a predator. See predation.

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Pribnow box

(minus 10 sequence) A consensus sequence of nucleotides – TATAAT – occurring in the promoter region of prokaryote genes about 10 nucleotides before the start of transcription. The predominance of adenine and thymine bases means that hydrogen bonding between the two DNA strands in this region is relatively weak, enabling the strands to be separated more easily to permit transcription by RNA polymerase. See also TATA box.

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prickle

A hard sharp protective outgrowth, many of which may cover the surface of a plant. It contains cortical and vascular tissue and is not regarded as an epidermal outgrowth. Compare spine; thorn.

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primary consumer

See consumer.

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primary growth

The increase in size of shoots and roots of plants that results from the activity of the apical (tip) meristems and subsequent expansion of the cells produced. The tissues thus produced are called primary tissues and the resultant plant parts constitute the primary plant body . Compare secondary growth.

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primary producer

See producer.

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primary productivity

The total amount of organic matter synthesized by the producers (e.g. green plants) of an ecosystem. See productivity.

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primary structure

See protein.

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primase

An enzyme that creates the short primer sequences needed for the discontinuous replication of DNA. It is an RNA polymerase that synthesizes a short segment of RNA, consisting of 10 to 12 nucleotides, attached to the lagging strand of the unwound DNA molecule. Each primer sequence provides the free 3' –OH group required for the initiation of synthesis of an Okazaki fragment by DNA polymerase. These primers are subsequently removed before the Okazaki fragments are united into a continuous new DNA strand.

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Primates

An order of mammals that includes the monkeys, apes, and man. Primates evolved from arboreal insectivores 130 million years ago. They are characterized by thumbs and big toes that are opposable (i.e. capable of facing and touching the other digits), which permits manual dexterity, and forward-facing eyes allowing binocular vision. The brain, particularly the cerebrum, is relatively large and well-developed, accounting for the intelligence and quick reactions of these mammals. The young are usually produced singly and undergo a long period of growth and development to the adult form.

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primitive streak

The longitudinal groove that develops in the gastrula during the development of bird and mammal embryos. The cells in the primitive streak proliferate rapidly to form mesoderm cells, which migrate to the interior of the embryo.

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primordium

(pl. primordia) A group of cells that represents the initial stages in development of a plant organ. Root and shoot primordia are present in a young plant embryo while leaf primordia (or leaf buttresses ) are seen as small bulges just below the shoot apex.

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prion

An abnormal form of a normal cell protein (PrP) found in the brain of mammals that is believed to be the agent responsible for the diseases scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, and Creutzfeldt–Jakob disease in humans. Produced by mutation of the normal PrP gene, the abnormal prion protein induces the normal protein to fold incorrectly, causing it to form aggregates. These accumulate in the brain and progressively damage and destroy brain cells. Prion protein can be transmitted to other individuals of the same or closely related species, by injection or ingestion of infected tissue, and appears to be transmissible between species that are not closely related, e.g. between cattle and humans.

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Proboscidea

The order of mammals that comprises the elephants. They are herbivorous, with a muscular trunk (proboscis) used for drinking, bathing, and collecting food. The tusks are continuously growing upper incisors and the enormous ridged molar teeth are produced in sequence to replace worn teeth throughout life. The order, which evolved in the Eocene epoch, was formerly much larger and more widespread than it is today and included the extinct mammoths. There are only two species of modern elephants: the African and Indian species.

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proboscis

1 The trunk of an elephant: a muscular and very flexible elongation of the nose, which has a finger-like extremity and is capable of picking up and moving objects, taking in water, collecting food, etc.

2 The elongated mouthparts of certain invertebrates, such as the two-winged flies (Diptera).
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procambium (provascular tissue)

A plant tissue formed by the apical meristems of shoots and roots. It consists of cells elongated parallel to the long axis of the plant. The procambium subsequently gives rise to the primary vascular tissue.

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procarcinogen

The precursor of an active carcinogen. The procarcinogen itself is not usually carcinogenic but is converted to the active carcinogen after it has been metabolized. For example, the drug diethylstilboestrol (a synthetic oestrogen no longer in clinical use) is metabolized to an epoxide intermediate, which can cause cervical cancer.

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procaryote

See prokaryote.

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Proconsul

A genus of extinct apes known from fossil remains, 50 million years old, found mostly in Kenya and assigned to the species P. africanus. A find made in 1948 was the first discovery of a skull of Miocene ape. It is believed that Proconsul was similar and possibly closely related to the genus Dryopithecus.

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prodrug

A drug that requires further metabolism in the liver before becoming biologically active. An example of a prodrug is the immunosuppressant azathioprine, which is metabolized first to cortisone and then to the pharmacologically active compound hydrocortisone.

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producer

An organism considered as a source of energy for those above it in a food chain (i.e. at the next trophic level). Green plants, which convert energy from sunlight into chemical energy, are primary producers ; herbivores are secondary producers , as they utilize energy from green plants and supply energy for carnivores. Compare consumer.

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productivity (production)

(in ecology) The rate at which an organism, population, or community assimilates energy (gross productivity ) or makes energy potentially available (as body tissue) to an animal that feeds on it (net productivity ). The difference between these two rates is due to the rate at which energy is lost through excretion and respiration. Thus gross primary productivity is the rate at which plants (or other producers) assimilate light energy, and net primary productivity is the rate at which energy is incorporated as plant tissue. It is measured in kilojoules per square metre per year (kJm–2y–1). In terrestrial plants, much of the net productivity is not actually available to consumers, e.g. tree roots are not eaten by herbivores. See also energy flow; secondary productivity.

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profundal

Occurring in or designating the deep-water zone of an inland lake. Light intensity, oxygen concentration, and (during summer and autumn) temperature are markedly lower than in the surface layer. Compare littoral; sublittoral.

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progenesis

The maturation of gametes in an organism that is still otherwise at the juvenile stage of development (see paedogenesis). Progenesis can lead to paedomorphosis.

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progeny

See offspring.

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progesterone

A hormone, produced primarily by the corpus luteum of the ovary but also by the placenta, that prepares the inner lining of the uterus for implantation of a fertilized egg cell. If implantation fails, the corpus luteum degenerates and progesterone production ceases accordingly. If implantation occurs, the corpus luteum continues to secrete progesterone, under the influence of luteinizing hormone and prolactin, for several months of pregnancy, by which time the placenta has taken over this function. During pregnancy, progesterone maintains the constitution of the uterus and prevents further release of eggs from the ovary. Small amounts of progesterone are produced by the testes. See also progestogen.

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progestogen

One of a group of naturally occurring or synthetic hormones that maintain the normal course of pregnancy. The best known is progesterone. In high doses progestogens inhibit secretion of luteinizing hormone, thereby preventing ovulation, and alter the consistency of mucus in the vagina so that conception tends not to occur. They are therefore used as major constituents of oral contraceptives.

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proglottid (proglottis)

See Cestoda.

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prokaryote (procaryote)

Any organism in which the genetic material is not enclosed in a cell nucleus. Prokaryotes consist exclusively of bacteria, i.e. archaebacteria and eubacteria, which some authorities place together in the kingdom Bacteria (or Prokaryotae); others classify them in separate domains. It is believed that eukaryotic cells (see eukaryote) probably evolved as symbiotic associations of prokaryotes (see endosymbiont theory).

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prolactin (lactogenic hormone; luteotrophic hormone; luteotrophin)

A hormone produced by the anterior pituitary gland. In mammals it stimulates the mammary glands to secrete milk (see lactation) and the corpus luteum of the ovary to secrete the hormone progesterone. Secretion of prolactin is increased by suckling. In birds prolactin stimulates secretion of crop milk by the crop glands.

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proline

See amino acid.

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prometaphase

The period during the early part of metaphase in which the chromosomes become attached to the spindle fibres and align themselves at right angles to the spindle poles in a plane through the centre of the cell.

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promoter

(in protein synthesis) The region of a DNA molecule that signals the start of transcription. The promoter region contains a sequence of nucleotides (the recognition site ) that induces binding between the DNA and the enzyme RNA polymerase and a further sequence of nucleotides (the initiation site ) that contains the first DNA nucleotide to be copied by the RNA polymerase. The tight binding between RNA polymerase and DNA at the recognition site destabilizes the DNA molecule, causing the double helix structure to unzip so that transcription can proceed.

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pronation

Rotation of the lower forearm so that the hand faces backwards or downwards with the radius and ulna crossed. Compare supination.

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propagation

1 (in botany) See vegetative propagation.

2 (in neurophysiology) The process whereby a nerve impulse travels along the axon of a neurone. Compare transmission.
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prophage

The DNA of a temperate bacteriophage following its incorporation into the host bacterium. The process of incorporation of the viral DNA is known as lysogeny.

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prophase

The first stage of cell division, during which chromosomes contract and divide along their length (except for the centromeres) into chromatids. In mitosis, the chromosomes remain separate from each other. In the first division of meiosis, homologous chromosomes become paired (see pairing). By the end of first prophase the two chromosomes begin to move apart.

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proplastid

See plastid.

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proprioceptor

Any receptor that is sensitive to movement, pressure, or stretching (see stretch receptor) within the body. Proprioceptors occurring in muscles, tendons, and ligaments are important for the coordination of muscular activity and the maintenance of balance and posture.

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prop root

Any of the modified roots that arise from the stem of certain plants and provide extra support. Such stems are usually tall and slender and the prop roots develop at successively higher levels as the stem elongates, as in the maize plant. Buttress roots , which develop at the base of the trunks of many tropical trees, are similar but tend to have a more flattened appearance. Stilt roots are stouter than prop roots. Those formed at the base of the mangrove tree provide firm anchorage in the soft mud of the swamps.

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prosencephalon

See forebrain.

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prosoma

The anterior section of the body (see tagma) of arachnids and other arthropods of the phylum Chelicerata, which consists of the fused head and thorax (cephalothorax) and bears the chelicerae and other appendages. Compare opisthosoma.

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prostaglandin

Any of a group of organic compounds derived from essential fatty acids and causing a range of physiological effects in animals. Prostaglandins have been detected in most body tissues. They act at very low concentrations to cause the contraction of smooth muscle; natural and synthetic prostaglandins are used to induce abortion or labour in humans and domestic animals. Two prostaglandin derivatives have antagonistic effects on blood circulation: thromboxane A_2 causes blood clotting while prostacyclin causes blood vessels to dilate. Prostaglandins are also involved in inflammation, being released from affected tissues. See also aspirin.

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prostate gland

A gland in male mammals that surrounds and opens into the urethra where it leaves the bladder. During ejaculation it secretes a slightly alkaline fluid into the semen that activates the sperms and prevents them from sticking together.

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prosthetic group

A tightly bound nonpeptide inorganic or organic component of a protein. Prosthetic groups may be lipids, carbohydrates, metal ions, phosphate groups, etc. Some coenzymes are more correctly regarded as prosthetic groups.

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protamine

Any of a group of proteins of relatively low molecular weight found in association with the chromosomal DNA of vertebrate male germ cells. They contain a single polypeptide chain comprising about 67% arginine. Protamines serve in packaging the highly condensed DNA of the germ-cell chromosomes.

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protandry

1 The condition in which the male reproductive organs (stamens) of a flower mature before the female ones (carpels), thereby ensuring that self-fertilization does not occur. Examples of protandrous flowers are ivy and rosebay willowherb. Compare protogyny; homogamy. See also dichogamy.

2 The condition in some hermaphrodite or colonial invertebrates in which the male gonads or individuals are sexually mature before the female ones. Compare protogyny.
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protanopia

See colour blindness.

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protease (peptidase; proteinase; proteolytic enzyme)

Any enzyme that catalyses the hydrolysis of proteins into smaller peptide fractions and amino acids, a process known as proteolysis . Examples are pepsin and trypsin. Several proteases, acting sequentially, are normally required for the complete digestion of a protein to its constituent amino acids. See also acid protease.

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proteasome

A large complex of proteins found in the cytoplasm of cells that degrades cell proteins into small peptide units. Proteasomes participate in the disposal of damaged proteins and the removal of proteins that have reached the end of their functional life in the cell. They thus play a vital part both in the cell's response to stress and in the day-to-day running of the cell. A proteasome consists of a hollow cylinder of protease enzymes, each with its catalytic site facing the internal cylinder wall. Another protein complex guards the entrance to the cylinder. Proteins destined for degradation are usually tagged by a small protein, ubiquitin, and unfolded before entering the proteasome, where they are cleaved into peptides. However, the proteasome is thought to identify 'normal' proteins with short half-lives according to their N-terminal amino acid sequence.

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protein

Any of a large group of organic compounds found in all living organisms. Proteins comprise carbon, hydrogen, oxygen, and nitrogen and most also contain sulphur; molecular weights range from 6000 to several million. Protein molecules consist of one or several long chains (polypeptides) of amino acids linked in a characteristic sequence. This sequence is called the primary structure of the protein. These polypeptides may undergo coiling (see alpha helix) or pleating (see beta sheet), the nature and extent of which is described as the secondary structure . The three-dimensional shape of the coiled or pleated polypeptides is called the tertiary structure , the functional unit of which is a domain. Quaternary structure specifies the structural relationship of the component polypeptides.
Proteins may be broadly classified into globular proteins and fibrous proteins. Globular proteins have compact rounded molecules and are usually water-soluble. Of prime importance are the enzymes, proteins that catalyse biochemical reactions. Other globular proteins include the antibodies, which combine with foreign substances in the body; the carrier proteins, such as haemoglobin; the storage proteins (e.g. casein in milk and albumin in egg white), and certain hormones (e.g. insulin). Fibrous proteins are generally insoluble in water and consist of long coiled strands or flat sheets, which confer strength and elasticity. In this category are keratin and collagen. Actin and myosin are the principal fibrous proteins of muscle, the interaction of which brings about muscle contraction. Blood clotting involves the fibrous protein called fibrin.
When heated over 50°C or subjected to strong acids or alkalis, proteins lose their specific tertiary structure and may form insoluble coagulates (e.g. egg white). This usually inactivates their biological properties.

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protein blotting

See Western blotting.

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protein engineering

The techniques used to alter the structure of proteins (especially enzymes) in order to improve their use to humans. This involves artificially modifying the DNA sequences that encode them so that, for example, new amino acids are inserted into existing proteins. Synthesized lengths of novel DNA can be used to produce new proteins by cells or other systems containing the necessary factors for transcription and translation. Alternatively, new proteins can be synthesized by solid state synthesis , in which polypeptide chains are assembled under the control of chemicals. One end of the chain is anchored to a solid support and the chemicals selectively determine which amino acids are added to the free end. The appropriate chemicals can be renewed during the process; when synthesized, the polypeptide is removed and purified. Protein engineering is used to synthesize enzymes (so-called `designer enzymes') used in biotechnology. Further understanding of the three-dimensional tertiary structure of proteins is required for future development in protein engineering; this is being assisted by computer-aided modelling.

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protein kinase

An enzyme that catalyses the transfer of a phosphate group from ATP to an intracellular protein, thereby affecting the biological activity of the protein (see kinase). Protein kinases phosphorylate specific amino-acid residues of their target proteins, usually either serine, threonine, or tyrosine. They play an important role in increasing or decreasing enzyme activity and in transmitting signals from receptors on the cell surface. The activity of the protein kinases is itself controlled by cyclic AMP, calcium ions, or other intracellular chemicals. It can be reversed by the action of phosphatase enzymes in the cell.

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protein sequencing

The process of determining the amino-acid sequence of a protein or its component polypeptides. The technique most commonly used is Edman degradation (devised by Pehr Edman), in which the terminal amino-acid residues are removed sequentially and identified chromatographically. Each step is automated and the whole process can now be performed by a single machine – the sequenator. Large polypeptides must be cleaved into smaller peptides before sequencing.
The results of this chemical sequencing can often be compared with the amino-acid sequence deduced by gene sequencing. The gene coding for the protein under investigation may be found by screening a gene library, for example by Western blotting. However, the base sequence of the gene gives only the amino-acid sequence of the nascent protein, i.e. before post-translational modification. The sequence of the functional protein can only be found by chemical analysis.

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protein synthesis

The process by which living cells manufacture proteins from their constituent amino acids, in accordance with the genetic information carried in the DNA of the chromosomes. This information is encoded in messenger RNA, which is transcribed from DNA in the nucleus of the cell (see genetic code; transcription): the sequence of amino acids in a particular protein is determined by the sequence of nucleotides in messenger RNA. At the ribosomes the information carried by messenger RNA is translated into the sequence of amino acids of the protein in the process of translation.

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protein targeting

The process whereby a newly synthesized protein is directed to its correct location within the cell. Protein targeting is determined by short sequences of amino acids in the protein (known as signal sequences ), which direct it to the correct destination; for example to a mitochondrion or (in the case of a plant cell) a chloroplast. This movement can occur either during synthesis of the protein (cotranslational) or after synthesis is complete (post-translational).

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proteoglycan

One of a group of glycoproteins found in the matrix of connective tissue, consisting of a polysaccharide chain (see glycosaminoglycan) bound to a small protein unit.

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proteolysis

The enzymic splitting of proteins. See protease.

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proteolytic enzyme

See protease.

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Proterozoic

The eon of geological time extending from the end of the Archaean, about 2600 million years ago, to the start of the present eon (see Phanerozoic), about 570 million years ago. Life in the early Proterozoic was dominated by bacteria, which flourished in shallow seas and muds. They depended on a wide variety of metabolic strategies, including photosynthesis, which were crucial in determining the composition of the earth's atmosphere and oceans. The oldest eukaryotic fossils date from after the middle Proterozoic, about 1200 million years ago. These early protoctists are thought to have arisen through symbiotic associations of various protoctists (see endosymbiont theory), probably on several independent occasions. Towards the end of this eon comes the first fossil evidence of multicellular animal life, the so-called Ediacara fauna, named after a rocky outcrop in Australia but also found elsewhere. These rocks, dated to around 650 million years ago, reveal traces of soft-bodied fanlike or quiltlike creatures, perhaps unrelated to any modern forms, as well as animals resembling jellyfish and worms.

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prothallus

A small flattened multicellular structure that represents the independent gametophyte generation of clubmosses, horsetails, and ferns. In some of these plants a single prothallus bears both male and female sex organs. In others there are separate male and female prothalli.

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prothoracic gland

See ecdysone.

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prothoracicotropic hormone (PTTH)

An insect hormone, released by glands (corpora allata) on either side of the brain, that stimulates release of the moulting hormone, ecdysone, from the prothoracic glands. PTTH isolated from the moth Manduca sexta is a 30-kDa protein consisting of two identical peptide chains.

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prothrombin (Factor II)

One of the blood clotting factors. It is the precursor of the enzyme thrombin, which catalyses the formation of the fibrin matrix of the blood clot. Prothrombin synthesis occurs in the liver and is dependent on adequate supplies of vitamin K. See also blood clotting.

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Protista

In some classifications, a kingdom into which all organisms of simple biological organization are classified. Originally proposed by Ernst Haeckel in 1866 to include the algae, bacteria, fungi, and protozoa, it was later restricted first to unicellular organisms, and then to protozoa, unicellular algae, and organisms then regarded as simple fungi. In most modern classifications it has been replaced by the Protoctista.

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Protoctista

A kingdom consisting of unicellular or simple multicellular organisms that possess nuclei and cannot be classified as animals, plants, or fungi. Protoctists include the protozoa, algae, Dinomastigota, Oomycota, and slime moulds. See also Protista.

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protoderm

A plant tissue formed by the apical meristem of shoots and roots that subsequently gives rise to the epidermis.

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protogyny

1 The condition in which the female reproductive organs (carpels) of a flower mature before the male ones (stamens), thereby ensuring that self-fertilization does not occur. Examples of protogynous flowers are plantain and figwort. Compare protandry; homogamy. See also dichogamy.

2 The condition in hermaphrodite or colonial invertebrates in which the female gonads or individuals are sexually mature before the male ones. Compare protandry.
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protonema

The first, usually filamentous, structure produced by a germinating moss or fern spore under dark conditions. The protonema of mosses bears buds that develop into the gametophyte plant; in ferns the protonema becomes the prothallus.

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protonephridium

See nephridium.

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proton pump

A carrier protein or complex of proteins that transports protons (H+) across biological membranes. Such pumps use energy, for example ATP, to establish a high concentration of protons on one side of the membrane compared to the other. This proton gradient is then exploited by the cell to drive various processes, including the transport of ions and small molecules across cell membranes. Proton pumps are also involved in secretion of gastric acid by mucosal cells in the stomach lining. The concept of a proton pump is central to the chemiosmotic theory of ATP formation by electron transport in mitochondria and chloroplasts. Here, the pump is driven not by ATP hydrolysis but by electron transport, and the proton gradient that is set up represents a means of conserving the energy released by respiration, or captured by photosynthetic pigments. This energy is then stored as ATP. See also acid growth theory.

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protophloem

The primary phloem tissue that arises from the elongating regions of shoot or root apices. Compare metaphloem.

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protoplasm

The material comprising the living contents of a cell, i.e. all the substances in a cell except large vacuoles and material recently ingested or to be excreted. The term is no longer used; the material of the cell is now referred to as the nucleoplasm (see nucleus) and the cytoplasm.

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protoplast (energid)

The living unit of a cell, consisting of the nucleus and cytoplasm bounded by the cell membrane. Protoplasts of bacterial and plant cells can be prepared by removing the cell wall; they are used to study the processes involved in cell metabolism and reproduction.

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Prototheria

A subclass of mammals – the monotremes – that lay large yolky eggs. It contains only the duckbilled platypus and the spiny anteater. After hatching, the young feed on milk from simple mammary glands inside a maternal abdominal pouch. In the anteater the eggs are also incubated in this pouch, while the platypus builds an underground nest. Adult monotremes have no true teeth. Their skeleton resembles that of a reptile, and although they are warm-blooded the body temperature is somewhat variable. They are believed to have originated at least 150 million years ago.

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protoxylem

The primary xylem tissue that is formed in the expanding region of roots and shoots before the shoot or root has completed the elongation process. In the secondary walls of these vessels the lignin is deposited in rings and spirals, which allows for further elongation. The protoxylem is succeeded by metaxylem.

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protozoa

A group of unicellular or acellular, usually microscopic, organisms now classified in various phyla of the kingdom Protoctista (see Apicomplexa; Ciliophora; Rhizopoda; Zoomastigota). They were formerly regarded either as a phylum of simple animals or as members of the kingdom Protista. They are very widely distributed in marine, freshwater, and moist terrestrial habitats; most protozoans are saprotrophs, but some are parasites, including the agents causing malaria (Plasmodium) and sleeping sickness (Trypanosoma), and a few contain chlorophyll and carry out photosynthesis, like plants. Protozoan cells may be flexible or rigid, with an outer pellicle or protective test . In some (such as Paramecium and Trypanosoma) undulipodia (cilia or flagella) are present for locomotion; others (such as Amoeba) have pseudopodia for movement and food capture. Contractile vacuoles occur in freshwater protozoans. Reproduction is usually asexual, by binary fission, but some protozoans undergo a form of sexual reproduction (see conjugation).

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Protura

An order of tiny wingless insects that live in soil and leaf litter. There are about 175 known species. The slender elongate adults are usually less than 2 mm long and lack eyes and antennae; the front pair of legs are held forwards as sensors. The mouthparts, which are largely entognathous (concealed within folds), are adapted for feeding on decaying organic matter. The young resemble the adults, but three segments are added to the end of the abdomen during development. The relationship of proturans to other arthropod groups is controversial. Although some authorities place them in the subclass Apterygota with the other wingless insects, other authors regard the Protura as a separate class within the superclass Hexapoda, most closely affiliated to the Collembola.

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provascular tissue

See procambium.

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proventriculus

1 The anterior part of the stomach of a bird, where digestive enzymes are secreted. Food passes from the proventriculus to the gizzard.

2 See gastric mill.
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provirus

The intermediate stage in the infection of a host cell by a virus, e.g. a retrovirus, in which the viral genome is integrated into the DNA of the host, where it can undergo successive replications before being transcribed. In the case of retroviruses the single RNA strand of the virus is converted into double-stranded DNA by the enzyme reverse transcriptase, then integrated into the host cell DNA and subsequently transcribed to form new RNA viruses. This integration also introduces retroviral oncogenes into the host, with the attendant risk of host-cell transformation to a cancer cell. A provirus, notably that of HIV, can remain dormant for long periods before being transcribed. See also prophage.

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proximal

Denoting the part of an organ that is nearest to the organ's point of attachment. For example, the knuckles are at the proximal end of the fingers. Compare distal.

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proximal convoluted tubule (first convoluted tubule)

The section of a nephron situated between Bowman's capsule and the loop of Henle in the vertebrate kidney. Reabsorption of salt, water, and glucose from the glomerular filtrate occurs in this tubule; at the same time certain substances, including uric acid and drug metabolites, are actively transferred from the blood capillaries into the tubule. Both activities are facilitated by finger-like projections (see brush border) on the inner surface of the tubule, which increase its effective surface area.

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pseudocarp (false fruit)

A fruit that incorporates, in addition to the ovary wall, other parts of the flower, such as the receptacle. For example, the fleshy part of the strawberry is formed from the receptacle and the `pips' on the surface are the true fruits. See also composite fruit; pome; sorosis; syconus.

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pseudocoelomate

Describing any invertebrate animal whose body cavity is a pseudocoel , a cavity between the gut and the outer body wall derived from a persistent blastocoel (see blastula), rather than a true coelom. Pseudocoelomate animals include the Rotifera and Nematoda.

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pseudogamy

A form of parthenogenesis in which the male gamete is required to activate development of the egg but fertilization does not occur. Generally, the sperm or pollen is from a member of the same species, although in some cases, for example in the fish Poeciliopsis, egg development is triggered by the sperm of a closely related species. Pseudogamous plants require pollination, and the endosperm undergoes fertilization even though the diploid embryo develops without it.

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pseudogene

A sequence of nucleotides in DNA that resembles a functional gene but is not transcribed. Pseudogenes are thought to arise by duplication of an existing gene through unequal crossing-over during meiosis, with accompanying loss of the promoter or other flanking regions required for transcription. For example, the - and ß-globin gene clusters in humans contain several pseudogenes.

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pseudoheart

Any one of a series of contractile blood vessels in annelid worms that pump blood from the dorsal vessel to the ventral vessel.

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pseudoparenchyma

A tissue that superficially resembles plant parenchyma but is made up of an interwoven mass of hyphae (in fungi) or filaments (in algae). Examples of pseudoparenchymatous structures are the fruiting bodies (mushrooms, toadstools, etc.) of certain fungi and the thalli of certain red and brown algae.

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pseudopodium

(pl. pseudopodia) A temporary outgrowth of the cell of some protozoans (e.g. Amoeba), which serves as a feeding and locomotory organ. Pseudopodia may be blunt or threadlike, form a branching network, or be stiffened with an internal supporting rod. Phagocytic white blood cells also form pseudopodia to engulf invading bacteria. See amoeboid movement.

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pseudopregnancy

A state resembling pregnancy that may occur in some mammals (e.g. rabbits and rodents) in which many of the phenomena of pregnancy are present but there is no fetus developing in the uterus. It is caused by an extended dioestrus (see oestrous cycle) in the absence of fertilization.

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Psilophyta

A phylum of primitive tracheophyte plants that contains two extant genera – Psilotum (whisk ferns) and Tmesipteris – as well as numerous extinct forms that flourished in the Devonian period. Psilophytes have rhizoids rather than roots; the stems, which show dichotomous branching, may be naked or bear scalelike leaves.

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psychrophilic

Describing an organism that lives and grows optimally at relatively low temperatures, usually below 15°C, and cannot grow above 20°C. Psychrophiles, which consist mainly of bacteria, algae, and fungi, are restricted to permanently cold climates. For example, the ice in polar regions often contains dense masses of algae. Such organisms are likely to be killed by even brief warming. Psychrophiles have various adaptations to enable survival at extremes of cold. These include a high content of unsaturated fatty acids in their cell membranes, which maintain the membrane in a semifluid state. They also have enzymes that function optimally at relatively low temperatures. Psychrotolerant organisms live and grow best at higher temperatures (20–40°C), but are able to tolerate cold conditions. Compare mesophilic; thermophilic.

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Pteridophyta

In traditional classification systems, a division of the plant kingdom that included ferns, horsetails, and clubmosses, i.e. the nonseed-bearing tracheophytes. These are now classified as separate phyla: Filicinophyta (ferns), Sphenophyta (horsetails), Lycophyta (clubmosses), and Psilophyta.

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Pteridospermales

See Cycadofilicales.

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pterodactyls

See Pterosauria.

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Pterophyta

See Filicinophyta.

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Pteropsida

In older classifications, a subdivision of tracheophytes that contained the ferns and seed plants, or a class of the Pteridophyta containing only the ferns.

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Pterosauria

An extinct order of flying reptiles – the pterodactyls – that lived in the Jurassic and Cretaceous periods (180–70 million years ago). Pterodactyls had beaked jaws and an elongated fourth finger that supported a membranous wing. They had long jointed tails, no feathers, and could probably only fly by soaring.

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Pterygota

A subclass of insects that typically possess wings, although some are secondarily wingless. It comprises the exopterygotes and the endopterygotes. Compare Apterygota.

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ptyalin

An enzyme that digests carbohydrates (see amylase). It is present in mammalian saliva and is responsible for the initial stages of starch digestion.

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puberty

See adolescence.

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pubis

One of the three bones that make up each half of the pelvic girdle. It is the most anterior of the three pelvic bones. In mammals and many reptiles the pubes are united at a slightly movable joint, the pubic symphysis . See also ilium; ischium.

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puff

(in genetics) See polyteny.

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pullulan

A water-soluble polysaccharide composed of glucose units that are polymerized in such a way as to make it viscous and impermeable to oxygen. Pullulan is used in adhesives, food packaging, and moulded articles. It is derived from the fungus Aureobasidium pullulans.

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pulmonary

Of or relating to the lungs.

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pulmonary artery

The artery that conveys deoxygenated blood from the right ventricle of the heart to the lungs, where it receives oxygen.

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pulmonary circulation

The part of the circulatory system of birds and mammals that transports deoxygenated blood from the right side of the heart to the lungs and returns oxygenated blood to the left side of the heart. Compare systemic circulation. See double circulation.

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pulmonary valve

See semilunar valve.

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pulmonary vein

The vein that conveys oxygenated blood from the lungs to the left atrium of the heart.

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pulp cavity

The central region of a tooth, which is connected by a narrow channel at the tip of the root with the surrounding tissues. The pulp cavity contains the pulp – connective tissue in which blood vessels and nerve fibres are embedded, and it is lined with odontoblasts, which produce the dentine.

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pulse

(in physiology) A series of waves of dilation that pass along the arteries, caused by pressure of blood pumped from the heart through contractions of the left ventricle. In man it can be felt easily where arteries pass close to the skin surface, e.g. at the wrist.

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pulvinus

A group of cells at the base of a leaf or leaflet in certain plants that, by rapidly losing water, brings about changes in the position of the leaves. In the sensitive plant (Mimosa pudica), the pulvinus is responsible for the folding of the leaves that occurs at nightfall or when the plant is touched or injured.

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punctuated equilibrium

A theory proposing that plant and animal species usually arise very quickly in terms of geological time (in less than 100000 years) and seldom through a process of gradual change. It thus questions the traditional Darwinian theory of evolution, citing as evidence the discontinuities observed in the fossil records of certain animal groups (e.g. the ammonites).

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Punnett square

A representation of all possible genotypes produced in the F2 generation of a Mendelian genetic cross; it was devised by R. C. Punnett. All possible male gametes are listed horizontally against a grid and all possible female gametes are listed vertically against the grid. The various combinations of these gametes are then written into the grid. See illustration at dihybrid cross.

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pupa

The third stage of development in the life cycle of endopterygote insects. During the pupal stage locomotion and feeding cease and metamorphosis from the larva to the adult form takes place. There are three types of pupa. The commonest is the exarate or free pupa, in which the wings and other appendages are visible and movable. In the obtect type the wings are stuck to the body and immovable, as in the chrysalis of a butterfly or moth; and in the co-arctate type an exarate pupa develops within a hard barrel-shaped puparium , as in the housefly and other Diptera.

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pupil

See iris.

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pupillary reflex

An alteration in the size of the pupil in response to a change in the light intensity. An increase in light intensity causes both pupils to contract (see iris) even if only one retina is directly stimulated. This is known as a consensual reflex. Conversely, a decrease in light intensity causes dilation of the pupils.

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pure line

A population of plants or animals all having a particular feature that has been retained unchanged through many generations. The organisms are homozygous and are said to `breed true' for the feature concerned.

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purine

An organic nitrogenous base (see formula), sparingly soluble in water, that gives rise to a group of biologically important derivatives, notably adenine and guanine, which occur in nucleotides and nucleic acids (DNA and RNA).

Purine

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Purkyne cell (Purkinje cell)

A type of nerve cell large numbers of which are found in the cortex of the cerebellum. Purkyne cells are multipolar neurones having large bodies from which arise several dendrons with highly branched dendrites, which form a flat fan extending towards the surface of the cerebellum. They are named after the Czech physiologist J. E. Purkyne (1787–1869).

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Purkyne fibres (Purkinje fibres)

Modified fibres in the mammalian heart that originate in the bundle of His and spread out in a network over the ventricles. Action potentials generated in the sinoatrial node (the pacemaker of the heart) are conducted extremely rapidly through the ventricles, due to the extensive branching of the Purkyne fibres, causing both ventricles to contract almost simultaneously.

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putrefaction

The microbial decomposition of organic matter, especially the anaerobic breakdown of proteinaceous material with the production of foul-smelling amines.

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Pycnogonida

See Chelicerata.

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pyknotic

Describing a nucleus of a damaged cell that has decreased in volume and become darker due to some degree of condensation of the nuclear chromatin. See also karyorrhexis.

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pyloric sphincter

See sphincter; stomach.

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pyramidal cell

A type of neurone found in the cerebral cortex of the brain. Pyramidal cells have a pyramid-shaped cell body and its dendrites extend from both the body and the axon.

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pyramid of biomass

A diagrammatic representation of the amount of organic material (see biomass), measured in grams of dry mass per square metre (gm–2), found in a particular habitat at ascending trophic levels of a food chain. Biomass decreases at each ascending level of the food chain. A pyramid of biomass is a more accurate representation of the flow of energy through a food chain than a pyramid of numbers, but seasonal variations in the rate of turnover of the organisms at a particular level may result in higher or lower values for the amount of biomass sampled at a particular time than the average amount over the whole year. The best representation of energy flow in a food chain is a pyramid of energy.

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pyramid of energy

A diagrammatic representation of the amount of energy, measured in kilojoules per square metre per year (kJm–2yr–1), available at ascending trophic levels of a food chain in a particular habitat (see illustration). A pyramid of energy is the most accurate representation of the energy flow through a food chain as it indicates how much energy is lost at each trophic level (through respiration, etc.). See also bomb calorimeter. Compare pyramid of biomass; pyramid of numbers.

Pyramid of energy

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pyramid of numbers

A diagramatic representation of the numbers of animals found in an area at ascending trophic levels of a food chain (see illustration). Because only a small proportion of the energy taken in by an organism is converted to tissue and is thus available to consumers at the next trophic level, the number of organisms that can be supported at each level is generally much less than the number at the level that supplies its food (i.e. the level below). See also pyramid of biomass; pyramid of energy.

Pyramid of numbers for a woodland food chain

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pyranose

A sugar having a six-membered ring containing five carbon atoms and one oxygen atom.

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pyrenocarp

See drupe.

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pyrenoid

A spherical protein body found in the chloroplasts of many algae and the hornworts (see Anthocerophyta). Pyrenoids are associated with the storage of starch: layers of starch are often found around them.

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pyrethrum

An insecticide (see pesticide) prepared from the flowers of Chrysanthemum cinerariifolium. It penetrates the insect's cuticle and is fast-acting, nontoxic to other animals and to plants, and readily biodegradable.

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pyridoxine

See vitamin B complex.

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pyrimidine

An organic nitrogenous base (see formula), sparingly soluble in water, that gives rise to a group of biologically important derivatives, notably uracil, thymine, and cytosine, which occur in nucleotides and nucleic acids (DNA and RNA).

Pyrimidine

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pyrrole

An organic nitrogen-containing compound (see formula) that forms part of the structure of porphyrins.

Pyrrole

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pyruvic acid (2-oxopropanoic acid)

A colourless liquid organic acid, CH_3COCOOH. Pyruvate is an important intermediate compound in metabolism, being produced during glycolysis and converted to acetyl coenzyme A, required for the Krebs cycle. Under anaerobic conditions pyruvate is converted to lactate or ethanol.

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Q

quadrat

An ecological sampling unit consisting of a small square area of ground within which all species of interest are noted or measurements taken. Quadrats may be spaced over a larger area to form an overall view when a total survey would be impracticable, or they may be used to sample along a transect.

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quadrate

A paired bone in the upper jaw of bony fishes, amphibians, reptiles, and birds that articulates with the lower jawbone. It is absent in mammals, being reduced to a small bone (the incus) in the middle ear (see ear ossicles).

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quadriceps

A group of extensor muscles that cover the anterior surface and sides of the mammalian femur. These muscles join at the base and are connected to the tibia by a single tendon. Muscle spindles in the quadriceps are responsible for the knee-jerk reflex (see stretch reflex).

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qualitative variation

See discontinuous variation.

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quantitative inheritance

See polygenic inheritance.

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quantitative structure–activity relationship (QSAR)

A statistical algorithm that quantitatively defines the relationship between the chemical structure of a drug and its effect on an organism. QSAR studies are often used to predict the activity or toxicity of new drugs. Similar methods can be used to predict the metabolism of new drugs (quantitative structure–metabolism relationships ).

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quantitative structure–metabolism relationship (QSMR)

See quantitative structure–activity relationship.

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quantitative variation

See continuous variation.

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quarantine

A period of isolation imposed on an animal that moves from an area where particular diseases are prevalent to an area where those diseases are not prevalent. In the UK domestic animals or livestock entering the country are quarantined for the necessary incubation period (see infection) in order to prevent the spread of a particular disease.

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Quaternary

The second period of the Cenozoic era, which began about 2 million years ago, following the Tertiary period, and includes the present. It is subdivided into two epochs – the Pleistocene and Holocene. The beginning of the Quaternary is usually based on the onset of a worldwide cooling. During the period four principal glacial phases occurred in Europe and North America, in which ice advanced towards the equator, separated by interglacials during which conditions became warmer and the ice sheets and glaciers retreated. The last glacial ended about 10000 years ago. Man became the dominant terrestrial species during the Quaternary. Among the fauna adapted to the cold conditions were the mammoth and the woolly rhinoceros.

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quaternary structure

See protein.

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queen substance

A pheromone, trans-9-keto-2-decenoic acid, that is secreted by the mandibular salivary glands of a queen honeybee and inhibits the development of ovaries in the worker bees in the colony. The queen substance also affects the behaviour of the workers, preventing them from maintaining special brood cells for larvae that develop into new queens. As the queen ages, the secretion of queen substance diminishes: the workers then construct brood cells for future queens, which they feed exclusively with royal jelly.

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quinone

Any of various compounds derived from benzene and containing C=O groups in an unsaturated ring. An example is plastoquinone.

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R

race

1 (in biology) A category used in the classification of organisms that consists of a group of individuals within a species that are geographically, ecologically, physiologically, or chromosomally distinct from other members of the species. The term is frequently used in the same sense as subspecies. Physiological races , for example, are identical in appearance but differ in function. They include strains of fungi adapted to infect different varieties of the same crop species.

2 (in anthropology) A distinct human type possessing several characteristics that are genetically inherited. The major races of man are Mongoloid, Caucasian, Negroid, and Australoid.
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raceme

A type of racemose inflorescence in which the main flower stalk is elongated and bears stalked flowers. An example is the lupin. See also panicle.

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racemose inflorescence (indefinite inflorescence)

A type of flowering shoot (see inflorescence) in which the growing region at the tip of the flower stalk continues to produce new flower buds during growth. As a result, the youngest flowers are at the top and the oldest flowers are at the base of the stalk. In a flattened inflorescence, the youngest flowers are in the centre and the oldest flowers are on the outside. Types of racemose inflorescence include the capitulum, catkin, corymb, raceme, spadix, spike, and umbel (see illustration). Compare cymose inflorescence.

Types of racemose inflorescence

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rachis (rhachis)

1 The main axis of a compound leaf or an inflorescence.

2 The shaft of a feather.
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3 The backbone.
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rad

See radiation units.

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radial symmetry

The arrangement of parts in an organ or organism such that cutting through the centre of the structure in any direction produces two halves that are mirror images of each other. The stems and roots of plants usually show radial symmetry, while all animals belonging to the Cnidaria (e.g. jellyfish) and Echinodermata (e.g. starfish) are radially symmetrical – and typically sessile – in their adult form. The term actinomorphy is used to describe radial symmetry in flowers (e.g. a buttercup flower). Compare bilateral symmetry.

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radiation

1 Energy travelling in the form of electromagnetic waves or photons. See electromagnetic spectrum.

2 A stream of particles, especially alpha- or beta-particles from a radioactive source or neutrons from a nuclear reactor. See also background radiation.
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radiation damage

Harmful changes that occur to inanimate materials and living organisms as a result of exposure to energetic electrons, nucleons, fission fragments, or high-energy electromagnetic radiation. In organisms this can cause changes to cells that alter their genetic structure, interfere with their division, or kill them. In humans, these changes can lead to radiation sickness , radiation burns (from large doses of radiation), or to long-term damage of several kinds, the most serious of which result in various forms of cancer (especially leukaemia).

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radiation units

Units of measurement used to express the activity of a radionuclide and the dose of ionizing radiation.
The becquerel (Bq), the SI unit of activity, is the activity of a radionuclide decaying at a rate, on average, of one spontaneous nuclear transition per second. Thus 1 Bq = 1 s–1. The former unit, the curie (Ci), is equal to 3.7 × 10^10 Bq. The curie was originally chosen to approximate the activity of 1 gram of radium–226.
The gray (Gy), the SI unit of absorbed dose, is the absorbed dose when the energy per unit mass imparted to matter by ionizing radiation is 1 joule per kilogram. The former unit, the rad (rd), is equal to 10–2 Gy.
The sievert (Sv), the SI unit of dose equivalent, is the dose equivalent when the absorbed dose of ionizing radiation multiplied by the stipulated dimensionless factors is 1 Jkg–1. As different types of radiation cause different effects in biological tissue a weighted absorbed dose, called the dose equivalent , is used in which the absorbed dose is modified by multiplying it by dimensionless factors stipulated by the International Commission on Radiological Protection. The former unit of dose equivalent, the rem (originally an acronym for roentgen equivalent man), is equal to 10–2 Sv.
In SI units, exposure to ionizing radiation is expressed in coulombs per kilogram, the quantity of X- or gamma-radiation that produces ion pairs carrying 1 coulomb of charge of either sign in 1 kilogram of pure dry air. The former unit, the roentgen (R), is equal to 2.58 × 10–4 Ckg–1.

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radicle

The part of a plant embryo that develops into the root system. The tip of the radicle is protected by a root cap and points towards the micropyle. On germination it breaks through the testa and grows down into the soil. Compare plumule.

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radioactive age

The age of an archaeological or geological specimen as determined by a process that depends on a radioactive decay. See carbon dating; fission-track dating; potassium–argon dating; rubidium–strontium dating; uranium–lead dating.

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radioactive tracing

See labelling.

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radioactive waste (nuclear waste)

Any solid, liquid, or gaseous waste material that contains radionuclides (radioactive atomic nuclei). These wastes are produced in the mining and processing of radioactive ores, the normal running of nuclear power stations and other reactors, the manufacture of nuclear weapons, and in hospitals and research laboratories. Because high-level radioactive wastes can be extremely dangerous to all living matter and because they may contain radionuclides having half-lives of many thousands of years, their disposal has to be controlled with great stringency.
High-level waste (e.g. spent nuclear fuel) requires to be cooled artificially and is therefore stored for several decades by its producers before it can be disposed of. Intermediate-level waste (e.g. processing plant sludge and reactor components) is solidified, mixed with concrete, packed in steel drums, and stored in special sites at power stations before being buried in concrete chambers in deep mines or below the seabed. Low-level waste (e.g. solids or liquids lightly contaminated by radioactive substances) is disposed of in steel drums in special sites in concrete-lined trenches. In the UK, a company (Nirex Ltd) was set up by the nuclear industry and the government in 1988 to handle the disposal of nuclear waste. There are also nuclear reprocessing plants at Dounreay and Sellafield (Thermal Oxide Reprocessing Plant). Other countries have similar arrangements. Since 1983 no wastes have been disposed of in steel drums cast in concrete in the Atlantic deeps.

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radioactivity

The spontaneous disintegration of certain atomic nuclei accompanied by the emission of alpha-particles (helium nuclei), beta-particles (electrons or positrons), or gamma radiation (short-wavelength electromagnetic waves). Natural radioactivity is the result of the spontaneous disintegration of naturally occurring radioisotopes. The rate of disintegration is uninfluenced by chemical changes or any normal changes in their environment. However, radioactivity can be induced in many nuclides by bombarding them with neutrons or other particles. See also radiation units.

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radiobiology

The branch of biology concerned with the effects of radioactive substances on living organisms and the use of radioactive tracers to study metabolic processes (see labelling).

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radiocarbon dating

See carbon dating.

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radiography

The process or technique of producing images of an opaque object on photographic film or on a fluorescent screen by means of radiation (either particles or electromagnetic waves of short wavelength, such as X-rays and gamma-rays). The photograph produced is called a radiograph . The process is widely used in diagnostic radiology, using X-rays. See also autoradiography.

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radioimmunoassay (RIA)

A sensitive quantitative method for detecting trace amounts of a biomolecule, based on its capacity to displace a radioactively labelled form of the molecule from combination with its antibody. See also immunoassay.

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radioisotope (radioactive isotope)

An isotope of an element that is radioactive. See labelling.

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radiology

The study and use of X-rays, radioactive materials, and other ionizing radiations for medical purposes, especially for diagnosis (diagnostic radiology ) and the treatment of cancer and allied diseases (therapeutic radiology or radiotherapy ).

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radiometric dating (radioactive dating)

See dating techniques; radioactive age.

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radio-opaque (radiopaque)

Describing a medium that is opaque to X-rays and gamma rays. Examples are barium salts, used in diagnostic radiology of the digestive tract.

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radiotherapy

See radiology.

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radius

(in anatomy) The smaller of the two bones in the lower section of the forelimb of a tetrapod vertebrate (compare ulna). The radius articulates with some carpal bones and the ulna at the wrist and with the humerus at the elbow. This sophisticated articulation of the radius enables man (and some other animals) to twist the forearm (see pronation; supination).

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radula

A tonguelike organ of molluscs, consisting of a horny strip whose surface is studded with rows of horny teeth for rasping food. In some species it is modified for scraping or boring.

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rainforest

Any major terrestrial biome in which trees form the dominant plants and annual rainfall is high (over 200cm). Tropical rainforest is restricted to equatorial regions, such as the Amazon basin, central west Africa, and Southeast Asia. It is dominated by broadleaved evergreens and shows a very rich species diversity (see biodiversity). The leafy crowns of the trees typically form three layers of canopy, since the trees grow to different heights, which prevents much sunlight from reaching ground level. This limits the number of herbaceous plants and small shrubs that grow on the forest floor, but epiphytes, vines, and creepers are abundant. The average temperature is about 27°C, which – together with the high humidity – encourages rapid decomposition of leaf litter, releasing minerals that replace those leached from the soil by the heavy rain. If the forest canopy is removed, the soil is destroyed rapidly due to leaching by rain. The soil of a rainforest, known as latosol , is acidic and typically red, due to the oxidation of iron oxide (Fe_2O_3) in the topsoil. Rainforests are thought to contain many undiscovered plant species that could be of benefit in the fields of medicine and biotechnology. The continued destruction of rainforest in many parts of the world, particularly in South America and Southeast Asia (see deforestation), will not only result in the loss of these and other species but also contributes to the greenhouse effect.

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r.a.m.

See relative atomic mass.

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Ramapithecus

A genus of extinct primates that lived between 8 and 14 million years ago. Fossil remains of ramapithecines have been found in India and Pakistan, the Near East, and East Africa. Jaw fragments suggest that they chewed from side to side and had fairly short muzzles, both of which are humanoid features. They were possibly the earliest representatives of the hominid line after this split from the pongids (apes) during the Miocene. See also Dryopithecus; Australopithecus.

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Ramón y Cajal, Santiago

See Golgi, Camillo.

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ramus communicans

(pl. rami communicantes) A structure containing nerve fibres that runs from a spinal nerve to a ganglion of the sympathetic nervous system (see autonomic nervous system). The sympathetic ganglia form a series down each side of the vertebral column; each is connected to the ventral root of a spinal nerve by a pair of rami communicantes.

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rank (category)

The position or status of a taxon in a classification hierarchy. Examples of ranks are the class, order, family, genus, and species.

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raphe

1 A ridge on a seed marking the line of fusion between an anatropous (inverted) ovule and the funicle.

2 A groove on the valve of certain diatoms.
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3 Any other groove, ridge, or suture line marking the line of fusion between two originally separate structures.
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rapid eye movement (REM)

See sleep.

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rate-limiting step

The slowest step in a metabolic pathway or series of chemical reactions, which determines the overall rate of the other reactions in the pathway. In an enzymatic reaction, the rate-limiting step is generally the stage that requires the greatest activation energy or the transition state of highest free energy.

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Ratitae (Palaeognathae)

A group comprising the flightless birds, including the ostrich, kiwi, and emu. They have long legs, heavy bones, small wings, a flat breastbone, and curly feathers. These birds are thought to have descended from a variety of flying birds and are not representatives of a single homologous group.

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ray

1 (in optics) A narrow beam of radiation.

2 (in botany) See medullary ray.
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reabsorption

(in excretion) See selective reabsorption.

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reactant

See chemical reaction.

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reaction

See chemical reaction.

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reaction time (latent period)

The period of time between the detection of a stimulus at a sensory receptor and the performance of the appropriate response by the effector organ. This delay is caused by the time taken for the impulse to travel across the synapses of adjacent neurones. The reaction time for a reflex response, involving only a single linking synapse, is very short.

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reading frame

A sequence of bases in messenger RNA (or deduced from DNA) that encodes for a polypeptide. Since each coding unit (codon) of the genetic code consists of three consecutive bases, the reading frame is established according to precisely where translation starts. For example, if translation starts one base either side of the correct base, an entirely different sequence of codons will be read, resulting in a faulty polypeptide or none at all. The hallmark of a functional gene is that it is transcribed to produce an open reading frame (ORF), consisting of a start codon to pinpoint exactly where translation should start, a stop codon to signal termination of translation, and typically a long sequence of codons that specify the constituent amino acids of the polypeptide (as well as introns in most eukaryote genes).

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readthrough

The continuation of transcription of DNA beyond a normal stop signal, or terminator sequence, due to failure of RNA polymerase to recognize the signal. Readthrough can also occur in translation, when a mutation has converted a normal stop codon into one encoding an amino acid. This results in extension of the polypeptide chain until the next stop codon is reached, producing a so-called readthrough protein .

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recapitulation

The theory that some stages of evolution are repeated in the development of an individual organism, i.e. that phylogeny is repeated in ontogeny. It was proposed by Ernst Haeckel. See also acceleration.

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Recent

See Holocene.

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receptacle

1 (thalamus or torus) The tip of a flower stalk, which bears the petals, sepals, stamens, and carpels. The way the receptacle develops determines the position of the flower parts. It can be dilated and dome-shaped, saucer-shaped, or hollow and enclosing the gynoecium (see epigyny; hypogyny; perigyny). In some plants it may become part of the fruit (see pseudocarp).

2 A swollen part of the thallus of some algae, e.g. Fucus, that bears the conceptacles in which the sex organs are situated.
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receptor

1 A cell or group of cells specialized to detect a particular stimulus and to initiate the transmission of impulses via the sensory nerves. The eyes, ears, nose, skin, and other sense organs all contain specific receptors responding to external stimuli (see exteroceptor); other receptors are sensitive to changes within the body (see interoceptor). See also baroreceptor; chemoreceptor; electroreceptor; magnetoreceptor; mechanoreceptor; osmoreceptor; proprioceptor.

2 An area of a cell membrane, consisting of a specially adapted membrane protein, that can bind with a specific hormone, neurotransmitter, drug, or other chemical, thereby initiating a change within the cell. For example, the binding of protein hormones to receptors of target cells initiates the synthesis of a second messenger inside the cell.
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recessive

The allele that is not expressed in the phenotype when two different alleles are present in the cells of an organism. The aspect of a characteristic controlled by a recessive allele only appears when two such alleles are present, i.e. in the double recessive condition. Compare dominant.

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recipient

An individual who receives tissues or organs of the body from another (the donor).

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reciprocal cross

A cross reversing the roles of males and females to confirm the results obtained from an earlier cross. For example, if the pollen (male) from tall plants is transferred to the stigmas (female) of dwarf plants in one cross, the reciprocal cross would use the pollen of dwarf plants to pollinate the stigmas of tall plants.

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recognition site

See promoter.

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recombinant DNA

DNA that contains genes from different sources that have been combined by the techniques of genetic engineering rather than by breeding experiments. Genetic engineering is therefore also known as recombinant DNA technology . Recombinant DNA is formed during gene cloning or in the creation of genetically modified organisms.

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recombination

The rearrangement of genes that occurs when reproductive cells (gametes) are formed (see crossing over). Recombination results in offspring that have a combination of characteristics different from that of their parents. Recombination can also be induced artificially by genetic engineering techniques.

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recombinational repair

See postreplicative repair.

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recon

The smallest unit of DNA in which a mutation can occur. A recon consists of a single pair of nucleotides.

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recruitment

The activation of extra motor neurones in order to bring about an increased response to a stimulus that is present at an even intensity.

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rectal gland

A salt-secreting gland found in the cloaca of cartilaginous fishes.

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rectum

The portion of the alimentary canal between the colon and the anus. Its main function is the storage of faeces prior to elimination.

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recycling

1 The recovery and processing of materials after they have been used, which enables them to be reused. For example, used paper, cans, and glass can be broken down into their constituents, which form the raw materials for the manufacture of new products.

2 The continual movement of essential elements between the biotic (living) and abiotic (nonliving) components of the environment. See carbon cycle; nitrogen cycle; oxygen cycle; phosphorus cycle; sulphur cycle.
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red algae

See Rhodophyta.

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red blood cell

See erythrocyte.

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redox

See oxidation–reduction.

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Red Queen hypothesis

A hypothesis, proposed by L. M. Van Valen in the early 1970s, that describes how the coevolution of competing species creates a dynamic equilibrium, in which the probability of extinction remains fairly constant over time. Hence, evolution is seen neither as `progressive' – with a species' chances of survival improving over time – nor as `escalatory' – with increasing vulnerability to extinction over time. Instead, as one species evolves improvements that make it more competitive, its competitors experience selection pressures that force them to evolve in order to keep pace with it. Ones that lag too far behind will become extinct. The hypothesis is named after the remark made by the Red Queen in Lewis Carroll's Alice Through the Looking Glass: “Here, you see, it takes all the running you can do, to keep in the same place.”

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red tide

A sudden often toxic proliferation of marine phytoplankton, notably dinomastigotes, that colours the sea red, brown, or yellowish due to the high concentration of the organisms' photosynthetic accessory pigments. Some dinomastigotes, such as Gonyaulax, produce potent toxins, which may kill fish and invertebrates outright or accumulate in the food chain, posing a hazard to humans eating shellfish and other seafood. These phytoplankton blooms may be related to nutrient-rich inputs from the land, or upwelling oceanic waters, and are initiated by the activation of cystlike forms lying on the seabed.

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reducing sugar

A monosaccharide or disaccharide sugar that can donate electrons to other molecules and can therefore act as a reducing agent. The possession of a free ketone (–CO–) or aldehyde (–CHO) group enables most monosaccharides and disaccharides to act as reducing sugars. Reducing sugars can be detected by Benedict's test. Compare nonreducing sugar.

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reduction

See oxidation–reduction.

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reduction division

See meiosis.

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reflex

An automatic and innate response to a particular stimulus. A reflex response is extremely rapid. This is because it is mediated by a simple nervous circuit called a reflex arc , which at its simplest involves only a receptor linked to a sensory neurone, which synapses with a motor neurone (supplying the effector) in the spinal cord. Such reflexes are known as monosynaptic spinal reflexes ; an example is the stretch reflex. Other spinal reflexes involve more than one synapse (see polysynaptic reflex); an example is the withdrawal reflex of the hand from a painful stimulus (such as fire). Cranial reflexes are mediated by pathways in the cranial nerves and brain; examples are the blinking and swallowing reflexes. See also conditioning.

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reflex action

An automatic movement produced in response to a stimulus (see reflex).

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reflex arc

See reflex.

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reforestation

The replanting of trees on areas of land where forests have been cleared by felling or burning (see deforestation) or by natural means. Reforestation is particularly important in countries, such as Brazil, where large areas of forest have been destroyed by deforestation, although planted forest has much less species diversity (see biodiversity) than the original forest.

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refractory period

The period after the transmission of an impulse in a nerve or muscle in which the membrane of the axon or muscle fibre regains its ability to transmit impulses (see action potential). This period lasts approximately 3 milliseconds and is divided into an absolute refractory period , during which a second impulse may not be generated; and a relative refractory period , during which it is possible to generate an impulse only if there is an abnormally strong stimulus.

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regeneration

The growth of new tissues or organs to replace those lost or damaged by injury. Many plants can regenerate a complete plant from a shoot segment or a single leaf, this being the basis of many horticultural propagation methods (see cutting). The capacity for regeneration in animals is less marked. Some planarians and sponges can regenerate whole organisms from small pieces, and crustaceans (e.g. crabs), echinoderms (e.g. brittlestars), and some reptiles and amphibians can grow new limbs or tails (see autotomy), but in mammals regeneration is largely restricted to wound healing.

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regma

A dry fruit that is characteristic of the geranium family. It is similar to the carcerulus but breaks up into one-seeded parts, each of which splits open to release a seed.

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regulation

(in embryology) The processes involved in animal embryonic development that counteract any abnormalities that may arise in the different developmental stages. Regulative (or regulation) embryos or eggs can compensate for the removal of sections of the embryo or egg at an early stage of development so that subsequent development is not affected; in such embryos the direction of development of the cells is not determined until cleavage is well advanced. Regulation also includes twinning : the formation of two embryos from the cleavage of a single embryo (see twins).

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regulator

Any organism that can maintain a constant internal environment largely independently of the external environment. This is generally achieved by homeostatic mechanisms (see homeostasis). Regulators tend to occupy habitats in which environmental conditions are variable.

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regulator gene

See operon.

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regulatory enzyme

Any enzyme that is involved in controlling the different metabolic pathways in the cell by switching them on or off. Regulatory enzymes exist in active and inactive forms; they include allosteric enzymes and those enzymes whose activity is controlled by kinases.

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regulatory protein (gene-regulatory protein)

Any protein that influences the regions of a DNA molecule that are transcribed by RNA polymerase during the process of transcription. These proteins, which include transcription factors, therefore help control the synthesis of proteins in cells.

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reinforcement

(in animal behaviour) Increasing (or decreasing) the frequency of a particular behaviour through conditioning, by arranging for some biologically important event (the reinforcer ) always to follow another event. In instrumental conditioning an appetitive reinforcer , or reward (e.g. food), given after a response made by the animal, increases that response; an aversive reinforcer , or punishment (e.g. an electric shock) decreases the response.

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Reissner's membrane

The membrane in the cochlea of the inner ear that separates the vestibular canal from the cochlear duct. It is named after the German anatomist Ernst Reissner (1824–78).

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relative atomic mass (atomic weight; r.a.m.)

Symbol Ar. The ratio of the average mass per atom of the naturally occurring form of an element to 1/12 of the mass of a carbon–12 atom.

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relative density (r.d.)

The ratio of the density of a substance to the density of some reference substance. For liquids or solids it is the ratio of the density (usually at 20°C) to the density of water (at its maximum density). This quantity was formerly called specific gravity .

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relative growth rate

A measurement of the productivity of a plant, defined as the increase in dry mass per unit of plant mass over a specified period of time.

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relative molecular mass (molecular weight)

Symbol Mr. The ratio of the average mass per molecule of the naturally occurring form of an element or compound to 1/12 of the mass of a carbon–12 atom. It is equal to the sum of the relative atomic masses of all the atoms that comprise a molecule.

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relative refractory period

See refractory period.

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relaxin

A mammalian hormone produced by the corpus luteum and placenta during the terminal stages of pregnancy. It relaxes the pubic symphysis and dilates the cervix of the uterus, thereby aiding parturition.

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release factor (RF)

A protein that terminates translation of messenger RNA (mRNA) during protein synthesis and releases the completed polypeptide chain from the ribosome. It recognizes a stop codon in the mRNA and attaches to the A site of the ribosome (the binding site for an aminoacyl tRNA molecule), blocking further elongation of the polypeptide chain. Then the mRNA is released and the ribosome complex dissociates, releasing the polypeptide. Prokaryotes have two principal release factors, RF-1 and RF-2, whereas eukaryotes have just one, eRF.

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releaser

See sign stimulus.

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releasing factor (releasing hormone)

A hormone that is produced by the hypothalamus and stimulates the release of a hormone from the anterior pituitary gland into the bloodstream. Each hormone has a specific releasing factor; for example, thyrotrophin-releasing hormone stimulates the release of thyroid-stimulating hormone.

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rem

See radiation units.

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renal

Of or relating to the kidney. For example, the renal artery and renal vein convey blood towards and away from the kidney, respectively.

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renal capsule

See Bowman's capsule.

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renal tubule

Any of the sections of a nephron of the vertebrate kidney that are concerned with reabsorption of water and solutes from the glomerular filtrate. See also proximal convoluted tubule; loop of Henle; distal convoluted tubule.

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renaturation

The reconstruction of a protein or nucleic acid that has been denatured such that the molecule resumes its original function. Some proteins can be renatured by reversing the conditions (of temperature, pH, etc.) that brought about denaturation.

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renin

A proteolytic enzyme (see protease) that is involved in the formation of the hormone angiotensin, which raises blood pressure. Renin is secreted into the blood by cells of the kidney glomeruli under the control of the sympathetic nervous system; its release also occurs in response to a fall in blood-sodium levels and to falling blood pressure. Renin catalyses cleavage of the circulating precursor angiotensinogen to produce angiotensin I, precursor of the active hormone.

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rennin (chymosin)

An enzyme secreted by cells lining the stomach in mammals that is responsible for clotting milk. It acts on a soluble milk protein (caseinogen ), which it converts to the insoluble form casein. This ensures that milk remains in the stomach long enough to be acted on by protein-digesting enzymes.

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repetitive DNA

Sequences of DNA that appear repeatedly in chromosomal DNA. A considerable proportion of eukaryotic chromosomes consists of recurring nucleotide sequences – amounting to 25% of the genome in the mouse, for example. Some of this is not transcribed and appears to have no function (see selfish DNA). The most extreme form is satellite DNA, which occurs near the centromeres and telomeres. In the mouse this consists of around a million copies of a 200-nucleotide sequence and constitutes highly repetitive DNA . A second category, moderately (or intermediately ) repetitive DNA , consists of sequences that have some 10^3–10^5 copies in the genome. This includes multiple copies of transcribed genes, e.g. for ribosomal RNA, as well as nontranscribed sequences dispersed throughout the genome.

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replacing bone

See cartilage bone.

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replicon

A DNA sequence that is replicated as a unit from a single initiation site (origin of replication). The genome of a bacterium or a virus comprises a single replicon; eukaryotes contain a number of replicons on each chromosome.

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repolarization

The restoration of the resting potential in neurones or muscle fibres following the passage of a nerve impulse. Repolarization is brought about by diffusion of potassium ions out of the neurone and by active elimination of sodium ions (see sodium pump).

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reporter gene

A gene that is used to `tag' another gene or DNA sequence of interest, such as a promoter. Expression of the reporter is easily monitored, and permits the function or whereabouts of the `target' sequence to be tracked. It helps pinpoint which cells contain the tagged gene, for instance among a population of genetically engineered bacteria, or shows the varying degrees of expression of the tagged gene within different tissues of the body. For example, the ß-galactosidase gene (lacZ) is a common reporter gene whose activity can be detected on indicator plates by causing a colour change in a dye. One use of reporter genes is to investigate the function of `foreign' promoters in transgenic organisms. The promoter is inserted in a vector `upstream' of the reporter gene, and the vector is allowed to transfect the organism. How well and in what tissues the promoter functions can then be assessed by expression of the reporter. Another striking example, obtained from the jellyfish Aequorea victoria, is the reporter gene that encodes green fluorescent protein (GFP). Tissues in which the gene is expressed emit a green fluorescent light, making them easily identifiable.

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repressor

A protein that can prevent the expression of a gene. See operon.

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reproduction

The production of new individuals more or less similar in form to the parent organisms. This may be achieved by a number of means (see sexual reproduction; asexual reproduction) and serves to perpetuate or increase a species.

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reproductive system

The organs that are involved in the process of sexual reproduction in an organism. The reproductive system of a flowering plant is found in the flower and consists of the stamens (male organs) and carpels (female organs). In mammals the reproductive system consists of the testes, epididymis, sperm duct, and penis in the male and the ovaries, fallopian tubes, and uterus in the female (see illustration).

Human male reproductive system

Human female reproductive system

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Reptilia

The class that contains the first entirely terrestrial vertebrates, which can live in dry terrestrial habitats as their skin is covered by a layer of horny scales, preventing water loss. They breathe atmospheric oxygen by means of lungs assisted by respiratory movements principally involving the ribs (there is no diaphragm). Reptiles are cold-blooded (see poikilothermy) but behavioural patterns make it possible for them to maintain a fairly even body temperature throughout the day. Fertilization is internal and the majority of reptiles lay eggs on land. These eggs have a porous shell to provide protection from desiccation and allow gas exchange. In some reptiles the eggs are retained within the body of the mother until the young are ready to hatch, thereby greatly reducing juvenile mortality (see ovoviviparity).
The class includes the modern crocodiles, lizards and snakes (see Squamata), and tortoises and turtles, as well as many extinct forms, such as the dinosaurs and Pterosauria.

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residual volume

The amount of air remaining in the lungs after maximum expiration, which cannot be expelled from the lungs voluntarily. An average human has a residual volume of about 1 litre. See also vital capacity.

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resin

A naturally occurring acidic polymer secreted by many trees (especially conifers) into ducts or canals. Resins are found either as brittle glassy substances or dissolved in essential oils. Their functions are probably similar to those of gums and mucilages, i.e. protective.

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resistance

1 (in microbiology) The degree to which pathogenic microorganisms remain unaffected by antibiotics and other drugs. Genes for antibiotic resistance are often carried on plasmids or transposons, which can spread across species barriers.

2 (in ecology)
a The degree to which a pest can withstand the effects of a pesticide. It depends on the selection and spread within a pest population of genes that confer the ability to destroy, or minimize the effects of, a pesticide.
b See environmental resistance.
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3 (in immunology) The degree of immunity to infection that an animal possesses.
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resistance response

A long-term response to any stimulus that potentially threatens the wellbeing of an organism. An example of a resistance response is the release of ACTH (adrenocorticotrophic hormone) in response to stress. This hormone stimulates the production of mineralocorticoid hormones (e.g. aldosterone), which promote the excretion of hydrogen ions produced in excess as a result of the increased metabolic activity resulting from the alarm response. Resistance responses diminish as normal conditions resume.

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resolving power

A measure of the ability of an optical instrument to form separable images of close objects or to separate close wavelengths of radiation. The resolving power of a microscope is usually taken as the minimum distance between two points that can be separated; the smaller the resolving power, the better the resolution.

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respiration

The metabolic process in animals and plants in which organic substances are broken down to simpler products with the release of energy, which is incorporated into special energy-carrying molecules (see ATP) and subsequently used for other metabolic processes. In most plants and animals respiration requires oxygen, and carbon dioxide is an end product. The exchange of oxygen and carbon dioxide between the body tissues and the environment is called external respiration (see ventilation). In many animals the exchange of gases takes place at respiratory organs (e.g. lungs in air-breathing vertebrates) and is assisted by respiratory movements (e.g. breathing). In plants oxygen enters through pores on the plant surface and diffuses through the tissues via intercellular spaces or dissolved in tissue fluids.
Respiration at the cellular level is known as internal (cellular or tissue ) respiration and can be divided into two stages. In the first, glycolysis, glucose is broken down to pyruvate. This does not require oxygen and is a form of anaerobic respiration. In the second stage, the Krebs cycle, pyruvate is broken down by a cyclic series of reactions to carbon dioxide and water. This is the main energy-yielding stage and requires oxygen. The processes of glycolysis and the Krebs cycle are common to all plants and animals that respire aerobically (see aerobic respiration).

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respiratory chain

See electron transport chain.

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respiratory movement

The muscular movement that enables the passage of air to and from the lungs or other respiratory organs of an animal. The mechanism of the movement varies with the species. In insects abdominal muscles relax and contract rhythmically to encourage the flow of air through the tracheae. In amphibians air is drawn into the lungs by a pumping action of the muscles in the floor of the mouth. Breathing in mammals involves the muscle of the diaphragm and the intercostal muscles between the ribs. Contraction of these muscles lowers the diaphragm and raises the ribs, so that the lungs expand and air is drawn in (see inspiration). Relaxation has the opposite effect and forces air out during expiration.

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respiratory organ

Any animal organ across which exchange of carbon dioxide and oxygen takes place. The surface membranes of such organs are always moist, thin, and well supplied with blood. Examples are the lungs of air-breathing vertebrates, the gills of fish, the tracheae of insects, and the lung books of arachnids.

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respiratory pigment

1 A coloured compound that is capable of reversibly binding with oxygen at high oxygen concentrations and releasing it at low oxygen concentrations. Most such pigments are present in the blood (blood pigments ), transporting oxygen within the circulatory system from the respiratory organs to the tissues of the body; an exception is myoglobin, which occurs in the muscles. In vertebrates the respiratory pigment is haemoglobin, contained in the erythrocytes (red blood cells). See also chlorocruorin; haemocyanin; haemoerythrin.

2 Any of the proteins involved in cellular respiration as components of an electron transport chain. They include the cytochromes.
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respiratory quotient (RQ)

The ratio of the volume of carbon dioxide produced by an organism during respiration to the volume of oxygen consumed. The RQ is usually about 0.8.

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respirometer

Any device that measures an organism's oxygen uptake. Simple respirometers consist of a chamber (in which the organism is placed) connected to a manometer. Carbon dioxide is chemically removed from the chamber so that only oxygen uptake is measured. Human oxygen consumption is generally measured by a device known as a spirometer , which can also be used to measure depth and frequency of breathing.

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response

The physiological, muscular, or behavioural activity that can be elicited by a stimulus.

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resting potential

The difference in electrical potential that exists across the membrane of a nerve cell that is not in the process of transmitting a nerve impulse. The resting potential is maintained by means of the sodium pump. Compare action potential.

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restriction enzyme (restriction endonuclease)

A type of enzyme that can cleave molecules of foreign DNA at a particular site. Restriction enzymes are produced by many bacteria and protect the cell by cleaving (and therefore destroying) the DNA of invading viruses. The bacterial cell is protected from attack by its own restriction enzymes by modifying the bases of its DNA during replication. Restriction enzymes are widely used in the techniques of genetic engineering (see DNA fingerprinting; gene cloning; gene library; gene sequencing; restriction mapping).

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restriction fragment length polymorphism (RFLP)

The occurrence of different cleavage sites for restriction enzymes in the DNA of different individuals of the same species. Cleavage of DNA from different individuals with restriction enzymes thus produces differing sets of restriction fragments. The deletion of existing restriction sites or the creation of new ones is the result of random base changes in the noncoding stretches of DNA (introns) between genes. RFLPs have provided geneticists with a powerful set of genetic markers for mapping the genome (see restriction mapping) and for identifying particular genes (see gene tracking).

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restriction mapping

A technique for determining the sites at which a length of DNA (e.g. from a chromosome) is cleaved by restriction enzymes. By cleaving the DNA with various such enzymes, both individually and in combination, and analysing the resultant number and size of fragments by electrophoresis, a restriction map , indicating the order of restriction sites in the original DNA, can be deduced. This can then be integrated with a classical linkage map (see chromosome map). Restriction mapping is routinely applied to organisms undergoing genetic investigation. Gene deletions or rearrangements that alter the restriction sites can be detected as changes in the pattern of fragments obtained. This may be used, for instance, to diagnose certain genetic abnormalities in the fetus. The fragments are separated by gel electrophoresis and identified using specific gene probes, as in the Southern blotting technique. The absence of a certain fragment in a fetal DNA digest can be diagnostic of a pathological change in the fetal gene containing the corresponding restriction site.

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restriction point

See cell cycle.

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reticular formation

See brainstem.

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reticuloendothelial system (mononuclear phagocyte system)

The network of macrophages and their precursors (monocytes) in the body's tissues, which is concentrated in the bone marrow, liver, spleen, and lymph nodes.

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reticulum

The first of four chambers that form the stomach of ruminants. See Ruminantia.

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retina

The light-sensitive membrane that lines the interior of the eye. The retina consists of two layers. The outer layer (pigment epithelium ) is pigmented, which prevents the back reflection of light and consequent decrease in visual acuity. The inner layer contains nerve cells, blood vessels, and two types of light-sensitive cells (rods and cones). Light passing through the lens stimulates individual rods and cones, which generates nerve impulses that are transmitted through bipolar and ganglion cells to the optic nerve, and hence to the brain, where the visual image is formed. Information can also be transferred horizontally within the retina via a network of horizontal and amacrine cells (see illustration).

Structure of the retina

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retinal (retinene)

See rhodopsin; vitamin A.

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retinol

See vitamin A.

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retrotransposon

A type of transposon found in the DNA of various organisms, including yeast, Drosophila, and mammals, that forms copies of itself using a mechanism similar to that of retroviruses. It undergoes transcription to RNA, then creates a DNA copy of the transcript with the aid of the enzyme reverse transcriptase. This DNA copy can then reintegrate into the cell's genome.

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retrovirus

An RNA-containing virus that converts its RNA into DNA by means of the enzyme reverse transcriptase; this enables it to become integrated into its host's DNA. Retroviruses can cause cancer in animals: they contain oncogenes (cancer-causing genes), which are activated when the virus enters its host cell and starts to replicate. The special properties of retroviruses make them potentially useful as vectors for inserting genetic material into eukaryotic cells. See also provirus.

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reverse transcriptase

An enzyme, occurring in retroviruses, that catalyses the formation of double-stranded DNA using the single RNA strand of the viral genome as template. This enables the viral genome to be inserted into the host's DNA and replicated by the host. Reverse transcriptase is thus an RNA-directed DNA polymerase. The enzyme is used in genetic engineering for producing complementary DNA from messenger RNA.

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reversion

(in genetics) The restoration of a mutation to the wild-type genotype or phenotype by a second mutation. The gene or organism affected in this way is called a revertant .

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RF

See release factor.

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RFLP

See restriction fragment length polymorphism.

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RF value

(in chromatography) The distance travelled by a given component divided by the distance travelled by the solvent front. For a given system at a known temperature, it is a characteristic of the component and can be used to identify components. For example, the photosynthetic pigments of an organism and the metabolites of a drug excreted in the urine can be identified by their RF values in paper or thin-layer chromatography.

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Rh

See rhesus factor.

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rhabdom

See compound eye.

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rhachis

See rachis.

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rhesus factor (Rh factor)

An antigen whose presence or absence on the surface of red blood cells forms the basis of the rhesus blood group system. (The factor was first recognized in rhesus monkeys.) Most people possess the Rh factor, i.e. they are rhesus positive (Rh+). People who lack the factor are Rh–. If Rh+ blood is given to an Rh– patient, the latter develops anti-Rh antibodies. Subsequent transfusion of Rh+ blood results in agglutination, with serious consequences. Similarly, an Rh– pregnant woman carrying an Rh+ fetus may develop anti-Rh antibodies in her blood; these will react with the blood of a subsequent Rh+ fetus, causing anaemia in the newborn baby.

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rhizoid

One of a group of delicate and often colourless hairlike outgrowths found in certain algae and the gametophyte generation of bryophytes and ferns. They anchor the plant to the substrate and absorb water and mineral salts.

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rhizome

A horizontal underground stem. It enables the plant to survive from one growing season to the next and in some species it also serves to propagate the plant vegetatively. It may be thin and wiry, as in couch grass, or fleshy and swollen, as in Iris. Compact upright underground stems, as in rhubarb, strawberry, and primrose, are often called rootstocks .

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Rhizopoda

A phylum of the Protoctista that contains the amoebas and cellular slime moulds. They are characterized by the possession of pseudopodia, which are used for locomotion and engulfing food particles. Rhizopods are found in freshwater and marine habitats and the soil. The amoebas reproduce by binary fission, whereas the cellular slime moulds aggregate into a slimy mass that produces spores. See also Amoeba; protozoa.

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Rhodophyta (red algae)

A phylum of algae that are often pink or red in colour due to the presence of the pigments phycocyanin and phycoerythrin. Members of the Rhodophyta may be unicellular or multicellular; the latter form branched flattened thalli or filaments. They are commonly found along the coasts of tropical areas. Sexual reproduction is by means of carpospores (see carpogonium).

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rhodopsin (visual purple)

The light-sensitive pigment found in the rods of the vertebrate retina. It consists of a protein component, opsin , linked to a nonprotein chromophore, retinal (or retinene ), a derivative of vitamin A. Light falling on the rod is absorbed by the retinal, which changes its form and separates from the opsin component; this initiates the transmission of a nerve impulse to the brain. The great sensitivity of rhodopsin allows vision in dim light (night vision). See also dark adaptation.

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rhombencephalon

See hindbrain.

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rhytidome

See bark.

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rib

One of a series of slender curved bones that form a cage to enclose, support, and protect the heart and lungs (see thorax). Ribs occur in pairs, articulating with the thoracic vertebrae of the spinal column at the back and (in reptiles, birds, and mammals) with the sternum (breastbone) in front. Movements of the rib cage, controlled by intercostal muscles between the ribs, are important in breathing (see respiratory movement).

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riboflavin

See vitamin B complex.

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ribonuclease

See RNAase.

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ribonucleic acid

See RNA.

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ribonucleoprotein (RNP)

Any complex of protein and RNA that forms during the synthesis of RNA in eukaryotes; the protein is involved in the packaging and condensation of the RNA. Certain RNPs are restricted to the nucleus whereas others are found in both the nucleus and the cytoplasm. The most common RNP occurring in the nucleus is heterogeneous nuclear RNP (hnRNP), which consists of protein bound to the primary transcript of DNA (see transcription). It may be associated with small nuclear RNP (snRNP), which is involved in the removal of intron sequences from the primary transcript to form messenger RNA, which eventually leaves the nucleus (see gene splicing). A third type, small cytoplasmic RNP (scRNP), is found in the cytoplasm; its function is unknown.

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ribose

A monosaccharide, C_5H_10O_5, rarely occurring free in nature but important as a component of RNA (ribonucleic acid). Its derivative deoxyribose , C_5H_10O_4, is equally important as a constituent of DNA (deoxyribonucleic acid), which carries the genetic code in chromosomes.

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ribosomal RNA

See ribosome; RNA.

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ribosome

A small spherical body within a living cell that is the site of protein synthesis. Ribosomes consist of two subunits, one large and one small, each of which comprises a type of RNA (called ribosomal RNA ) and protein. Ribosomes are described in terms of their sedimentation coefficients (i.e. their rates of sedimentation in an ultracentrifuge), which are measured in Svedberg units (symbol S). The prokaryote (70S) ribosome comprises a 50S (large) subunit and a 30S (small) subunit; the eukaryote (80S) ribosome has large 60S and small 40S subunits. Usually there are many ribosomes in a cell, either attached to the endoplasmic reticulum or free in the cytoplasm. During protein synthesis they are associated with messenger RNA as polyribosomes in the process of translation.

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ribozyme (catalytic RNA)

Any RNA molecule that can catalyse changes to its own molecular structure. Self-splicing introns (see gene splicing) are examples of ribozymes.

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ribulose

A ketopentose sugar (see monosaccharide), C_5H_11O_5, that is involved in carbon dioxide fixation in photosynthesis as a component of ribulose bisphosphate.

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ribulose bisphosphate (RuBP)

A five-carbon sugar that is combined with carbon dioxide to form a six-carbon intermediate in the first stage of the dark reaction of photosynthesis (see Calvin cycle). The enzyme that mediates the carboxylation of ribulose bisphosphate, ribulose bisphosphate carboxylase/oxygenase (rubisco ), is also involved in photorespiration.

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rickets

A childhood condition caused by decalcification of bone, resulting in deformed bones. Rickets is associated with chronic deficiency of vitamin D or calcium and with disorders that cause poor phosphate reabsorption from the kidney nephrons.

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rickettsia

A very small coccoid or rod-shaped Gram-negative bacterium belonging to the phylum Proteobacteria. With one exception, rickettsias are obligate parasites, being unable to reproduce outside the cells of their hosts. Rickettsias can infect such arthropods as ticks, fleas, lice, and mites, through which they can be transmitted to vertebrates, including humans. The group includes the causal agents of Q fever, Rocky Mountain spotted fever, and forms of typhus. The only genus that can be grown in culture outside host cells is Rochalimaea, which includes R. quintana, the causal agent of trench fever.

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rigor mortis

The stiffening of the body of an animal after death, due to a temporary rigidity of the muscles. This condition arises because ATP, which is no longer synthesized after death, is required to break down the bridges that form between actin and myosin filaments in muscle tissue during contraction.

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Ringer's solution

See physiological saline.

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ring species

Two species with a looped or ringlike distribution pattern, for example circumpolar, which comprises a series of interbreeding forms that are intermediate between the two species. The latter occur where the two ends of the `ring' meet. Ring species thus demonstrate how the evolution of differences among the different populations or subspecies leads eventually to the formation of new species.

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ritualization

An evolutionary process in which the form or context of an action is altered because it comes to play a role in social communication. For example, many courtship and greeting ceremonies in animals include ritual food presentation (though the quantities of food may be negligible), derived from the action of feeding the young.

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river continuum concept

The concept of a river as an ecosystem whose character changes continually between its source and its mouth according to the nature of its energy inputs. The headwaters are likely to be narrow, fast flowing, and shaded by trees and other vegetation, so that virtually all the energy enters in the form of leaves, twigs, and other debris from the surroundings. The fauna is dominated by detritivores and filter feeders. Further downstream, as the river broadens and becomes less shaded, it is colonized by algae and plants, which contribute significant energy to the community and are exploited by grazers. Towards the mouth, increased sediment loading reduces light penetration and in situ photosynthesis may decline. Energy, in the form of biomass and detritus, is constantly flowing downstream, hence the energetics of any particular section of the river are influenced by events upstream. The result is a longitudinal continuum of ecosystem structure, with certain predictable properties.

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RNA (ribonucleic acid)

A complex organic compound (a nucleic acid) in living cells that is concerned with protein synthesis. In some viruses, RNA is also the hereditary material. Most RNA is synthesized in the nucleus and then distributed to various parts of the cytoplasm. An RNA molecule consists of a long chain of nucleotides in which the sugar is ribose and the bases are adenine, cytosine, guanine, and uracil (see illustration; compare DNA). Messenger RNA (mRNA) is responsible for carrying the genetic code transcribed from DNA to specialized sites within the cell (known as ribosomes), where the information is translated into protein composition (see transcription; translation). Ribosomal RNA (rRNA) is present in ribosomes; it is single-stranded but helical regions are formed by base pairing within the strand. Transfer RNA (tRNA, soluble RNA , sRNA) is involved in the assembly of amino acids in a protein chain being synthesized at a ribosome. Each tRNA is specific for an amino acid and bears a triplet of bases complementary with a triplet on mRNA (see anticodon; elongation).

Molecular structure of RNA

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RNAase (RNase; ribonuclease)

Any enzyme that catalyses the cleavage of nucleotides in RNA. Each RNAase has a specificity for a different cleavage site. For example, RNAase A is a digestive enzyme secreted by the pancreas that hydrolyses phosphodiester bonds in the nucleotide chain. Other RNAases are active at the cellular level, for instance in modifying transfer RNA and ribosomal RNA after transcription.

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RNA polymerase

See polymerase.

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RNP

See ribonucleoprotein.

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rod

A type of light-sensitive receptor cell present in the retinas of vertebrates. Rods contain the pigment rhodopsin and are essential for vision in dim light. They are not evenly distributed on the retina, being absent in the fovea and occupying all of the retinal margin. See also dark adaptation. Compare cone.

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Rodentia

An order of mammals characterized by a single pair of long curved incisors in each jaw. These teeth are specialized for gnawing: they continue growing throughout life and have enamel only on the front so that they wear to a chisel-shaped cutting edge. Rodents often breed throughout the year and produce large numbers of quickly maturing young. The order includes the squirrels, beavers, rats, mice, and porcupines.

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roentgen

The former unit of dose equivalent (see radiation units). It is named after the discoverer of X-rays, W. K. Roentgen (1845–1923).

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root

1 (in botany) The part of a vascular plant that grows beneath the soil surface in response to gravity and water. It anchors the plant in the soil and absorbs water and mineral salts. Unlike the stem, it never produces leaves, buds, or flowers and never contains chlorophyll. The radicle (embryonic root) may give rise either to a tap root system with a single main tap root from which lateral roots develop, or a fibrous root system , with many roots of equal size. The apical meristem at the root tip gives rise to a protective sheath, the root cap, and to the primary tissues of the root. The vascular tissues usually form a central core (see illustration). This distinguishes roots from stems, in which the vascular tissue often forms a ring. A short distance behind the root tip root hairs develop from the epidermis and greatly increase the surface area for absorption of water and minerals. Beyond this, lateral roots develop.
Roots may be modified in various ways. Some are swollen with food to survive the winter, as in the carrot. Certain plants, such as orchids, have absorptive aerial roots; others, such as ivy, have short clasping roots for climbing. The roots of leguminous plants, such as beans and peas, contain root nodules, which have an important role in nitrogen fixation. Other modifications include prop roots, stilt roots, and buttress roots, which support the plant; and pneumatophores.

Section through the tip of a plant root

2 (in dentistry) The portion of a tooth that is not covered with enamel and is embedded in a socket in the jawbone. Incisors, canines, and premolars have single roots; molars normally have several roots.
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3 (in anatomy) The point of origin of a nerve in the central nervous system. There are two roots for every spinal nerve (see dorsal root; ventral root).
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root cap (calyptra)

A cone-shaped structure that covers the root tip and develops as a result of cell division by a meristem at the root apex (see calyptrogen). It protects the root tip as it grows between the soil particles. The cells are constantly worn away by friction and are replaced by the meristem.

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root hair

See root.

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root nodule

A swelling on the roots of certain plants, especially those of the family Fabaceae (Leguminosae), that contains bacteria (notably Rhizobium) capable of fixing atmospheric nitrogen into ammonia, which is subsequently converted to nitrates and amino acids (see bacteroid; nitrogen fixation). Plants that possess root nodules increase soil fertility by increasing the nitrate content of the soil. The practice of crop rotation will normally include the cultivation of a leguminous species. Certain nonleguminous plants, such as alder (Alnus) and bayberry (Myrica), also form root nodules, although the bacteria are filamentous actinomycetes (Frankia), not rhizobia.

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root pressure

The pressure that forces water, absorbed from the soil, to move through the roots and up the stem of a plant. This pressure can be demonstrated by cutting a stem, from which water will exude. A manometer can be attached to a plant stem to measure the root pressure. Root pressure is believed to be due to both the osmosis of water, from the soil into the root cells, and the active pumping of salts into the xylem tissue, which maintains a concentration gradient along which the water will move. See also transpiration.

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rootstock

See rhizome.

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Rotifera

A phylum of microscopic (0.04–2.00mm) pseudocoelomate aquatic animals characterized by a crown of cilia at the head end. These are used in locomotion and in some species for feeding: the crown resembles a rotating wheel when the cilia are beating. Rotifers possess jaws and are covered with a layer of chitin (the lorica ). There is no circulatory system and gas exchange occurs across the body surface. Some rotifers reproduce by parthenogenesis.

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roughage

See (dietary) fibre.

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rough endoplasmic reticulum (rough ER)

See endoplasmic reticulum.

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Roundup

See glyphosate.

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round window (fenestra rotunda)

A membrane-covered opening between the middle ear and the inner ear (see ear), situated below the oval window. Pressure waves transmitted through the perilymph in the cochlea are released into the middle ear through the round window, which bulges out into the middle-ear cavity.

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roundworms

See Nematoda.

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royal jelly

A highly nutritious food substance that is secreted from the hypopharyngeal and mandibular glands of worker honeybees and fed to the newly hatched larvae. For the first three days after hatching, larvae that will develop into worker bees are fed on royal jelly, after which their diet changes to pollen and nectar. However, larvae that will develop into queen bees continue to be fed exclusively on royal jelly throughout the larval stage.

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RQ

See respiratory quotient.

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rRNA

See RNA.

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r selection

A type of selection that favours organisms with a high rate of reproduction (r value ). Organisms that are r selected (r strategists ) are able to colonize a habitat rapidly, utilizing the food and other resources before other organisms are established and begin to compete. The r strategists tend to be relatively small organisms with short life spans (e.g. bacteria) and often live in temporary or unstable environments; characteristically their survival depends on their ability to produce large numbers of offspring rather than on their ability to compete. Compare K selection. See survival curve.

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rubidium–strontium dating

A method of dating geological specimens based on the decay of the radioisotope rubidium–87 into the stable isotope strontium–87. Natural rubidium contains 27.85% of rubidium–87, which has a half-life of 4.7 × 10^11 years. The ratio 87Rb/87Sr in a specimen gives an estimate of its age (up to several thousand million years).

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rubisco

See ribulose bisphosphate.

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ruderal

Describing a plant species that is characteristic of land with a high level of disturbance (e.g. resulting from environmental disaster or human activity) but rich in resources (water, nutrients, etc.). Such species tend to quickly colonize bare patches and reproduce rapidly, but are poor competitors against more robust species. Compare fugitive species.

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rumen

The second of four chambers that form the stomach of ruminants. See Ruminantia.

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Ruminantia

A suborder of hooved mammals (see Artiodactyla) comprising the sheep, cattle, goats, deer, and antelopes. They are characterized by a four-chambered stomach (see illustration). The oesophagus empties into the first and most forward chamber, the reticulum . This communicates freely with the second and largest chamber, the rumen . Here food is fermented by the large population of anaerobic bacteria and protoctists; cellulose and other normally indigestible plant materials are broken down by the microbial enzymes, such as cellulase. Periodically the animal regurgitates material from the rumen and chews it before swallowing it once again; this process, known as `chewing the cud', helps to break down fibrous food. The digesta enters the third chamber, the omasum , from the reticulum. Here water and some nutrients are absorbed, before the contents pass to the fourth and final chamber, the abomasum which functions rather like a normal stomach, secreting acidic abomasal juice and digestive enzymes from its walls.

Section of the stomach of a ruminant

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runner

A stem that grows horizontally along the soil surface and gives rise to new plants either from axillary or terminal buds. Runners are seen in the creeping buttercup and the strawberry. Offsets , e.g. those of the houseleek, are short runners.

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rusts

A group of parasitic fungi of the phylum Basidiomycota. Many of these species attack the leaves and stems of cereal crops: characteristic rust-coloured streaks of spores appear on infected plants. The life cycles of some rusts may be complex; many form a number of different types of spore and some require two different host plants. Compare smuts.

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S

saccharide

See sugar.

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Saccharomyces

An industrially important genus of yeasts. S. cerevisiae, of which there are at least 1000 strains, is used in baking (see baker's yeast), brewing, and wine making; it is also used in the production of single-cell protein and ergosterol, a steroid. The other main yeast used in the production of beer is S. uvarum (or carlsbergensis); it is distinguished from S. cerevisiae by its ability to ferment the disaccharide melibose using -galactosidase, an enzyme not produced by S. cerevisiae.

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saccharose

See sucrose.

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sacculus (saccule)

A chamber of the inner ear from which the cochlea arises in reptiles, birds, and mammals. It bears patches of sensory epithelium concerned with balance (see macula).

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sacral vertebrae

The vertebrae that lie between the lumbar and the caudal vertebrae in the vertebral column. The function of the sacral vertebrae is to articulate securely with the pelvic girdle, and they are usually fused to form a single bone (the sacrum ) to provide a firm support. The number of sacral vertebrae varies from animal to animal. Amphibians have a single sacral vertebra, reptiles have two, and mammals have three or more.

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safranin

A stain used in optical microscopy that colours lignified tissues, cutinized tissues, and nuclei red and chloroplasts pink. It is used mainly for plant tissues, in conjunction with a green or blue counterstain.

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sagittal

Describing a section through an organism that is bisected in the longitudinal plane.

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salicylic acid (1-hydroxybenzoic acid)

A naturally occurring carboxylic acid, HOC_6H_4COOH, found in certain plants. It is used in making aspirin and in the foodstuffs and dyestuffs industries.

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saline

Describing a chemical compound that is a salt, or a solution containing a salt. See also physiological saline.

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salinization

An increase in the salt content of the soil (or of a body of fresh water), which can lead to the retardation of plant growth and eventually renders the soil infertile. This problem is particularly acute in hot areas, where water readily evaporates from the soil, and it may also occur as a consequence of irrigation.

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saliva

A watery fluid secreted by the salivary glands in the mouth. Production of saliva is stimulated by the presence of food in the mouth and also by the smell or thought of food. Saliva contains mucin, which lubricates food and eases its passage into the oesophagus, and in some animals salivary amylase (or ptyalin), which begins the digestion of starch. The saliva of insects is rich in digestive enzymes, and that of bloodsucking animals contains an anticoagulant.

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salivary glands

Glands in many terrestrial animals that secrete saliva into the mouth. In man there are three pairs: the sublingual, submandibular, and the submaxillary glands. The salivary gland cells of some insect larvae produce giant chromosomes (see polyteny), which are widely used in the study of genetics and protein synthesis.

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Salmonella

A genus of rod-shaped Gram-negative bacteria that inhabit the intestine and cause disease (salmonellosis ) in humans and animals. They are aerobic or facultatively anaerobic, and most are motile. Salmonellae can exist for long periods outside their host, and may be found, for example, in sewage and surface water. Humans may become infected by consuming contaminated water or food, especially animal products, such as eggs, meat, and milk, or vegetables that have been fertilized with contaminated manure. The bacteria can also be transmitted from human or animal carriers by unhygienic food preparation. Various species of Salmonella cause gastroenteritis and septicaemia; typhoid fever and paratyphoid fever are caused by S. typhi and S. paratyphi, respectively.

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salt

A compound formed by reaction of an acid with a base, in which the hydrogen of the acid has been replaced by metal or other positive ions. Typically, salts are crystalline ionic compounds, such as Na+Cl– (sodium chloride).

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samara

A dry single-seeded indehiscent fruit in which the fruit wall hardens and extends to form a long membranous winglike structure that aids dispersal. Examples are ash and elm fruits. The sycamore fruit is a double samara and technically a schizocarp. See also achene.

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sampling

The selection of small groups of entities to represent a large number of entities in statistics. In random sampling each individual of a population has an equal chance of being selected as part of the sample. In stratified random sampling , the population is divided into strata, each of which is randomly sampled and the samples from the different strata are pooled. In systematic sampling , individuals are chosen at fixed intervals; for example, every tenth animal in a population. In sampling with replacement , each individual chosen is replaced before the next selection is made.

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Sanger, Frederick

(1918–)
British biochemist who worked at Cambridge University and the Medical Research Council. He was awarded two Nobel Prizes for chemistry. The first was for his discovery of the amino-acid sequence in bovine insulin, which enabled insulin to be synthesized. His second Nobel Prize was for discovering the sequence of 5400 nucleotides in a strand of viral DNA. His technique for sequencing nucleotides has been widely applied (see gene sequencing).

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sap

1 The sugary fluid that is found in the phloem tissue of plants. Sap is the medium in which carbohydrates, produced in photosynthesis, and other organic molecules are transported and stored in plants.

2 (cell sap) The fluid that is contained in the vacuoles of plant cells. It is a solution of organic and inorganic compounds, including sugars, amino acids, salts, pigments, and waste products.
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saponin

Any of a class of glycosides, found widely in plants, that have detergent properties and form a lather when shaken with water. They are especially concentrated in the soapwort (Saponaria officinalis), whose foliage was formerly boiled and used as a soap substitute. Chemically saponins consist of a sugar group (e.g. glucose) linked to a steroid or triterpene group; a related group of compounds, the sapogenins , have no sugar group. Their presence in plants is thought to act as a deterrent to herbivores – they are bitter-tasting and cause gastric irritation if ingested. They are also highly toxic to fish. If injected into the bloodstream they disrupt red cells, through their effects on cell membranes.

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saprotroph (saprobe; saprobiont)

Any organism that feeds by absorbing dead organic matter. Most saprotrophs are bacteria and fungi. Saprotrophs are important in food chains as they bring about decay and release nutrients for plant growth. Compare parasitism.

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sapwood (alburnum)

The outer wood of a tree trunk or branch. It consists of living xylem cells, which both conduct water and provide structural support. Compare heartwood.

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sarcolemma

The contractile membrane that surrounds a muscle fibre. See also transverse tubules.

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sarcoma

See cancer.

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sarcomere

Any of the functional units that make up the myofibrils of voluntary muscle. Each sarcomere is bounded by two membranes (Z lines), which provide the points of attachment of actin filaments; another membrane (the M band or line ) is the point of attachment of the myosin filaments. The sarcomere is divided into various bands reflecting the arrangement of the filaments (see illustration). During muscle contraction the actin and myosin filaments slide over each other (see sliding filament theory) and the length of the sarcomere shortens: the Z lines are drawn closer together and the I and H bands become narrower.

Structure of a sarcomere

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sarcoplasm

The cytoplasm of a muscle fibre. Sarcoplasm contains chemicals that are required for muscle contraction, including glycogen, ATP, and phosphocreatine. In addition the sarcoplasm of active muscles tends to be rich in mitochondria.

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sarcoplasmic reticulum

The specialized endoplasmic reticulum in a muscle fibre, which forms a network of membrane-lined cavities surrounding the contractile myofibrils that run through the fibre. The release of calcium ions from the sarcoplasmic reticulum into the cytosol following stimulation from an action potential (see transverse tubules) causes subsequent contraction of the myofibrils (see sarcomere).

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satellite DNA

The proportion of the DNA of a eukaryotic cell that consists of very large numbers (approximately 10^6) of copies of a short nucleotide sequence. It occurs mainly around the centromeres and telomeres of the chromosomes. The highly repetitive nature of this DNA fraction gives it a distinctive base composition, and consequently when samples of DNA are centrifuged it forms so-called `satellite bands' quite separate from the band representing the bulk of the cell's DNA. See repetitive DNA. Compare microsatellite DNA.

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saturated

Denoting a compound consisting of molecules that have only single bonds (i.e. no double or triple bonds). Saturated compounds can undergo substitution reactions but not addition reactions. See fatty acid. Compare unsaturated.

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savanna

See grassland.

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scala

Any one of three fluid-filled canals of the cochlea in the inner ear: the scala media (cochlear duct), scala tympani (tympanic canal), and scala vestibuli (vestibular canal).

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scales

The small bony or horny plates forming the body covering of fish and reptiles. The wings of some insects, notably the Lepidoptera (butterflies and moths), are covered with tiny scales that are modified cuticular hairs.
In fish there are three types of scales. Placoid scales (denticles ), characteristic of cartilaginous fish, are small and toothlike, with a projecting spine and a flattened base embedded in the skin. They are made of dentine, have a pulp cavity, and the spine is covered with a layer of enamel. Teeth are probably modified placoid scales. Cosmoid scales , characteristic of lungfish and coelacanths, have an outer layer of hard cosmin (similar to dentine) covered by modified enamel (ganoine ) and inner layers of bone. The scale grows by adding to the inner layer only. In modern lungfish the scales are reduced to large bony plates. Ganoid scales are characteristic of primitive ray-finned fishes, such as sturgeons. They are similar to cosmoid scales but have a much thicker layer of ganoine and grow by the addition of material all round. The scales of modern teleost fish are reduced to thin bony plates.
In reptiles there are two types of scales: horny epidermal corneoscutes sometimes fused with underlying bony dermal osteoscutes .

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scanning electron microscope

See electron microscope.

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scapula (shoulder blade)

The largest of the bones that make up each half of the pectoral (shoulder) girdle. It is a flat triangular bone, providing anchorage for the muscles of the forelimb and an articulation for the humerus at the glenoid cavity. It is joined to the clavicle (collar bone) in front.

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scavenger

An animal that feeds on dead organic matter. Scavengers (such as hyenas) may feed on animals killed by predators or they may be detritivores.

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Schiff's reagent

A reagent used for testing for aldehydes and ketones; it consists of a solution of fuchsin dye that has been decolorized by sulphur dioxide. Aliphatic aldehydes restore the pink immediately, whereas aromatic ketones have no effect on the reagent. Aromatic aldehydes and aliphatic ketones restore the colour slowly. It is named after the German chemist Hugo Schiff (1834–1915).

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schizocarp

A dry indehiscent fruit formed from carpels that develop into separate one-seeded fragments called mericarps , which may be dehiscent, as in the regma, or indehiscent, as in the cremocarp and carcerulus.

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schizogeny

The localized separation of plant cells to form a cavity (surrounded by the intact cells) in which secretions accumulate. Examples are the resin canals of some conifers and the oil ducts of caraway and aniseed fruits. Compare lysigeny.

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Schleiden, Matthias Jakob

(1804–81)
German botanist, who became professor of botany at the University of Jena in 1839. A year earlier he had introduced the idea that plants consisted of cells; this theory was later extended to animals by Theodor Schwann (see cell theory). Schleiden recognized the existence and importance of the cell's nucleus, although he mistakenly believed that new cells were formed by budding from the surface of the nucleus.

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Schwann, Theodor

(1810–82)
German physiologist, who trained in medicine. After working in Berlin, he moved to Belgium. In 1838 Matthias Schleiden had stated that plant tissues were composed of cells. Schwann demonstrated the same fact for animal tissues, and in 1839 concluded that all tissues are made up of cells: this laid the foundations for the cell theory. Schwann also worked on fermentation and discovered the enzyme pepsin. Schwann cells are named after him.

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Schwann cell (neurilemma cell)

A cell that forms the myelin sheath of a nerve fibre. Each cell is responsible for a single length of the fibre (called an internode ); adjacent internodes are separated by small gaps (nodes of Ranvier ). During its development the cell wraps itself around the fibre, so the sheath consists of concentric layers of Schwann cell membrane. These cells are named after Theodor Schwann.

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scintillation counter

A type of particle or radiation counter that makes use of the flash of light (scintillation) emitted by an excited atom falling back to its ground state after having been excited by a passing photon or particle. The scintillating medium is usually either solid or liquid and is used in connection with a photomultiplier, which produces a pulse of current for each scintillation. The pulses are counted to enable the radioactivity of the source to be calculated. The distribution of a radiolabelled compound, such as a drug, in an organism can be determined in this way by testing tissue samples from different organs after drug administration.

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scion

See graft.

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sclera

See sclerotic.

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sclereid

A type of sclerenchyma cell that is shorter than a fibre; its lignified walls typically contain branched pits. Sclereids are abundant in seed coats, nut shells, and in pear fruits.

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sclerenchyma

A plant tissue whose cell walls have become impregnated with lignin. Due to the added strength that this confers, sclerenchyma plays an important role in support; it is found in the stems and also in the midribs of leaves. The cell walls contain pits, enabling the exchange of substances between adjacent cells. Mature sclerenchyma cells are dead, since the lignin makes the cell wall impermeable to water and gases. Sclerenchyma cells take the form of fibres or sclereids. Compare collenchyma; parenchyma.

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sclerophyllous

Describing scrub or woodland in which the plants have small leathery evergreen leaves. Such leaves are an adaptation to conserve water, and sclerophyllous vegetation is characteristic of Mediterranean-type climates, which have summer drought and occur in California, South Australia, South Africa, and Chile, as well as the Mediterranean region itself.

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scleroprotein

Any of a group of proteins found in the exoskeletons of some invertebrates, notably insects. Scleroproteins are formed by conversion of the relatively soft elastic larval protein by a natural tanning process (sclerotization ) involving orthoquinones. These are secreted and form cross linkages between polypeptides of the proteins, producing a hard rigid covering.

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sclerotic (sclera)

The tough external layer of the vertebrate eye. At the front of the eye, the sclera is modified to form the cornea.

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scolex

See Cestoda.

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scorpions

See Arachnida.

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scotopic vision

The type of vision that occurs when the rods in the eye are the principal receptors, i.e. when the level of illumination is low. With scotopic vision colours cannot be identified. Compare photopic vision.

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SCP

See single-cell protein.

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scrotum

The sac of skin and tissue that contains and supports the testes in most mammals. It is situated outside the body cavity and allows sperm to develop at the optimum temperature, which is slightly lower than body temperature.

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scurvy

A disease caused by deficiency of vitamin C, which results in poor collagen formation. Symptoms include anaemia, skin discoloration, and tooth loss. Scurvy was a common disease among sailors in the 16th–18th centuries, when no fresh food was available on long sea voyages.

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scutellum

The tissue in a grass seed that lies between the embryo and the endosperm. It is the modified cotyledon of grasses, being specialized for the digestion and absorption of the endosperm.

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Scyphozoa

See Cnidaria.

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seasonal isolation

See isolating mechanism.

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seaweeds

Large multicellular algae living in the sea or in the intertidal zone. They are commonly species of the Chlorophyta, Phaeophyta, and Rhodophyta.

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sebaceous gland

A small gland occurring in mammalian skin. Its duct opens into a hair follicle, through which it discharges sebum onto the skin surface.

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sebum

The substance secreted by sebaceous glands onto the surface of the skin. It is a fatty mildly antiseptic material that protects, lubricates, and waterproofs the skin and hair and helps prevent desiccation.

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second

Symbol s. The SI unit of time equal to the duration of 9192631770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium–133 atom.

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secondary consumer

See consumer.

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secondary growth (secondary thickening)

The increase in thickness of plant shoots and roots through the activities of the vascular cambium and cork cambium. It is seen in most dicotyledons and gymnosperms but not in monocotyledons. The tissues produced by secondary growth are called secondary tissues and the resultant plant or plant part is the secondary plant body . Compare primary growth.

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secondary productivity

The rate of biomass formation or energy fixation by heterotrophic organisms, such as grazers and decomposers (see heterotrophic nutrition). These derive all their energy from photosynthetic plants and other autotrophs, either directly or indirectly, and their productivity determines the number of trophic levels and the lengths of the food chains within an ecosystem – both of which are likely to be increased by greater secondary productivity. See productivity.

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secondary sexual characteristics

External features of a sexually mature animal that, although not directly involved in copulation, are significant in reproductive behaviour. The development of such features is controlled by sex hormones (androgens or oestrogens); they may be seasonal (e.g. the antlers of male deer or the body colour of male sticklebacks) or permanent (e.g. breasts in women or facial hair in men). In humans they develop during adolescence.

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secondary structure

See protein.

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secondary thickening

See secondary growth.

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second convoluted tubule

See distal convoluted tubule.

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second messenger

A chemical within a cell that is responsible for initiating the response to a signal from a chemical messenger (such as a hormone, neurotransmitter, or growth factor) that cannot enter the target cell itself, for example because it is not lipid-soluble and is therefore unable to cross the plasma membrane. A common second messenger is cyclic AMP; the signal for its formation within the cell is transmitted from hormone receptors on the cell surface by G-protein. Inositol 1,4,5-trisphosphate (see inositol) is another example of a second messenger.

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secretin

A hormone produced by the anterior part of the small intestine (the duodenum and jejunum) in response to the presence of hydrochloric acid from the stomach. It causes the pancreas to secrete alkaline pancreatic juice and stimulates bile production in the liver. Secretin, whose function was first demonstrated in 1902, was the first substance to be described as a hormone.

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secretion

1 The manufacture and discharge of specific substances into the external medium by cells in living organisms. (The substance secreted is also called the secretion.) Secretory cells are often specialized and organized in groups to form glands. The substances produced may be released directly into the blood (endocrine secretion ; see endocrine gland) or through a duct (exocrine secretion ; see exocrine gland). Secretions can be classified according to the manner of their discharge. Merocrine (eccrine) secretion occurs without the secretory cells sustaining any permanent change; in apocrine secretion the cells release a secretory vesicle incorporating part of the secretory cell membrane; and holocrine secretion involves the disruption of the entire cell to release its accumulated secretory vesicles.

2 The process by which a substance is pumped out of a cell against a concentration gradient. Secretion has an important role in adjusting the composition of urine as it passes through the nephrons of the kidney.
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secretion vector

See expression vector.

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seed

The structure in angiosperms and gymnosperms that develops from the ovule after fertilization. Occasionally seeds may develop without fertilization taking place (see apomixis). The seed contains the embryo and nutritive tissue, either as endosperm or food stored in the cotyledons. Angiosperm seeds are contained within a fruit that develops from the ovary wall. Gymnosperm seeds lack an enclosing fruit and are thus termed naked . The seed is covered by a protective layer, the testa. During development of the testa the seed dries out and enters a resting phase (dormancy) until conditions are suitable for germination.
Annual plants survive the winter or dry season as seeds. The evolution of the seed habit enabled plants to colonize the land, since seed plants do not depend on water for fertilization (unlike the lower plants).

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seed coat

See testa.

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seed ferns

See Cycadofilicales.

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seed leaf

See cotyledon.

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seed plant

Any plant that produces seeds. Most seed plants belong to the phyla Anthophyta (flowering plants) or Coniferophyta (conifers).

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segmentation

1 See metameric segmentation.

2 See cleavage.
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segregation

The separation of pairs of alleles during the formation of reproductive cells so that they contain one allele only of each pair. Segregation is the result of the separation of homologous chromosomes during meiosis. See also Mendel's laws.

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Selachii

The major subclass of the Chondrichthyes (cartilaginous fishes), containing the sharks, rays, skates, and similar but extinct forms. Their sharp teeth develop from the toothlike placoid scales (denticles) and are rapidly replaced as they wear out.

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selection

The process by which one or more factors acting on a population produce differential mortality and favour the transmission of specific characteristics to subsequent generations. See artificial selection; directional selection; disruptive selection; natural selection; sexual selection; stabilizing selection.

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selection coefficient

Symbol s or t. A measure of the sum of the forces acting to oppose the reproductive success of a particular organism, phenotype, or genotype. It varies between 0 and 1 and is related to fitness (W) by the equation: W=1–s. Hence, as the selection coefficient increases, fitness decreases.

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selection pressure

The extent to which organisms possessing a particular characteristic are either eliminated or favoured by environmental demands. It indicates the degree of intensity of natural selection.

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selective breeding

See breeding.

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selective reabsorption

The absorption of some of the components of the glomerular filtrate back into the blood as the filtrate flows through the nephrons of the kidney. Glucose, amino acids, and salts can be reabsorbed against a concentration gradient and their transport across the nephron into the capillaries requires energy (see active transport). Other components, such as ammonia and urea, are secreted rather than absorbed (see secretion), while certain ions, including potassium, can be both secreted and absorbed by the tubules according to the overall ionic balance throughout the body.

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self-fertilization

See fertilization.

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selfish DNA

Regions of DNA that apparently have no function (it is also known as `junk' DNA) and exist between those regions of DNA that represent the genes. Transposons are good examples; certain types of repetitive DNA also have `selfish' characteristics. Selfish DNA is so called as it seemingly exists only to pass copies of itself from one generation to another; it does so by using the organism in which it is contained as a survival machine. This is known as the selfish DNA theory . The greatest amounts of selfish DNA are found in vertebrates and higher plants. The presence of selfish DNA may be due to an unrecognizable function that it performs or because the cell has no way of halting its increase in the genome.

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self-pollination

See pollination.

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self-sterility

The condition found in many hermaphrodite organisms in which male and female reproductive cells produced by the same individual will not fuse to form a zygote, or if they do, the zygote is unable to develop into an embryo. In plants this is usually termed self-incompatibility (see incompatibility).

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Seliwanoff's test

A biochemical test to identify the presence of ketonic sugars, such as fructose, in solution. It was devised by the Russian chemist F. F. Seliwanoff. A few drops of the reagent, consisting of resorcinol crystals dissolved in equal amounts of water and hydrochloric acid, are heated with the test solution and the formation of a red precipitate indicates a positive result.

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semelparity

The strategy of reproducing only once during a lifetime, after which death is inevitable, even if the reproductive event is unfruitful. Semelparous organisms include most annual and biennial plants and representatives from most invertebrate animal taxa; vertebrate examples include the Pacific salmon and Atlantic eel. Compare iteroparity.

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semen

A slightly alkaline fluid (pH 7.2–7.6) containing sperm and various secretions that is produced by a male mammal during copulation and is introduced into the body of the female by ejaculation. Spermatozoa are produced by the testes and the secretions by the prostate gland, seminal vesicles, and Cowper's glands. Semen also contains enzymes that activate the sperm after ejaculation.

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semicircular canals

The sense organ in vertebrates that is concerned with the maintenance of physical equilibrium (sense of balance). It occurs in the inner ear and consists of three looped canals set at right angles to each other and attached to the utriculus. The canals contain a fluid (endolymph ) that flows in response to movements of the head and body. A swelling (ampulla) at one attachment point of each canal contains sensory cells that respond to movement of the endolymph in any of the three planes. These sensory cells initiate nervous impulses to the brain.

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semiconservative replication

The generally accepted method of DNA replication, in which the two strands of the DNA helix separate and free nucleotides pair with the exposed bases on the single chains to form two new DNA molecules, each containing one original and one newly synthesized strand of DNA. Compare conservative replication; dispersive replication.

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semilunar valve

Either of two valves in the heart, found in the pulmonary artery (pulmonary valve ) and in the aorta (aortic valve ), that prevent the backflow of blood into the right and left ventricles from the pulmonary artery and the aorta, respectively, thus maintaining blood flow in a single direction. Each semilunar valve consists of three pockets, which close by filling with blood when the force of contraction that expels the blood from the heart diminishes.

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seminal receptacle

See spermatheca.

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seminal vesicle

1 A pouch or sac in many male invertebrates and lower vertebrates that is used for storing sperm.

2 One of a pair of glands in male mammals that secrete a liquid component of semen into the vas deferens. This secretion is alkaline, which neutralizes the acidic conditions in the female genital tract, and contains fructose, used by the sperm as a source of energy.
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seminiferous tubules

See testis.

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semiochemical

A chemical that affects the behaviour of an organism. Such chemicals include pheromones, which are used for communication between members of the same species, and allelochemicals, which act as chemical signals between members of different species. Terrestrial organisms may respond to chemical signals both in the air and on the ground, using their senses of smell or taste; whereas aquatic organisms are influenced very largely by chemicals dissolved or dispersed in their watery medium. Synthetic semiochemicals, especially artificial sex pheromones, are now widely used to control populations of agricultural and horticultural pests, notably insects pests. For example, they can be used to lure male insects to field traps, or to raise the general level of sex attractants in the area and so confuse males and thereby disrupt mating.

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semipermeable membrane

See partially permeable membrane.

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senescence

The changes that occur in an organism (or a part of an organism) between maturity and death, i.e. ageing. Characteristically there is a deterioration in functioning as the cells become less efficient in maintaining and replacing vital cell components. In animals this results in a decline in physical ability and, in man, there is also often a reduction in mental ability. Not all the parts of the body necessarily become senescent at the same time or age at the same rate. For example, in deciduous trees the shedding of senescent leaves in the autumn is a normal physiological process.

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sensation

The raw data detected by the senses. For example, red is a colour sensation. Compare perception.

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sense organ

A part of the body of an animal that contains or consists of a concentration of receptors that are sensitive to specific stimuli (e.g. sound, light, pressure, heat). Stimulation of these receptors initiates the transmission of nervous impulses to the brain, where sensory information is analysed and interpreted. Examples of sense organs are the eye, ear, nose, and taste bud.

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senses

The faculties that enable animals to perceive information about their external environment or about the state of their bodies in relation to this environment (see sense organ; vision; hearing; balance; olfaction; taste; touch). Specific receptors are sensitive to pain, temperature, chemicals, etc.

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sensillum

(pl. sensilla) Any of various hairlike or peglike sense organs found in insects and other arthropods, comprising a cluster of basal receptor cells whose dendrites extend inside a sheath and hairshaft. They respond to a wide range of chemical and mechanical stimuli, acting as chemoreceptors of both `smell' and `taste', touch receptors, mechanoreceptors to detect movement at joints in the skeleton, and auditory organs. They may protrude from the surface, for example as setae, or they may be embedded in the cuticle. All the constituent cells of any one sensillum are derived from a single mother cell; they may include receptor cells responsive to several different classes of stimuli. The antennae, in particular, bear large numbers of olfactory sensilla. Chemicals enter the hairshaft via one or numerous pores and elicit nerve impulses in the dendrites. For example, males of the silkworm moth (Bombyx mori) have in each antenna about 17000 sensilla that respond to the female sex pheromone; each sensillum has some 3000 pores, giving sensitivity to just a few molecules of this vital odour.

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sensitivity (irritability)

One of the fundamental properties of all organisms: the capacity to detect, interpret, and respond to changes in the environment (e.g. the stimuli of light, touch, chemicals, etc.). Multicellular animals have specialized sense organs and effector organs for this purpose; in unicellular organisms, which lack a nervous system, the reception of, and response to, a stimulus occur in the same cell.

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sensitization

1 (of a cell) The alteration of the integrity of a cell membrane resulting from the reaction of specific antibodies with antigens on the surface of the cell. In the presence of complement, the cell ruptures.

2 (of an individual) Initial exposure to a specific antigen such that re-exposure to the same antigen causes a severe immune response (see anaphylaxis).
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sensory cell

See receptor.

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sensory neurone

A nerve cell (see neurone) that transmits information about changes in the internal and external environment to the central nervous system. Sensory neurones are of two types. Somatic sensory neurones occur in peripheral nerves in the skin, skeletal muscle, joints, and bones. Visceral sensory neurones are located in sympathetic and parasympathetic nerves in the heart, lungs, and other organs.

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sepal

One of the parts of a flower making up the calyx. Sepals are considered to be modified leaves with a simpler structure. They are usually green and often hairy but in some plants, e.g. monk's hood, they may be brightly coloured.

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sepsis

Infection of the soft tissues or blood by pathogenic microorganisms, arising usually after these have entered the body through a skin wound. It results in destruction of the tissues by the pathogens or their toxins.

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septum

(pl. septa) Any dividing wall in a plant or animal. Examples are the septa that separate the chambers of the heart.

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sere

A complete succession of plant communities, which results in the climax community. A sere is composed of a series of different plant communities that change with time. These communities are known as seral stages or seral communities .

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serine

See amino acid.

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serology

The laboratory study of blood serum and its constituents, particularly antibodies and complement, which play a part in the immune response.

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serotonin (5-hydroxytryptamine; 5-HT)

A compound (see formula), derived from the amino acid tryptophan, that affects the diameter of blood vessels and also functions as a neurotransmitter. It is released by mast cells, basophils, and platelets and acts as a mediator of inflammation and allergic reactions. In the brain it is thought to influence mood: many antidepressant drugs act by blocking the reabsorption of serotonin by nerve endings in the brain and thus increase its concentration in the brain. See also lysergic acid diethylamide.

Serotonin

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serous membrane (serosa)

A tissue consisting of a layer of mesothelium attached to a surface by a thin layer of connective tissue. Serous membrane lines body cavities that do not open to the exterior; the peritoneum, pleura, and serous pericardium are examples.

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Sertoli cells (sustentacular cells)

Cells that line the seminiferous tubules in the testis. Named after Italian histologist Enrico Sertoli (1842–1910), Sertoli cells protect the spermatids and convey nutrients to both the developing and mature spermatozoa. They also produce a hormone, inhibin , which can inhibit follicle-stimulating hormone and thereby regulate production of spermatozoa.

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serum

See blood serum.

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sessile

1 Describing animals that live permanently attached to a surface, i.e. sedentary animals. Many marine animals, e.g. sea anemones and limpets, are sessile.

2 Describing any organ that does not possess a stalk where one might be expected. For example, the leaves of the oak (Quercus robur) are attached directly to the twigs.
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seta

(pl. setae) 1 A bristle or hair in many invertebrates. Setae are produced by the epidermis and consist either of a hollow projection of cuticle containing all or part of an epidermal cell (as in insects) or are composed of chitin (as in the chaetae of annelid worms).

2 See sporogonium.
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sewage

Waste matter from industrial and domestic sources that is dissolved or suspended in water. Raw (untreated) sewage is a pollutant. It has a high content of organic matter (notably faeces and nitrogenous waste) and therefore provides a rich source of food for many decomposers (bacteria, fungi) and detritivores, some of which are pathogenic to humans. The release of raw (untreated) sewage into a river causes eutrophication (see eutrophic); there is a sudden increase in the biochemical oxygen demand (BOD), as the organisms that feed on sewage proliferate and use up the available dissolved oxygen in the river. Oxygen-sensitive organisms, such as fish, will die. Certain organisms can proliferate in particular concentrations of sewage, depending on their tolerance, and can be used as markers of the extent to which a river is polluted by sewage. For example, Tubifex worms are able to tolerate high concentrations of sewage.
Sewage can be treated before release. This involves a number of stages, including filtration, sedimentation, and microbial degradation (notably by methanogens). When most of the solid waste has been removed, the remaining liquid (effluent ) is discharged into rivers, etc. During sedimentation, particulate organic matter accumulates at the bottom of large tanks. This material, known as sludge , is periodically removed, further decomposed by microorganisms (see activated sludge process), and then sold as fertilizer or dumped.

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Sewall Wright effect

See genetic drift

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sex chromosome

A chromosome that operates in the sex-determining mechanism of a species. Many animals have two different types of sex chromosome. For example, in mammals there is a large X chromosome and a much smaller Y chromosome; the female has two matching X chromosomes, making it the homogametic sex, whereas the male has one X chromosome and one Y chromosome, making it the heterogametic sex. In other animal groups, such as butterflies and moths, birds, and reptiles, the situation is reversed: males have two matching W chromosomes, while females have one W chromosome and one Z chromosome. Sex chromosomes carry genes governing the development of sex organs and secondary sexual characteristics. They also carry other genes unrelated to sex (see sex linkage). See also sex determination.

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sex determination

The method by which the distinction between males and females is established in a species. It is usually under genetic control. Equal numbers of males and females are produced when sex is determined by sex chromosomes or by a contrasting pair of alleles. In some species (e.g. bees) females develop from fertilized eggs and males from unfertilized eggs. This does not produce equal numbers. Environmental factors can also be crucial in governing the sex of developing individuals. For example, temperature can affect the sex ratios of broods of certain turtles. High incubation temperatures (>28°C) produce a preponderance of males, while lower temperatures (<26°C) give rise to more females.

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sex factor

A plasmid found in the cytoplasm of certain bacteria that is responsible for initiating conjugation and gene transfer. The sex factor in Escherichia coli is known as the F factor ; it can be transferred from a donor cell (F+) to a recipient (F–) by conjugation. Occasionally the F factor may become integrated with the chromosome of an F+ bacterium to form a high-frequency recombinant (Hfr) cell. This enables the part of the host chromosome nearest to the F factor to be transferred to the recipient by conjugation.

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sex hormones

Steroid hormones that control sexual development. The most important are the androgens and oestrogens.

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sex linkage

The tendency for certain inherited characteristics to occur far more frequently in one sex than the other. For example, red–green colour blindness and haemophilia affect men more often than women. This is because the genes governing normal colour vision and blood clotting occur on the X sex chromosome. Women have two X chromosomes. If one carries an abnormal allele it is likely that its effects will be masked by a normal allele on the other X chromosome. However, men only have one X chromosome and any abnormal alleles therefore will not be masked. See also carrier.

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sex ratio

The ratio of the number of females to the number of males in a population. Because the mortality rates in the two sexes may be different, the sex ratios in different age classes may differ.

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sexual cycle

See menstrual cycle; oestrous cycle.

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sexual intercourse (coitus; copulation; mating)

The process by which spermatozoa from a male are deposited in the body of a female during sexual reproduction. In mammals the penis of the male becomes erect and stiff as its tissues become filled with blood, enabling it to be inserted into the vagina of the female. Thrusting movements of the penis result in ejaculation, in which semen, containing spermatozoa, is deposited in the vagina.

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sexually transmitted disease (STD)

Any disease that is passed from one individual to another during sexual intercourse or other types of sexual activity. These diseases have been traditionally referred to as venereal diseases . They include gonorrhoea, caused by the bacterium Neisseria gonorrhoeae; syphilis, due to infection by the bacterium Treponema pallidum; genital herpes, which is caused by a herpesvirus; and AIDS, resulting from infection with HIV, a retrovirus. The transmission of sexually transmitted diseases can be reduced by limiting the number of sexual partners and by the use of condoms (see birth control), which reduces the risk of contact with body fluids that harbour the microorganisms that cause these diseases.

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sexual reproduction

A form of reproduction that involves the fusion of two reproductive cells (gametes) in the process of fertilization. Normally, especially in animals, it requires two parents, one male and the other female. However, most plants bear both male and female reproductive organs and self-fertilization may occur, as it does in hermaphrodite animals. Gametes are formed by meiosis, a special kind of cell division in the parent reproductive organs that both reassorts the genetic material and halves the chromosome number. Meiosis thus ensures genetic variability in the gametes and therefore in the offspring resulting from their subsequent fusion. Sexual reproduction, unlike asexual reproduction, therefore generates variability within a species. However, it depends on there being reliable means of bringing together male and female gametes, and many elaborate mechanisms have evolved to ensure this.

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sexual selection

The means by which it is assumed that certain secondary sexual characteristics, particularly of male animals, have evolved. Females presumably choose to mate with the male that gives the best courtship display and therefore has the brightest coloration, etc.: these features would be inherited by its male offspring and would thus tend to become exaggerated down the generations.

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Sherrington, Sir Charles Scott

(1857–1952)
British physiologist, who became professor of physiology at Oxford University in 1913. After early work on antitoxins he began studying human reflex reactions. Like Pavlov in Russia, he discovered conditioned reflexes. His subsequent research concerned the functioning of neurones, for which he shared the 1932 Nobel Prize for physiology or medicine with Sir Edgar Adrian.

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shikimic acid pathway

The key metabolic pathway in the biosynthesis of the aromatic amino acids – tyrosine, phenylalanine, and tryptophan – which occurs in plants, bacteria, and fungi, but not in animals. This series of reactions makes these amino acids available to animals, for which they are essential amino acids, required in the diet for proteins and as precursors for a wide range of aromatic molecules. The initial step of the pathway is the condensation of erythrose-4-phosphate (from the pentose phosphate pathway) and phosphoenolpyruvate (from glycolysis). The product of this is then cyclized and reduced to form the intermediate compound shikimate, with the phenolic ring structure characteristic of the aromatic amino acids. Combination with a further molecule of phosphoenolpyruvate produces chorismate, from which alternative pathways lead to either phenylalanine/tyrosine or to tryptophan. An enzyme responsible for a step in the conversion of shikimate to its immediate derivative, 3-enolpyruvyl-shikimate-5-phosphate, is inhibited by the herbicide glyphosate, thus blocking the biosynthesis of aromatic amino acids by the plant.

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Shine–Dalgarno sequence

A sequence of five to nine (typically seven) nucleotides preceding the start codon in prokaryotic messenger RNA (mRNA) that is recognized by the ribosome as the correct site for binding the mRNA molecule prior to the start of translation. The sequence (AGGAGGU) binds a complementary sequence on the 16S ribosomal subunit, helping to form a stable complex between the ribosome and mRNA. The role of this sequence was first proposed by John Shine (1946– ) and Lynn Dalgarno (1935– ).

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shoot

The aerial part of a vascular plant. It develops from the plumule and consists of a stem supporting leaves, buds, and flowers.

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short-day plant

A plant in which flowering can be induced or enhanced by short days, usually of less than 12 hours of daylight. Examples are strawberry and chrysanthemum. See photoperiodism. Compare day-neutral plant; long-day plant.

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short interspersed element

See SINE.

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short-sightedness

See myopia.

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shotgun cloning

A method for cloning the entire genome of an organism in which the DNA is broken into small fragments at random using restriction enzymes. Each fragment is then incorporated with a cloning vector and introduced into a host organism. The fragments are maintained as a gene library. See gene cloning.

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shoulder girdle

See pectoral girdle.

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shunt vessel

A blood vessel that links an artery directly to a vein, allowing the blood to bypass the capillaries in certain areas. Shunt vessels can control blood flow by constriction and dilation. In endotherms the shunt vessels dilate in response to cold, thereby cutting off the blood flow to the extremities and preventing heat loss.

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siblings

Individuals that have both parents in common.

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sickle-cell anaemia

See polymorphism.

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sieve element

A type of plant cell occurring within the phloem. Sieve elements combine to form a series of tubes (sieve tubes ) connecting the leaves, shoots, and roots in a fine network. Food materials are transported from one element to another via perforations termed sieve areas or sieve plates (see mass flow). Sieve elements contain little cytoplasm and no nucleus. It is believed that their metabolic activities are supplemented by companion cells in angiosperms and by albuminous cells in gymnosperms. See also P-protein.

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sievert

The SI unit of dose equivalent (see radiation units). It is named after the Swedish physicist Rolf Sievert (1896–1966).

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sieve tube

A tube within the phloem tissue of a plant, composed of joined sieve elements.

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sigmoid growth curve

See population growth.

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signal hypothesis

A hypothesis to explain how ribosomes become attached to membranes within cells in order to deliver the appropriate proteins to cell organelles, such as mitochondria and chloroplasts, or transport proteins outside the cell membrane. It proposes that the leading end of the nascent polypeptide chain consists of a signal peptide . This sticks out from the ribosome and is recognized by a ribonucleoprotein particle called a signal recognition particle (SRP). When the complex of ribosome and SRP encounters a membrane, the SRP binds to a docking protein (signal recognition particle receptor ) on the membrane surface. Synthesis of the polypeptide, which has hitherto been stalled, now resumes, and the polypeptide (or fully formed protein) passes into the membrane, where the signal peptide is removed by a signal peptidase enzyme. Once translation is completed, the ribosome dissociates and is freed from the membrane. It is thought that the signal sequence tags the protein for insertion at particular sites, by interacting with membrane-bound glycoproteins (signal sequence receptors ). If the signal sequence is not the correct one, the ribosome is released before delivering its protein. The hypothesis, which was formulated in the early 1970s by workers including Gunter Blobel (1936– ) and César Milstein (1927– ), is now widely accepted.

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sign stimulus (releaser)

The essential feature of a stimulus, which is necessary to elicit a response. For example, a red belly (characteristic of courting male sticklebacks) is the sign stimulus necessary to provoke an attack from a rival male; even a very crude model fish is attacked if it has a red undersurface. Similarly, territorial fighting in male robins is triggered by the sign stimulus of a red breast; a bunch of red feathers is enough to induce territorial behaviour. See also stimulus filtering.

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silent mutation

An alteration in the genetic code that has no apparent effect on the phenotype of an organism.

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silicula

A type of capsule formed from a bicarpellary ovary. It is longitudinally flattened and divided lengthwise into two cavities (loculi ). It is broader than a siliqua. Examples include the fruits of Alyssum and candytuft.

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siliqua

A type of capsule formed from a bicarpellary ovary. It resembles a silicula but is longer than it is broad; an example is the fruit of the wallflower. See also lomentum.

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Silurian

A geological period of the Palaeozoic era following the Ordovician period and extending until the beginning of the Devonian period. It began about 438 million years ago and lasted for about 30 million years. The Silurian was named by Roderick Murchison (1792–1871) after an ancient British tribe that inhabited South Wales, where he observed rocks of this period. The majority of Silurian life was marine but during the later part of the period primitive plants began to make their appearance on land. Trilobites and graptolites became less common, brachiopods were numerous and varied, crinoids became common for the first time, and corals also increased. The only known vertebrates during the Silurian were primitive fish; the first jawed fish appeared later in the period. The Caledonian orogeny (mountain-building period) reached its peak towards the end of the Silurian.

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SINE (short interspersed element)

Any of a class of dispersed moderately repetitive DNA found in eukaryotes, consisting of numerous copies (>10^5) of relatively short (<500 bp) sequences scattered throughout the genome. SINEs are not translated into proteins, occur mostly in introns, and are thought to be degenerate copies of retrotransposons, with unknown function. The most notable example in humans and other primates is the Alu family.

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single-cell protein (SCP)

Protein produced by microorganisms, such as bacteria, yeasts, and unicellular algae, that is extracted for use as a component of human and animal foods.

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single circulation

The type of circulatory system that occurs in fishes, in which the blood passes only once through the heart in each complete circuit of the body Compare double circulation.

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sinoatrial node

See pacemaker.

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sinus

A saclike cavity or organ in an animal, such as the sinus venosus.

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sinusoid

A tiny blood vessel or blood-filled space in an organ. Sinusoids replace capillaries in certain organs, notably the liver; they allow more direct contact between the blood and the tissue it is supplying.

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sinus venosus

A thin-walled chamber of the heart of vertebrate embryos that conveys deoxygenated blood to the single atrium. It is retained in adult fishes but in higher vertebrates it becomes incorporated into the left atrium.

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siphonaceous (siphonous)

Describing algae and other protoctists in which the thallus is coenocytic (see coenocyte) and often takes the form of a tubular structure.

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Siphonaptera

An order of secondarily wingless insects comprising the fleas. The body of a flea is laterally compressed and bears numerous backward-directed spines. Fleas live as blood-sucking ectoparasites of mammals and birds, having mouthparts adapted to piercing their host, injecting saliva to prevent clotting, and sucking up the blood. The long bristly legs can transmit energy stored in the elastic body wall to leap relatively long distances (over 300mm horizontally). Apart from causing irritation, fleas can transmit disease organisms, most notably bubonic plague bacteria, which can be carried from rats to humans by the rat flea (Xenopsylla cheopsis). The whitish wormlike legless larvae feed on organic matter. After two moults the larva spins a cocoon and undergoes metamorphosis into the adult.

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Siphunculata (Anoplura)

An order of secondarily wingless insects comprising the sucking lice: blood-sucking ectoparasites of mammals, with piercing and sucking mouthparts forming a snoutlike proboscis. They constitute an irritating pest to humans and domestic animals and can transmit diseases, including typhoid. The human louse (Pediculus humanus) exists in two forms: the head louse (P. humanus capitis) and the body louse (P. humanus corporis).

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sister species

Either of the two descendant species formed when one species splits during evolution. Hence, the sister species (or sister group) is the one most closely related to any given species (or group), since both share an ancestral species (or group) not shared by any other species (or group). In classification systems based on the principles of cladistics, sister species are always grouped together. Moreover the sister group is the prime choice for use in outgroup comparison.

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site-directed mutagenesis

A technique used in genetic engineering for introducing particular changes to the base sequence of a gene at a specific site. It allows precise and specific mutations to be made and is done, for example, to modify the amino-acid sequence of the protein expressed by the gene to investigate how such a change affects the protein's structure and function. First, the gene of interest is cloned and made available as single-stranded DNA – a widely used vector for this purpose is the bacteriophage M13. Then an artificial oligonucleotide, containing perhaps 20–30 nucleotides, is constructed containing the desired change in base sequence. This is allowed to hybridize with the complementary (apart from the mutation site) single-stranded DNA and is then extended at either end by the enzyme DNA polymerase using the single-stranded DNA as template. The two strands, each with their vector, are then separated and cloned, and clones containing the mutant gene are selected.

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Site of Special Scientific Interest

See SSSI.

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SI units

Système International d'Unités: the international system of units now recommended for all scientific purposes. A coherent and rationalized system of units derived from the m.k.s. units (a metric system based on the metre, kilogram, and second), SI units have now replaced c.g.s. units and Imperial units. The system has seven base units and two dimensionless units (formerly called supplementary units ), all other units being derived from these nine units. There are 18 derived units with special names. Each unit has an agreed symbol (a capital letter or an initial capital letter if it is named after a scientist, otherwise the symbol consists of one or two lower-case letters). Decimal multiples of the units are indicated by a set of prefixes; when possible a prefix representing 10 raised to a power that is a multiple of three should be used.

TABLE 1.1 Base and dimensionless SI units

Physical quantity

Name

Symbol

length

metre

m

mass

kilogram

kg

time

second

s

electric current

ampere

A

thermodynamic temperature

kelvin

K

luminous intensity

candela

cd

amount of substance

mole

mol

plane angle

radian

rad

solid angle

steradian

sr

dimensionless units

TABLE 1.2 Derived SI units with special names

Physical quantity

Name of SI unit

Symbol of SI unit

frequency

hertz

Hz

energy

joule

J

force

newton

N

power

watt

W

pressure

pascal

Pa

electric

charge

coulomb

C

electric potential difference

volt

V

electric resistance

ohm

electric conductance

siemens

S

electric capacitance

farad

F

magnetic flux

weber

Wb

inductance

henry

H

magnetic flux density (magnetic induction)

tesla

T

luminous flux

lumen

lm

illuminance

lux

lx

absorbed dose

gray

Gy

activity

becquerel

Bq

dose equivalent

sievert

Sv

TABLE 1.3 Decimal multiples and submultiples to be used with SI units

Submultiple

Prefix

Symbol

Multiple

Prefix

Symbol

10-1

deci

d

10

deca

da

10-2

centi

c

10^2

hecto

h

10-3

milli

m

10^3

kilo

k

10-6

micro

µ

10^6

mega

M

10-9

nano

n

10^9

giga

G

10-12

pico

p

10^12

tera

T

10-15

femto

f

10^15

peta

P

10-18

atto

a

10^18

exa

E

10-21

zepto

z

10^21

zetta

Z

10-24

yocto

y

10^24

yotta

Y

TABLE 1.4 Conversion of units of SI units

From

To

Multiply by

in

m

2.54 × 10-2

ft

m

0.3048

sq. in

m^2

6.4516 × 10-4

sq. ft

m^2

9.2903 × 10-2

cu. in

m^3

1.63871 × 10-5

cu. ft

m^3

2.83168 × 10-2

l(itre)

m^3

10-3

gal(lon)

l(itre)

4.546 09

miles/hr

m s-1

0.477 04

km/hr

m s-1

0.277 78

lb

kg

0.453 592

g cm-3

kg m-3

10^3

lb/in^3

kg m-3

2.767 99 × 10^4

dyne

N

10-5

poundal

N

0.138 255

lbf

N

4.448 22

mmHg

Pa

133.322

atmosphere

Pa

1.013 25 × 10^5

hp

W

745.7

erg

J

10-7

eV

J

1.602 10 × 10-19

kW h

J

3.6 × 10^6

cal

J

4.1868

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skeletal muscle

See voluntary muscle.

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skeleton

The structure in an animal that provides mechanical support for the body, protection for internal organs, and a framework for anchoring the muscles. The skeleton may be external (see exoskeleton) or internal (see endoskeleton). Both types require joints to allow locomotion. The skeleton of higher vertebrates consists of a system of bones (see appendicular skeleton; axial skeleton). Soft-bodied animals have a hydrostatic skeleton.

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skin

The outer layer of the body of a vertebrate (see illustration). It is composed of two layers, the epidermis and dermis, with a complex nervous and blood supply. The skin may bear a variety of specialized structures, including hair, scales, and feathers. This skin has an important role in protecting the body from mechanical injury, water loss, and the entry of harmful agents (e.g. disease-causing bacteria). It is also a sense organ, containing receptors sensitive to pain, temperature, and pressure (see Meissner's corpuscles; Pacinian corpuscle). In warm-blooded animals it helps regulate body temperature by means of hair, fur, or feathers and sweat glands.

Structure of mammalian skin

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skull

The skeleton of the head. In mammals it consists of a cranium enclosing the brain and the bones of the face and jaw. All the joints between the individual bones of the skull are immovable (see suture) except for the joint between the mandible (lower jaw) and the rest of the skull. There is a large opening (foramen magnum ) at the base of the skull through which the spinal cord passes from the brain.

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sleep

A readily reversible state of reduced awareness and metabolic activity that occurs periodically in many animals. Usually accompanied by physical relaxation, the onset of sleep in humans and other mammals is marked by a change in the electrical activity of the brain, which is recorded by an electroencephalogram as waves of low frequency and high amplitude (slow-wave sleep ). This is interspersed by short bouts of high-frequency low-amplitude waves (similar to wave patterns produced when awake) associated with restlessness, dreaming, and rapid eye movement (REM); this is called REM (or paradoxical ) sleep and is often accompanied by an increased pulse rate and dilation of the pupils. Several regions of the brain are involved in sleep, especially the reticular formation of the brainstem.

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sleep movements

See nyctinasty.

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sliding filament theory

A proposed mechanism of muscle contraction in which the actin and myosin filaments of striated muscle slide over each other to shorten the length of the muscle fibres (see sarcomere). Myosin-binding sites on the actin filaments are exposed when calcium ions bind to troponin molecules in these filaments. This allows bridges to form between actin and myosin, which requires ATP as an energy source. Hydrolysis of ATP in the heads of the myosin molecules causes the heads to change shape and bind to the actin filaments. The release of ADP from the myosin heads causes a further change in shape and generates mechanical energy that causes the actin and myosin filaments to slide over one another (see illustration).

Sliding filament theory of muscle contraction

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slime moulds

Small simple organisms widely distributed in damp habitats on land. They exist either as free cells (myxamoebas ) or as multinucleate aggregates of cells (see plasmodium), depending on the stage of the life cycle. In the plasmodial (or true ) slime moulds the nuclei of the plasmodia are not separated by cell membranes; in cellular slime moulds the myxamoebas retain their cell membranes after aggregating to form a `pseudoplasmodium'. Cell walls are generally absent. Slime moulds show amoeboid movement and feed by ingesting small particles of food. They reproduce by means of spores. Slime moulds were formerly classified as fungi. They are now usually classified in two phyla, Myxomycota (plasmodial slime moulds) and Rhizopoda (cellular slime moulds plus other amoebas), of the kingdom Protoctista.

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slow virus

Any of a group of quasi-viral or subviral agents formerly thought to be responsible for the diseases scrapie in sheep and bovine spongiform encephalopathy (BSE) in cattle. These diseases are now generally regarded as being caused by abnormal prion proteins.

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slow-wave sleep

See sleep.

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sludge

See sewage.

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small intestine

The portion of the alimentary canal between the stomach and the large intestine. It is subdivided into the duodenum, jejunum, and ileum. It plays an essential role in the final digestion and absorption of food.

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smell

See olfaction.

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smooth endoplasmic reticulum (smooth ER)

See endoplasmic reticulum.

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smooth muscle

See involuntary muscle.

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smuts

A group of parasitic fungi of the phylum Basidiomycota. Many of these species attack the ears of cereal crops, replacing the grain by a mass of dark spores. Compare rusts.

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snakes

See Squamata.

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snRNP

See ribonucleoprotein.

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social behaviour

Any behaviour exhibited by a group of animals that interact with each other. Social behaviour ranges from moving as a herd in order to minimize the effects of predators to performing designated roles in highly organized societies. For example, within a colony of bees specific tasks, including tending the larvae, foraging for food, and controlling the temperature within the colony by wing fanning, are performed by different individuals (see caste). See also eusocial.

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sodium

Symbol Na. A soft silvery element that is a major essential element required by animals. It is important in maintaining the acid–base balance and in controlling the volume of extracellular fluid and functions in the transmission of nerve impulses (see sodium pump).

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sodium chloride (common salt)

A colourless crystalline solid, NaCl, that is soluble in water. Sodium chloride has a key role in biological systems in maintaining electrolyte balances. It is used as a food preservative (see food preservation).

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sodium fluoride

A crystalline compound, NaF. It is highly toxic but in very dilute solution (less than 1 part per million) it is used in the fluoridation of water for the prevention of tooth decay on account of its ability to replace OH groups with F groups in the material of dental enamel.

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sodium pump

A mechanism by which sodium ions are transported out of a eukaryotic cell across the cell membrane. The process requires energy in the form of ATP, being a form of active transport. The most important type is sodium/potassium ATPase , which is a membrane protein that exchanges sodium ions (Na+) for potassium ions (K+), thus maintaining the differential concentrations of each ion across the cell membrane. This differential is vital to cellular function, e.g. in establishing the resting potential of a neurone.

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softwood

See wood.

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soil

The layer of unconsolidated particles derived from weathered rock, organic material (humus), water, and air that forms the upper surface over much of the earth and supports plant growth. The formation of soil depends on the parent material (i.e. the original material from which the soil is derived), the climate and topography of the area, the organisms present in the soil, and the time over which the soil has been developing. Soils are often classified in terms of their structure and texture. The structure of a soil is the way in which the individual soil particles are bound together to form aggregates or peds. The structure types include platy, blocky, granular, and crumbs. The texture of a soil denotes the proportion of the various particle sizes that it contains. The four main texture classes are sand, silt, clay, and loam, of which loams are generally the best agricultural soils as they contain a mixture of all particle sizes. A number of distinct horizontal layers can often be distinguished in a vertical section (profile) of soil – these are known as soil horizons . Four basic horizons are common to most soils: an uppermost A horizon (or topsoil ) containing the organic matter; an underlying B horizon (or subsoil ), which contains little organic material and is strongly leached; a C horizon consisting of weathered rock; and a D horizon comprising the bedrock.

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soil erosion

The removal and thinning of the soil layer due to climatic and physical processes, such as high rainfall, which is greatly accelerated by certain human activities, such as deforestation. Soil erosion can lead to a loss of agricultural land and if unchecked, eventually results in desertification.

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sol

A colloid in which small solid particles are dispersed in a liquid continuous phase.

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solenocytes

See flame cells.

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solute

The substance dissolved in a solvent in forming a solution.

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solute potential

Symbol s. The component of water potential that is due to the presence of solute molecules. It always has a negative value as solutes lower the water potential of the system.

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solution

A homogeneous mixture of a liquid (the solvent) with a gas or solid (the solute ). In a solution, the molecules of the solute are discrete and mixed with the molecules of solvent. There is usually some interaction between the solvent and solute molecules. For example, when sodium chloride dissolves in water the sodium ions attract polar water molecules, with the negative oxygen ion pointing towards the positive Na+ ion. This is called hydration .

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solvent

A liquid that dissolves another substance or substances to form a solution. Polar solvents are compounds, such as water, in which there is some separation of charge in the chemical bonds. These solvents are capable of dissolving ionic compounds or covalent compounds that ionize. Nonpolar solvents are compounds, such as benzene, that do not dissolve ionic compounds but will dissolve nonpolar covalent compounds.

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somatic

1 Relating to all the cells of an animal or plant other than the reproductive cells. Thus a somatic mutation is one that is not heritable.

2 Relating to organs and tissues of the body other than the gut and its associated structures. The term is applied especially to voluntary muscles, the sense organs, and the nervous system. Compare visceral.
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somatic cell hybridization

See cell fusion.

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somatic sensory neurone

See sensory neurone.

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somatostatin (growth hormone inhibiting hormone; GHIH)

A hormone, secreted by the hypothalamus, that inhibits the release of growth hormone from the anterior pituitary gland. The secretion of somatostatin is stimulated by various factors, including very high blood glucose levels, which result from the effect that growth hormone has on glucose metabolism. It is also produced by the (or D) cells of the islets of Langerhans in the pancreas, where it can inhibit the release of glucagon and insulin.

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somatotrophin

See growth hormone.

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somite

Any of a series of segmented blocks of tissue in animal embryos that develop from the mesoderm. In vertebrate embryos they lie on the dorsal side of the notochord and give rise to the vertebral column, ribs, dermis, and striated muscle. In invertebrates showing metameric segmentation the somites give rise to the body segments.

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sonicator

A device used to break down cells using ultrasonic waves. Ultrasonic waves are transmitted through a metal rod, which is placed in a suspension of the cells. The vibrations cause the cell membranes to rupture and the cellular contents are released into the surrounding medium.

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soredium

(pl. soredia) A small body, consisting of a core of algal cells surrounded by fungal hyphae, that functions as a structure of vegetative propagation in lichens. Soredia, which range from 25 to 100 µm in diameter, are released like spores and are dispersed in air currents. Compare isidium.

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sorosis

A type of composite fruit formed from an entire inflorescence spike. Mulberry and pineapple fruits are examples.

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sorus

1 Any of the spore-producing structures on the undersurface of a fern frond, visible as rows of small brown dots.

2 A reproductive area on the thallus of some algae, e.g. Laminaria.
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3 Any of various spore-producing structures in certain fungi.
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SOS response

A suite of metabolic mechanisms that are invoked in a bacterial cell in response to damage by ultraviolet light, certain chemicals, or other mutagens. Such agents interfere with DNA replication, causing large gaps in newly synthesized DNA strands. Hence the main functions of the SOS response are to fill these gaps and maintain the integrity of the cell's DNA, and to suspend cell division, and hence further rounds of DNA replication until repairs have been made. In E. coli the key to this response is the RecA protein. This is involved in postreplicative repair, which is one of the SOS repair mechanisms. Moreover, activation of RecA by DNA damage causes it to cleave another protein, LexA, which is a repressor for several DNA repair genes. Inactivation of LexA causes these genes to become activated, and their products are instrumental in filling gaps in damaged DNA. However, some of the SOS repair mechanisms are error-prone, introducing mutations in the nucleotide sequences, and it is these repair mechanisms that are the direct cause of mutations in such circumstances. In cells infected with lambda prophage, activation of RecA protein causes derepression of the prophage genes, leading to vegetative growth of the phage and ultimately host cell lysis. Hence the phage has evolved a means of detecting the SOS response, and is able to `escape' from a damaged host.

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Southern blotting

A chromatographic technique for isolating and identifying specific fragments of DNA, such as the fragments formed as a result of DNA cleavage by restriction enzymes. The mixture of fragments is subjected to electrophoresis through an agarose gel, followed by denaturation to form single-stranded fragments. These are transferred, or `blotted', onto a nitrocellulose filter where they are immobilized in their relative positions. Specific gene probes labelled with a radioisotope are then added. These hybridize with any complementary fragments on the filter, which are subsequently revealed by autoradiography. The technique was devised by US biologist E. M. Southern (1938– ). A similar technique for detecting RNA fragments is called Northern blotting , by analogy. See also Western blotting.

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spadix

A flowering shoot (a type of spike) with a large fleshy floral axis bearing small, usually unisexual, flowers. It is protected by a large petal-like bract, the spathe , and is characteristic of plants of the family Araceae (e.g. calla lily).

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spathe

See spadix.

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special creation

The belief, in accordance with the Book of Genesis, that every species was individually created by God in the form in which it exists today and is not capable of undergoing any change. It was the generally accepted explanation of the origin of life until the advent of Darwinism. The idea has recently enjoyed a revival, especially among members of the fundamentalist movement in the USA, partly because there still remain problems that cannot be explained entirely by Darwinian theory. However, special creation is contradicted by fossil evidence and genetic studies, and the pseudoscientific arguments of creation science cannot stand up to logical examination.

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specialization

1 Increasing adaptation of an organism to a particular environment.

2 See physiological specialization.
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speciation

The development of one or more species from an existing species. It occurs when sympatric or allopatric populations diverge so much from the parent population that interbreeding can no longer occur between them. See also parapatric speciation.

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species

A category used in the classification of organisms that consists of a group of similar individuals that can usually breed among themselves and produce fertile offspring. Similar or related species are grouped into a genus. Within a species groups of individuals may become reproductively isolated because of geographical or behavioural factors. Such populations may, because of different selection pressures, develop different characteristics from the main population and so form a distinct subspecies. See also binomial nomenclature.

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species diversity

See biodiversity.

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spectrin

A protein that forms part of the fibrous network (cytoskeleton) underlying the plasma membrane of erythrocytes and is thought to be involved in maintaining the biconcave shape of these blood cells.

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spectroscopy

The study of methods of producing and analysing spectra using spectroscopes and other instruments. Spectroscopy is employed in a wide range of biological research areas, such as biochemistry and toxicology, for the identification of metabolites and other compounds of biological significance. See mass spectroscopy.

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spectrum

(pl. spectra) A range of electromagnetic energies arrayed in order of increasing or decreasing wavelength or frequency. The emission spectrum of a body or substance is the characteristic range of radiations it emits when it is heated, bombarded by electrons or ions, or absorbs photons. The absorption spectrum of a substance is produced by examining, through the substance and through a spectroscope, a continuous spectrum of radiation. The energies removed from the continuous spectrum by the absorbing medium show up as black lines or bands; with a substance capable of emitting a spectrum these are in exactly the same positions in the spectrum as the emission lines and bands would occur in the emission spectrum.
Emission and absorption spectra may show a continuous spectrum , a line spectrum , or a band spectrum . A continuous spectrum contains an unbroken sequence of frequencies over a relatively wide range; it is produced by incandescent solids, liquids, and compressed gases. Line spectra are discontinuous lines produced by excited atoms and ions as they fall back to a lower energy level. Band spectra (closely grouped bands of lines) are characteristic of molecular gases or chemical compounds. Absorption spectra of chlorophylls and other photosynthetic pigments are important in the study of photosynthesis. See action spectrum.

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sperm

1 A single spermatozoon.

2 Spermatozoa, collectively.
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spermatheca (seminal receptacle)

A sac or receptacle in some female or hermaphrodite animals (e.g. earthworms) in which sperm from the mate is stored until the eggs are ready to be fertilized.

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spermatid

A nonmotile cell, produced during spermatogenesis, that subsequently differentiates into a mature spermatozoon. Four spermatids are formed after two meiotic divisions of a primary spermatocyte and therefore contain the haploid number of chromosomes.

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spermatium

(pl. spermatia) A type of nonmotile male sex cell that is produced by the red algae (Rhodophyta) and by certain fungi (e.g. the rusts).

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spermatocyte

A diploid cell in the testis that divides by meiosis to give rise to four spermatids (see spermatogenesis). A primary spermatocyte develops from a spermatogonium. Two secondary spermatocytes result from the first meiotic division of a primary spermatocyte; each of these produces two spermatids after the second meiotic division.

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spermatogenesis

The series of cell divisions in the testis that results in the production of spermatozoa. Within the seminiferous tubules of the testis germ cells grow and divide by mitosis to produce spermatogonia. These divide by mitosis to produce spermatocytes, which divide by meiosis to produce spermatids. The spermatids, which thus have half the number of chromosomes of the original germ cells, then develop into spermatozoa.

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spermatogonium

(pl. spermatogonia) Any of the diploid cells in the walls of the seminiferous tubules of the testis that give rise to the primary spermatocytes. See also spermatogenesis.

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Spermatophyta

In traditional classifications, a division of the plant kingdom containing plants that reproduce by means of seeds. In modern systems seed plants are grouped into separate phyla, the most important of which are the Anthophyta and Coniferophyta.

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spermatozoid

See antherozoid.

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spermatozoon (sperm)

The mature mobile reproductive cell (see gamete) of male animals, which is produced by the testis (see spermatogenesis). It consists of a head section containing a haploid nucleus and an acrosome , which allows the sperm to penetrate the egg at fertilization; a middle section containing mitochondria to provide the energy for movement; and a tail section, which contains a single undulipodium that lashes to drive the sperm forward.

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sperm competition

Competition between sperm from different males to reach and fertilize the egg cell of a single female. Sperm competition can occur among rodents in which a male mates a number of times with the same female, with a rest period between successive matings during which the sperm journeys towards the egg. If a second male mates with the female during a rest period its own sperm may disrupt the movement of sperm from the first male and succeed in fertilizing the egg cell. Certain animals in which sperm competition is possible have evolved features to minimize this interference. For example, in moths and butterflies the male cements the opening of the female genitalia after mating, thereby preventing further matings with other males. An ingenious mechanism operates in the fly Johannseniella nitida, in which the female eats a copulating male except for his genitalia, which remain in the body of the female and prevent further mating.

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Sphenophyta

A phylum of tracheophyte plants, the only living members of which are the horsetails (Equisetum). Horsetails have a perennial creeping rhizome supporting erect jointed stems bearing whorls of thin leaves. Spores are produced by terminal conelike structures. The group has a fossil record extending back to the Palaeozoic with its greatest development in the Carboniferous period, when giant tree forms were the dominant vegetation with the Lycophyta.

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spherosome (oleosome)

A small spherical organelle found in the cytoplasm of plant cells. Up to 1mm in diameter and bounded by a single membrane, it synthesizes and stores lipids.

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sphincter

A specialized muscle encircling an opening or orifice. Contraction of the sphincter tends to close the orifice. Examples are the anal sphincter (round the opening of the anus) and the pyloric sphincter (at the lower opening of the stomach).

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sphingolipid

See phospholipid.

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spiders

See Arachnida.

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spike

A type of racemose inflorescence in which stalkless flowers arise from an undivided floral axis, as in plantain and Orchis. In the family Gramineae (Poaceae; sedges and grasses) the flowers are grouped in clusters called spikelets (see illustration), which may be arranged to form a compound spike (as in wheat). See glume; lemma.

Structure of a spikelet

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spinal column

See vertebral column.

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spinal cord

The part of the vertebrate central nervous system that is posterior to the brain and enclosed within the vertebral column. It consists of a hollow core of grey matter (H-shaped in cross section) surrounded by an outer layer of white matter; the central cavity contains cerebrospinal fluid. The white matter contains numerous longitudinal nerve fibres organized into distinct tracts: ascending tracts consist of sensory neurones, conducting impulses towards the brain; descending tracts consist of motor neurones, transmitting impulses from the brain. Paired spinal nerves arise from the spinal cord.

Transverse section of the spinal cord

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spinal nerves

Pairs of nerves that arise from the spinal cord (compare cranial nerves). In man there are 31 pairs (one from each of the vertebrae). Each nerve arises from a dorsal root and a ventral root and contains both motor and sensory fibres (i.e. they are mixed nerves). The spinal nerves form an important part of the peripheral nervous system.

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spinal reflex

See reflex.

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spindle

1 A structure formed from microtubules in the cytoplasm of cells at the beginning of the metaphase stage of cell division (see meiosis; mitosis). Two poles become established at diametrically opposite points close to the nucleus. The spindle microtubules, or `fibres', radiate from each pole towards the opposite one and diverge to their greatest extent midway between. This widest part of the spindle is the equator . Chromosomes are attached by their centromeres to the spindle fibres at the equator.

2 See muscle spindle.
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spindle attachment

See centromere.

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spine

1 See vertebral column.

2 A hard pointed protective structure on a plant that is formed through modification of a leaf, part of a leaf, or a stipule. The edge of the holly leaf is drawn out into spines, but in cacti the whole leaf is modified as a spine. Compare prickle; thorn.
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spinneret

A small tubular appendage from which silk is produced in spiders and some insects. Spiders have four to six spinnerets on the hind part of the abdomen, into which numerous silk glands open. The silk is secreted as a fluid and hardens on contact with the air. Various types of silk are produced depending on its use (e.g. for webs, egg cocoons, etc.). The spinnerets that produce the cocoons of insects are not homologous with those of spiders. For example, the spinneret of the silkworm is in the pharynx and the silk is produced by modified salivary glands.

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spinocerebellar tracts

Pathways of neurones that transfer sensory information from the spinal cord to the cerebellum of the brain.

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spiracle

1 A small paired opening that occurs on each side of the head in cartilaginous fish. It is the reduced first gill slit, its small size resulting from adaptations of the skeleton for the firm attachment of the jaws. In modern teleosts (bony fish) the spiracle is closed up. In tetrapods the first gill slit develops into the middle ear cavity.

2 Any of the external openings of the tracheae along the side of the body of an insect.
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spirillum

Any rigid spiral-shaped bacterium. Generally, spirilla are Gram-negative (see Gram's stain), aerobic, and highly motile, bearing flagella either in tufts or singly. They occur in soil and water, feeding on organic matter.

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spirochaete

Any nonrigid corkscrew-shaped bacterium that moves by means of flexions of the cell, produced by a series of rotatory axial fibrils underlying the cell's outer sheath. Most spirochaetes are Gram-negative (see Gram's stain), anaerobic, and feed on dead organic matter. They are particularly common in sewage-polluted waters. Some, however, cause disease in humans and other animals; Treponema, the agent of syphilis, is an example.

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spirometer

See respirometer.

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spleen

A vertebrate organ, lying behind the stomach, that is basically a collection of lymphoid tissue. Its functions include producing lymphocytes and destroying foreign particles. It acts as a reservoir for erythrocytes and can regulate the number in circulation. It is also the site for the breakdown of worn-out erythrocytes and it stores the iron they contain.

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spliceosome

See intron.

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splicing

See gene splicing.

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sponges

See Porifera.

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spongy bone

See bone.

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spongy mesophyll

See mesophyll.

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spontaneous generation

The discredited belief that living organisms can somehow be produced by nonliving matter. For example, it was once thought that microorganisms arose by the process of decay and even that vermin spontaneously developed from household rubbish. Controlled experiments using sterilized media by Pasteur and others finally disproved these notions. Compare biogenesis. See also biopoiesis.

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sporangiophore

The simple or branched stalk that bears one or more sporangia.

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sporangium

(pl. sporangia) A reproductive structure in plants that produces asexual spores. A megasporangium produces megaspores, which give rise to the female gametophyte; in seed plants it is represented by the ovule. A microsporangium produces microspores, which give rise to the male gametophyte; it is represented in seed plants by the pollen sac. See also sporophyll.

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spore

A reproductive cell that can develop into an individual without first fusing with another reproductive cell (compare gamete). Spores are produced by plants, fungi, bacteria, and some protoctists. A spore may develop into an organism resembling the parent or into another stage in the life cycle, either immediately or after a period of dormancy. In plants showing alternation of generations, spores are formed by the sporophyte generation and give rise to the gametophyte generation. In ferns, the rows of brown reproductive structures on the undersurface of the fronds are spore-producing bodies.

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spore mother cell (sporocyte)

A diploid cell that gives rise to four haploid spores by meiosis. See also megaspore mother cell; microspore mother cell.

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sporocyte

See spore mother cell.

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sporogonium

The sporophyte generation in mosses and liverworts. It is made up of an absorptive foot , a stalk (seta ), and a spore-producing capsule . It may be completely or partially dependent on the gametophyte.

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sporophore (fructification)

The aerial spore-producing part of certain fungi; for example, the stalk and cap of a mushroom.

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sporophyll

A leaf that bears sporangia (spore-producing structures). In ferns the sporophylls are the normal foliage leaves, but in other plants the sporophylls are modified and arise in specialized structures such as the strobilus (cone) of clubmosses and horsetails and gymnosperms and the flower of angiosperms. Most plants produce spores of two different sizes (small microspores and large megaspores ). The sporophylls bearing these are called microsporophylls and megasporophylls respectively.

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sporophyte

The generation in the life cycle of a plant that produces spores. The sporophyte is diploid but its spores are haploid. It is either completely or partially dependent on the gametophyte generation in mosses and liverworts, but is the dominant plant in the life cycle of clubmosses, horsetails, ferns, and seed plants. See also alternation of generations.

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Sporozoa

See Apicomplexa.

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sport

An individual organism that shows the effects of a mutation. The term is usually applied to atypical forms of plants in horticulture.

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squalene

An intermediate compound formed in the synthesis of cholesterol; it is a hydrocarbon containing 30 carbon atoms. The immediate oxidation of squalene to squalene 2,3-epoxide is the last common step in the synthesis of sterols in animals, plants, and fungi.

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Squamata

An order of reptiles comprising the lizards and snakes. They appeared at the end of the Triassic period, about 170 million years ago, and have invaded a wide variety of habitats. Most lizards have four legs and a long tail, eardrums, and movable eyelids. Snakes are limbless reptiles that lack eardrums; the eyes are covered by transparent immovable eyelids and the articulation of the jaws is very loose, enabling a wide gape to facilitate swallowing prey whole.

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squamous epithelium

A type of epithelium that is made up of flattened cells and therefore presents a small distance over which substances have to pass. This type of epithelium is found lining the alveoli in the lungs and Bowman's capsule in the kidney.

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SSSI (Site of Special Scientific Interest)

The legal designation for an area of land in England, Scotland, or Wales that has been identified by English Nature, Scottish National Heritage, or the Countryside Council for Wales as being of special interest because of its flora, fauna, or geological or physiographical features. Such sites are protected from development activities and funds are available for their conservation and management. There are over 6000 SSSIs in Britain; similar sites in Northern Ireland are designated Areas of Special Scientific Interest (ASSIs).

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stabilizing selection

Natural selection that acts to maintain the constancy of a species over successive generations. It involves selection against the extremes of the range of phenotypes for a particular characteristic. For example, babies whose birth weight is substantially below or above the average of 3.6 kg historically have a greater mortality than babies of average birth weight (although medical advances have now greatly reduced this pattern of selection in humans). Compare directional selection; disruptive selection.

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staining

A technique in which cells or thin sections of biological tissue that are normally transparent are immersed in one or more coloured dyes (stains ) to make them more clearly visible through a microscope. Staining heightens the contrast between the various cell or tissue components. Stains are usually organic salts with a positive and negative ion. If the colour comes from the negative ion (organic anion), the stain is described as acidic , e.g. eosin. If the colour comes from the positive ion (organic cation), the stain is described as basic , e.g. haematoxylin. Neutral stains have a coloured cation and a coloured anion; an example is Leishman's stain. Cell constituents are described as being acidophilic if they are stained with acidic dyes, basophilic if receptive to basic dyes, and neutrophilic if receptive to neutral dyes. Vital stains are used to colour the constituents of living cells without harming them (see vital staining); nonvital stains are used for dead tissue.
Counterstaining involves the use of two or more stains in succession, each of which colours different cell or tissue constituents. Temporary staining is used for immediate microscopical observation of material, but the colour soon fades and the tissue is subsequently damaged. Permanent staining does not distort the cells and is used for tissue that is to be preserved for a considerable period of time.
Electron stains , used in the preparation of material for electron microscopy, are described as electron-dense as they interfere with the transmission of electrons. Examples are lead citrate, phosphotungstic acid (PTA), and uranyl acetate (UA).

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stamen

One of the male reproductive parts of a flower. It consists of an upper fertile part (the anther) on a thin sterile stalk (the filament ).

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staminode

A sterile stamen.

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standard deviation

A measure of the dispersion of data in statistics. For a set of values a_1, a_2, a_3, … an, the mean m is given by (a_1 + a_2 + … + an)/n. The deviation of each value is absolute value of the difference from the mean |m – a_1|, etc. The standard deviation is the square root of the mean of the squares of these values, i.e.
[(|m – a_1|2 + … |m – an|2)/n]

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standard error of the mean (SEM)

The standard deviation divided by the square root of the number of samples. Whereas the standard deviation of a population provides a measure of the scatter of the data, the SEM is a measure of the accuracy of the calculated population mean.

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standing biomass

See standing crop.

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standing crop

The total amount of living material in a specified population at a particular time, expressed as biomass (standing biomass ) or its equivalent in terms of energy. The standing crop may vary at different times of the year; for example, in a population of deciduous trees between summer and winter.

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stapes (stirrup)

The third of the three ear ossicles of the mammalian middle ear.

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Staphylococcus

A genus of spherical nonmotile Gram-positive bacteria that occur widely as saprotrophs or parasites. The cells occur in grapelike clusters. Many species inhabit the skin and mucous membranes, and some cause disease in humans and animals. S. aureus infection can lead to boils and abscesses in humans; this species also produces toxins that irritate the gastrointestinal tract and result in staphylococcal food poisoning. Certain strains are resistant to antibiotics, and infection with these is very difficult to treat.

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starch

A polysaccharide consisting of various proportions of two glucose polymers, amylose and amylopectin. It occurs widely in plants, especially in roots, tubers, seeds, and fruits, as a carbohydrate energy store. Starch is therefore a major energy source for animals. When digested it ultimately yields glucose. Starch granules are insoluble in cold water but disrupt if heated to form a gelatinous solution.

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start codon (initiation codon)

The triplet of nucleotides on a messenger RNA molecule (see codon) at which the process of translation is initiated. In eukaryotes the start codon is AUG (see genetic code), which codes for the amino acid methionine; in bacteria the start codon can be either AUG, coding for N-formyl methionine, or GUG, coding for valine. Compare stop codon.

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startle display

A response by an animal to discovery by a predator in which the potential victim exposes some previously hidden markings, for example an eyespot, in order to surprise its attacker. This may enable the victim to escape or divert the attentions of the predator elsewhere. Some camouflaged insects use this as a second line of defence, flicking their forewings forwards when disturbed to reveal a pair of vividly coloured eyespots on the hindwings.

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stationary phase

1 See bacterial growth curve.

2 See chromatography.
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statoblast

A chitinous bud that is produced asexually by members of the Bryozoa. It stores food and is able to remain dormant for long periods, withstanding drought and extreme temperatures. Statoblasts may be liberated and dispersed after the death of the parent zooid, and subsequently `germinate' to produce a zooid capable of founding a new colony.

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statocyst (otocyst)

A balancing organ found in many invertebrates. It consists of a fluid-filled sac lined with sensory hairs and contains granules of calcium carbonate, sand, etc. (statoliths ). As the animal moves the statoliths stimulate different hairs, giving a sense of the position of the body or part of it. The semicircular canals in the ears of vertebrates act on the same principle and have a similar function.

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statocyte

Any plant cell that contains statoliths.

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statolith

1 See statocyst.

2 A membrane-bound group of starch grains (see amyloplast) in plant cells that is believed to act as a sensor to gravity. Starch statoliths are found in cells at the root tips and in the tissues close to the vascular bundles in shoots; under the influence of gravity they sink to the bottom of the cell. Their mechanism of action in triggering the transport of growth substances across the cell membrane is not understood. One theory is that the statoliths exert pressure on membrane systems inside the cell, e.g. the endoplasmic reticulum, which in turn causes differential growth.
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stearic acid (octadecanoic acid)

A solid saturated fatty acid, CH_3(CH_2)16COOH, that occurs widely (as glycerides) in animal and vegetable fats.

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stele

The vascular tissue (i.e. xylem and phloem) of tracheophyte plants, together with the endodermis and pericycle (when present). The arrangement of stelar tissues is very variable. In roots the stele often forms a solid core, which better enables the root to withstand tension and compression. In stems it is often a hollow cylinder separating the cortex and pith. This arrangement makes the stem more resistant to bending stresses. Monocotyledons and dicotyledons can usually be distinguished by the pattern of their stelar tissue. In monocotyledons the vascular bundles are scattered throughout the stem whereas in dicotyledons (and gymnosperms) they are arranged in a circle around the pith.

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stem

The part of a plant that usually grows vertically upwards towards the light and supports the leaves, buds, and reproductive structures (see illustration). The leaves develop at the nodes and side or branch stems develop from buds at the nodes. The stems of certain species are modified as bulbs, corms, rhizomes, and tubers. Some species have twining stems; others have horizontal stems, such as runners. Another modification is the cladode. Erect stems may be cylindrical or angular; they may be covered with hairs, prickles, or spines and many exhibit secondary growth and become woody (see growth ring). In addition to its supportive function, the stem contains vascular tissue that conducts food, water, and mineral salts between the roots and leaves. It may also contain chloroplasts and carry out photosynthesis.

Transverse section through a herbaceous stem

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stem cell

A cell that is not differentiated itself but can undergo unlimited division to form other cells, which either remain as stem cells or differentiate to form specialized cells. For example, stem cells in the bone marrow divide to produce daughter cells that differentiate into various types of immune cell (e.g. monocytes, lymphocytes, mast cells). See also haemopoietic tissue.

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steno-

A prefix denoting narrowness. For example, stenohaline aquatic organisms can tolerate only small variations in the salinity of the water. Compare eury-.

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stereoisomerism

The existence of chemical compounds (stereoisomers ) that have the same molecular formulae and functional groups, but differ in the arrangement of groups in space. Optical isomerism is one form of this (see optical activity).

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sterigma

(pl. sterigmata) A small spore-bearing stalk found in certain fungi. In the Basidiomycota, sterigmata develop as outgrowths of a basidium, each bearing a basidiospore at its tip.

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sterile

1 (of organisms) Unable to produce offspring. See also hybrid; incompatibility; self-sterility; sterilization.

2 (of objects, food, etc.) Free from contaminating microorganisms. See sterilization.
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sterilization

1 The process of destroying microorganisms that contaminate food, wounds, surgical instruments, etc. Common methods of sterilization include heat treatment (see autoclave; pasteurization) and the use of disinfectants and antiseptics.

2 The operation of making an animal or human incapable of producing offspring. Men are usually sterilized by tying and then cutting the vas deferens (vasectomy ); in women the operation often involves permanently blocking the fallopian tubes by means of clips (tubal occlusion ). See also birth control.
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sternum (breastbone)

1 A shield-shaped or rod-shaped bone in terrestrial vertebrates, on the ventral side of the thorax, that articulates with the clavicle (collar bone) of the pectoral girdle and with most of the ribs. It is absent in fish, and in birds it bears a keel.

2 The ventral portion of each segment of the exoskeleton of arthropods.
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steroid

Any of a group of lipids derived from a saturated compound called cyclopentanoperhydrophenanthrene, which has a nucleus of four rings (see formulae). Some of the most important steroid derivatives are the steroid alcohols, or sterols. Other steroids include the bile acids, which aid digestion of fats in the intestine; the sex hormones (androgens and oestrogens); and the corticosteroid hormones, produced by the adrenal cortex. Vitamin D is also based on the steroid structure.

Steroid structure

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sterol

Any of a group of steroid-based alcohols having a hydrocarbon side-chain of 8–10 carbon atoms. Sterols exist either as free sterols or as esters of fatty acids. Animal sterols (zoosterols ) include cholesterol and lanosterol. The major plant sterol (phytosterol ) is beta-sitosterol, while fungal sterols (mycosterols ) include ergosterol.

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sticky end (cohesive end)

A single unpaired strand of nucleotides protruding from the end of a double-stranded DNA molecule. It is able to join with a complementary single strand, e.g. the sticky end of another DNA molecule, thus forming a single large double-stranded molecule. Sticky ends provide a means of annealing segments of DNA in genetic engineering, e.g. in the packaging of vectors.

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stigma

1 The glandular sticky surface at the tip of a carpel of a flower, which receives the pollen. In insect-pollinated plants the stigmas are held within the flower, whereas in wind-pollinated species they hang outside it.

2 See eyespot.
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stilt root

See prop root.

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stimulus

Any change in the external or internal environment of an organism that provokes a physiological or behavioural response in the organism. In an animal specific receptors are sensitive to stimuli.

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stimulus filtering

The process of separating useful sensory information from the many thousands of stimuli present in the external environment, so that only potentially useful information is sent to the brain. Many sensory organs are adapted so that they receive stimuli only within a certain range. For example, the human eye only detects colours in the visible region of the spectrum and not in the ultraviolet or infrared regions. Another method of stimulus filtering involves sign stimuli, which are more important than other stimuli in eliciting a response and, once detected, may lead to other stimuli being ignored.

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stipe

1 The stalk that forms the lower portion of the fruiting body of certain fungi, such as mushrooms, and supports the umbrella-shaped cap.

2 The stalk between the holdfast and blade (lamina ) of certain brown algae, notably kelps.
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stipule

An outgrowth from the petiole or leaf base of certain plants. Those of the garden pea are leaflike photosynthetic organs. The stipules of the lime tree are scalelike and protect the winter buds, whereas those of the false acacia (Robinia) are modified as spines.

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stock

See graft.

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stolon

A long aerial side stem that gives rise to a new daughter plant when the bud at its apex touches the soil. Plants that multiply in this way include blackberry and currant bushes. Gardeners often pin down stolons to the soil to aid the propagation of such plants. This process is termed layering .

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stoma

(pl. stomata) A pore, large numbers of which are present in the epidermis of leaves (especially on the undersurface) and young shoots. Stomata function in gas exchange between the plant and the atmosphere. Each stoma is bordered by two semicircular guard cells (specialized epidermal cells), whose movements (due to changes in water content) control the size of the aperture. The term stoma is also used to mean both the pore and its associated guard cells.

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stomach

The portion of the vertebrate alimentary canal between the oesophagus and the small intestine. It is a muscular organ, capable of dramatic changes in size and shape, in which ingested food is stored and undergoes preliminary digestion. Cells lining the stomach produce gastric juice, which is thoroughly mixed with the food by muscular contractions of the stomach. The resultant acidic partly digested food mass (chyme) is discharged into the duodenum through the pyloric sphincter for final digestion and absorption. Some herbivorous animals (see Ruminantia) have multichambered stomachs from which food is regurgitated, rechewed, and swallowed again.

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stomium

A region of thin-walled cells in certain spore-producing structures that ruptures to release the spores. For example, in the sporangium of the fern Dryopteris the stomium ruptures when the annulus dries out.

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stop codon (termination codon)

The triplet of nucleotides on a messenger RNA molecule (see codon) at which the process of translation ends. It cannot code for an amino acid as there is no anticodon that complements it: this effectively stops the formation of a polypeptide chain at that point. The three stop codons are UGA, UAA, and UAG (see genetic code). Compare start codon.

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storage compound

See food reserves.

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stratification

1 The arrangement of the components of an entity in layers (strata ). Stratification is a feature of sedimentary rocks and soils. It is also seen in stratified epithelium, and thermal stratification can occur in some lakes (see thermocline).

2 The practice of placing certain seeds between layers of peat or sand and then exposing them to low temperatures for a period, which is required before they will germinate. See vernalization.
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stratified epithelium

Epithelium that is made up of a number of layers of cells and is present in areas of the body that are subject to wear and tear. Stratified epithelium containing keratin forms the outer layer of the skin; stratified epithelial cells that do not contain keratin are found in moist areas, such as the mouth and vagina.

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stratosphere

The layer of the earth's atmosphere that lies above the troposphere and extends to about 50 km above the earth's surface. The temperature within the stratosphere remains fairly constant but can rise in the upper regions of this layer due to absorption of ultraviolet radiation by ozone. See ozone layer.

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stratum corneum

The layers of dead keratinized cells that form the outermost layers of mammalian epidermis. The stratum corneum provides a water-resistant barrier between the external environment and the living cells of the skin.

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Streptococcus

A genus of spherical Gram-positive bacteria occurring widely in nature, typically as chains or pairs of cells. Many are saprotrophic and exist as usually harmless commensals inhabiting the skin, mucous membranes, and intestine of humans and animals. Others are parasites, some of which cause diseases, including scarlet fever (S. pyogenes; group A streptococci), endocarditis (S. viridans), and pneumonia (S. pneumoniae).

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streptomycin

See Actinobacteria; antibiotics.

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stretch receptor

A specialized group of cells found in muscles (see muscle spindle) or tendons that is sensitive to mechanical stress. When muscles or tendons are stretched, the strain is recorded by the stretch receptors.

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stretch reflex (myotactic reflex)

The reflex initiated when a muscle is stretched; an example is the knee-jerk reflex . Stretching of a muscle causes impulses to be generated in the muscle spindles. These impulses are transmitted by sensory neurones to the spinal cord, where the sensory neurones synapse with motor neurones; these initiate contraction of the same muscle so that it returns to its original length. Since the reflex action involves the transmission of impulses across only one set of synapses, the response is rapid and described as monosynaptic .

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striated muscle

See voluntary muscle.

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stridulation

The production of sounds by insects rubbing one part of the body against another. The parts of the body involved vary from species to species. Stridulation is typical of the Orthoptera (grasshoppers, crickets, cicadas), in which the purpose of the sounds is usually to bring the sexes together, although they are also used in territorial behaviour, warning, etc.

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strobilus

1 A type of composite fruit that is formed from a complete inflorescence. It produces achenes enclosed in bracts and when mature becomes cone-shaped. The hop fruit is an example.

2 See cone.
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stroke volume

The volume of blood ejected at each stroke of the heart during systole. A typical stroke volume for a normal person at rest is 70ml; this increases to a maximum of 120ml during strenuous exercise. See cardiac output.

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stroma

Tissue that forms the framework of an organ; for example, the tissue of the ovary that surrounds the reproductive cells, or the gel-like matrix of chloroplasts that surrounds the grana.

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stromatolite

A rocky cushion-like mass formed by the unchecked growth of millions of lime-secreting blue-green bacteria (see Cyanobacteria). Stromatolites are found only in areas where other organisms that would normally keep down the bacterial numbers cannot survive, such as extremely salty bays. Such bacteria were abundant during the Proterozoic and Archaean eons, perhaps from as early as 3900 million years ago. The white rings of fossilized microorganisms found in rocks of this age are the remains of stromatolites.

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strontium

Symbol Sr. A soft yellowish metallic element. The isotope strontium–90 is present in radioactive fallout (half-life 28 years), and can be metabolized with calcium so that it collects in bone.

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structural gene

See operon.

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strychnine

A colourless poisonous crystalline alkaloid found in certain plants.

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style

The stalk of a carpel, between the stigma and the ovary. In many plants it is elongated to aid pollination.

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subarachnoid space

The space between the arachnoid membrane and the pia mater, two of the membranes (meninges) that surround the brain and spinal cord. It is filled with cerebrospinal fluid.

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subclavian artery

A paired artery that passes beneath the collar bone (clavicle) and branches to supply blood to the arm. The left subclavian artery arises from the aorta; the right from the innominate artery.

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subcutaneous tissue

The tissue that underlies the dermis (see skin). It is made up of loose fibrous connective tissue, muscle, and fat (see adipose tissue), which in some animals (e.g. whales and hibernating mammals) forms an insulating layer or an important food store.

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suberin

A mixture of waxy substances, similar to cutin, present in the thickened cell walls of many trees and shrubs, particularly in corky tissues. The deposition of suberin (suberization ) provides a protective water-impermeable layer. See also Casparian strip.

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sublittoral

1 Designating or occurring in the shallow-water zone of a sea, over the continental shelf and below the low tide mark.

2 Designating or occurring in the zone of a lake below the littoral zone, to a depth of 6–10 metres.
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submucosa

A layer of areolar connective tissue that lies beneath the mucosa, for example in the stomach and intestine.

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subsoil

See soil.

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subspecies

A group of individuals within a species that breed more freely among themselves than with other members of the species and resemble each other in more characteristics. Reproductive isolation of a subspecies may become so extreme that a new species is formed (see speciation). Subspecies are sometimes given a third Latin name, e.g. the mountain gorilla, Gorilla gorilla beringei (see also binomial nomenclature).

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substitution

(in genetics) A point mutation in which one base pair in the DNA sequence is replaced by another. There are two types of substitution mutation: transition mutations, in which a pyrimidine base (i.e. thymine or cytosine) is replaced by another pyrimidine base or a purine base (adenine or guanine) is replaced by another purine base; and transversion mutations, in which a pyrimidine base is replaced by a purine base, or vice versa. Most substitutions tend to alter one amino acid in a protein chain, which may or may not affect the functioning of that protein. Sickle-cell anaemia is an example of a substitution mutation in which thymine is replaced by adenine in the triplet coding for the sixth amino acid in the ß-chain of haemoglobin.

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substrate

1 (in biochemistry) The substance upon which an enzyme acts in biochemical reactions.

2 (in biology) The material on which a sedentary organism (such as a barnacle or a plant) lives or grows. The substrate may provide nutrients for the organism or it may simply act as a support.
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subtilisin

Any of a group of protein-digesting enzymes (see protease) produced by Bacillus subtilis and related species. Subtilisins have broad specificities and work over a wide range of pH values. Subtilisin Carlsberg is an alkaline protease, derived from Bacillus licheniformis, that is used widely in detergents. Roughly 30–40% of the peptide bonds in casein, a milk protein, are hydrolysed by subtilisin Carlsberg.

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succession

(in ecology) The sequence of communities that develops in an area from the initial stages of colonization until a stable mature climax community is achieved. Many factors, including climate and changes brought about by the colonizing organisms, influence the nature of a succession; for example, after many years shrubs produce soil deep enough to support trees, which then shade out the shrubs. See also sere.

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succinate

A salt of succinic (butanedioic) acid, HOOC(CH_2)2COOH, a four-carbon fatty acid. Succinate occurs in living organisms as an intermediate in metabolism, especially in the Krebs cycle.

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succulent

A plant that conserves water by storing it in fleshy leaves or stems. Succulents are found either in dry regions or in areas where there is sufficient water but it is not easily obtained, as in salt marshes. Such plants are often modified to reduce water loss by transpiration. For example, the leaves of cacti are reduced to spines. See also crassulacean acid metabolism; xerophyte.

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succus entericus

The alkaline secretion produced by glands in the wall of the duodenum, consisting of water, mucoproteins, and hydrogencarbonate ions. It helps to counteract the highly acidic and proteolytic chyme entering the small intestine from the stomach, and thus protects the duodenum from damage. It was formerly believed to contain digestive enzymes, but duodenal enzymes are now known to be confined to cells of the brush border.

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sucker (turion)

A shoot that arises from an underground root or stem and grows at the expense of the parent plant. Suckers can be dug up with a portion of root attached and used to propagate a plant. If, however, a plant is grafted onto a different rootstock, as many roses are, any suckers will be of the wild rootstock, rather than the ornamental scion, and must be removed.

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sucrase

A carbohydrate-digesting enzyme, produced in the brush border of the small intestine, that breaks down the disaccharide sucrose into the monosaccharides glucose and fructose.

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sucrose (cane sugar; beet sugar; saccharose)

A sugar comprising one molecule of glucose linked to a fructose molecule. It occurs widely in plants and is particularly abundant in sugar cane and sugar beet (15–20%), from which it is extracted and refined for table sugar. If heated to 200°C, sucrose becomes caramel.

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sugar (saccharide)

Any of a group of water-soluble carbohydrates of relatively low molecular weight and typically having a sweet taste. The simple sugars are called monosaccharides. More complex sugars comprise between two and ten monosaccharides linked together: disaccharides contain two, trisaccharides three, and so on. The name is often used to refer specifically to sucrose (cane or beet sugar).

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sulpha drugs

See sulphonamides.

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sulphonamides

Organic compounds containing the group –SO_2.NH_2. The sulphonamides are amides of sulphonic acids. Many have antibacterial action and are also known as sulpha drugs , including sulphadiazine, NH_2C_6H_4SO_2NHC_4H_3N_2, sulphathiazole, NH_2C_6H_4SO_2NHC_5H_2NS, and several others. They act by preventing bacteria from reproducing and are used to treat a variety of bacterial infections, especially of the gut and urinary system.

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sulphur

Symbol S. A yellow nonmetallic element that is an essential element in living organisms, occurring in the amino acids cysteine and methionine and therefore in many proteins. It is also a constituent of various cell metabolites, e.g. coenzyme A. Sulphur is absorbed by plants from the soil as the sulphate ion (SO_42–). See sulphur cycle.

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sulphur bacteria

Any of various groups of unrelated bacteria that utilize sulphur, sulphide, or sulphate in their metabolism. The anaerobic photoautotrophic green sulphur bacteria and purple sulphur bacteria can use hydrogen sulphide as a source of electrons in photosynthesis, producing sulphur as a by-product:
2H_2S + CO_2 H_2O + CH_2O + 2S

The chemoautotrophic sulphur-oxidizing bacteria , such as Thiobacillus, derive energy from the oxidation of sulphur or its compounds (e.g. sulphide), producing sulphate (SO_42–). The anaerobic heterotrophic sulphate-reducing bacteria , such as Desulfovibrio, require sulphate for respiration, deriving energy from its reduction to sulphur or hydrogen sulphide. The aerobic filamentous sulphur bacteria (e.g. Beggiatoa), which belong to the phylum Myxobacteria (gliding bacteria), can grow by oxidizing sulphides to sulphates.

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sulphur bridge

See disulphide bridge.

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sulphur cycle

The cycling of sulphur between the biotic (living) and abiotic (nonliving) components of the environment (see biogeochemical cycle). Most of the sulphur in the abiotic environment is found in rocks, although a small amount is present in the atmosphere as sulphur dioxide (SO_2), produced by combustion of fossil fuels. Sulphate (SO_42–), derived from the weathering and oxidation of rocks, is taken up by plants and incorporated into sulphur-containing proteins. In this form sulphur is passed along food chains to animals. Decomposition of dead organic matter and faeces by anaerobic sulphate-reducing bacteria returns sulphur to the abiotic environment in the form of hydrogen sulphide (H_2S). Hydrogen sulphide can be converted back to sulphate or to elemental sulphur by the action of different groups of photosynthetic and sulphide-oxidizing bacteria (see sulphur bacteria). Elemental sulphur becomes incorporated into rocks.

The sulphur cycle

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sulphur dioxide (sulphur(IV) oxide)

A colourless liquid or pungent gas, SO_2, formed by sulphur burning in air. Sulphur dioxide is produced by burning fossil fuels in power stations; it dissolves in water to give a mixture of sulphuric and sulphurous acids. See also acid rain; air pollution.

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summation

1 (in neurophysiology) The combined effect of the changes in electric potential elicited in one or more postsynaptic membranes by the transmission of impulses at synapses that is sufficient to trigger an action potential in the postsynaptic neurone. Summation occurs when one or a few postsynaptic potentials alone are insufficient to elicit a response in the postsynaptic neurone; it may consist of the effect of two or more potentials evoked simultaneously at different synapses on the same neurone (spatial summation ) or in rapid succession at the same synapse (temporal summation ).

2 See synergism.
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supercoiling

A form of DNA in which the double helix is further twisted about itself, forming a tightly coiled structure. This is the form generally adopted by DNA in nature, and enables it to condense sufficiently to be packaged into living cells. In negative supercoiling the DNA is twisted about an axis in a direction opposite to that of the clockwise turns of the (right-handed) double helix; this decreases the number of turns of one helix around the other. In positive supercoiling the twist of the supercoils is in the same direction as that of the double helix, which increases the number of turns of one helix around the other. Supercoiling must be temporarily removed when DNA replication takes place, and the degree of supercoiling can affect gene transcription. Changes in supercoiling are performed by topoisomerase enzymes.

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supercooling

The cooling of a liquid below its normal freezing point without changing from a liquid to a solid. Water is relatively amenable to supercooling, and many organisms living in cold environments exploit this property to avoid freezing of their body water and consequent tissue damage. Supercooling of extracellular fluids requires first the exclusion of potential ice-nucleating agents, such as food particles, that could promote the formation of ice crystals. Hence, animals preparing for winter frequently void their gut contents. Second, the animal manufactures antifreeze molecules, such as glycerol, which depress the freezing point. These measures can be extremely effective in achieving supercooling.

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supergene

A cluster of tightly linked genes that affect the same trait and are inherited apparently as a single unit. For example, supergenes are known to determine shell colour and patterning in terrestrial snails, and wing shape and colouring essential to mimicry in certain butterflies. This tight linkage is advantageous since it ensures that the appropriate combination of alleles for the character concerned is virtually always transmitted intact to the animal's offspring. Any rare recombinants will be selected against. A supergene can arise through inversion of part of a chromosome; crossing over within the inversion produces unbalanced recombinant chromatids, and hence zygotes containing these are nonviable. Thus, with respect to the inversion, only nonrecombinant zygotes are generally produced, giving a very low apparent rate of recombination between the loci in this region.

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superior

Describing a structure that is positioned above or higher than another structure in the body. For example, in flowering plants the ovary is described as superior when located above the other organs of the flower (see hypogyny). Compare inferior.

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supernormal stimulus

1 (in animal behaviour) A stimulus that produces a more vigorous response than the normal stimulus eliciting that particular response. For example, a female herring gull will brood a giant egg in preference to its own eggs, which are smaller. A supernormal stimulus is an exaggerated sign stimulus.

2 (in neurophysiology) A stimulus that is more intense than a normal stimulus and is capable of inducing a response in a nerve fibre during the relative refractory period.
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superoxide dismutase (SOD)

A widely distributed enzyme that removes the superoxide radical (O_2·), with the formation of molecular oxygen and hydrogen peroxide, in the following reaction:
O_2· + O_2· + 2H+ O_2 + H_2O_2

The hydrogen peroxide formed is removed by the action of catalase. The superoxide anion, which damages tissues, is a free radical formed by the partial reduction of molecular oxygen; it is produced during the metabolic breakdown of various toxins (including drugs and chemical poisons) and as part of the immune response to the presence of bacteria and virus-infected cells.

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supination

Rotation of the lower forearm so that the hand faces forwards or upwards with the radius and ulna parallel. Compare pronation.

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supplementary units

See SI units.

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suprarenal glands

See adrenal glands.

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surface tension

Symbol . The property of a liquid that makes it behave as if its surface is enclosed in an elastic skin. The property results from intermolecular forces: a molecule in the interior of a liquid experiences a force of attraction from other molecules equally from all sides, whereas a molecule at the surface is only attracted by molecules below it in the liquid. The surface tension is defined as the force acting over the surface per unit length of surface perpendicular to the force. It is measured in newtons per metre. It can equally be defined as the energy required to increase the surface area by one square metre, i.e. it can be measured in joules per metre squared (which is equivalent to Nm–1).
The surface tension of water is very strong, due to the intermolecular hydrogen bonding, and is responsible for the formation of drops, bubbles, and meniscuses (the curved surfaces of columns of liquid), as well as the rise of water in a capillary tube (capillarity ), the absorption of liquids by porous substances, and the ability of liquids to wet a surface. Capillarity is very important in plants as it contributes to the transport of water, against gravity, within the plant.

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surfactant (surface active agent)

A substance, such as a detergent, added to a liquid to increase its spreading or wetting properties by reducing its surface tension.

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survival curve

A graph that shows the relationship between the survival and age of a population of a particular species. Survival can be represented as the percentage of individuals of the original population that have survived after a specified time. Some types of organism, notably those that produce many offspring (e.g. bacteria), have a low survival rate in the early stages of development (line A on graph; see r selection). Other organisms, which have a low rate of reproduction, tend to be long-lived and their population numbers are maintained over a long period (line B on graph; see K selection). This type of survival curve is typical of a human population in a developed country.

Survival curves

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suspension culture

A type of culture in which intact cells are maintained in suspension in the culture medium so that they are distributed evenly within it. Compare monolayer culture.

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suspensor

The chain of cells that anchors a plant embryo in the surrounding gametophyte tissue. In flowering plants the suspensor attaches the embryo to the embryo sac and extends to push the embryo into the endosperm.

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suspensory ligaments

See lens.

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suture

The line marking the junction of two body structures. Examples are the immovable joints between the bones of the skull and, in plants, the seam along the edge of a pea or bean pod.

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SV40

A DNA-containing virus belonging to the papovavirus group that is much used in cancer research for its ability to induce transformations in cells. It was originally isolated from monkeys – hence its name, which is an abbreviation of Simian Virus 40 .

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swallowing

See deglutition.

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swamp

See hydrosere.

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sweat

The salty fluid secreted by the sweat glands onto the surface of the skin. Excess body heat is used to evaporate sweat, thereby resulting in cooling of the skin surface. Small amounts of urea are excreted in sweat.

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sweat gland

A small gland in mammalian skin that secretes sweat. The distribution of sweat glands on the body surface varies between species: they occur over most of the body surface in man and higher primates but have a more restricted distribution in other mammals.

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swim bladder (air bladder; gas bladder)

An air-filled sac lying above the alimentary canal in bony fish that regulates the buoyancy of the animal. Air enters or leaves the bladder either via a pneumatic duct opening into the oesophagus or stomach or via capillary blood vessels, so that the specific gravity of the fish always matches the depth at which it is swimming. This makes the fish weightless, so less energy is required for locomotion. In lungfish it also has a respiratory function. The lungs of tetrapods are homologous with the swim bladder, which has developed its hydrostatic function by specialization.

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syconus

A type of composite fruit formed from a hollow fleshy inflorescence stalk inside which tiny flowers develop. Small drupes, the `pips', are produced by the female flowers. An example is the fig.

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symbiont

An organism that is a partner in a symbiotic relationship (see symbiosis).

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symbiosis

An interaction between individuals of different species (symbionts ). The term symbiosis is usually restricted to interactions in which both species benefit (see cooperation; mutualism), but it may be used for other close associations, such as commensalism, inquilinism, and parasitism. Many symbioses are obligatory (i.e. the participants cannot survive without the interaction); for example, a lichen is an obligatory symbiotic relationship between an alga or a blue-green bacterium and a fungus.

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symmetry

Regularity in the arrangement of the parts of an organism. See bilateral symmetry; radial symmetry.

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sympathetic nervous system

Part of the autonomic nervous system. Its nerve endings release noradrenaline or adrenaline as a neurotransmitter and its actions tend to antagonize those of the parasympathetic nervous system, thus achieving a balance in the organs they serve. For example, the sympathetic nervous system decreases salivary gland secretion, increases heart rate, and constricts blood vessels, while the parasympathetic nervous system has opposite effects.

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sympathetic tone

The condition of a muscle when the tone is maintained predominantly by impulses from the sympathetic nervous system.

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sympatric

Describing groups of similar organisms that, although in close proximity and theoretically capable of interbreeding, do not interbreed because of differences in behaviour, flowering time, etc. See isolating mechanism. Compare allopatric.

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symphysis

A joint that is only slightly movable; examples are the joints between the vertebrae of the vertebral column and that between the two pubic bones in the pelvic girdle. The bones at a symphysis articulate by means of smooth layers of cartilage and strong fibres.

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symplast

The system of protoplasts in plants, which are interconnected by plasmodesmata. This effectively forms a continuous system of cytoplasm bounded by the plasma membranes of the cells. The movement of water through the symplast is known as the symplast pathway . It is one of the routes by which water travels across the root cortex from the root hairs and is the only means by which water crosses the endodermis. Compare apoplast.

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sympodium

The composite primary axis of growth in such plants as lime and horse chestnut. After each season's growth the shoot tip of the main stem stops growing (sometimes terminating in a flower spike); growth is continued by the tip of one or more of the lateral buds. Compare monopodium.

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symporter

A membrane protein that effects the transport of a solute across a membrane but only when it also transports another type of solute in the same direction. For example, the protein that carries the sugar lactose across membranes in bacteria functions as a hydrogen-ion symport. Compare antiporter; uniporter.

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synapomorphy

See apomorphy.

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synapse

The junction between two adjacent neurones (nerve cells), i.e. between the axon ending of one (the presynaptic neurone ) and the dendrites of the next (the postsynaptic neurone ). The swollen tip of the axon of the presynaptic neurone, called the synaptic knob , contains vesicles of neurotransmitter substance. At a synapse, the membranes of the two cells (the pre- and postsynaptic membranes ) are in close contact, with only a minute gap (the synaptic cleft ) between them. A nerve impulse is transmitted across the synapse by the release from the presynaptic membrane of neurotransmitter, which diffuses across the synaptic cleft to the postsynaptic membrane. This triggers the propagation of the impulse from the dendrite along the length of the postsynaptic neurone. Most neurones have more than one synapse. See also excitatory postsynaptic potential.

Structure of a synapse

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synapsis

See pairing.

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synaptic cleft

See synapse.

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synaptic knob

See synapse.

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syncarpy

The condition in which the female reproductive organs (carpels) of a flower are joined to each other. It occurs, for example, in the primrose. Compare apocarpy.

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syncytium

(pl. syncytia) A group of animal cells in which cytoplasmic continuity is maintained. For example, the cells of striated muscle form a syncytium. In some syncytia the cells remain discrete but are joined together by cytoplasmic bridges. For example, developing male germ cells in the testes of mammals are linked throughout the process of differentiation until an individual sperm is released into the lumen of the seminiferous tubule (see spermatogenesis). Compare plasmodium.

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synecology

The study of ecology at the level of the community. A synecological study aims to investigate the relationships between different species that form a community and their interactions with the surrounding environment. Synecology involves both biotic and abiotic factors. Compare autecology.

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synergids

The two nuclei in the embryo sac of flowering plants that are closely associated with the oosphere, or egg cell, to form the egg apparatus .

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synergism (summation)

1 The phenomenon in which the combined action of two substances (e.g. drugs or hormones) produces a greater effect than would be expected from adding the individual effects of each substance. See also potentiation.

2 The combined action of one muscle (the synergist ) with another (the agonist ) in producing movement. Compare antagonism.
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syngamy

See fertilization.

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synomone

See allelochemical.

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synovial membrane

The membrane that lines the ligament surrounding a freely movable joint (such as that at the hip or elbow). It secretes a fluid (synovial fluid ) that lubricates the layers of cartilage forming the articulating surfaces of the joint.

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synthesis

The formation of chemical compounds from more simple compounds. See biosynthesis.

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syphilis

See sexually transmitted disease.

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syrinx

The sound-producing organ of a bird, situated at the lower end of the trachea where it splits into the bronchi. It has a complex structure with a number of vibrating membranes.

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systematics

The study of the diversity of organisms and their natural relationships. It is sometimes used as a synonym for taxonomy. The term biosystematics (or experimental taxonomy ) describes the experimental study of diversity, especially at the species level. Biosystematic methods include breeding experiments, field work, biochemical work (known as chemosystematics ), and cytotaxonomy. See also molecular systematics.

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Système International d'Unités

See SI units.

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systemic acquired resistance (SAR)

A generalized state of enhanced immunity to infection demonstrated by plants following an initial localized injury. It is thought that the hypersensitive response of the plant to local infection (see hypersensitivity) produces certain substances that, over a period of hours to days, evoke resistance throughout the plant, even in parts quite distant from the original injury site. There is now strong evidence that one such substance is salicylic acid (2-hydroxybenzoic acid), whose acetyl derivative, aspirin, is widely used as an analgesic. One model proposes that salicyclic acid synthesized at the initial injury site is translocated through the plant via the phloem to distant parts. Here it activates genes encoding so-called pathogenesis-related proteins , which include enzymes capable of degrading microbial cell walls (e.g. chitinase) and other proteins that block microbial enzymes. The related compound, methylsalicylic acid, is more volatile and may be given off by injured parts to act as an airborne signal.

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systemic arch

A paired blood vessel in the embryos of tetrapods that carries blood from the aorta to the trunk and hind limbs. It is derived from the fourth aortic arch. Adult amphibians and reptiles retain both arches, while birds and mammals only have one.

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systemic circulation

The part of the circulatory system of birds and mammals that transports oxygenated blood from the left ventricle of the heart to the tissues in the body and returns deoxygenated blood from the tissues to the right atrium of the heart. Compare pulmonary circulation. See double circulation.

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systems ecology

The use of mathematical models and relationships in the study of ecology.

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systole

The phase of the heart beat during which the ventricles of the heart contract to force blood into the arteries. Compare diastole. See blood pressure.

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T

2,4,5-T

2,4,5-trichlorophenoxyacetic acid (2,4,5-trichlorophenoxyethanoic acid): a synthetic auxin formerly widely used as a herbicide and defoliant. It is now banned in many countries, as it tends to become contaminated with the toxic chemical dioxin.

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tachycardia

An increase in heart rate. The heart rate is controlled by the opposing actions of the sympathetic nervous system, which accelerates the heartbeat; and the parasympathetic nervous system, which slows it down. The heart rate is normally approximately 70 beats per minute. During exercise there is an increase in sympathetic activity and tachycardia occurs: the heart rate increases to up to 110 beats per minute. Immediately after exercise, at rest, the heart rate decreases to around 60 beats per minute (bradycardia ), due to an increase in parasympathetic activity.

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tachymetabolism

Metabolism that is sustained at a relatively high rate. Characteristic of `warm-blooded' animals (see endotherm), it generates the heat required to maintain the body temperature of such animals at a constant high level. A tachymetabolic animal typically has a metabolic rate at least five times that of an ectothermic (`cold-blooded') animal of equivalent size. Compare bradymetabolism.

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tactic movement

See taxis.

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tagma

(pl. tagmata) A section of the body of an arthropod that is formed by the fusion of mesodermal somites and has a distinct function and structure. The basic tagmata are the head, thorax, and abdomen, but the form of the tagmata (known as tagmosis ) varies between arthropod groups, each group having its characteristic tagmosis. For example, many crustaceans have a cephalothorax and abdomen, while arachnids have a prosoma and opisthosoma.

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tagmosis

See tagma.

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taiga

A terrestrial biome consisting mainly of evergreen coniferous forests (mainly pine, fir, and spruce), which occurs across subarctic North America and Eurasia. In certain parts, such as northeastern Siberia, deciduous conifers and broadleaved trees, such as larch and birch, are dominant. Over most of the taiga the ground is permanently frozen within about one metre of the surface, which prevents water from filtering down to deeper levels in the soil. This means that bogs may form in depressions. For at least six months of the year temperatures are below freezing but there is a short growing season lasting 3–5 months. The soil in taiga areas is acidic and infertile. Compare tundra.

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tannin

One of a group of complex organic chemicals commonly found in leaves, unripe fruits, and the bark of trees. Their function is uncertain: the unpleasant taste may discourage grazing animals, or they may help form barriers against the entry of pathogens. Some tannins have commercial uses, notably in the production of leather and ink.

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tapetum

A reflecting layer, containing crystals of guanine, in the choroid of the eye of many nocturnal vertebrates. It reflects light back onto the retina, thus improving vision and causing the eyes to shine in the dark.

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tapeworms

See Cestoda.

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taphonomy

The study of the biological, chemical, and physical processes that change organisms after death, leading ultimately to their preservation as fossils in rock. The initial phase in this process is removal or decay of the organism's soft parts by scavengers or microbes. Thereafter the remaining hard parts, such as bones and shells, may undergo disarticulation and fragmentation, and are often moved from the site of death, for example by water currents or the wind. Exposure to such movements causes abrasion against other solid particles, and consequent erosion of sharp edges. Burial underneath sediment is common, resulting in flattening or collapse of shell or skeletal cavities, depending on their mechanical strength. The chemical components of the remains may also change; for example, the calcium carbonate of a shell may be converted from aragonite to calcite. Also, concretions of carbonates often form with shell cavities, sometimes protecting them from collapse. The physical and chemical processes occurring after burial of an organism's remains are termed diagenesis . Knowledge of all these processes, which may take thousands or millions of years, enables a fuller and more accurate interpretation of fossil remains.

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tap root

See root.

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tarsal (tarsal bone)

One of the bones that form the ankle (see tarsus) in terrestrial vertebrates.

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tarsus

The ankle (or corresponding part of the hindlimb) in terrestrial vertebrates, consisting of a number of small bones (tarsals ). The number of tarsal bones varies with the species: humans, for example, have seven.

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tartrazine

A food additive (E102) that gives foods a yellow colour. Tartrazine can cause a toxic response in the immune system and is banned in certain countries.

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taste

1 The sense that enables the flavour of different substances to be distinguished (see taste bud).

2 The flavour of a substance.
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taste bud

A small sense organ in most vertebrates, specialized for the detection of taste. In terrestrial animals taste buds are concentrated on the upper surface of the tongue. They contain gustatory receptors , which are sensitive to four types of taste: sweet, salt, bitter, or sour. The taste bud transmits information about a particular type of taste to the brain via nerve fibres. The four types of taste bud show distinct distribution patterns on the surface of the human tongue.
In fishes, taste buds are distributed over the entire surface of the body and provide information about the surrounding water.

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TATA box (Hogness box)

A sequence of nucleotides that serves as the main recognition site for the attachment of RNA polymerase in the promoter region of eukaryotic genes. Located at around 25 nucleotides before the start of transcription, it consists of the seven-base consensus sequence TATAAAA, and is analogous to the Pribnow box in prokaryotic promoters.

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Tatum, Edward

See Beadle, George Wells.

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taxis (taxic response; tactic movement)

The movement of a cell (e.g. a gamete) or a microorganism in response to an external stimulus. Certain microorganisms have a light-sensitive region that enables them to move towards or away from high light intensities (positive and negative phototaxis respectively). Many bacteria move in response to chemical stimuli (chemotaxis ); a specific example is aerotaxis , in which atmospheric oxygen is the stimulus. Taxic responses are restricted to cells that possess cilia, flagella, or some other means of locomotion. The term is usually not applied to the movements of higher animals. See also geotaxis. Compare kinesis; tropism.

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taxon

Any named taxonomic group of any rank in the hierarchical classification of organisms. Thus the taxa Papilionidae, Lepidoptera, Hexapoda, and Uniramia are named examples of a family, order, class, and phylum, respectively.

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taxonomy

The study of the theory, practice, and rules of classification of living and extinct organisms. The naming, description, and classification of a given organism draws on evidence from a number of fields. Classical taxonomy is based on morphology and anatomy. Cytotaxonomy compares the size, shape, and number of chromosomes of different organisms. Numerical taxonomy uses mathematical procedures to assess similarities and differences and establish taxonomic groups. See also systematics.

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TCA cycle

See Krebs cycle.

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T cell (T lymphocyte)

Any of a population of lymphocytes that are the principal agents of cell-mediated immunity. T cells are derived from the bone marrow but migrate to the thymus to mature (hence T cell). Subpopulations of T cells play different roles in the immune response and can be characterized by their surface antigens (see CD). Helper T cells recognize foreign antigens provided these are presented by cells (such as macrophages and B lymphocytes) bearing class II histocompatibility (MHC) proteins (see histocompatibility). The helper T cell carries T-cell receptors , which recognize the class II MHC proteins on the antigen-presenting cell. Interleukin-1 released by inducer cells stimulates helper T cells to release interleukin-2, which in turn triggers the release of other cytokines (see interleukin). Consequently there is a proliferation of B lymphocytes and the generation of effector T cells , i.e. cytotoxic T cells, delayed hypersensitivity T cells, and suppressor T cells.
Cytotoxic T cells recognize foreign antigen on the surface of virus-infected cells and destroy the cell by releasing cytolytic proteins. Suppressor T cells are important in regulating the activity of other lymphocytes and are crucial in maintaining tolerance to self tissues. Delayed hypersensitivity T cells mediate delayed hypersensitivity by releasing various lymphokines.

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tectorial membrane

See organ of Corti.

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teeth

See deciduous teeth; permanent teeth; dentition; tooth.

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Teleostei

The major superorder of the Osteichthyes (bony fish), containing about 20000 species. Teleosts have colonized an extensive variety of habitats and show great diversity of form. The group includes the eel, seahorse, plaice, and salmon. They have been the dominant fish since the Cretaceous period (about 70 million years ago).

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telocentric

See centromere.

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telomere

The end of a chromosome, which consists of repeated sequences of DNA that perform the function of ensuring that each cycle of DNA replication has been completed. Each time a cell divides some sequences of the telomere are lost; eventually (after 60–100 divisions in an average cell) the cell dies. Replication of telomeres is directed by telomerase , an enzyme consisting of RNA and protein that is inactive in normal cells of higher organisms. Its presence in tumours is linked to the uncontrolled multiplication of cancer cells. In yeasts, protoctists, and other single-celled eukaryotes, telomerase is normally active, so that the chromosomes are kept at their appropriate lengths.

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telophase

A stage of cell division. In mitosis the chromatids that separated from each other at anaphase collect at the poles of the spindle. A nuclear membrane forms around each group, producing two daughter nuclei with the same number and kind of chromosomes as the original cell nucleus. In the first telophase of meiosis, complete chromosomes from the pairs that separated at first anaphase form the daughter nuclei. The number of chromosomes in these nuclei is therefore half the number in the original one. In the second telophase, daughter nuclei are formed from chromatids (as in mitosis).

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temperature inversion

An abnormal increase in air temperature that occurs in the troposphere, the lowest level of the earth's atmosphere. This can lead to pollutants becoming trapped in the troposphere (see air pollution).

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template

Any molecule that acts as a pattern for the synthesis of a new molecule. For example, the two nucleotide chains of a DNA molecule can separate and each acts as a template for the synthesis of the missing chain (see DNA replication).

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tendon

A thick strand or sheet of tissue that attaches a muscle to a bone. Tendons consist of collagen fibres and are therefore inelastic: they ensure that the force exerted by muscular contraction is transmitted to the relevant part of the body to be moved.

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tendril

A slender branched or unbranched structure found in many climbing plants. It may be a modified stem, leaf, leaflet, or petiole. Tendrils respond to contact with solid objects by twining around them (see thigmotropism). The cells that touch the object lose water and decrease in volume in comparison to the outer cells, thus causing the tendril to curve.

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tentacle

Any of the soft flexible appendages in aquatic invertebrate animals that are used principally for feeding. Water flows over the tentacles, which are able to capture food and direct it to the oral aperture. Tentacles are possessed by many cnidarians, some echinoderms (including sea cucumbers), and by cephalopod molluscs, in which the tentacles bear rows of suckers.

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tera-

Symbol T. A prefix used in the metric system to denote one million million times. For example, 10^12 volts = 1 teravolt (TV).

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teratogen

Any environmental factor that acts on a fetus to cause congenital abnormality. Examples include ionizing radiation (e.g. X-rays), nutritional deficiencies, drugs (e.g. thalidomide), toxic chemicals, and virus infections (e.g. rubella).

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tergum

The protective cuticle found on the dorsal surface of body segments in arthropods.

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termination codon

See stop codon.

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terpenes

A group of unsaturated hydrocarbons present in plants (see essential oil). Terpenes consist of isoprene units, CH_2:C(CH_3)CH:CH_2. Monoterpenes have two units, C_10H_16, sesquiterpenes three units, C_15H_24, diterpenes four units, C_20H_32, etc. Derivatives of terpenes, known as terpenoids , include the plant growth substances abscisic acid and gibberellin, and the carotenoid and chlorophyll pigments used in photosynthesis.

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territory

A fixed area that an animal or group of animals defends against intrusion from others of its species by various types of territorial behaviour . Outside the territory (which may contain food sources, hiding places, and nesting sites) others are not threatened (compare home range). Many mammals indicate their territory boundaries with scent markings, while birds sing territorial songs that repel would-be intruders. Animals in neighbouring territories normally respect each other's boundaries, which reduces overt aggression. Some animals are territorial only at certain times of the year, usually the breeding season (see courtship; lek).

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Tertiary

The older geological period of the Cenozoic era (compare Quaternary). It began about 65 million years ago, following the Cretaceous period, and extended to the beginning of the Quaternary, about 2 million years ago. It is subdivided into the Palaeocene, Eocene, Oligocene, Miocene, and Pliocene epochs in ascending order. The Tertiary period was characterized by the rise of the modern mammals and the development of shrubs, grasses, and other flowering plants.

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tertiary consumer

See consumer.

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tertiary structure

See protein.

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testa (seed coat)

The lignified or fibrous protective covering of a seed that develops from the integuments of the ovule after fertilization. See also hilum; micropyle.

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test cross

A mating (cross) made to identify hidden recessive alleles in an individual of unknown genotype. This individual is crossed with one that is homozygous for the allele being investigated (i.e. a homozygous recessive). The homozygous recessive individual may be the parent of the individual being investigated (see back cross).

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testicle

See testis.

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testis (testicle)

The reproductive organ in male animals in which spermatozoa are produced. In vertebrates there are two testes; as well as sperm, they produce steroid hormones (see androgen). In most animals the testes are within the body cavity but in mammals, although they develop within the body near the kidneys, they come to hang outside the body cavity in a scrotum. Most of the vertebrate testis is made up of a mass of seminiferous tubules , lined with Sertoli cells, in which the sperms develop (see spermatogenesis). It is connected to the outside by means of the vas deferens. See reproductive system.

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testosterone

The principal male sex hormone. See androgen.

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tetanus

1 A powerful sustained contraction of a voluntary muscle resulting from the summation of a series of rapid muscular contractions (twitches ) that are induced by repeated stimulation of the muscle.

2 A disease caused by the bacterium Clostridium tetani, which generally enters the body via an open wound. A toxin produced by the bacterium irritates the nerves, which induce muscle spasms that begin in the jaw muscles (giving the disease the informal name of lockjaw ).
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tetrad

A group of four haploid cells formed at the end of the second division of meiosis.

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tetraploid

Describing a nucleus, cell, or organism that has four times (4n) the haploid number (n) of chromosomes. See also polyploid.

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Tetrapoda

In some classifications, a superclass of jawed chordates (Gnathostomata) comprising all vertebrate animals with four limbs, i.e. the amphibians, reptiles, birds, and mammals. The skeleton of the limbs of all tetrapods is based on the same five-digit pattern (see pentadactyl limb).

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tetraspore

Any one of four spores produced by meiosis in an independent diploid sporophyte (tetrasporophyte ) in the life cycle of certain red algae. The tetraspores are produced within a tetrasporangium and germinate to produce the sexual haploid generation. The tetrasporophyte develops from a carpospore.

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thalamus

1 (in anatomy) Part of the vertebrate forebrain that lies above the hypothalamus. It relays sensory information to the cerebral cortex and is also concerned with the translation of impulses into conscious sensations.

2 (in botany) See receptacle.
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thalassaemia

A blood disease caused by a substitution mutation in the DNA sequence that controls the synthesis of the - or ß-chains of haemoglobin. Thalassaemia results in anaemia and growth retardation.

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Thallophyta

A former division of the plant kingdom containing relatively simple plants, i.e. those with no leaves, stems, or roots. It included the algae, bacteria, fungi, and lichens.

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thallus

A relatively undifferentiated vegetative body with no true roots, stems, leaves, or vascular system. It is found in the algae, fungi, mosses, and liverworts and in the gametophyte generation of clubmosses, horsetails, and ferns.

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theca

See capsule.

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thelytoky

The phenomenon occurring in the reproduction of certain animals in which fertilized eggs give rise to males and unfertilized eggs to females. It is found among aphids and certain other insects, and in some mites. The males are diploid, whereas females are haploid and transmit only the maternal genome – their production represents thelytokous parthenogenesis. Compare arrhenotoky.

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therapeutic half-life

(in pharmacology) The time taken for half the dose of a drug to be excreted: used to calculate the most effective and nontoxic dosing intervals. It can be determined by administering a therapeutic dose of the drug labelled with a radioisotope (see labelling) and measuring the time for half of it to be excreted in the urine.

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therapeutic index (therapeutic window)

The therapeutic usefulness of a drug, expressed as a ratio of the maximum nontoxic dose and the minimum effective dose of the drug. The therapeutic index is commonly used as a measure of the range of the dose considered to be safe. However, due to the variation in individual response to a given drug, the therapeutic index cannot be calculated accurately.

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thermal denaturation

See denature.

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thermal hysteresis protein

See antifreeze molecule.

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thermocline

A steep temperature gradient that exists in the middle zone (the metalimnion ) of a lake and gives rise to thermally induced vertical stratification of the water. The metalimnion lies between the relatively warm epilimnion above and the cold hypolimnion below. The thermocline may represent a temperature change of 1°C for every incremental depth of 1 metre of water. It may be short-lived, especially in shallow lakes where wind action can mix the water from different levels. However, it can exist for most of the summer period in temperate lakes and sometimes nearly all year in tropical lakes. A thermocline can speed up the process of eutrophication by preventing the diffusion of oxygen from the epilimnion to the hypolimnion (see eutrophic).

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thermogenesis

The production of heat, for example in the tissues of hibernating animals in order to maintain their body temperature. Heat is generated by preventing synthesis of ATP in the mitochondria, which results in an increase in oxygen consumption and heat production.

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thermography

A medical technique that makes use of the infrared radiation from the human skin to detect an area of elevated skin temperature that could be associated with an underlying cancer. The heat radiated from the body varies according to the local blood flow, thus an area of poor circulation produces less radiation. A tumour, on the other hand, has an abnormally increased blood supply and is revealed on the thermogram (or thermograph ) as a `hot spot'. The technique is used particularly in mammography, the examination of the infrared radiation emitted by human breasts in order to detect breast cancer.

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thermoluminescence

Luminescence produced in a solid when its temperature is raised. It arises when free charge carriers, trapped in a solid as a result of exposure to ionizing radiation, unite and emit photons of light. The process is made use of in thermoluminescent dating , which assumes that the number of charge carriers trapped in a sample of pottery is related to the length of time that has elapsed since the pottery was fired. By comparing the luminescence produced by heating a piece of pottery of unknown age with the luminescence produced by heating similar materials of known age, a fairly accurate estimate of the age of an object can be made.

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thermoluminescent dating

See thermoluminescence.

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thermonasty

See nastic movements.

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thermoneutral zone

The range of environmental temperatures over which the heat produced by a `warm-blooded' animal (see endotherm) remains fairly constant. Hence, it is the range in which the animal is `comfortable', having neither to generate extra heat to keep warm nor expend metabolic energy on cooling mechanisms, such as panting. Animals adapted to cold environments tend to have broader thermoneutral zones than ones living in hot environments.

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thermophilic

Describing an organism that lives and grows optimally at extremely high temperatures, typically over 40°C. The majority are prokaryotes, such as the archaebacteria found in hot springs and in undersea hydrothermal vents (thermoacidophils ), some of these (called hyperthermophiles ) thrive at temperatures above 80°C and can survive beyond the boiling point of water. Some eukaryotes, especially certain protoctists and fungi, are capable of surviving temperatures up to about 60°C. Thermophiles have various adaptations in order to thrive at such high temperatures. For example, their proteins and nucleic acids have structural modifications that give them much greater heat stability, so that the cell machinery is able to function. Also, the chemical makeup of their cell membranes is adapted , for example by inclusion of lipids rich in saturated fatty acids. Compare mesophilic; psychrophilic.

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thermoreceptor

A group of specialized cells (see receptor) in the skin that is sensitive to temperature.

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thiamin(e)

See vitamin B complex.

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thigmotropism (haptotropism)

The growth of an aerial plant organ in response to localized physical contact. For example, when a tendril of sweet pea touches a supporting structure, it curves in the direction of the support and coils around it. See tropism.

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thin-layer chromatography

A technique for the analysis of liquid mixtures using chromatography. The stationary phase is a thin layer of an absorbing solid (e.g. alumina) prepared by spreading a slurry of the solid on a plate (usually glass) and drying it in an oven. A spot of the mixture to be analysed is placed near one edge and the plate is stood upright in a solvent. The solvent rises through the layer by capillary action carrying the components up the plate at different rates (depending on the extent to which they are absorbed by the solid). After a given time, the plate is dried and the location of spots noted. It is possible to identify constituents of the mixture by the distance moved in a given time. The technique needs careful control of the thickness of the layer and of the temperature.

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thoracic cavity

The space within the thorax, which in vertebrates contains the heart, lungs, and rib cage.

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thoracic duct

The main collecting vessel of the lymphatic system, running longitudinally in front of the backbone. The thoracic duct drains its lymph into the superior vena cava.

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thoracic vertebrae

The vertebrae of the upper back, which articulate with the ribs. They lie between the cervical vertebrae and the lumbar vertebrae and are distinguished by a number of articulating facets for attachment of the ribs. In man there are 12 thoracic vertebrae.

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thorax

The anterior region of the body trunk of animals. In vertebrates it contains the heart and lungs within the rib cage. It is particularly well-defined in mammals, being separated from the abdomen by the diaphragm. In insects the thorax is divided into an anterior prothorax , a middle mesothorax , and a posterior metathorax , each of which bears a pair of legs; the hindmost two segments also both carry a pair of wings. In other arthropods, especially crustaceans and arachnids, the thorax is fused with the head to form a cephalothorax .

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thorn

A hard side stem with a sharp point at the tip, replacing the growing point. In some plants the development of thorns and subsequent suppression of the growing points may be a response to dry conditions. Examples are the thorns of gorse and hawthorn. Compare prickle; spine.

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thread cell (nematoblast; cnidoblast)

A specialized cell found only in the ectoderm of the Cnidaria. It contains a nematocyst , a fluid-filled sac within which lies a long hollow coiled thread. When a small sensory projection (cnidocil ) on the surface of the thread cell is touched, e.g. by prey, the thread is shot out and adheres to the prey, coils round it, or injects poison into it. Numerous thread cells on the tentacles of jellyfish produce their sting.

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threat display

See agonistic behaviour.

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threonine

See amino acid.

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threshold

(in physiology) The minimum intensity of a stimulus that is necessary to initiate a response.

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thrombin

An enzyme that catalyses the conversion of fibrinogen to fibrin. See blood clotting; prothrombin.

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thrombocyte

See platelet.

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thromboplastin (tissue factor; Factor III)

A membrane glycoprotein expressed on the surface of damaged tissue cells that initiates the cascade of reactions leading to formation of a blood clot. It forms a complex with Factor VIIa in the presence of phospholipid and calcium ions; this complex converts Factor X to Xa, which in turn converts prothrombin to thrombin. See blood clotting.

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thrombosis

The obstruction of a blood vessel by a mass of blood cells and fibrin (thrombus ), which can result from excessive blood clotting.

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thylakoid

Any of the flattened saclike membranous structures that are stacked on top of one another to form the grana (see granum) of plant chloroplasts. Chlorophyll and other photosynthetic pigments are situated in the thylakoid membranes, which are the site for the light reaction of photosynthesis (see also photophosphorylation). The photolysis of water (see photosystems I and II) occurs in the space between the thylakoid membranes.

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thymidine

A nucleoside consisting of one thymine molecule linked to a D-ribose sugar molecule.

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thymine

A pyrimidine derivative and one of the major component bases of nucleotides and the nucleic acid DNA.

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thymus

An organ, present only in vertebrates, that is concerned with development of lymphoid tissue, particularly the white blood cells involved in cell-mediated immune responses (see T cell). In mammals it is a bilobed organ in the region of the lower neck, above and in front of the heart. It shrinks in size after sexual maturity and is therefore believed to function only during early life.

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thyrocalcitonin

See calcitonin.

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thyroglobulin (TGB)

A glycoprotein, made in the thyroid gland, that consists of about 5000 amino acids, some of which are tyrosine residues. TGB is the precursor of the thyroid hormones, thyroxine and triiodothyronine. Iodine binds to the tyrosine residues in thyroglobulin, which is then hydrolysed into iodotyrosines that combine to form triiodothyronine (T_3) or thyroxine (tetraiodothyronine or T_4).

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thyroid gland

A bilobed endocrine gland in vertebrates, situated in the base of the neck. It secretes two iodine-containing thyroid hormones , thyroxine (T_4; see formula) and triiodothyronine (T_3), which are formed in the gland from thyroglobulin; they control the rate of all metabolic processes in the body and influence physical development and activity of the nervous system. Growth and activity of the thyroid is controlled by thyroid-stimulating hormone, secreted by the anterior pituitary gland. The thyroid gland also contains C cells , which secrete calcitonin.

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thyroid-stimulating hormone (TSH; thyrotrophin)

A hormone, secreted by the anterior pituitary gland, that controls the synthesis and secretion of the two thyroid hormones, thyroxine and triiodothyronine, in the thyroid gland. The secretion of thyroid-stimulating hormone is controlled by thyrotrophin-releasing hormone (TRH) from the hypothalamus. The release of TRH depends on many factors, including the levels of TSH, glucose, and thyroxine in the blood and the rate of metabolism in the body.

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thyrotrophin-releasing hormone (TRH)

See thyroid-stimulating hormone.

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thyroxine (T_4)

The principal hormone of the thyroid gland. See also thyroglobulin.

Thyroxine

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Thysanura

An order of medium-sized wingless insects traditionally placed in the subclass Apterygota and comprising the bristletails and silverfish. These typically have slender bodies with long threadlike antenna and a segmented tail-like extension to the abdomen bordered on either side by the elongated paired cerci. The 350 or so species of bristletails are mostly nocturnal, feeding on algae, lichens, and other plant material and sheltering in litter or beneath bark during the day. They can spring some distance by arching the thorax and flexing the abdomen. The other major group includes the silverfish and firebrats, which comprise about 370 species. These are mostly detritivores living in litter or under bark, although some species are found in deserts and others are familiar inhabitants of human dwellings. These domestic species feed on food debris, paper, cotton, and similar materials. Some authorities now place the bristletails in a separate order, the Archaeognatha.

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tibia

1 The larger of the two bones of the lower hindlimb of terrestrial vertebrates (compare fibula). It articulates with the femur at the knee and with the tarsus at the ankle. The tibia is the major load-bearing bone of the lower leg.

2 The fourth segment of an insect's leg, which is attached to the femur.
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ticks

See Acarina.

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tidal volume

The volume of air taken in or expelled by an animal breathing normally at rest during each cycle of ventilation. The average human has a tidal volume of approximately 500 cm^3.

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tight junction (zonula occludens)

The region between the plasma membranes of two adjacent cells that are so closely positioned that there is no intercellular space between them. This type of junction fuses cells together and provides a selective barrier to the diffusion of substances between cells. Tight junctions are common between cells of the intestinal epithelium, where they act to seal off the intestinal lumen from the intercellular fluid. Compare desmosome; gap junction.

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tiller

A shoot that develops at the base of a plant stem from an axillary bud. Tillers are often produced in response to injury of the main stem, as occurs when a tree is lopped.

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time-lapse photography

A form of photography used to record a slow process, such as plant growth. A series of single exposures of the object is made on film at predetermined regular intervals. The film produced can then be projected at normal ciné speeds and the process appears to be taking place at an extremely high rate.

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Tinbergen, Niko(laas)

(1907–88)
Dutch-born British zoologist and ethologist. Working first at Leiden University, he moved to Oxford in 1947 becoming professor of animal behaviour in 1966. With Konrad Lorenz he was a pioneer of ethology, working with animals in their natural setting. In later years Tinbergen attempted to apply ethological principles to human beings, especially autistic children. He shared the Nobel Prize for physiology or medicine with Lorenz and Karl von Frisch.

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Ti plasmid

See Agrobacterium tumefaciens.

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tissue

A collection of cells of similar structure organized to carry out one or more particular functions. For example, in animals nervous tissue is specialized to perceive and transmit stimuli. An organ, such as a lung or kidney, contains many different types of tissues.

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tissue culture

The growth of the tissues of living organisms outside the body in a suitable culture medium. Culture (or nutrient) media contain a mixture of nutrients either in solid form (e.g. in agar) or in liquid form (e.g. in physiological saline). Tissue culture has proved to be invaluable for gaining information about factors that control the growth and differentiation of cells. Culture of plant tissues has resulted in the regeneration of complete plants, enabling commercial propagation (e.g. of orchids) and – through culture of meristem tissues – the production of virus-free crop plants. See also explantation; micropropagation.

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tissue fluid

The fluid, consisting of water, ions, and dissolved gases and food substances, that is formed when blood is ultrafiltered (see ultrafiltration) from the capillaries into the intercellular spaces. The pressure in the arterial capillaries causes most components of the blood to pass across the capillary walls; blood cells and most of the plasma proteins are retained in the capillaries. The tissue fluid surrounds the body cells, facilitating the exchange of nutrients and waste materials. At the venous end of the capillaries, the tissue fluid is drawn into the capillaries by osmosis.

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tissue typing

The process of identifying the human leucocyte antigens (see HLA system) in both the recipient and the potential donor before organ transplantation. If the donor and recipient tissue types do not match closely, the transplanted organ will be rejected.

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titre

1 The number of infectious virus particles present in a suspension.

2 A measure of the amount of antibody present in a sample of serum, given by the highest dilution of the sample that results in the formation of visible clumps with the appropriate antigen (see agglutination).
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T lymphocyte

See T cell.

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toads

See Amphibia.

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tobacco mosaic virus (TMV)

A rigid rod-shaped RNA-containing virus that causes distortion and blistering of leaves in a wide range of plants, especially the tobacco plant. It is transmitted by insects when they feed on plant tissue. TMV was the first virus to be discovered.

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tocopherol

See vitamin E.

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tolerance

1 The ability of an organism to withstand extreme variations in environmental conditions, such as drought.

2 The build-up of resistance to drugs or other chemicals (such as pesticides), which occurs after prolonged use or application. Increasingly large doses of the chemical are required to produce the desired effect in the organism.
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3 (immunological tolerance) The phenomenon by which the cells of the immune system are constrained from mounting an immune response against `self' tissues. During their development and maturation, lymphocyte precursors (i.e. precursors of both B cells and T cells) undergo a series of selection processes to ensure that they are capable of recognizing the body's own tissue markers, particularly the histocompatibility proteins, and that they do not respond to the wide range of other `self' antigens when the latter are combined with these marker proteins. Any precursor lymphocytes that fail these selection procedures are eliminated, ensuring that only tolerant clones are produced. The term `tolerance' also embraces failure of the immune response in an animal exposed to a foreign antigen to which an immune response would normally be mounted. This commonly follows exposure to the antigen during fetal life, presumably when the immune system is developing self-tolerance.
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Tollens reagent

A reagent used in testing for aldehydes, named after German chemist B. C. G. Tollens (1841–1918). It is made by adding sodium hydroxide to silver nitrate to give silver(I) oxide, which is dissolved in aqueous ammonia (giving the complex ion [Ag(NH_3)2]+). The sample is warmed with the reagent in a test tube. Aldehydes reduce the complex Ag+ ion to metallic silver, forming a bright silver mirror on the inside of the tube (hence the name silver-mirror test ). Ketones give a negative result.

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tomography

The use of X-rays to photograph a selected plane of a human body with other planes eliminated. The CT (computerized tomography ) scanner is a ring-shaped X-ray machine that rotates through 180° around the horizontal patient, making numerous X-ray measurements every few degrees. The vast amount of information acquired is built into a three-dimensional image of the tissues under examination by the scanner's own computer. The patient is exposed to a dose of X-rays only some 20% of that used in a normal diagnostic X-ray.

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tone (tonus)

The state of sustained tension in muscles that is necessary for the maintenance of posture. In a tonic muscle contraction, only a certain proportion of the muscle fibres are contracting at any given time; the rest are relaxed and recovering for subsequent contractions. The fibres involved in tone contract more slowly than the fast fibres used for rapid responses by the same muscle. The proportions of slow and fast fibres depend on the function of the muscle.

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tongue

A muscular organ of vertebrates that in most species is attached to the floor of the mouth. It plays an important role in manipulating food during chewing and swallowing and in terrestrial species it bears numerous taste buds on its upper surface. In some advanced vertebrates the tongue is used in the articulation of sounds, particularly in human speech.

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tonoplast (vacuole membrane)

The single membrane that bounds the vacuole of plant cells.

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tonsil

A mass of lymphoid tissue, several of which are situated at the back of the mouth and throat in higher vertebrates. In humans there are the palatine tonsils at the back of the mouth, lingual tonsils below the tongue, and pharyngeal tonsils (or adenoids ) in the pharynx. They are concerned with the production of lymphocytes and therefore with defence against infection.

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tonus

See tone.

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tooth

Any of the hard structures in vertebrates that are used principally for biting and chewing food but also for attack, grooming, and other functions. In fish and amphibians the teeth occur all over the palate, but in higher vertebrates they are concentrated on the jaws. They evolved in cartilaginous fish as modified placoid scales, and this is reflected in their structure: a body of bony dentine with a central pulp cavity and an outer covering of enamel on the exposed surface (crown ). The portion of the tooth embedded in the jawbone is the root (see illustration).
In mammals there are four different types of teeth, specialized for different functions (see canine tooth; incisor; molar; premolar). Their number varies with the species (see dental formula). See also deciduous teeth; permanent teeth.

Section through an incisor tooth

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top carnivore

See consumer.

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topoisomer

An isomer of, typically, a large complex molecule that is distinct from other similar isomers by virtue of its topology, such as degree of twisting or interlocking of ring structures. For example, topoisomers of DNA may be distinguished according to the degree of supercoiling of the double helix.

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topoisomerase

An enzyme that creates or changes the state of a topoisomer, particularly a topoisomer of DNA. Such enzymes change the way in which DNA is packaged in living cells by altering the degree of supercoiling of the DNA molecules. For example, DNA gyrase, a topoisomerase found in E. coli, introduces negative supercoiling by breaking both strands of the DNA double helix and passing another stretch of the double helix through the gap, which is then sealed. This creates a twist in the circular DNA of the bacterial chromosome. Other topoisomerases can remove supercoiling by introducing a temporary break in one DNA strand and passing the other strand through it.

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topsoil

See soil.

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tornaria

A ciliated larva produced by some acorn worms (see Hemichordata).

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torus

See receptacle.

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totipotent

1 Describing differentiated plant cells that, when isolated, have the ability to develop into an entire new plant if provided with the suitable growing medium.

2 Describing embryonic cells at a stage before their fate is irreversibly determined, when they have the ability to develop into any differentiated cell given the appropriate stimulation.
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touch

The sense that enables the texture of objects and substances to be perceived. Touch receptors occur in the skin, being concentrated in the tips of the finger in humans (see Meissner's corpuscles).

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toxicology

The science of the study of poisons. Originally developed by Paracelsus (1493–1541), toxicology is concerned with the investigation of the deleterious effects of all foreign substances (xenobiotics) on living organisms.

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toxin

A poison produced by a living organism, especially a bacterium. An endotoxin is released only when the bacterial cell dies or disintegrates. An exotoxin is secreted by a bacterial cell into the surrounding medium. In the body a toxin acts as an antigen, producing an immune response.

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trace element

See essential element.

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trace fossil

See fossil.

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trachea

1 (or windpipe) The tube in air-breathing vertebrates that conducts air from the throat to the bronchi. It is strengthened with incomplete rings of cartilage.

2 An air channel in insects and most other terrestrial arthropods. Tracheae occur as ingrowths of the body wall. They open to the exterior by spiracles and branch into finer channels (tracheoles ) that terminate in the tissues (see also air sac). Pumping movements of the abdominal muscles cause air to be drawn into and out of the tracheae.
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tracheid

A type of cell occurring within the xylem of conifers, ferns, and related plants. Tracheids are elongated and their walls are usually extensively thickened by deposits of lignin. Water flows from one tracheid to another through unthickened regions (pits) in the cell walls. Compare vessel element.

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tracheole

See trachea.

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tracheophyte

Any plant that has elaborate tissues, including vascular tissue; a conspicuous sporophyte generation; and complex leaves with waterproof cuticles. Tracheophytes include plants of the phyla Psilophyta, Lycophyta, Sphenophyta, Filicinophyta, Coniferophyta, and Anthophyta. In traditional classification systems these were regarded as classes of the division Tracheophyta.

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tracing (radioactive tracing)

See labelling.

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transaminase

An enzyme that catalyses the transfer of an amino group from one molecule to another. Transaminases play an important role in the synthesis of amino acids (see transamination).

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transamination

A biochemical reaction in amino acid metabolism in which an amine group is transferred from an amino acid to a keto acid to form a new amino acid and keto acid. The coenzyme required for this reaction is pyridoxal phosphate.

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transcriptase

See reverse transcriptase.

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transcription

The process in living cells in which the genetic information of DNA is transferred to a molecule of messenger RNA (mRNA) as the first step in protein synthesis (see also genetic code). Transcription takes place in the cell nucleus or nuclear region and is regulated by transcription factors. It involves the action of RNA polymerase enzymes in assembling the nucleotides necessary to form a complementary strand of mRNA from the DNA template (see also promoter), and eukaryote cells) the subsequent removal of the noncoding sequences from this primary transcript (see gene splicing) to form a functional mRNA molecule. The term is also applied to the assembly of single-stranded DNA from an RNA template by the enzyme reverse transcriptase. Compare translation.

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transcription factor

Any of a group of proteins that can increase or decrease the binding of RNA polymerases to the DNA molecule during the process of transcription. This is achieved by the ability of the transcription factors to bind to the DNA molecule (see DNA-binding proteins). Transcription factors contain finger domains, which are often in repeated sequences called multifinger loops .

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transduction

1 The transfer of genetic material from one bacterial cell to another by means of a bacteriophage.

2 The conversion of stimuli detected in the receptor cells into electric impulses, which are transported by the nervous system.
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transect

A straight line across an expanse of ground along which ecological measurements are taken, continuously or at regular intervals. Thus an ecologist wishing to study the numbers and types of organisms at different distances above the low-tide line might sample at five-metre intervals along a number of transects perpendicular to the shore.

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transfection

1 (in genetic engineering) Any process by which eukaryotic cells take up foreign DNA in a form that is replicated and passed to daughter cells, thereby changing the genetic constitution of the cell line or organism concerned. It is synonymous with transformation in bacteria. Various techniques have been devised, depending on the nature of the recipient cell. The most popular are electroporation, microinjection, and biolistics. See genetically modified organisms.

2 (in microbiology) The uptake by a bacterium of DNA from a bacterial virus (bacteriophage). Compare transformation.
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transferase

Any of a class of enzymes that catalyse the transfer of a group of atoms from one molecule to another.

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transferrin

A plasma protein (a ß-globulin) that can bind to iron and transport it to the liver for storage (see ferritin) or to the cells of bone marrow, where it is used for the formation of haemoglobin. One transferrin molecule can bind to two iron ions.

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transfer RNA

See RNA.

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transformation

1 A permanent heritable change in a cell, particularly a bacterial cell, that occurs as a result of its acquiring foreign DNA. Nonvirulent bacterial cells can be transformed into virulent forms if cultured in a medium containing killed virulent bacteria.

2 The conversion of a normal cell into a malignant cell (see cancer), which can be brought about by the action of carcinogens or oncogenic viruses.
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transgene

A gene that is taken from one organism and inserted into the germ line of another organism so that it is replicated as part of the genome and present in all the recipient's cells. The resulting organism is described as transgenic . See genetically modified organisms.

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transgenic

Describing an organism whose genome incorporates and expresses genes from another species. Transgenic individuals are created by genetic engineering, using suitable vectors to insert the desired foreign gene into the fertilized egg or early embryo of the host. For example, the gene for rat growth hormone can be inserted into fertilized mouse eggs to produce mice with cells that produce rat growth hormone. Transgenic organisms offer considerable commercial potential; see genetically modified organisms.

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transient polymorphism

See polymorphism.

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transition

(in genetics) See substitution.

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transition zone

See hypocotyl.

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translation

The process in living cells in which the genetic information encoded in messenger RNA (mRNA) in the form of a sequence of nucleotide triplets (codons) is translated into a sequence of amino acids in a polypeptide chain during protein synthesis (see illustration). Translation takes place on ribosomes in the cell cytoplasm (see initiation factor). The ribosomes move along the mRNA `reading' each codon in turn. Molecules of transfer RNA (tRNA), each bearing a particular amino acid, are brought to their correct positions along the mRNA molecule: base pairing occurs between the bases of the codons and the complementary base triplets of tRNA (see anticodon). In this way amino acids are assembled in the correct sequence to form the polypeptide chain (see elongation). Translation is terminated by the release factor.

The stages of translation in protein synthesis

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translocation

1 (in botany) The movement of minerals and chemical compounds within a plant. There are two main processes. The first is the uptake of soluble minerals from the soil and their passage upwards from the roots to various organs by means of the water-conducting vessels (xylem). The second is the transfer of organic compounds, synthesized by the leaves, both upwards and downwards to various organs, particularly the growing points. This movement occurs within the phloem tubes. See also mass flow.

2 (in genetics) A type of chromosome mutation in which a section of one chromosome is broken off and becomes attached to another chromosome, resulting in a loss of genetic information from the first chromosome.
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transmission

1 (in neurophysiology) The one-way transfer of a nerve impulse from one neurone to another across a synapse. See also neurotransmitter. Compare propagation.

2 (in medicine) The spread of an infection from person to person. This can occur in various ways, such as close contact with an infected person, including sexual contact (see sexually transmitted disease); contact with a vector or a carrier of the disease; consuming food or drink contaminated with the infecting microorganism; and breathing in contaminated droplets of moisture, produced by coughing and sneezing.
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transmission electron microscope

See electron microscope.

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transmitter

See neurotransmitter.

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transpiration

The loss of water vapour by plants to the atmosphere. It occurs mainly from the leaves through pores (stomata) whose primary function is gas exchange. The water is replaced by a continuous column of water (and dissolved nutrients) moving upwards from the roots within the xylem vessels. The flow of this column of water is known as the transpiration stream , which is maintained by root pressure and a combination of cohesive and adhesive forces in the xylem vessels according to the cohesion–tension theory (see cohesion). See also potometer.

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transplantation

See graft.

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transposon (transposable genetic element)

A mobile genetic element, known informally as a `jumping gene', that can become integrated at many different sites in the genome, either by moving from place to place or by producing copies of itself that insert elsewhere in the genome. The simplest types are known as insertion sequences , typically consisting of some 700–1500 base pairs and with numerous short repeated nucleotide sequences at either end. Larger and more complex are the composite transposons , which consist of a central portion, possibly containing functional genes, flanked by insertion sequences at either end. Transposons were first discovered by Barbara McClintock in maize in the 1940s and have since been found in other eukaryotes and in prokaryotes. They can disrupt gene expression or cause deletions and inversions, and hence affect both the genotype and phenotype of the organisms concerned. They also account for a sizable proportion of the repetitive DNA in eukaryotes.

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transverse tubules (T tubules)

A series of infoldings of the plasma membrane (sarcolemma) of muscle fibres that envelop each myofibril at the Z line (see sarcomere). The T tubules transfer action potentials from the sarcolemma to the sarcoplasmic reticulum, which releases calcium ions into the cytosol causing contraction of the muscle fibres.

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transversion

(in genetics) See substitution.

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Trematoda

A class of parasitic flatworms (see Platyhelminthes) comprising the flukes, such as Fasciola (liver fluke). Flukes have suckers and hooks to anchor themselves to the host and their body surface is covered by a protective cuticle. The whole life cycle may either occur within one host or require one or more intermediate hosts to transmit the infective eggs or larvae (see cercaria; miracidium). Fasciola hepatica, for example, undergoes larval development in a land snail (the intermediate host) and infects sheep (the primary host) when contaminated grass containing the larvae is swallowed.

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trial-and-error learning

See learning.

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Triassic

The earliest period of the Mesozoic era. It began about 248 million years ago, following the Permian, the last period of the Palaeozoic era, and extended until about 213 million years ago when it was succeeded by the Jurassic. It was named, by F. von Alberti in 1834, after the sequence of three divisions of strata that he studied in central Germany – Bunter, Muschelkalk, and Keuper. The Triassic rocks are frequently difficult to distinguish from the underlying Permian strata and the term New Red Sandstone is often applied to rocks of the Permo-Triassic. During the period marine animals diversified: molluscs were the dominant invertebrates – ammonites were abundant and bivalves replaced the declining brachiopods. Reptiles were the dominant vertebrates and included turtles, phytosaurs, dinosaurs, and the marine ichthyosaurs.

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triazines

A group of organic compounds based on a heterocyclic ring containing three carbon atoms and three nitrogen atoms and including several commonly used agricultural herbicides, such as simazine and atrazine. The triazine herbicides work by inhibiting electron transport of photosynthesis. They are absorbed primarily through the plant root and bind to a particular site in photosystem II, thereby preventing binding of plastoquinone, a key electron acceptor in the electron transport chain. Certain crop plants are resistant to triazines – for example, maize roots contain an enzyme that renders them inactive – hence their usefulness as selective weedkillers. Attempts have been made to introduce the resistance gene into other crop species, to broaden the applicability of triazine herbicides. However, these efforts have been hampered because the target gene is part of the chloroplast genome, not the nuclear genome. Also, many weeds have now evolved triazine resistance.

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tribe

A category used in the classification of plants and animals that consists of several similar or closely related genera within a family. For example the Bambuseae, Oryzeae, Paniceae, and Aveneae are tribes of grasses.

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tricarboxylic acid cycle

See Krebs cycle.

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triceps

A muscle that runs parallel to the humerus and works antagonistically with the biceps, causing the arm to extend. See antagonism; voluntary muscle.

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trichogyne

See carpogonium.

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trichome

A hairlike projection from a plant epidermal cell. Examples include root hairs and the stinging hairs of nettle leaves.

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trichromatic theory

See colour vision.

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tricuspid valve (right atrioventricular valve)

A valve, consisting of three flaps, situated between the right atrium and the right ventricle of the mammalian heart. When the right ventricle contracts, forcing blood into the pulmonary artery, the tricuspid valve closes the aperture to the atrium, thereby preventing any backflow of blood. The valve reopens to allow blood to flow from the atrium into the ventricle. Compare bicuspid valve.

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triglyceride (triacylglycerol)

An ester of glycerol (propane-1,2,3-triol) in which all three hydroxyl groups are esterified with a fatty acid. Triglycerides are the major constituent of fats and oils and provide a concentrated food energy store in living organisms as well as cooking fats and oils, margarines, etc. Their physical and chemical properties depend on the nature of their constituent fatty acids. In simple triglycerides all three fatty acids are identical; in mixed triglycerides two or three different fatty acids are present.

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triiodothyronine (T_3)

A hormone secreted by the thyroid gland. See also thyroglobulin.

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trilobite

An extinct marine arthropod belonging to the class Trilobita (some 4000 species), fossils of which are found in deposits dating from the Precambrian to the Permian period (590–280 million years ago). Trilobites were typically small (1–7 cm long); the oval flattened body comprised a head (covered by a semicircular dorsal shield) and a thorax and abdomen, which were protected by overlapping dorsal plates with a raised central part and flattened lateral portions, presenting a three-lobed appearance. The head bore a pair of antenna-like appendages and a pair of compound eyes; nearly all body segments bore a pair of Y-shaped (biramous) appendages – one branch for locomotion and the other fringed for respiratory exchange. Trilobites were bottom-dwelling scavengers.

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trimethylamine

A nitrogenous compound, (CH_3)3N, that acts as an osmolyte in the cells of the kidney. This compound can also be converted to trimethylamine oxide , which is excreted by many marine fish and gives them their characteristic odour.

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triose

A sugar molecule that contains three carbon atoms. See monosaccharide.

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triplet code

See codon; genetic code.

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triploblastic

Describing an animal having a body composed of three embryonic cell layers: the ectoderm, mesoderm, and endoderm. Most multicellular animals are triploblastic; the coelenterates, which are diploblastic, are an exception.

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triploid

Describing a nucleus, cell, or organism that has three times (3n) the haploid number (n) of chromosomes (see also polyploid). Triploid organisms are normally sterile as their lack of homologous chromosomes prevents pairing during meiosis. This can be useful to plant breeders, for example in banana cultivation: sterile triploid bananas can be propagated asexually and will not contain any seeds.

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trisomy

The condition of a nucleus, cell, or organism in which one of the pairs of homologous chromosomes has gained an additional chromosome, resulting in a chromosome number of 2n + 1 (see aneuploid). Trisomy is the cause of a number of human genetic abnormalities, including Down's syndrome; Patau's syndrome, in which there is an extra chromosome 13 (trisomy 13 ); and Edwards' syndrome, in which there is an extra chromosome 18 (trisomy 18 ).

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tritanopia

See colour blindness.

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tritiated compound

See labelling.

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tRNA

See RNA.

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trochanter

1 Any of several bony knobs on the femur of vertebrates to which muscles are attached.

2 The second segment of an insect's leg, between the coxa and the femur.
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trochophore

The pelagic planktonic larva of polychaete worms, some molluscs, and certain other invertebrates. It is top-shaped and usually has two bands of cilia encircling the body.

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trophic level

The position that an organism occupies in a food chain. For example, green plants (which obtain their energy directly from sunlight) are the primary producers; herbivores are primary consumers (and secondary producers). A carnivore that eats only herbivores is a secondary consumer and a tertiary producer. Many animals feed at several different trophic levels.

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trophoblast

The layer of epithelial cells that forms the outer wall of a blastocyst (see blastula). It embeds the blastocyst into the wall of the uterus (see implantation) and contributes to the chorion.

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tropism

The directional growth of a plant organ in response to an external stimulus, such as light, touch, or gravity. Growth towards the stimulus is a positive tropism ; growth away from the stimulus is a negative tropism . See also geotropism; hydrotropism; orthotropism; phototropism; plagiotropism; thigmotropism. Compare nastic movements; taxis.

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tropomyosin

A protein found in the actin filaments in muscles. The molecule consists of two elongated strands that run along the length of the filament. When the muscle is at rest, the tropomyosin molecule covers the binding site of the actin molecule, where interaction with myosin occurs. On contraction of the muscle, the tropomyosin is displaced by another protein, troponin, allowing the interaction of actin with myosin. See sliding filament theory.

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troponin

A complex of three polypeptide chains that are found at regular intervals along the length of an actin filament. During muscle contraction, troponin binds to calcium ions, displacing tropomyosin and exposing the binding site on the actin filament. This allows the interaction of actin and myosin to occur. See sliding filament theory.

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troposphere

The lowest level of the earth's atmosphere, extending from the earth's surface to a height of about 10 km (its thickness varies from 7 km at the poles to 28 km at the equator). Within the troposphere temperature falls with increasing height, although temperature inversions can occur.

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trypsin

An enzyme that digests proteins (see endopeptidase; protease). It is secreted in an inactive form (trypsinogen ) by the pancreas into the duodenum. There, trypsinogen is acted on by an enzyme (enterokinase ) produced in the brush border of the duodenum to yield trypsin. The active enzyme plays an important role in the digestion of proteins in the anterior portion of the small intestine. It also activates other proteases in the pancreatic juice (see carboxypeptidase; chymotrypsin).

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trypsinogen

See trypsin.

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tryptophan

See amino acid.

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TSH

See thyroid-stimulating hormone.

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T tubules

See transverse tubules.

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tube feet

See Echinodermata.

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tube nucleus

One of the two nuclei produced when the haploid nucleus of a pollen grain divides by mitosis (compare generative nucleus). The tube nucleus is thought to control growth of the pollen tube.

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tuber

A swollen underground stem or root in certain plants. It enables the plant to survive the winter or dry season and is also a means of propagation. A stem tuber , such as the potato, forms at the end of an underground stem. Each tuber represents several nodes and internodes. The following season several new plants develop from the terminal and axillary buds (eyes). Root tubers , such as those of the dahlia, are modified food-storing adventitious roots and may also give rise to new plants.

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tubicolous

Describing invertebrate animals that live in tubes that they have themselves constructed. For example, the polychaete fanworms (Sabella) build tubes of sand particles.

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tubulin

A protein of which the microtubules of cells are formed.

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Tullgren funnel

A device used to remove and collect small animals, such as insects, from a sample of soil or leaf litter. The sample is placed on a coarse sieve fixed across the wide end of a funnel and a 100-watt light bulb, in a metal reflector, is placed about 25cm above the funnel. The heat from the bulb dries and warms the sample, causing the animals to move downwards and fall through the sieve into the funnel, which directs them into a collecting dish or tube below. The dish can contain water or alcohol to prevent the animals from escaping.

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tumour

See neoplasm.

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tumour-suppressor gene (anti-oncogene)

Any of a class of genes that suppress the development of cancers in living cells. Hence, mutation and consequent loss of activity of such genes leads to the formation of tumours. For example, development of the childhood cancer of retinal cells, known as retinoblastoma, is caused by loss of the chromosome segment containing the tumour suppressor gene RB. This codes for a protein, retinoblastoma protein, that restrains cell proliferation. Another common example is the p53 gene, mutations of which are linked to about half of all human cancers. Its product, a 53-kDa protein, is thought to act as a key control element in the cell cycle. If genetic damage has occurred, the p53 protein halts the cell cycle to allow repair of the faulty DNA. If the damage is severe, it can trigger cell death.

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tundra

A terrestrial biome characterized by a lack of trees and a permanently frozen subsoil. Tundra lies to the north of the taiga in North America and Eurasia; the vegetation is dominated by grasses, sedges, lichens, mosses, heathers, and low shrubs. The growing season, which occurs during the warmest part of the year when the average daily mean temperature is about 10°C, lasts only 2–4 months, during which the topsoil thaws to a depth of 30cm, allowing roots to penetrate it. However, below this level the soil is permenently frozen (permafrost ); water cannot filter through the soil and may lie in surface depressions during the growing season. Compare taiga.

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tunicates

See Urochordata.

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Turbellaria

A class of free-living flatworms (see Platyhelminthes) comprising the planarians, which occur in wet soils, fresh water, and marine environments. Their undersurface is covered with cilia, used for gliding over stones and weeds. Planarians can also swim by means of undulations of the body.

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turgor

The condition in a plant cell when its vacuole is distended with water, pushing the protoplast against the cell wall. In this condition the force causing water to enter the cell by osmosis is balanced by the hydrostatic pressure (pressure potential) that has built up in the cell sap due to the pressure exerted by the cell wall on the protoplast (see also water potential). Turgidity assists in maintaining the rigidity of plants; a decrease in turgidity leads to wilting. Compare plasmolysis.

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turion

1 A winter bud, covered with scale leaves and mucilage, that is produced by certain aquatic plants, such as frogbit. Turions become detached and remain dormant on the pond or lake bottom during the winter before developing into new plants the following season.

2 See sucker.
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Turner's syndrome

A genetic disorder of women caused by the absence of the second sex chromosome (such women are XO, rather than the normal XX). It is characterized by a lack of ovaries and menstrual cycle. Affected women are sterile and lack secondary sexual characteristics, although the external genitalia are present. The syndrome is named after the US endocrinologist H. H. Turner (1892–1970), who first described it.

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twins

Two individuals born to the same mother at the same time. Twins can develop from the same egg (see identical twins) or from two separately fertilized eggs (see fraternal twins).

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tylose

A balloon-like extension of a parenchyma cell that protrudes into the lumen of a neighbouring xylem vessel or tracheid through a pit in the cell wall. Tyloses form most commonly in older woody tissue, possibly in response to injury; they may eventually block the vessels and thus help prevent the spread of fungi and other pathogens within the plant. Tyloses may become filled with tannins, gums, pigments, etc., giving heartwood its dark colour, and their walls can remain thin or become lignified.

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tympanic cavity

See middle ear.

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tympanum (tympanic membrane; eardrum)

The membrane that separates the outer ear from the middle ear. It vibrates in response to sound waves and transmits these vibrations via the ear ossicles of the middle ear to the site of hearing (the cochlea of the inner ear). In amphibians and some reptiles there is no external ear and the tympanum is exposed at the skin surface.

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type specimen

The specimen used for naming and describing a species or subspecies. If this is the original specimen collected by the author who named the species it is termed a holotype . The type specimen is not necessarily the most characteristic representative of the species. The term type is also applied to any taxon selected as being representative of the rank to which it belongs. For example, the genus Solanum (potato) is said to be the type genus of the family Solanaceae.

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tyrosine

See amino acid.

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U

ubiquinone (coenzyme Q)

Any of a group of related quinone-derived compounds that serve as electron carriers in the electron transport chain reactions of cellular respiration. Ubiquinone molecules have side chains of different lengths in different types of organisms but function in similar ways.

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ubiquitin

A small protein (consisting of 76 amino acid residues), found universally in prokaryotes and eukaryotes, that tags proteins destined for degradation by proteasomes. It forms a covalent bond with lysine residues in an ATP-dependent reaction termed ubiquitination . Ubiquitin is important in both the normal life of the cell and in a cell's response to stress; it is considered to be a heat-shock protein.

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ulna

The larger of the two bones in the forearm of vertebrates (compare radius). It articulates with the outer carpals at the wrist and with the humerus at the elbow.

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ultimobranchial bodies

See C cell.

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ultracentrifuge

A high-speed centrifuge used to measure the rate of sedimentation of colloidal particles or to separate macromolecules, such as proteins or nucleic acids, from solutions. Ultracentrifuges are electrically driven and are capable of speeds up to 60000 rpm.

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ultrafiltration

The process in which hydrostatic pressure causes water and small dissolved molecules and ions to move across a membrane against a concentration gradient. Ultrafiltration is responsible for the formation of tissue fluid and glomerular filtrate from blood. In both these processes the ultrafiltered fluid has the same composition as the plasma except that it does not contain blood cells or large protein molecules.

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ultramicroscope

A form of microscope that reveals the presence of particles that cannot be seen with a normal optical microscope. Colloidal particles, smoke particles, etc., are suspended in a liquid or gas in a cell with a black background and illuminated by an intense cone of light that enters the cell from the side and has its apex in the field of view. The particles then produce diffraction-ring systems, appearing as bright specks on the dark background.

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ultramicrotome

See microtome.

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ultrasonics

The study and use of pressure waves that have a frequency in excess of 20000 Hz and are therefore inaudible to the human ear. Ultrasound is used in medical diagnosis, particularly in conditions such as pregnancy, in which X-rays could have a harmful effect.

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ultrastructure

The submicroscopic, almost molecular, structure of living cells, which is revealed by the use of an electron microscope.

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ultraviolet microscope

A microscope that has quartz lenses and slides and uses ultraviolet radiation as the illumination. The use of shorter wavelengths than the visible range enables the instrument to resolve smaller objects and to provide greater magnification than the normal optical microscope. The final image is either photographed or made visible on a fluorescent screen by means of an image converter.

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ultraviolet radiation (UV)

Electromagnetic radiation having wavelengths between that of violet light and long X-rays, i.e. between 400 nanometres and 4 nm. In the range 400–300 nm the radiation is known as the near ultraviolet . In the range 300–200 nm it is known as the far ultraviolet . Below 200 nm it is known as the extreme ultraviolet or the vacuum ultraviolet , as absorption by the oxygen in the air makes the use of evacuated apparatus essential. The sun is a strong emitter of UV radiation but only the near UV reaches the surface of the earth as the ozone layer of the atmosphere absorbs all wavelengths below 290 nm. Ultraviolet radiation is classified in three ranges according to its effect on the skin: UV-A (320–400 nm), UV-B (290–320 nm), and UV-C (230–290 nm).
The longest-wavelength range, UV-A, is not harmful in normal doses and is used clinically in the treatment of certain skin complaints, such as psoriasis. It is also used to induce vitamin D formation in patients that are allergic to vitamin D preparations. UV-B causes reddening of the skin followed by pigmentation (tanning). Excessive exposure can cause severe blistering. UV-C, with the shortest wavelengths, is particularly damaging. It is thought that short-wavelength ultraviolet radiation causes skin cancer and that the risk of contracting this has been increased by the depletion of the ozone layer.
Most UV radiation for practical use is produced by various types of mercury-vapour lamps. Ordinary glass absorbs UV radiation and therefore lenses and prisms for use in the UV are made from quartz.

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umbel

A type of racemose inflorescence in which stalked flowers arise from the same point on the flower axis, resembling the spokes of an umbrella. An involucre (cluster) of bracts may occur at the point where the stalks emerge. This arrangement is characteristic of the family Umbelliferae (Apiaceae; e.g. carrot, hogweed, parsley, parsnip), in which the inflorescence is usually a compound umbel.

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umbilical cord

The cord that connects the embryo to the placenta in mammals. It contains a vein and two arteries that carry blood between the embryo and placenta. It is severed after birth to free the newly born animal from the placenta, and shrivels to leave a scar, the navel, on the animal.

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undernourishment

See malnutrition.

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undulipodium

A slender flexible outgrowth of a eukaryote cell used for locomotion or propelling fluids over the surface of the cell. The term `undulipodium' is used to designate a eukaryotic `flagellum' or a cilium (which have the same structure), to emphasize the distinction between these structures and the flagellum of a bacterium. Many protoctists and sperm cells swim by means of undulipodia, and various organisms use them to establish feeding currents, or to clear debris from epithelial surfaces. All undulipodia have a shaft, about 0.25 µm in diameter, consisting of a longitudinal array of microtubules, the axoneme, which is surrounded by an extension of the cell's plasma membrane. The axoneme has two singlet microtubules running down the middle and nine doublet microtubules around the periphery, giving a characteristic 9 + 2 array. At its base the axoneme connects with a basal body (or kinetosome ). This is a short cylinder that, like a centriole, consists of nine triplet microtubules; it organizes assembly of the axoneme microtubules. Cilia are shorter than flagella, and move by a whiplike power stroke followed by a recovery stroke in the opposite direction. Flagella generate successive waves that pass from the base to the tail. In both cases, flexing of the shaft is produced by a sliding motion of the microtubule doublets relative to each other. This involves the successive formation and breakage of molecular bridges between adjacent doublets. The bridges are composed of a protein, dynein, and their formation requires energy in the form of ATP.

Cross section of an undulipodium

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ungulate

A herbivorous mammal with hoofed feet (see unguligrade). Ungulates are grouped into two orders: Artiodactyla and Perissodactyla.

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unguligrade

Describing the gait of ungulates (e.g. horses and cows), in which only the tips of the digits (i.e. the hooves) are on the ground and the rest of the foot is off the ground. Compare digitigrade; plantigrade.

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unicellular

Describing tissues, organs, or organisms consisting of a single cell. For example, the reproductive organs of some algae and fungi are unicellular. Unicellular organisms include bacteria, protozoans, and certain algae. Compare acellular; multicellular.

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unipolar neurone

A neurone that has one main process, the axon, extending from its cell body. Unipolar neurones include many sensory neurones and many vertebrate motor neurones and interneurones. Compare bipolar neurone; multipolar neurone.

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uniporter

A membrane protein that transports molecules across a membrane in one direction only. Compare antiporter; symporter.

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Uniramia (Mandibulata)

A phylum of arthropods that contains the classes Chilopoda (centipedes), Diplopoda (millipedes), Pauropoda and Symphyla (centipede-like animals), and Hexapoda (insects). Members are characterized by uniramous appendages.

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uniramous appendage

A type of appendage that is characteristic of insects and other members of the subphylum Uniramia. It consists of an unbranched series of segments (see coxa; trochanter; femur). Compare biramous appendage.

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unisexual

Describing animals or plants with either male or female reproductive organs but not both. Most of the more advanced animals are unisexual but plants are often hermaphrodite. Flowers that contain either stamens or carpels but not both are also described as unisexual. See also monoecious; dioecious.

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unit

A specified measure of a physical quantity, such as length, mass, time, etc., specified multiples of which are used to express magnitudes of that physical quantity. For scientific purposes previous systems of units have now been replaced by SI units.

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unsaturated

Denoting a compound having double or triple bonds in its molecules. Unsaturated compounds can undergo addition reactions as well as substitution. See fatty acid. Compare saturated.

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upper critical temperature

The maximum body temperature that can be tolerated by an organism. When body temperature exceeds the upper critical point, cellular components and process become disrupted, and the organism eventually dies. For most animals the upper critical temperature lies in the range 30–45°C, while few plants can survive leaf temperatures in excess of 50°C, exceptions being desert plants such as agaves and cacti, which can tolerate temperatures of 60°C or more. Some specialized bacteria, notably certain archaebacteria (see Archaea), live near deep-sea vents at temperatures exceeding 100°C (see thermophilic). Compare lower critical temperature.

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uracil

A pyrimidine derivative and one of the major component bases of nucleotides and the nucleic acid RNA.

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uranium–lead dating

A group of methods of dating certain rocks that depends on the decay of the radioisotopes uranium–238 to lead–206 (half-life 4.5 × 10^9 years) or the decay of uranium–235 to lead–207 (half-life 7.1 × 10^8 years). One form of uranium–lead dating depends on measuring the ratio of the amount of helium trapped in the rock to the amount of uranium present (since the decay 238U 206Pb releases eight alpha-particles). Another method of calculating the age of the rocks is to measure the ratio of radiogenic lead (206Pb, 207Pb, and 208Pb) present to nonradiogenic lead (204Pb). These methods give reliable results for ages of the order 10^7–10^9 years.

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urea (carbamide)

A white crystalline water-soluble solid, CO(NH_2)2. Urea is the major end product of nitrogen excretion in mammals, being synthesized by the urea cycle. Urea is synthesized industrially from ammonia and carbon dioxide for use in urea–formaldehyde resins and pharmaceuticals, as a source of nonprotein nitrogen for ruminant livestock, and as a nitrogen fertilizer.

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urea cycle (ornithine cycle)

The series of biochemical reactions that converts ammonia to urea during the excretion of metabolic nitrogen. Urea formation occurs in mammals and, to a lesser extent, in some other animals. The liver converts ammonia to the much less toxic urea, which is excreted in solution in urine.

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ureotelic

Describing animals that excrete nitrogenous waste in the form of urea. Most terrestrial vertebrates are ureotelic, converting ammonium ions formed during the breakdown of amino acids into urea. See urea cycle. Compare ammonotelic; uricotelic.

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ureter

The duct in vertebrates that conveys urine from the kidney to the bladder.

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urethra

The duct in mammals that conveys urine from the bladder to be discharged to the outside of the body. In males the urethra passes through the penis and is joined by the vas deferens; it therefore also serves as a channel for sperm.

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uric acid

The end product of purine breakdown in most mammals, birds, terrestrial reptiles, and insects and also (except in mammals; see urea) the major form in which metabolic nitrogen is excreted. Being fairly insoluble, uric acid can be expelled in solid form, which conserves valuable water in arid environments. The accumulation of uric acid in the synovial fluid of joints causes gout.

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uricotelic

Describing animals that excrete nitrogenous waste in the form of uric acid. Uricotelic animals include birds and reptiles. Compare ammonotelic; ureotelic.

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uridine

A nucleoside consisting of one uracil molecule linked to a D-ribose sugar molecule. The derived nucleotide uridine diphosphate (UDP) is important in carbohydrate metabolism.

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urinary system

The collection of organs and tissues that perform osmoregulation and excretion. The mammalian urinary system consists of two kidneys each linked to the bladder by a ureter.

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urine

The aqueous fluid formed by the excretory organs of animals for the removal of metabolic waste products. In higher animals, urine is produced by the kidneys, stored in the bladder, and excreted through the urethra or cloaca. Apart from water, the major constituents of urine are one or more of the end products of nitrogen metabolism – ammonia, urea, uric acid, and creatinine. It may also contain various inorganic ions, the pigments urochrome and urobilin, amino acids, and purines. Precise composition depends on many factors, especially the habitat of a particular species: aquatic animals produce copious volumes; terrestrial animals need to conserve water and produce much less (about 1.0–1.5 litres per day in humans).

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uriniferous tubule

See nephron.

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Urochordata (Tunicata)

A subphylum of marine nonvertebrate chordates in which the body is typically enclosed in a protective tunic of a cellulose-like material and the notochord and dorsal hollow nerve cord are evident only in the free-swimming tadpole-like larval stage. The pharynx is specialized for filter feeding (see endostyle). There are three classes: the sessile Ascidiacea (sea squirts), the pelagic Thaliacea (salps), and the pelagic Larvacea (in which the larval form is retained into adulthood).

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uterus (womb)

The organ of female mammals in which the embryo develops. Paired in most mammals but single in humans, it is situated between the bladder and rectum and is connected to the fallopian tubes and to the vagina. The lining (see endometrium) shows cyclical changes (see menstrual cycle; oestrous cycle) associated with egg production and provides a thick spongy layer in which the fertilized egg becomes embedded. The outer wall of the uterus (myometrium ) is thick and muscular; by contracting, it forces the fully grown fetus through the vagina to the outside.

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utriculus (utricle)

A chamber of the inner ear from which the semicircular canals arise. It bears patches of sensory epithelium concerned with detecting changes in the direction and speed of movement (see macula).

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UV

See ultraviolet radiation.

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V

vaccination

See immunization.

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vaccine

A liquid preparation of treated disease-producing microorganisms or their products used to stimulate an immune response in the body and so confer resistance to the disease (see immunization). Vaccines are administered orally or by injection (inoculation ). They take the form of dead viruses or bacteria that can still act as antigens, live but weakened microorganisms (see attenuation), specially treated toxins, or antigenic extracts of the microorganism.

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vacuole

A space within the cytoplasm of a living cell that is filled with air, water or other liquid, cell sap, or food particles. In plant cells there is usually one large vacuole bounded by a single-layered membrane (tonoplast or vacuole membrane ); animal cells usually have several small vacuoles. See also contractile vacuole.

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vacuole membrane (tonoplast)

See vacuole.

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vagal tone

The effect produced on the heart when only the parasympathetic nerve fibres (which are carried in the vagus nerve) are controlling the heart rate. The parasympathetic nerve fibres slow the heart rate from approximately 70 beats per minute to 60 beats per minute.

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vagina

The tube leading from the uterus to the outside. Sperm are deposited in the vagina during copulation and the fully developed fetus is born through it. In a number of mammals the vagina may be sealed when the animal is not sexually receptive and only open during oestrus. Its lining produces mucus, which prevents friction and the entry of infective organisms.

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vagus nerve

The tenth cranial nerve: a paired nerve that supplies branches to many major internal organs. It carries motor nerve fibres to the heart, lungs, and viscera and sensory fibres from the viscera.

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valine

See amino acid.

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valve

1 Any of various structures for restricting the flow of a fluid through an aperture or along a tube to one direction. Valves in the heart (see bicuspid valve; semilunar valve; tricuspid valve), veins, and lymphatic vessels consist of two or three flaps of tissue (cusps ) fastened to the walls. The cusps are flattened to the walls to allow the normal passage of blood or lymph, but a reverse flow causes them to block the vessel or aperture, so preventing further backflow.

2 Any of the parts that make up a capsule or other dry fruit that sheds its seeds.
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3 One of the two halves of the cell wall of a diatom.
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4 Either of the two hinged portions of the shell of a bivalve mollusc.
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variable number tandem repeats (VNTR)

A family of genetic loci found in eukaryotes consisting of short (10–60 bp) sequences of DNA repeated in tandem. The alleles for any particular locus all have the same sequence but differ as to how many times the sequence is repeated. VNTR loci contribute to repetitive DNA and have proved valuable in DNA fingerprinting, especially in human forensic science. The VNTR sequences can be released intact from a DNA sample using restriction endonuclease enzymes and identified using gene probes with Southern blotting. Alternatively, the VNTR alleles can be amplified by the polymerase chain reaction, separated by gel electrophoresis, and the resultant patterns compared without the need for special gene probes. Since each VNTR locus typically has many different alleles, the likelihood of two individuals having identical sets of alleles for even a few such loci is very remote. Hence, a DNA sample can be ascribed to a particular person with a high degree of certainty.

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variation

The differences between individuals of a plant or animal species. Variation may be the result of environmental conditions; for example, water supply and light intensity affect the height and leaf size of a plant. Differences of this kind, acquired during the lifetime of an individual, are not transmitted to succeeding generations since the genes are not affected. Genetic variation , due to differences in genetic constitution, is inherited (see continuous variation; discontinuous variation). The most important sources of genetic variation are mutation and recombination (see also crossing over). It is also increased by outbreeding. Wide genetic variation improves the ability of a species to survive in a changing environment, since the chances that some individuals will tolerate a particular change are increased. Such individuals will survive and transmit the advantageous genes to their offspring.

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variegation

The occurrence of differently coloured patches, spots, or streaks in plant leaves, petals, or other parts, due to absence of pigment or different combinations of pigment in the affected area of the part. Variegation may be brought about by infection, for example tobacco mosaic virus infection, or by genetic differences between the cells of the variegated part.

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variety

A category used in the classification of plants and animals below the species level. A variety consists of a group of individuals that differ distinctly from but can interbreed with other varieties of the same species. The characteristics of a variety are genetically inherited. Examples of varieties include breeds of domestic animals and the races of man. See also cultivar. Compare subspecies.

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varve dating (geochronology)

An absolute dating technique using thin sedimentary layers of clays called varves . The varves, which are particularly common in Scandinavia, have alternate light and dark bands corresponding to winter and summer deposition. Most of them are found in the Pleistocene series, where the edges of varve deposits can be correlated with the annual retreat of the ice sheet, although some varve formation is taking place in the present day. By counting varves it is possible to establish an absolute time scale for fossils up to about 20000 years ago.

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vasa recta

Thin-walled blood vessels that branch from the efferent arterioles leaving each glomerulus in the vertebrate kidney (see nephron). The vasa recta form U-shaped loops adjacent to the loop of Henle and eventually drain into the renal vein.

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vascular bundle (fascicle)

A long continuous strand of conducting (vascular) tissue in tracheophyte plants that extends from the roots through the stem and into the leaves. It consists of xylem and phloem, which are separated by a cambium in plants that undergo secondary thickening. See vascular tissue.

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vascular cambium

See cambium.

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vascular plants

All plants possessing organized vascular tissue. See tracheophyte.

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vascular system

1 A specialized network of vessels for the circulation of fluids throughout the body tissues of an animal. All animals, apart from simple invertebrate groups, possess a blood vascular system , which enables the passage of respiratory gases, nutrients, excretory products, and other metabolites into and out of the cells. In vertebrates it consists of a muscular heart, which pumps blood through major blood vessels (arteries) into increasingly finer branches until in the capillaries it is in intimate contact with tissues. It then returns to the heart via another network of vessels (the veins). This circulation also enables a stable internal environment for tissue function (see homeostasis), the transmission of chemical messengers (hormones) around the body, and a means of defending the body against pathogens and damage via the immune system. A water vascular system is characteristic of the Echinodermata.

2 The system of vascular tissue in plants.
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vascular tissue (vascular system)

The tissue that conducts water and nutrients through the plant body in higher plants (tracheophytes). It consists of xylem and phloem. Since the xylem and phloem tissues are always in close proximity to each other, distinct regions of vascular tissue can be identified (see vascular bundle). The possession of vascular tissue has enabled the higher plants to attain a considerable size and dominate most terrestrial habitats.

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vas deferens

One of a pair of ducts carrying sperm from the testis (or epididymis) to the outside, in mammals through the urethra.

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vas efferens

Any of various small ducts carrying sperm. In reptiles, birds, and mammals they convey sperm from the seminiferous tubules of the testis to the epididymis; in invertebrates they carry sperm from the testis to the vas deferens.

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vasoactive intestinal peptide

See VIP.

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vasoconstriction

The reduction in the internal diameter of blood vessels, especially arterioles or capillaries. The constriction of arterioles is mediated by the action of nerves on the smooth muscle fibres of the arteriole walls and results in an increase in blood pressure.

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vasodilation (vasodilatation)

The increase in the internal diameter of blood vessels, especially arterioles or capillaries. The vasodilation of arterioles is mediated by the action of nerves on the smooth muscle fibres of the arteriole walls and results in a decrease in blood pressure.

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vasomotor centre

The region of the medulla oblongata of the brain that mediates the diameter of the blood vessels (via the action of the vasomotor nerves) and hence controls blood pressure (see vasoconstriction; vasodilation).

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vasomotor nerves

The nerves of the autonomic nervous system that control the diameter of blood vessels. Vasoconstrictor nerves decrease the diameter (see vasoconstriction); vasodilator nerves increase it (see vasodilation).

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vasopressin

See antidiuretic hormone.

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vector

1 An animal, usually an insect, that passively transmits disease-causing microorganisms from one animal or plant to another or from an animal to man. Compare carrier.

2 (cloning vector) A vehicle used in gene cloning to insert a foreign DNA fragment into the genome of a host cell. For bacterial hosts three different types of vector are used: bacteriophages, plasmids, and their hybrid derivatives, cosmids. The foreign DNA is spliced into the vector using specific restriction enzymes and DNA ligases to cleave the vector DNA and join the foreign DNA to the two ends created (insertional vectors ). In some phage vectors, part of the viral genome is enzymically removed and replaced with the foreign DNA (replacement vectors ). Retroviruses can be effective vectors for introducing recombinant DNA into mammalian cells. In plants, derivatives of the tumour-inducing (Ti) plasmid of the crown gall bacterium, Agrobacterium tumefaciens, are used as vectors. See also expression vector.
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vegetative propagation (vegetative reproduction)

1 A form of asexual reproduction in plants whereby new individuals develop from specialized multicellular structures (e.g. tubers, bulbs) that become detached from the parent plant. Examples are the production of strawberry plants from runners and of gladioli from daughter corms. Artificial methods of vegetative propagation include grafting (see graft), budding, and making cuttings.

2 Asexual reproduction in animals, e.g. budding in Hydra.
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vein

1 A blood vessel that carries blood towards the heart. Most veins carry deoxygenated blood (the pulmonary vein is an exception). The largest veins are fed by smaller ones, which are formed by the merger of venules. Veins have thin walls and a relatively large internal diameter. Valves within the veins ensure that the flow of blood is always towards the heart. Compare artery.

2 A vascular bundle in a leaf (see venation).
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3 Any of the tubes of chitin that strengthen an insect's wing.
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velamen

A whitish spongy sheath of dead empty cells that surrounds the aerial roots of epiphytic plants, such as certain orchids. It absorbs any surface water on the roots.

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velum

See annulus.

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vena cava

Either of the two large veins that carry deoxygenated blood into the right atrium of the heart. The precaval vein (anterior or superior vena cava ) receives blood from the head and forelimbs; the postcaval vein (posterior or inferior vena cava ) drains blood from the trunk and hindlimbs.

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venation

1 The arrangement of veins (vascular bundles) in a leaf. The leaves of dicotyledons have a central main vein (midrib) with side branches that themselves further subdivide to form a network (net or reticulate venation ). The leaves of monocotyledons have parallel veins (parallel venation ).

2 The arrangement of the veins in an insect's wing, which is often important in classification.
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venter

The swollen base of an archegonium, in which the egg cell (oosphere) develops.

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ventilation

The process by which a continuous exchange of gases is maintained across respiratory surfaces. Often called external respiration, this is achieved by respiratory movements; in air-breathing vertebrates it is movement of air into and out of the lungs (see also air sac; expiration; inspiration; trachea; ventilation centre). The ventilation rate (or respiration rate ) of an animal is the volume of air breathed per minute, i.e. tidal volume × number of breaths per minute. It can be measured with the aid of a respirometer.

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ventilation centre

The group of neurones in the medulla oblongata of the brain that controls the process of ventilation. The partial pressure of carbon dioxide in the blood and the pH of the blood are monitored by chemoreceptors in the arteries. These include the carotid bodies in the carotid arteries and the aortic bodies in the wall of the aorta close to the heart. The ventilation centre responds to an increase in the amount of carbon dioxide in the blood by increasing the rate of breathing. Within the ventilation centre are subcentres that control inspiration (see inspiratory centre) and expiration (expiratory centre ).

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ventral

Describing the surface of a plant or animal that is nearest or next to the ground or other support, i.e. the lower surface. In bipedal animals, such as man, it is the forward-directed (anterior) surface. Compare dorsal.

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ventral aorta

The artery in vertebrate embryos that carries blood from the ventricle of the heart to the aortic arches. In adult fish it branches into afferent branchial arteries supplying the gills; in adult tetrapods it is represented by the ascending part of the aorta. Compare dorsal aorta.

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ventral root

The part of a spinal nerve that leaves the spinal cord on the ventral side and contains motor fibres. Compare dorsal root. See spinal cord.

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ventricle

1 A chamber of the heart that receives blood from an atrium and pumps it into the arterial system. Amphibians and fish have a single ventricle, but mammals, birds, and reptiles have two, pumping deoxygenated blood to the lungs and oxygenated blood to the rest of the body, respectively.

2 Any of the four linked fluid-filled cavities in the brain of vertebrates. One of these cavities is in the medulla oblongata, two are in the cerebral hemispheres (see cerebrum), and the fourth is in the posterior part of the forebrain. The ventricles contain cerebrospinal fluid filtered from the blood by the choroid plexus.
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venule

A small blood vessel that receives blood from the capillaries and transports it to a vein.

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vermiform appendix

See appendix.

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vernalization

The application of a cold treatment to germinating seeds or seedlings to ensure flowering. Many plants, including winter cereals, will not flower unless subjected to a period of chilling early in their development. Winter cereals are therefore sown in the autumn for flowering the following year. However, if germinating seeds are artificially vernalized they can be sown in the spring for flowering the same year.

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vertebra

Any of the bones that make up the vertebral column. In mammals each vertebra typically consists of a main body, or centrum , from which arises a neural arch through which the spinal cord passes, and transverse processes projecting from the side. There are five groups of vertebrae, specialized for various functions and varying in number with the species. In man, for example, there are 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 fused sacral vertebrae, and 5 fused caudal vertebrae (forming the coccyx).

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vertebral column (backbone; spinal column; spine)

A flexible bony column in vertebrates that extends down the long axis of the body and provides the main skeletal support. It also encloses and protects the spinal cord and provides attachment for the muscles of the back. The vertebral column consists of a series of bones (see vertebra) separated by discs of cartilage (intervertebral discs). It articulates with the skull by means of the atlas vertebra, with the ribs at the thoracic vertebrae, and with the pelvic girdle at the sacrum (see sacral vertebrae).

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vertebrate

Any one of a large group of animals comprising all those members of the phylum Chordata that have backbones (see vertebral column). Vertebrates include the fishes, amphibians, reptiles, birds, and mammals.

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Vesalius, Andreas

(1514–64)
Belgian physician and anatomist, who was a professor at Padua for six years before becoming a physician to the Habsburg court. He is remembered for producing in 1538–43 definitive text and anatomical drawings of the human body, which were made from actual dissections.

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vesicle

A small, usually fluid-filled, membrane-bound sac within the cytoplasm of a living cell. Vesicles form part of the Golgi apparatus.

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vessel

1 (in botany) A tube within the xylem composed of joined vessel elements. Vessels facilitate the efficient movement of water from the roots to the shoots and leaves of a plant.

2 (in zoology) Any of various tubular structures through which substances are transported, especially a blood vessel or a lymphatic vessel.
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vessel element

A type of cell occurring within the xylem of flowering plants, many of which, end to end, form water-conducting vessels . Vessel elements are frequently very broad and have side walls thickened by deposits of lignin over most of the surface area. However, the end walls are broken down to provide connections with the cells both above and below them. Compare tracheid.

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vestibular apparatus

The part of the inner ear that is responsible for balance. The vestibular apparatus is continuous with the cochlea. It consists of the three semicircular canals, which detect movements of the head (see ampulla), and the utriculus and sacculus, which detect the position of the head (see macula). See ear.

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vestibular canal

A canal in the cochlea of the inner ear that connects with the oval window. It contains perilymph, through which pressure waves are transmitted from the oval window via Reissner's membrane to the endolymph within the cochlear duct. See also organ of Corti.

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vestibule

A chamber that leads to a body cavity or that links one cavity to another. For example, a vestibule leads from the vulva into the vagina in the mammalian reproductive system.

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vestigial organ

Any part of an organism that has diminished in size during its evolution because the function it served decreased in importance or became totally unnecessary. Examples are the human appendix and the wings of the ostrich.

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viable count

A measure of the number of living cells within a culture.

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vibrio

Any comma-shaped bacterium. Generally, vibrios are Gram-negative (see Gram's stain), motile, and aerobic. They are widely distributed in soil and water and while most feed on dead organic matter some are parasitic, e.g. Vibrio cholerae, the causal agent of cholera.

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villus

(pl. villi) A microscopic outgrowth from the surface of some tissues and organs, which serves to increase the surface area of the organ. Numerous villi line the interior of the small intestine. Their shape may vary from finger-like (in the duodenum) to spadelike (in the ileum). Intestinal villi are specialized for the absorption of soluble food material: each contains blood vessels and a lymph vessel (see lacteal).
Chorionic villi occur on the chorion of the mammalian placenta, where they increase the surface area for the exchange of materials between the fetal and maternal blood.

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viologen dyes (bipyridylium dyes)

A group of organic compounds based on two linked pyridine rings and including certain agricultural herbicides, notably Paraquat and diquat . These generally kill all nonwoody plants and are used as nonselective weedkillers. They act by interfering with electron transport in photosynthesis and by generating superoxide anions, which damage chloroplasts and other cell components. These herbicides are also highly toxic to animals, and their use is strictly controlled.

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VIP (vasoactive intestinal peptide)

A peptide hormone secreted by endocrine cells of the upper part of the small intestine in response to the entry of partially digested food from the stomach. VIP, along with secretin, stimulates the pancreas to produce a thin watery secretion containing bicarbonate. This raises the pH in the intestine in preparation for secretion of pancreatic enzymes. VIP also occurs as a neuropeptide in central nervous tissue and has a range of other physiological roles.

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virion

See virus.

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viroid

Any of various small naked single-stranded RNA molecules that infect plant cells and cause disease. Smaller than viruses, viroids are not enclosed in a protein coat of any kind: they generally consist of less than 400 nucleotides and do not contain any genes. The circular RNA strand undergoes extensive base pairing within itself, forming a double-stranded structure that mimics DNA and is apparently replicated by the host cell's enzymes. This behaviour is similar to that of certain introns, prompting the suggestion that viroids are escaped introns. Viroids include many commercially important disease agents, such as coconut cadang-cadang, citrus exocortis, and potato spindle tuber viroid.

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virology

The scientific study of viruses. See microbiology.

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virulence

The disease-producing ability of a microorganism. See also pathogen.

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virus

A particle that is too small to be seen with a light microscope or to be trapped by filters but is capable of independent metabolism and reproduction within a living cell. Outside its host cell a virus is completely inert. A mature virus (a virion ) ranges in size from 20 to 400 nm in diameter. It consists of a core of nucleic acid (DNA or RNA) surrounded by a protein coat (see capsid). Some (the enveloped viruses ) bear an outer envelope consisting of proteins and lipids. Inside its host cell the virus initiates the synthesis of viral proteins and undergoes replication. The new virions are released when the host cell disintegrates. Viruses are parasites of animals, plants, and some bacteria (see bacteriophage). Viral diseases of animals include the common cold, influenza, AIDS, herpes, hepatitis, polio, and rabies (see adenovirus; arbovirus; herpesvirus; HIV; myxovirus; papovavirus; picornavirus; poxvirus); some viruses are also implicated in the development of cancer (see retrovirus). Plant viral diseases include various forms of yellowing and blistering of leaves and stems (see tobacco mosaic virus). Antiviral drugs are effective against certain viral diseases and vaccines (if available) provide protection against others. See also interferon.

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visceral

Relating to the internal organs (the viscera ) that lie in the coelomic cavities of animals, i.e. in the thoracic and abdominal cavities of mammals. Compare somatic.

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visceral sensory neurone

See sensory neurone.

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vision

The sense that enables perception of objects in the environment by means of the eyes.

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visual acuity

Sharpness of vision: the ability of the eye to distinguish between objects that lie close together. This hinges on the ability of the eye to focus incoming light to form a sharp image on the retina. Visual acuity depends on the cone cells, which are most densely packed in the fovea, close to the centre of the retina, and are therefore in the optimum position to receive focused light. In addition, each cone cell synapses with a single bipolar cell in the retina and is thus able to send a separate signal, via the optic nerve fibres, to the brain.

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visual cortex (striate cortex)

The region of the cerebral cortex of the brain where sensory information from the eyes is interpreted.

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visual purple

See rhodopsin.

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vital capacity

The total amount of air that can be exhaled after maximum inspiration. The vital capacity of an average human is about 4.5 litres; in trained male athletes it can be 6 litres or more. However, some air always remains in the lungs (see residual volume).

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vital staining

A technique in which a harmless dye is used to stain living tissue for microscopical observation. The stain may be injected into a living animal and the stained tissue removed and examined (intravital staining ) or the living tissue may be removed directly and subsequently stained (supravital staining ). Microscopic organisms, such as protozoa, may be completely immersed in the dye solution. Vital stains include trypan blue, vital red, and Janus green, the latter being especially suitable for observing mitochondria.

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vitamin

One of a number of organic compounds required by living organisms in relatively small amounts to maintain normal health. There are some 14 generally recognized major vitamins: the water-soluble vitamin B complex (containing 9) and vitamin C and the fat-soluble vitamin A, vitamin D, vitamin E, and vitamin K. Most B vitamins and vitamin C occur in plants, animals, and microorganisms; they function typically as coenzymes. Vitamins A, D, E, and K occur only in animals, especially vertebrates, and perform a variety of metabolic roles. Animals are unable to manufacture many vitamins themselves and must have adequate amounts in the diet. Foods may contain vitamin precursors (called provitamins ) that are chemically changed to the actual vitamin on entering the body. Many vitamins are destroyed by light and heat, e.g. during cooking.

VITAMINS

1897

Dutch physician Christiaan Eijkman (1858–1930) cures beriberi in chickens with diet of whole rice.

1906–07

British biochemist Sir Frederick Hopkins demonstrates existence of accessory dietary elements essential for growth.

1912

Polish-born US biochemist Casimir Funk (1884–1967) extracts antiberiberi factor (an amine) from rice husks and coins the term ‘vitamine’ (vital amine; later changed to ‘vitamin’).

1913

US biochemist Elmer McCollum (1879–1967) discovers and names vitamin A (retinol) and names antiberiberi factor vitamin B.

1920

McCollum names antirachitic factor vitamin D.

1922

US embryologist Herbert Evans (1882–1971) discovers vitamin E (tocopherol).

1926

German chemist Adolf Windaus (1876–1959) discovers that ergosterol is converted to vitamin D in the presence of sunlight.

1931

German chemist Paul Karrer (1889–1971) determines the structure of (and synthesizes) vitamin A.

1932

Hungarian-born US biochemist Albert Szent-Györgyi (1893–1986) and US biochemist Charles King (1896–1986) independently isolate vitamin C (ascorbic acid).

1933

Polish-born Swiss chemist Tadeus Reichstein (1897–1996) and British chemist Walter Haworth (1883–1950) independently synthesize vitamin C.

US chemist Roger Williams (1893–1988) discovers the B vitamin pantothenic acid.

1934

Danish biochemist Carl Dam (1895–1976) discovers vitamin K.

1935

Karrer and Austrian-born German chemist Richard Kuhn (1900–67) independently synthesize vitamin B2 (riboflavin).

1937

US chemist Robert Williams (1886–1965) synthesizes vitamin B1 (thamin).

1938

Karrer synthesizes vitamin E.

Kuhn isolates and synthesizes vitamin B6 (pyridoxine).

1939

Dam and Karrer isolate vitamin K.

1940

Szent-Györgyi and US biochemist Vincent Du Vigneaud (1901–78) discover ‘vitamin H’ (the B vitamin biotin).

Roger Williams determines the structure of pantothenic acid.

US biochemist Edward Doisey synthesizes vitamin K.

1948

US biochemist Karl Folkers (1906– ) isolates vitamin B12 (cyanocobalamin).

1956

British chemist Dorothy Hodgkin (1910–94) determines the structure of vitamin B12.

1971

US chemist Robert Woodward (1917–79) and Swiss chemist Albert Eschenmoser (1925– ) synthesize vitamin B12.

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vitamin A (retinol)

A fat-soluble vitamin that cannot be synthesized by mammals and other vertebrates and must be provided in the diet. Green plants contain precursors of the vitamin, notably carotenes, that are converted to vitamin A in the intestinal wall and liver. The aldehyde derivative of vitamin A, retinal , is a constituent of the visual pigment rhodopsin. Deficiency affects the eyes, causing night blindness, xerophthalmia (dryness and thickening of the cornea), and eventually total blindness. The role of vitamin A in other aspects of metabolism is less clear; it may be involved in controlling ATP production and the growth of epithelial cells.

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vitamin B complex

A group of water-soluble vitamins that characteristically serve as components of coenzymes. Plants and many microorganisms can manufacture B vitamins but dietary sources are essential for most animals. Heat and light tend to destroy B vitamins.
Vitamin B_1 (thiamin(e) ) is a precursor of the coenzyme thiamine pyrophosphate, which functions in carbohydrate metabolism. Deficiency leads to beriberi in humans and to polyneuritis in birds. Good sources include brewer's yeast, wheatgerm, beans, peas, and green vegetables.
Vitamin B_2 (riboflavin ) occurs in green vegetables, yeast, liver, and milk. It is a constituent of the coenzymes FAD and FMN, which have an important role in the metabolism of all major nutrients as well as in the oxidative phosphorylation reactions of the electron transport chain. Deficiency of B_2 causes inflammation of the tongue and lips and mouth sores.
Vitamin B_6 (pyridoxine ) is widely distributed in cereal grains, yeast, liver, milk, etc. It is a constituent of a coenzyme (pyridoxal phosphate) involved in amino acid metabolism. Deficiency causes retarded growth, dermatitis, convulsions, and other symptoms.
Vitamin B_12 (cyanocobalamin or cobalamin ) is manufactured only by microorganisms and natural sources are entirely of animal origin. Liver is especially rich in it. One form of B_12 functions as a coenzyme in a number of reactions, including the oxidation of fatty acids and the synthesis of DNA. It also works in conjunction with folic acid (another B vitamin) in the synthesis of the amino acid methionine and it is required for normal production of red blood cells. Vitamin B_12 can only be absorbed from the gut in the presence of a glycoprotein called intrinsic factor ; lack of this factor or deficiency of B_12 results in pernicious anaemia.
Other vitamins in the B complex include nicotinic acid, pantothenic acid, biotin, and lipoic acid. See also choline; inositol.

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vitamin C (ascorbic acid)

A colourless crystalline water-soluble vitamin found especially in citrus fruits and green vegetables. Most organisms synthesize it from glucose but man and other primates and various other species must obtain it from their diet. It is required for the maintenance of healthy connective tissue; deficiency leads to scurvy. Vitamin C is readily destroyed by heat and light.

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vitamin D

A fat-soluble vitamin occurring in the form of two steroid derivatives: vitamin D_2 (ergocalciferol , or calciferol ), found in yeast; and vitamin D_3 (cholecalciferol ), which occurs in animals. Vitamin D_2 is formed from a steroid by the action of ultraviolet light and D_3 is produced by the action of sunlight on a cholesterol derivative in the skin. Fish-liver oils are the major dietary source. The active form of vitamin D is manufactured in response to the secretion of parathyroid hormone, which occurs when blood calcium levels are low. It causes increased uptake of calcium from the gut, which increases the supply of calcium for bone synthesis. Vitamin D deficiency causes rickets in growing animals and osteomalacia in mature animals. Both conditions are characterized by weak deformed bones.

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vitamin E (tocopherol)

A fat-soluble vitamin consisting of several closely related compounds, deficiency of which leads to a range of disorders in different species, including muscular dystrophy, liver damage, and infertility. Good sources are cereal grains and green vegetables. Vitamin E prevents the oxidation of unsaturated fatty acids in cell membranes, so maintaining their structure (see antioxidants).

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vitamin K

A fat-soluble vitamin consisting of several related compounds that act as coenzymes in the synthesis of several proteins (including prothrombin) necessary for blood clotting. Deficiency of vitamin K, which leads to extensive bleeding, is rare because a form of the vitamin is manufactured by intestinal bacteria. Green vegetables and egg yolk are good sources.

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vitelline membrane

See egg membrane.

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vitreous humour

The colourless jelly that fills the space between the lens and the retina of the vertebrate eye.

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viviparity

1 (in zoology) A form of reproduction in animals in which the developing embryo obtains its nourishment directly from the mother via a placenta or by other means. Viviparity occurs in some insects and other arthropods, in certain fishes, amphibians, and reptiles, and in the majority of mammals. Compare oviparity; ovoviviparity.

2 (in botany)
a A form of asexual reproduction in certain plants, such as the onion, in which the flower develops into a budlike structure that forms a new plant when detached from the parent.
b The development of young plants on the inflorescence of the parent plant, as seen in certain grasses and the spider plant.
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VNTR

See variable number tandem repeats.

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vocal cords

A pair of elastic membranes that project into the larynx in air-breathing vertebrates. Vocal sounds are produced when expelled air passing through the larynx vibrates the cords. The pitch of the sound produced depends on the tension of the cords, which is controlled by muscles and cartilages in the larynx.

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voltage clamp

An experimental apparatus for measuring the flow of ions through membrane channels in cells. As the flow of ions amounts to a flow of current this can easily be measured. Typically a cell is placed in solution with three electrodes, two intracellular and one extracellular. One of the intracellular electrodes is used to measure voltage across the membrane relative to the extracellular electrode; the second intracellular electrode passes current into the cell. When the membrane becomes either less negative (depolarization) or more negative (hyperpolarization) these effects can be detected by the electrodes.

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voluntary

Controlled by conscious thought. See voluntary muscle. Compare involuntary.

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voluntary muscle

(skeletal, striped, or striated muscle) Muscle that is under the control of the will and is generally attached to the skeleton. An individual muscle consists of bundles of long muscle fibres , each bounded by a sarcolemma and containing sarcoplasm, sarcoplasmic reticulum, and many nuclei. The whole muscle is covered with a strong connective tissue sheath (epimysium ) and attached at each end to a bone by inextensible tendons. Running through each fibre are smaller fibres (myofibrils ) having alternate light and dark bands, which contain protein filaments responsible for the muscle's contractile ability and give the muscle its typical striped appearance under the microscope. The functional unit of a myofibril is the sarcomere. See illustration.
The end of the muscle that is attached to a nonmoving bone is called the origin of the muscle; the end attached to a moving bone is the insertion . As a muscle contracts it becomes shorter and fatter, moving one bone closer to the other. Since a muscle cannot expand, another muscle (the extensor ) is required to move the bone in the opposite direction and stretch the first muscle (known as the flexor ). The flexor and extensor are described as antagonistic muscles . See illustration.

Structure and action of a voluntary muscle

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volutin

Polyphosphate granules that are used as food reserves by certain bacteria, including the blue-green bacteria.

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vulva

The female external genitalia, comprising in women two pairs of fleshy folds of tissue, the labia (see labium); the clitoris; and the vaginal opening.

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W

waggle dance

See dance of the bees.

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Wallace, Alfred Russel

(1823–1913)
British naturalist, who in 1848 went on an expedition to the Amazon, and in 1854 travelled to the Malay Archipelago. There he noticed the differences between the animals of Asia and Australasia and devised Wallace's line, which separates them. This led him to develop a theory of evolution through natural selection, which coincided with the views of Charles Darwin; their theories were presented jointly to the Linnaean Society in 1858.

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Wallace's line

An imaginary line that runs between the Indonesian islands of Bali and Lombok and represents the separation of the Australian and Oriental faunas. It was proposed by the A. R. Wallace, who had noted that the mammals in Southeast Asia are different from and more advanced than their Australian counterparts. He suggested this was because the Australian continent had split away from Asia before the better adapted placental mammals evolved in Asia. Hence the isolated Australian marsupials and monotremes were able to thrive while those in Asia were driven to extinction by competition from placental mammals. See also zoogeography.

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warfarin

3-(alpha-acetonylbenzyl)-4-hydroxycoumarin: a coumarin derivative used as a synthetic anticoagulant both therapeutically in clinical medicine and, in lethal doses, as a rodenticide (see pesticide).

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warm-blooded animal

See endotherm.

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warning coloration (aposematic coloration)

The conspicuous markings of an animal that make it easily recognizable and warn would-be predators that it is a poisonous, foul-tasting, or dangerous species. For example, the yellow-and-black striped abdomen of the wasp warns of its sting. See also mimicry.

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waste product

Any product of metabolism that is not required for further metabolic processes and is therefore excreted from the body. Common products include nitrogenous wastes (such as urea and ammonia), carbon dioxide, and bile.

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water

A colourless liquid, H_2O. In the gas phase water consists of single H_2O molecules in which the H–O–H angle is 105°. The structure of liquid water is still controversial; hydrogen bonding of the type H_2O…H–O–H imposes a high degree of structure (see hydrogen bond) and current models supported by X-ray scattering studies have short-range ordered regions, which are constantly disintegrating and re-forming. This ordering of the liquid state is sufficient to make the density of water at about 0°C higher than that of the relatively open-structured ice; the maximum density occurs at 3.98°C. This accounts for the well-known phenomenon of ice floating on water and the contraction of water below ice, a fact of enormous biological significance for all aquatic organisms. Water is a powerful solvent for both polar and ionic compounds; molecules or ions in solution are frequently strongly hydrated. Pure liquid water is very weakly dissociated into H_3O+ (hydroxonium) and OH– (hydroxyl) ions by self-ionization and consequently any compound that increases the concentration of the positive ion, H_3O+, is acidic and compounds increasing the concentration of the negative ion, OH–, are basic (see acid). The phenomena of ion transport in water and the division of materials into hydrophilic (water loving) and hydrophobic (water hating) substances are central features of almost all biological chemistry.

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water cycle

See hydrological cycle.

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water potential

Symbol . The difference between the chemical potential of the water in a biological system and the chemical potential of pure water at the same temperature and pressure. It is manifested as a force acting on water molecules in a solution separated from pure water by a membrane that is permeable to water molecules only and can be expressed as the sum of the solute potential and the pressure potential:
= s + p

Water potential is measured in kilopascals (kPa). The water potential of pure water is zero; aqueous solutions of increasing concentration have increasingly negative values. Water tends to move from areas of high (less negative) water potential to areas of low (more negative) water potential. Osmosis in plants is now described in terms of water potential.

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water vascular system

See Echinodermata.

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Watson, James Dewey

(1928–)
US biochemist, who moved to the Cavendish Laboratory, Cambridge, in 1951 to study the structure of DNA. In 1953 he and Francis Crick announced the now accepted two-stranded helical structure for the DNA molecule. In 1962 they shared the Nobel Prize for physiology or medicine with Maurice Wilkins (1916– ), who with Rosalind Franklin (1920–58) had made X-ray diffraction studies of DNA.

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Watson–Crick model

The double-stranded twisted ladder-like molecular structure of DNA as determined by James Watson and Francis Crick at Cambridge, England, in 1953. It is commonly known as the double helix .

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wax

(in biology) Esters of fatty acids, usually having a protective function. Examples are the beeswax forming part of a honeycomb and the wax coating on some leaves, fruits, and seed coats, which acts as a protective water-impermeable layer supplementing the functions of the cuticle. The seeds of a few plants contain wax as a food reserve.

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weed

See pest.

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Weismannism

The theory of the continuity of the germ plasm published by August Weismann (1834–1914) in 1886. It proposes that the contents of the reproductive cells (sperms and ova) are passed on unchanged from one generation to the next, unaffected by any changes undergone by the rest of the body. It thus rules out any possibility of the inheritance of acquired characteristics, and has become fundamental to neo-Darwinian theory.

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Western blotting (protein blotting)

An immunoassay for determining very small amounts of a particular protein in tissue samples or cells. The sample is subjected to electrophoresis on SDS-polyacrylamide gel to separate constituent proteins. The resultant protein bands are then `blotted' onto a polymer sheet. A radiolabelled antibody specific for the target protein is added; this binds to the protein, which can then be detected by autoradiography. A variation of this technique is used to screen bacterial colonies containing cDNA clones in order to isolate those colonies expressing a particular protein. The name is derived by analogy to that of Southern blotting.

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whalebone (baleen)

Transverse horny plates hanging down from the upper jaw on each side of the mouth of the toothless whales (see Cetacea), forming a sieve. Water, containing plankton on which the whale feeds, enters the open mouth and is then expelled with the mouth slightly closed, so that food is retained on the baleen plates.

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whales

See Cetacea.

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whey

See curd.

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white blood cell

See leucocyte.

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white matter

Part of the tissue that makes up the central nervous system of vertebrates. It consists chiefly of nerve fibres enclosed in whitish myelin sheaths. Compare grey matter.

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wild type

Describing the form of an allele possessed by most members of a population in their natural environment. Wild-type alleles are usually dominant.

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wilting

The condition that arises in plants when more water is lost by evaporation than is absorbed from the soil. This causes the cells to lose their turgor and the plant structure droops. Plants can normally recover from wilting if water is added to the soil, but permanent wilting and possible death can result if the plant does not have access to water for a long period of time. In certain plants wilting is important as a mechanism to avoid overheating: when the leaves droop they are taken out of direct contact with the sun's rays. When the sun sets the plant can begin to transpire at the normal rate and the cells of the leaves regain their turgor.

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windpipe

See trachea.

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wind pollination

See anemophily.

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wing

See flight.

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withdrawal reflex

See polysynaptic reflex; reflex.

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womb

See uterus.

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wood

The hard structural and water-conducting tissue that is found in many perennial plants and forms the bulk of trees and shrubs. It is composed of secondary xylem and associated cells, such as fibres. The wood of angiosperms is termed hardwood , e.g. oak and mahogany, and that of gymnosperms softwood , e.g. pine and fir. New wood is added to the outside of the old wood each growing season by divisions of the vascular cambium (see growth ring). Only the outermost new wood (sapwood) functions in water conduction; the inner wood (heartwood) provides only structural support.

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wort

See brewing.

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X

xanthophyll

A member of a class of oxygen-containing carotenoid pigments, which provide the characteristic yellow and brown colours of autumn leaves.

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xanthophyll cycle

A cyclic series of interconversions involving certain xanthophylls, which plays a vital role in the photoprotection of plant chloroplasts. Under intense illumination, violaxanthin is converted, via an intermediate, to zeaxanthin. The latter is thought to receive excess energy from chlorophyll and dissipate this as heat, thereby avoiding possible damage to the photosynthetic apparatus. In dim light, the zeaxanthin is reconverted to violaxanthin, so that all incident light energy is used for photosynthesis.

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Xanthophyta

A phylum of mostly freshwater organisms of the kingdom Protoctista, traditionally known as yellow-green algae, that possess carotenoid pigments (including xanthins), which are responsible for their colour, in addition to chlorophylls. Xanthophytes occur in a variety of forms – unicellular, colonial, filamentous, and siphonaceous; motile cells have two unequal-sized undulipodia (flagella). Storage products are oil and the polysaccharide chrysolaminarin .

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X chromosome

See sex chromosome.

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xenobiotic

Any substance foreign to living systems. Xenobiotics include drugs, pesticides, and carcinogens. Detoxification of such substances occurs mainly in the liver.

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xeric

Denoting conditions characterized by an inadequate supply of water. Xeric conditions exist in arid habitats, extremely cold habitats, and in salt marshes. Certain plants are adapted to live in such conditions. See halophyte; xerophyte.

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xeromorphic

Describing the structural modifications of certain plants (xerophytes) that enable them to reduce water loss, particularly from their leaves and stems.

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xerophthalmia

See vitamin A.

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xerophyte

A plant that is adapted to live in conditions in which there is either a scarcity of water in the soil, or the atmosphere is dry enough to provoke excessive transpiration, or both. Xerophytes have special structural (xeromorphic ) and functional modifications, including swollen water-storing stems or leaves (see succulent) and specialized leaves that may be hairy, rolled, or reduced to spines or have a thick cuticle to lower the rate of transpiration. Examples of xerophytes are desert cacti and many species growing on sand dunes and exposed moorlands. Some halophytes have xeromorphic features. Compare mesophyte; hydrophyte.

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X-ray crystallography

The use of X-ray diffraction to determine the structure of crystals or molecules, such as nucleic acids. The technique involves directing a beam of X-rays at a crystalline sample and recording the diffracted X-rays on a photographic plate. The diffraction pattern consists of a pattern of spots on the plate, and the crystal structure can be worked out from the positions and intensities of the diffraction spots. X-rays are diffracted by the electrons in the molecules and if molecular crystals of a compound are used, the electron density distribution in the molecule can be determined.

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X-rays

Electromagnetic radiation of shorter wavelength than ultraviolet radiation and longer wavelength than gamma radiation. The range of wavelengths is 10–11m to 10–9m. X-rays can pass through many forms of matter and they are therefore used medically and industrially to examine internal structures. X-rays are produced for these purposes by an X-ray tube.

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xylem

A tissue that transports water and dissolved mineral nutrients in vascular plants. In flowering plants it consists of hollow vessels that are formed from cells (vessel elements) joined end to end. The end walls of the vessel elements are perforated to allow the passage of water. In less advanced vascular plants, such as conifers and ferns, the constituent cells of the xylem are called tracheids. In young plants and at the shoot and root tips of older plants primary xylem is formed by the apical meristems (see protoxylem; metaxylem). In plants showing secondary growth this xylem is replaced in most of the plant by secondary xylem , formed by the vascular cambium. The walls of the xylem cells are thickened with lignin, the extent of this thickening being greatest in secondary xylem. Xylem contributes greatly to the mechanical strength of the plant: wood is mostly made up of secondary xylem. See also fibre. Compare phloem.

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xylenes

See dimethylbenzenes.

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Y

YAC

See yeast artificial chromosome.

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Y chromosome

See sex chromosome.

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yeast artificial chromosome (YAC)

A highly modified plasmid vector used for cloning large segments of DNA in yeast cells. It is derived from a circular bacterial plasmid into which are inserted various elements required for its replication by the yeast, such as a centromere region and an origin of replication site. It can be cut with an endonuclease enzyme and telomeres added to each end, making a linear chromosome. Long stretches of DNA, up to 500 kb, can be inserted, making YACs suitable for cloning large eukaryotic genes in their entirety.

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yeasts

A group of unicellular fungi within the class Hemiascomycetae of the phylum Ascomycota. They occur as single cells or as groups or chains of cells; yeasts reproduce asexually by budding and sexually by producing ascospores. Yeasts of the genus Saccharomyces ferment sugars and are used in the baking and brewing industries (see baker's yeast).

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yellow body

See corpus luteum.

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yolk

The food stored in an egg for the use of the embryo. It can consist mainly of protein (protein yolk ) or of phospholipids and fats (fatty yolk ). The eggs of oviparous animals (e.g. birds) contain a relatively large yolk.

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yolk sac

One of the protective membranes surrounding the embryos of birds, reptiles, and mammals (see extraembryonic membranes). The embryo derives nourishment from the yolk sac via a system of blood vessels. In birds and reptiles the yolk sac encloses the yolk; in most mammals a fluid replaces the yolk.

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Y organ

Either of a pair of glands in the head of certain crustaceans that secrete a hormone resembling ecdysone and play a role in the moulting process.

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Z

zeatin

A naturally occurring cytokinin that was first identified in 1963 in the grains of the maize plant (Zea mais).

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zinc

Symbol Zn. A blue-white metallic element that is a trace element (see essential element) required by living organisms. It functions as the prosthetic group of a number of enzymes.

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zinc finger

A structural motif characteristic of certain proteins that bind to DNA, notably transcription factors. It consists of a finger-like fold of amino acids at the base of which lie two cysteine and two histidine residues. These four residues bind a single zinc ion in a tetrahedral array. The finger interacts with about five base pairs in the nucleic acid molecule. Unlike other DNA-binding motifs, such as the helix-turn-helix and the leucine zipper, the zinc finger is also found in proteins that bind to RNA, such as RNA-directed RNA polymerase.

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Z line (Z band)

The thin membrane that separates adjacent sarcomeres in the fibres of striated muscle. It is visible under an electron microscope as a thin dark line in the middle of a lighter band. The centre point of each thin (actin) filament is anchored to the Z line and exerts a pull on the membrane when the fibre contracts.

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zona pellucida

A layer of glycoprotein that surrounds the plasma membrane of a mammalian egg cell. It develops as a jelly coat around the primary oocyte and is surrounded by the granulosa cells.

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zonation

The distribution of the different species of a community into separate zones, which are created by variations in the environment. A clear example of zonation occurs on a rocky shore, where different species of seaweed (Fucus) occupy different zones, according to their ability to withstand desiccation. For example, the species found in the splash zone, which is never completely submerged in water, is better adapted to exposure than those found in zones lower down the shore, where they are submerged for longer periods. Animals, particularly stationary species, such as barnacles, also exhibit zonation on a rocky shore; as with the seaweeds, this may depend on the ability of different species to withstand desiccation. Competition between species may also contribute to zonation.

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zone fossil

See index fossil.

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zoogeography

The study of the geographical distributions of animals. The earth can be divided into several faunal regions separated by natural barriers, such as oceans, deserts, and mountain ranges. The characteristics of the fauna of each region are believed to depend particularly (but not wholly) on the process of continental drift and the stage of evolution reached when the various land masses became isolated. For example Australia, which has been isolated since Cretaceous times, has the most primitive native mammalian fauna, consisting solely of marsupials and monotremes. See also Wallace's line.

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zooid

An individual member of a colony of invertebrate animals, especially an individual of the phylum Bryozoa.

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zoology

The scientific study of animals, including their anatomy, physiology, biochemistry, genetics, ecology, evolution, and behaviour.

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Zoomastigota (Zoomastigina)

A phylum of the Protoctista containing heterotrophic protozoa that can be parasitic or free-living. They possess one or more undulipodia (flagella) for locomotion. In some recent classifications this phylum is revised: members lacking mitochondria are placed in the Archaeprotista, while others, including Trypanosoma (the sleeping-sickness parasite) and Naegleria, which have mitochondria with flattened cristae, are placed in the Discomitochondria, together with the Euglenida.

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zoonosis

(pl. zoonoses) An infectious disease of nonhuman vertebrates that can be transmitted to humans. Rabies and anthrax are well-known examples, and certain midges and the tsetse fly act as carriers for a variety of nematode-worm zoonoses.

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zooplankton

The animal component of plankton. All major animal phyla are represented in zooplankton, as adults, larvae, or eggs; some are just visible to the naked eye but most cannot be seen without magnification. Near the surface of the sea there may be many thousands of such animals per cubic metre.

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zoosporangium

See zoospore.

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zoospore

A spore that possesses one or more flagella and is therefore motile. Released from a sporangium (called a zoosporangium ), zoospores are produced by many algae and certain other protoctists, such as the potato blight (Phytophthora infestans) and other members of the Oomycota.

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zwitterion (ampholyte ion)

An ion that has a positive and negative charge on the same group of atoms. Zwitterions can be formed from compounds that contain both acid groups and basic groups in their molecules. For example, the amino acid glycine has the formula H_2N.CH_2.COOH. However, under neutral conditions, it exists in the different form of the zwitterion +H_3N.CH_2.COO–, which can be regarded as having been produced by an internal neutralization reaction (transfer of a proton from the carboxyl group to the amino group). Glycine therefore has some properties characteristic of ionic compounds, e.g. a high melting point and solubility in water. In acid solutions, the positive ion +H_3NCH_2COOH is formed. In basic solutions, the negative ion H_2NCH_2COO– predominates. The name comes from the German zwei, two.

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zygomorphy

See bilateral symmetry.

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Zygomycota

A phylum of saprotrophic or parasitic fungi that includes the bread mould (Mucor). Their hyphae lack cross walls and they can reproduce asexually by sporangiospores formed within a sporangium or sexually by means of zygospores.

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zygospore

A zygote with a thick resistant wall, formed by some algae and fungi (see Zygomycota). It results from the fusion of two gametes, neither of which is retained by the parent in any specialized sex organ (such as an oogonium). It enters a resting phase before germination. Compare oospore.

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zygote

A fertilized female gamete: the product of the fusion of the nucleus of the ovum or ovule with the nucleus of the sperm or pollen grain. See fertilization.

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zygotene

The second phase of the first prophase of meiosis, in which pairing (synapsis) of homologous chromosomes takes place. Intimate contact is made between identical regions of homologues, in a process involving proteins and DNA organized to form a synaptonemal complex .

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zymase

An extract of brewer's yeast containing a mixture of glycolytic enzymes (see glycolysis) that can perform fermentation.

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zymogen

Any inactive enzyme precursor that, following secretion, is chemically altered to the active form of the enzyme. For example, the protein-digesting enzyme trypsin is secreted by the pancreas as the zymogen trypsinogen. This is changed in the small intestine by the action of another enzyme, enterokinase, to the active form.

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OXFORD

UNIVERSITY PRESS
A Dictionary of Biology

Reproduced by kind permission of Market House Books Ltd.
Editors

Elizabeth Martin MA

Robert S. Hine BSc, MSc

Advisers

B. S. Beckett BSc, BPhil, MA Ed)

R. A. Hands BSc

Michael Lewis MA

W. D. Phillips PhD

Contributors

Tim Beardsley BA

Lionel Bender BSc

Belinda Cupid MSc, PhD

John Clark BSc

H. M. Clarke MA, MSc

E. K. Daintith BSc

Malcolm Hart BSc, MIBiol

Robert S. Hine BSc, MSc

Elaine Holmes BSc, PhD

Anne Lockwood BSc

J. Valerie Neal BSc, PhD

R. A. Prince MA

Michael Ruse BSc, PhD

Brian Stratton BSc, MSc

Elizabeth Tootill BSc, MSc

Over 4,700 entries
The perfect guide for those studying biology, either at school or university. Including biographies of key scientists, it provides comprehensive coverage of biology, biophysics, and biochemistry, as well as key terms from medicine and palaeontology. There are also feature articles on important topics, such as genetically modified organisms.

ISBN 198605838

Fourth Edition, 2000

E-book copyright © 2003.

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