The Biology glossary

The dark band of a sarcomere, equal to the length of the thick myosin filaments; its length does not change during contraction.

The aminoacyl site of a ribosome, where an incoming tRNA carrying an amino acid binds by codon–anticodon pairing.

Movement of a limb or body part away from the midline of the body, such as raising the leg sideways at the hip.

The origin of living cells from non-living matter through a series of chemical stages, thought to have happened on the early Earth.

A non-living physical or chemical feature of the environment that affects organisms, such as temperature, light intensity, water availability, salinity or pH.

A classification of human blood into groups A, B, AB and O according to the A and B carbohydrate antigens on the glycoproteins and glycolipids of red blood cells.

A phytohormone produced in leaves and roots when water is scarce that causes stomata to close, and that keeps seeds dormant.

In nutrition, the movement of the small molecules produced by digestion across the wall of the gut into the blood or lymph.

A graph of the amount of light absorbed by a pigment (absorbance or percentage absorption) against the wavelength of light.

A photosynthetic pigment other than chlorophyll a, such as chlorophyll b or a carotenoid, that absorbs additional wavelengths and passes the energy to chlorophyll a.

How close a measurement or the mean of several measurements is to the true value of the quantity being measured.

An acetyl group (2C) attached to coenzyme A, formed from pyruvate in the link reaction or from fatty acids, which delivers the acetyl group to the Krebs cycle.

A neurotransmitter (ACh) released at many synapses, including neuromuscular junctions, that binds to receptors that open Na⁺ channels; it is broken down by acetylcholinesterase.

An enzyme in the synaptic cleft that hydrolyses acetylcholine into choline and acetate, ending its effect on the postsynaptic membrane.

The elongation of plant cells caused by auxin: proton pumps move H⁺ into the cell wall, and the low pH loosens the cross-links between cellulose microfibrils so that turgor pressure stretches the wall.

A change in an organism's body or behaviour that develops during its lifetime through the environment or use, and is not inherited because it does not alter the genes in its gametes.

A vesicle over the front of a sperm head containing hydrolytic enzymes, released when the sperm meets the egg so that it can digest a path through the zona pellucida.

The release of hydrolytic enzymes from the acrosome of a sperm when it binds to the zona pellucida, which digest a path through the zona pellucida to the egg.

A globular protein that polymerises into the thin filaments of a sarcomere; myosin heads bind to sites on actin to form cross-bridges.

A rapid, temporary reversal of the membrane potential of a neuron, from about −70 mV to about +30 mV and back, caused by the movement of Na⁺ and K⁺ ions across the membrane.

A graph of the rate of photosynthesis, measured as oxygen release or carbon dioxide uptake, against the wavelength of light.

The energy that must be supplied to reactants to break or weaken their bonds so that a reaction can take place; enzymes lower it.

The region of an enzyme, usually a pocket or groove on its surface, to which the substrate binds and where catalysis takes place.

The movement of particles across a membrane against their concentration gradient, from a lower to a higher concentration, by pump proteins using energy from ATP.

A heritable characteristic that increases the chance of an organism surviving and reproducing in its environment; adaptations arise by natural selection over many generations.

The part of the immune system that responds specifically to particular pathogens, using lymphocytes and antibodies, and builds up a memory so that later responses are faster and stronger.

The rapid diversification of one ancestral species into many new species, each adapted to a different niche, often after colonising a new habitat or after other species became extinct.

Movement of a limb or body part towards the midline of the body, such as bringing a raised leg back to the side.

A purine base (A) found in DNA and RNA, which pairs with thymine in DNA and with uracil in RNA by two hydrogen bonds.

An enzyme in the plasma membrane that, when activated by a G protein, converts ATP into the second messenger cyclic AMP.

The attraction between water molecules and molecules of a different substance, caused by hydrogen bonds or other attractions to polar or charged surfaces.

Animal tissue made of cells that store large droplets of triglyceride; it is used for long-term energy storage, thermal insulation and protection of organs.

Adenosine diphosphate: the nucleotide with two phosphate groups formed when ATP is hydrolysed; energy from respiration is used to add a phosphate to ADP to remake ATP.

An endocrine gland on top of each kidney; its inner medulla secretes epinephrine when the body prepares for vigorous activity or faces a threat.

Cell respiration that uses oxygen and releases a large amount of energy from glucose or other substrates, producing carbon dioxide and water; it takes place in the cytoplasm and mitochondria.

Establishing a forest, usually by planting trees, on land that has not been forested in recent times.

The clumping together of cells or pathogens when antibodies bind to antigens on their surfaces, for example when mismatched red blood cells are transfused.

Acquired immunodeficiency syndrome: the condition that develops when HIV has destroyed so many helper T-lymphocytes that the body can no longer fight opportunistic infections.

The proportion of incoming solar radiation reflected by a surface; snow and ice have a high albedo, while open ocean, forest and bare soil have a low albedo.

A rapid increase in the population of algae or cyanobacteria in a body of water, usually caused by an increased supply of nutrients such as nitrate and phosphate.

The property of an action potential that it either occurs fully, with the same size, once the threshold potential is reached, or does not occur at all.

One of the different forms of a gene, differing from other forms of the same gene by small differences in base sequence.

The proportion of all the copies of a gene in a population's gene pool that are a particular allele, expressed as a decimal between 0 and 1.

The release by a plant of chemical substances into the soil or air that inhibit the germination, growth or survival of other plants nearby.

The formation of new species from populations that are separated geographically, so that gene flow between them is prevented by a physical barrier.

Polyploidy in which the chromosome sets come from two different species after hybridisation; doubling of the chromosome number makes the otherwise sterile hybrid fertile.

A binding site on an enzyme, separate from the active site, to which a specific regulatory molecule binds reversibly, changing the shape of the enzyme and its active site.

A secondary structure in which a polypeptide coils into a spiral held by hydrogen bonds between the C=O of one peptide bond and the N–H four amino acids along.

The ring form of glucose in which the hydroxyl group on carbon 1 lies below the plane of the ring; the monomer of starch and glycogen.

A statement that there is a difference, effect or association between the variables being studied; it is accepted when the null hypothesis is rejected.

Splicing of different combinations of exons from the same pre-mRNA, so that one gene can code for several different polypeptides.

A tiny air sac at the end of a bronchiole in the lungs, with a wall one cell thick and a dense capillary network, where gas exchange takes place.

The –NH₂ group of an amino acid, which forms part of a peptide bond; it can gain a hydrogen ion to become –NH₃⁺.

A molecule with a central carbon atom bonded to an amine group, a carboxyl group, a hydrogen atom and a variable R-group; the monomer of polypeptides.

Describes a molecule with both a hydrophilic region and a hydrophobic region, such as a phospholipid with its hydrophilic phosphate head and hydrophobic fatty acid tails.

The branched component of starch: chains of α-glucose joined by 1,4 glycosidic bonds, with 1,6 glycosidic bonds at the branch points.

The unbranched component of starch: a chain of α-glucose units joined by 1,4 glycosidic bonds that coils into a helix.

The metabolic reactions that build macromolecules from smaller molecules, mainly by condensation reactions; these reactions need an input of energy.

Cell respiration without oxygen, releasing a small amount of energy (2 ATP per glucose) in the cytoplasm; the product is lactate in humans and ethanol and carbon dioxide in yeast.

A structure that performs a similar function in different species but has a different evolutionary origin and basic structure, because it evolved independently.

The phase of mitosis in which sister chromatids separate at the centromere and are pulled to opposite poles of the cell.

A pair of muscles that produce opposite movements at a joint or structure: when one contracts, the other relaxes and is stretched.

The part of the stamen that produces pollen grains containing the male gametes.

Caused or produced by human activity, such as anthropogenic extinction or anthropogenic climate change.

A chemical that kills bacteria or inhibits their growth by blocking processes that occur in prokaryotic cells but not in eukaryotic cells.

The ability of a strain of bacteria to survive and reproduce in the presence of an antibiotic that would normally kill it; it evolves by natural selection.

A Y-shaped protein (immunoglobulin) produced by plasma cells that binds specifically to one antigen by its variable region.

A sequence of three bases on tRNA that is complementary to a codon on mRNA and pairs with it by hydrogen bonds during translation.

A hormone made in the hypothalamus and secreted by the pituitary gland when the blood is too concentrated; it causes aquaporins to be inserted into collecting duct cells so more water is reabsorbed.

A molecule, usually a protein or glycoprotein on the surface of a pathogen or cell, that is recognised by the immune system and stimulates the production of specific antibodies.

Gradual change in the surface antigens of a virus, such as influenza haemagglutinin and neuraminidase, caused by the accumulation of point mutations in their genes.

A sudden major change in the surface antigens of influenza virus, caused by reassortment of RNA segments when two different strains infect the same cell.

Running side by side in opposite directions; the two strands of DNA are antiparallel, so the 5′ end of one lies opposite the 3′ end of the other.

Treatment of HIV infection with a combination of drugs, usually three, that act on different viral targets, so that resistance to all of them at once is very unlikely to evolve.

A predator at the top of a food chain that is not preyed upon by any other species as an adult.

The inhibition of side-bud growth by auxin produced in the main shoot tip, so that the main shoot grows more strongly than the side shoots.

An integral channel protein forming a narrow pore through which water molecules pass in single file, greatly increasing the permeability of a membrane to water.

One of the three domains of life: single-celled prokaryotes that differ from bacteria in their membrane lipids and other molecules, and that obtain energy from light, inorganic chemicals or carbon compounds.

Succession prevented from reaching the climax community by a continuing factor, often a human activity such as grazing, burning or drainage, so the community is held at an earlier stage (a plagioclimax).

A small branch of an artery leading into a capillary bed, whose smooth muscle can narrow or widen the lumen to control blood flow to a tissue.

A blood vessel that carries blood away from the heart, at high pressure, with a thick wall of smooth muscle and elastic fibres.

Reproduction by a single parent, without the fusion of gametes, producing offspring that are genetically identical to the parent and to each other.

The uptake and use of absorbed food molecules by body cells, either to build new cell components or as respiratory substrates.

The scientific study of the origin, evolution and possible distribution of life in the universe, including the search for liquid water beyond Earth.

The build-up of fatty plaques containing cholesterol in the walls of arteries, which narrows the lumen, reduces elasticity and restricts blood flow.

Adenosine triphosphate: a nucleotide made of adenine, ribose and three phosphate groups that distributes energy within cells; hydrolysis of the terminal phosphate releases energy.

An enzyme spanning the inner mitochondrial or thylakoid membrane that uses the energy of protons diffusing through it to phosphorylate ADP to ATP.

A valve between an atrium and a ventricle (tricuspid on the right, bicuspid or mitral on the left) that closes during ventricular systole to prevent backflow into the atrium.

One of the two thin-walled upper chambers of the mammalian heart, which receive blood from veins and pump it into the ventricles.

A small signalling molecule released by bacteria in quorum sensing; its concentration rises with population density, and above a threshold it binds receptors that change gene expression.

Polyploidy in which all the sets of chromosomes come from a single species, usually after unreduced gametes form or the chromosome number doubles.

Any chromosome that is not a sex chromosome; humans have 22 pairs of autosomes.

An organism that uses an external energy source to synthesise its carbon compounds from simple inorganic substances such as carbon dioxide and water.

A phytohormone, mainly indole-3-acetic acid (IAA), made in shoot tips, that promotes cell elongation in shoots and controls tropisms, apical dominance and root formation.

A membrane protein (PIN protein) that moves auxin ions out of a plant cell; carriers on only one side of each cell make auxin flow in one direction through a tissue.

The single long nerve fibre of a neuron that carries impulses away from the cell body to the axon terminals.

A lymphocyte that, when activated by its specific antigen and a helper T-lymphocyte, divides to form plasma cells that secrete one type of antibody, and memory cells.

The normal, low rate at which species become extinct between mass extinctions, estimated from the fossil record.

The domain of prokaryotes with peptidoglycan cell walls and ester-linked membrane lipids, including organisms such as Escherichia coli and cyanobacteria.

A virus that infects bacteria, such as bacteriophage lambda, which infects Escherichia coli; often shortened to phage.

A synovial joint in which a rounded bone end fits into a cup-shaped socket, allowing movement in three planes, as at the hip and shoulder.

A sedimentary rock of alternating iron-oxide and silica-rich layers, mostly deposited 2.5–1.8 billion years ago when oxygen from photosynthesis reacted with dissolved iron.

A stretch receptor in the wall of the aorta or the carotid arteries that detects changes in blood pressure.

Two complementary bases on opposite strands joined by hydrogen bonds (A–T or C–G); DNA length is measured in base pairs (bp).

A gene mutation in which one base in the DNA sequence is replaced by a different base.

The average number of people infected by one infected person in a population where everyone is susceptible (R₀); the herd immunity threshold is 1 − 1/R₀.

Reproductive isolation caused by differences in courtship behaviour, songs, signals or mate preferences, so that members of two populations do not mate with each other.

A tumour that grows slowly, stays in one place and does not invade neighbouring tissue or spread; it is not cancerous.

The ring form of glucose in which the hydroxyl group on carbon 1 lies above the plane of the ring; the monomer of cellulose.

A secondary structure in which sections of polypeptide lie side by side in a pleated sheet, held by hydrogen bonds between backbone C=O and N–H groups of neighbouring sections.

The division of a prokaryote cell into two genetically identical daughter cells after its circular DNA has been copied; mitochondria and chloroplasts divide in a similar way.

The international system of naming each species with two words: the genus name followed by the specific name, for example Homo sapiens.

The build-up of a substance, such as a pollutant, in the tissues of an individual organism over its lifetime, because it is absorbed faster than it is broken down or excreted.

The mass of dissolved oxygen used by aerobic microorganisms to decompose the organic matter in a water sample, usually over five days at 20 °C in the dark; a high value indicates organic pollution.

The variety of life in all its forms, levels and combinations, including ecosystem diversity, species diversity and genetic diversity.

The current rapid loss of biodiversity worldwide at the genetic, species and ecosystem levels, at rates far above the background rate, caused mainly by human activity.

The use of computers to store, retrieve and analyse large amounts of biological data, such as base sequences of genes and amino acid sequences of proteins.

The definition of a species as a group of organisms that can interbreed and produce fertile offspring, and that are reproductively isolated from other such groups.

The production of light by a living organism through a chemical reaction catalysed by the enzyme luciferase, as in Vibrio fischeri at high population density.

The increase in the concentration of a persistent pollutant in the tissues of organisms at successively higher trophic levels of a food chain.

The total mass of organic matter in organisms, usually given as dry mass (or mass of carbon) per unit area.

A group of ecosystems with similar communities of organisms, resulting from similar abiotic conditions, especially temperature and rainfall, and from convergent evolution.

An effect of other living organisms on an organism or population, such as competition, predation, parasitism, disease or food supply.

A pair of homologous chromosomes held together during prophase I and metaphase I of meiosis; it consists of four chromatids.

A hollow ball of cells formed about five days after fertilisation, with an outer layer (trophoblast), an inner cell mass and a fluid-filled cavity; the stage that implants in the endometrium.

The conversion of liquid blood into a solid clot at a wound, in which a mesh of fibrin traps blood cells and seals the damaged vessel.

The liquid part of blood, about 90% water, which carries dissolved substances such as glucose, amino acids, ions, urea, hormones and carbon dioxide.

The force per unit area exerted by blood on the walls of blood vessels, measured in mmHg and recorded as systolic over diastolic pressure, such as 120/80 mmHg.

The movement of the oxygen dissociation curve to the right when carbon dioxide concentration rises, because haemoglobin's affinity for oxygen falls, so more oxygen is released.

Regulation of the populations in a community by the supply of resources at lower trophic levels, such as nutrients and light limiting producers and so the consumers that depend on them.

An 80S ribosome attached to the rough endoplasmic reticulum, synthesising a protein that enters the ER lumen for secretion, the plasma membrane or lysosomes.

The cup-shaped start of a nephron that surrounds the glomerulus; its inner wall of podocytes allows filtrate to pass into the tubule during ultrafiltration.

A graph showing the distribution of a continuous variable by its minimum, lower quartile, median, upper quartile and maximum, with outliers plotted separately.

At constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume, so increasing the volume lowers the pressure.

One of the narrow, branching airways in the lungs that carry air from the bronchi to the alveoli.

Fat tissue whose cells contain many mitochondria with thermogenin, specialised for producing heat by uncoupled respiration; abundant in newborn babies and present in the neck of many adults.

A form of asexual reproduction, as in yeast, in which a small outgrowth of the parent cell receives a nucleus after mitosis and is separated by unequal cytokinesis.

The upward force on an object in a fluid, equal to the weight of fluid it displaces; water gives far more buoyancy than air because it is much denser.

A disease in which cells divide uncontrollably, forming malignant tumours that can invade tissues and spread; it results from mutations in genes that control cell division.

The upper layer of a forest formed by the crowns of the tallest trees, which absorbs most of the light.

The narrowest type of blood vessel, with a wall made of a single layer of thin endothelial cells, in which materials are exchanged between blood and the surrounding tissue or environment.

The movement of water along narrow spaces, even against gravity, caused by adhesion of water to the walls together with cohesion between water molecules.

The protein coat of a virus, made of many repeating protein subunits, which encloses and protects the viral genome.

Breeding animals of a threatened species in zoos or other controlled conditions, choosing mates to maintain genetic diversity, often to reintroduce them to the wild.

A method of estimating the size of a population of motile animals by capturing, marking and releasing a sample, then capturing a second sample and recording how many of its members are marked.

A compound of carbon, hydrogen and oxygen, usually with hydrogen and oxygen in the ratio 2 : 1; carbohydrates include monosaccharides, disaccharides and polysaccharides.

The continuous movement of carbon atoms between the atmosphere, oceans, living organisms, dead organic matter and fossil stores by photosynthesis, feeding, respiration, decomposition, diffusion and combustion.

The conversion of inorganic carbon dioxide into an organic compound; in the Calvin cycle Rubisco combines CO₂ with RuBP to form two molecules of glycerate 3-phosphate.

The rate of transfer of carbon from one carbon pool to another, such as by photosynthesis or respiration, measured as mass of carbon per unit time.

The total amount of greenhouse gases, expressed as the equivalent mass of carbon dioxide, released directly and indirectly by an activity, product, organisation or person.

A reservoir in which carbon is held, such as the atmosphere, the oceans, living organisms, soils, peat or fossil fuels.

The capture of carbon dioxide from the atmosphere and its long-term storage, for example in the biomass and soil of forests or in peat.

A pool or ecosystem that absorbs more carbon dioxide than it releases, so that carbon accumulates in it, because photosynthesis exceeds respiration (and combustion).

A pool or ecosystem that releases more carbon dioxide than it absorbs, because respiration and combustion exceed photosynthesis.

A type of stony meteorite from water-rich asteroids, containing water in clay minerals and carbon compounds; its D/H ratio is close to that of Earth's oceans.

The –COOH group of an amino acid or fatty acid; in amino acids it forms part of a peptide bond, and it can lose a hydrogen ion to become –COO⁻.

An agent that causes cancer; most carcinogens are mutagens that change genes controlling cell division.

The sequence of contraction (systole) and relaxation (diastole) of the atria and ventricles that makes up one heartbeat, together with the opening and closing of the valves.

Striated muscle of the heart, made of short, branched cells with usually one nucleus each, joined end to end by intercalated discs.

The region of the medulla oblongata that receives impulses from baroreceptors and chemoreceptors and controls heart rate through the sympathetic and vagus nerves.

An animal that feeds on other animals; carnivorous mammals typically have long pointed canines and blade-like cheek teeth.

A yellow, orange or red accessory pigment, such as carotene or a xanthophyll, that absorbs blue and blue-green light and protects chlorophyll from excess light.

The female part of a flower, made up of the stigma, style and ovary containing one or more ovules.

An individual who is heterozygous for a recessive allele that causes a disease, and so can pass the allele on without showing the disease.

The maximum population size of a species that an environment can support indefinitely, given the resources available.

A smooth, tough, flexible connective tissue; at synovial joints it covers the ends of bones, reducing friction and absorbing shock.

The metabolic reactions that break down macromolecules into monomers by hydrolysis, or oxidise substrates into simpler molecules; many release energy.

A substance that increases the rate of a chemical reaction without being changed or used up by the reaction.

A relationship in which a change in one variable directly produces a change in another; correlation alone does not demonstrate it.

The part of a neuron that contains the nucleus and most of the cytoplasm, from which the nerve fibres (dendrites and axon) project.

The sequence of events between one cell division and the next, consisting of interphase (G1, S and G2), mitosis and cytokinesis.

The process by which a parent cell (mother cell) divides to form two daughter cells.

The breaking open of cells and separation of their organelles into different fractions, usually by centrifugation at increasing speeds, so that each type can be studied.

A membrane-bound structure formed at the equator of a dividing plant cell by the fusion of vesicles from the Golgi apparatus; it develops into the new plasma membranes and cell wall.

An increase in the number of cells by repeated cycles of cell growth and division by mitosis, used for growth, cell replacement and tissue repair.

The controlled release of energy from organic compounds in cells to produce ATP, by a series of enzyme-catalysed reactions.

The theory that all living organisms are composed of one or more cells, that the cell is the smallest unit of life, and that cells arise only from pre-existing cells.

A strong, freely permeable layer outside the plasma membrane that gives shape and support and stops the cell bursting; made of peptidoglycan in bacteria, cellulose in plants and chitin in fungi.

A protein in the plasma membrane that binds to a matching molecule on a neighbouring cell, attaching cells to each other to form tissues; different CAMs form different junctions.

The ability of cells to identify other cells, or to distinguish self from non-self, by binding to specific molecules such as the carbohydrate chains of glycoproteins on their surfaces.

A structural polysaccharide of plant cell walls made of β-glucose joined by 1,4 glycosidic bonds, with alternate units inverted, forming straight chains bundled into microfibrils.

The brain and spinal cord, which receive information from receptors, integrate it and send impulses to effectors.

A short cylinder of microtubules found in pairs near the nucleus of animal cells, which organises microtubules and forms the base of cilia and flagella.

The region of a chromosome where the two sister chromatids are held together and where spindle microtubules attach during cell division.

The part of the brain at the back of the head, below the cerebral hemispheres, that coordinates the timing and force of skeletal muscle contractions and maintains balance and posture.

One of the two halves of the cerebrum, the largest part of the brain, whose outer cortex processes sensory information, initiates voluntary movement and carries out thought, learning and memory.

An integral membrane protein with a hydrophilic pore through which specific ions or polar molecules diffuse across the membrane; many channels are gated and can open or close.

In double-stranded DNA the amount of adenine equals thymine and guanine equals cytosine, so purines equal pyrimidines, while the proportions of A + T and G + C vary between species.

A point in the cell cycle, such as the end of G1 or G2, at which the cell continues only if conditions are right, for example when DNA replication is complete.

The formation of increasingly complex carbon compounds from simple inorganic molecules without living organisms, using energy such as lightning, UV or heat.

The movement of protons down their concentration gradient across a membrane through ATP synthase, which couples the energy released to the synthesis of ATP.

An autotroph that obtains the energy it needs to make carbon compounds from oxidation reactions involving inorganic substances, such as the oxidation of iron(II) ions or hydrogen sulfide.

A receptor that detects chemicals; chemoreceptors in the carotid bodies, aortic bodies and medulla detect blood pH and CO₂ and O₂ concentrations.

A statistical test that compares observed frequencies with the frequencies expected if a null hypothesis is true, to determine whether the difference between them is statistically significant.

A point where non-sister chromatids of a bivalent have crossed and exchanged sections of DNA during crossing over in prophase I.

A tough nitrogen-containing polysaccharide that forms the cell walls of fungi and the exoskeletons of arthropods.

The main photosynthetic pigment, a green molecule that absorbs red and blue light and reflects green; plants contain chlorophyll a, the main pigment, and chlorophyll b, an accessory pigment.

A double-membrane organelle of plants and algae in which photosynthesis takes place; it contains thylakoids stacked into grana in a fluid stroma, with 70S ribosomes and circular DNA.

A steroid lipid found among the phospholipids of animal cell membranes, which regulates membrane fluidity by reducing it at high temperatures and preventing stiffening at low temperatures.

One of the two identical DNA molecules that make up a chromosome after DNA replication; sister chromatids are held together at the centromere until anaphase.

A technique that separates the substances in a mixture by the different distances they are carried by a solvent through a stationary phase such as paper or a thin layer of silica.

A DNA molecule carrying genes; in eukaryotes chromosomes are linear and bound to histone proteins, while the main prokaryote chromosome is a naked circular loop.

A short hair-like projection from the surface of a eukaryote cell, with a core of microtubules, that beats to move the cell or to move fluid over the cell surface; cilia are usually many and short.

A cycle of changes in an organism's body or behaviour that repeats about every 24 hours and continues even without external cues, although it is normally reset each day by light.

One of the flattened, membrane-bound sacs that make up the Golgi apparatus or the endoplasmic reticulum.

The collection of scientific data by members of the public, usually volunteers following a standard method, in projects designed and analysed by scientists.

The six-carbon compound formed in the Krebs cycle when an acetyl group combines with oxaloacetate.

A group of organisms that consists of a common ancestor and all of its descendants, which share characteristics inherited from that ancestor.

A method of classification that places organisms into clades based on shared characteristics inherited from a common ancestor, so that groups reflect evolutionary relationships.

A tree diagram showing the most probable sequence of divergence in a group of organisms, based on shared derived characteristics; each branch point represents a common ancestor.

A protein that assembles into a curved, cage-like coat on the cytoplasmic side of a membrane, bending it into a pit that buds off as a vesicle.

The groove that forms around the equator of a dividing animal cell as a contractile ring of actin and myosin pulls the plasma membrane inwards during cytokinesis.

A long-term change in the average weather patterns of a region or of the whole Earth, including temperature, rainfall and extreme events; current climate change is caused mainly by human greenhouse gas emissions.

The relatively stable community that forms at the end of succession, in balance with the climate and soil of the area, and which persists until a major disturbance occurs.

A graph showing climate: either the monthly temperature and precipitation of one place, or the mean annual temperature and precipitation at which each biome occurs.

The activation of only those B-lymphocytes whose antibodies match a particular antigen, followed by their division by mitosis to form a large clone of identical cells.

A system that exchanges energy but not matter with its surroundings, such as a sealed mesocosm; matter inside it must be recycled.

The strand of DNA that is not transcribed; it has the same base sequence as the mRNA, with thymine in place of uracil. Also called the sense strand.

A pattern of inheritance in which both alleles are expressed in a heterozygote, giving a dual phenotype, for example both A and B antigens in blood group AB.

A sequence of three bases on mRNA that codes for one amino acid or acts as a start or stop signal.

The square of the correlation coefficient (R²), a value from 0 to 1 giving the proportion of the variation in the dependent variable explained by its linear relationship with the independent variable.

A carrier molecule (CoA) that accepts the two-carbon acetyl group in the link reaction and from fatty acids, and delivers it to the Krebs cycle as acetyl-CoA.

The attraction between molecules of the same substance; in water it is caused by hydrogen bonds between water molecules.

The explanation that water is pulled up the xylem as a continuous column under tension, created by evaporation from leaf cell walls and transmitted down the column by cohesion between water molecules.

The protective sheath that covers the young shoot of a grass seedling as it grows through the soil; it is widely used in experiments on phototropism.

A fibrous structural protein made of three polypeptides wound into a triple helix; it gives tendons, skin and bone high tensile strength and is the most abundant protein in mammals.

The tube that receives filtrate from many nephrons and carries it through the medulla, where water is reabsorbed by osmosis to an extent controlled by ADH.

The theory that a reaction occurs only when particles collide with enough energy and in the correct orientation; the rate depends on the frequency of successful collisions.

A technique that measures how much light of a chosen wavelength a coloured solution absorbs, giving a numerical value (absorbance) that increases with the concentration of the coloured substance.

The burning of carbon compounds in oxygen, releasing energy as heat and light and releasing carbon dioxide and water.

A word or phrase in an assessment question, such as state, explain or evaluate, with a fixed meaning that sets what kind of answer is expected and how much depth it needs.

Descent of different species from a shared ancestor; the near-universal genetic code is evidence that all living organisms share a common ancestor.

All the populations of different species that live and interact with one another in the same area at the same time.

A phloem cell with a nucleus and many mitochondria that lies beside a sieve tube element, is connected to it by plasmodesmata and keeps it alive.

The division of a cell into separate membrane-bound compartments, each with its own conditions, enzymes and substrates, which concentrates reactants and keeps incompatible processes apart.

The principle that two species with identical niches cannot coexist indefinitely in the same place, because the better competitor will eventually eliminate the other.

Inhibition in which a molecule similar to the substrate binds reversibly to the active site, preventing substrate binding; it is overcome at high substrate concentrations.

The specific pairing of bases by hydrogen bonds between nucleic acid strands: adenine with thymine (or uracil in RNA) and cytosine with guanine.

A difference in the concentration of a substance between two regions; particles diffuse down it from higher to lower concentration, and moving them against it requires energy.

A reaction in which two molecules are joined by a covalent bond and a molecule of water is released; used to build polysaccharides, polypeptides, nucleic acids and lipids.

The precise three-dimensional shape of a protein, produced by the folding of its polypeptide chain or chains.

A change in the three-dimensional shape of a protein, for example when a substrate, signalling molecule or phosphate group binds to it.

A variable that is associated with both the factor being studied and the outcome, so that it can create a misleading correlation between them.

A protein that contains a non-polypeptide component, a prosthetic group, in addition to its polypeptides; haemoglobin, with its haem groups, is an example.

The state of awareness of oneself and one’s surroundings; an emergent property arising from the interactions of very large numbers of neurons in the brain.

A base or amino acid sequence that is identical or similar across a species or a group of species.

A heterotroph that obtains its energy and carbon compounds by feeding on other living organisms.

A table showing the frequencies of two categorical variables in combination, such as the presence or absence of two species in a set of quadrats.

The persistence of biological structures, information and conditions over time, for example through DNA replication, inheritance, homeostasis and the stability of ecosystems.

Variation in which a trait can take any value within a range, with many intermediates, such as human height; usually controlled by many genes and affected by the environment.

A vacuole in freshwater protists such as Paramecium that collects excess water entering by osmosis and expels it from the cell.

A variable other than the independent variable that could affect the dependent variable and is kept constant so that the experiment is a fair test.

The independent evolution of similar characteristics in unrelated lineages exposed to similar selection pressures, producing analogous structures.

Binding in which each oxygen molecule that binds to haemoglobin changes its conformation and increases the affinity of the remaining haem groups for oxygen (and the reverse on release).

The loss of colour in corals caused by expulsion of their symbiotic zooxanthellae, usually in response to high water temperature; prolonged bleaching leads to starvation and death of the coral.

A marine ecosystem built on a calcium carbonate framework deposited by reef-building corals, found in warm, clear, shallow tropical seas of normal salinity.

One of the arteries that branch from the aorta and supply the heart muscle itself with oxygenated blood.

Disease caused by narrowing or blockage of the coronary arteries, which reduces the blood supply to heart muscle; also called ischaemic heart disease (CHD).

The structure that develops from the follicle after ovulation and secretes progesterone (and oestradiol), maintaining the endometrium.

A relationship between two variables in which a change in one is associated with a change in the other; it may be positive or negative.

A number from −1 to +1 (r) that measures the strength and direction of a linear relationship between two variables; values near 0 show little or no correlation.

In a plant stem or root, the region of mainly parenchyma cells between the epidermis and the vascular tissue, used for support and storage.

A vesicle just beneath the plasma membrane of an egg whose contents, released after fertilisation, harden the zona pellucida and prevent other sperm entering.

The release of enzymes from cortical granules of the egg after a sperm fuses with it, which harden the zona pellucida so that no other sperm can pass through.

A membrane protein that moves two different particles across a membrane together in the same direction, using the movement of one down its concentration gradient to move the other against its gradient.

A chemical bond in which two atoms share a pair of electrons; covalent bonds are strong and hold the atoms of a molecule together.

A form of photosynthesis in which stomata open at night and CO₂ is stored as organic acids, then released for photosynthesis during the day while the stomata are closed, reducing water loss.

The shrinkage of an animal cell, such as a red blood cell, in a hypertonic solution as it loses water by osmosis, giving it a shrivelled outline with spiky projections.

A gene-editing system in which a guide RNA directs the nuclease Cas9 to a complementary DNA sequence, where Cas9 cuts both strands so the gene can be disrupted or changed.

One of the folds of the inner mitochondrial membrane that project into the matrix and increase the surface area for the reactions that produce ATP.

The value of a test statistic at the chosen significance level (usually p = 0.05); a calculated value greater than it leads to rejection of the null hypothesis.

The link formed when a myosin head binds to a binding site on an actin filament during muscle contraction.

The transfer of pollen from the anther of one plant to the stigma of a different plant of the same species.

The exchange of sections of DNA between non-sister chromatids of a pair of homologous chromosomes during prophase I of meiosis.

Electron microscopy of samples frozen so rapidly that water forms glass-like ice, with many images combined by computer to give three-dimensional structures of proteins at near-atomic resolution.

A photosynthetic bacterium that releases oxygen; chloroplasts are thought to have evolved from a cyanobacterium taken in by endosymbiosis.

Cyclic adenosine monophosphate (cAMP), a second messenger made from ATP by adenylyl cyclase, which activates protein kinase A, for example in the response to adrenaline.

ATP production in which excited electrons from photosystem I return to the electron transport chain and then to photosystem I, making ATP but no reduced NADP or oxygen.

A pattern of succession in which a community passes through a series of stages and then returns to an earlier stage, repeating the cycle instead of progressing to a single stable climax.

One of a family of regulatory proteins whose concentrations rise and fall during the cell cycle; a cyclin activates a cyclin-dependent kinase, allowing the cell to pass a checkpoint.

An enzyme that is active only when bound to a cyclin; it phosphorylates target proteins that trigger the next stage of the cell cycle.

A small signalling protein secreted by a cell, especially a cell of the immune system, that acts locally on the same or nearby cells to regulate growth, division and immune responses.

The division of the cytoplasm of a parent cell between the two daughter cells, after nuclear division.

A phytohormone made mainly in root tips and carried to the shoot in the xylem that promotes cell division and the growth of side shoots.

Everything inside the plasma membrane except the nucleus: a watery fluid (cytosol) containing dissolved substances, enzymes, ribosomes and, in eukaryotes, organelles.

A pyrimidine base (C) found in DNA and RNA, which pairs with guanine by three hydrogen bonds.

A network of protein filaments (microtubules and microfilaments) in the cytoplasm of eukaryote cells that maintains shape, holds organelles in place and moves structures.

An electronic device that records measurements from one or more sensors at set intervals over a period of time.

An early model of membrane structure, proposed in 1935, in which a phospholipid bilayer is sandwiched between two continuous layers of protein; falsified by later evidence.

The removal of a carboxyl group from a molecule, releasing carbon dioxide, as in the link reaction and the Krebs cycle.

An organism that breaks down dead organic matter and releases carbon dioxide and mineral ions that can be reused; mainly saprotrophic fungi and bacteria.

Reasoning from a general statement or theory to a specific prediction, such as predicting that a newly discovered organism will be made of cells.

The permanent removal of forest and its conversion to another land use, such as agriculture, plantations or urban development.

The feature of the genetic code that most amino acids are coded by more than one codon.

The number of values in a statistical calculation that are free to vary; for a contingency table it equals (rows − 1) × (columns − 1).

A gene mutation in which one or more nucleotides are removed from the base sequence of DNA.

A change in the three-dimensional conformation of a protein, caused by heat or pH breaking the bonds that hold its shape, so that it loses its function; usually permanent.

One of the multiple short, branched nerve fibres of a neuron that receive signals and carry impulses towards the cell body.

A factor whose effect on a population becomes greater as the population density increases, such as competition, predation and disease; it controls population size by negative feedback.

A factor that affects the same proportion of a population whatever its density, such as a frost, flood, drought or fire.

The number, type, shape and arrangement of the teeth of an animal, which reflect its diet.

The pentose sugar in DNA nucleotides, which has a hydrogen atom instead of an –OH group on carbon 2′.

The variable that is measured in an investigation, because it is expected to change when the independent variable changes.

A reduction in the potential difference across a membrane, making the inside less negative, as when Na⁺ ions diffuse into a neuron.

An animal that ingests dead organic matter (detritus) and digests it internally in its gut, such as an earthworm or woodlouse.

Dead organic matter, including faeces, dead parts of organisms such as leaves and shed skin, and the remains of whole dead organisms.

The ratio of deuterium (heavy hydrogen) to ordinary hydrogen atoms in a sample of water, used to identify where water came from; abbreviated D/H.

A dome-shaped sheet of muscle below the lungs; it contracts and flattens during inspiration, increasing the volume of the thorax.

The phase of the cardiac cycle in which the heart muscle relaxes and the chambers fill with blood.

An identification tool made of a series of steps, each offering a choice between two contrasting descriptions of an observable feature, leading to the name of an organism.

A flowering plant whose seeds have two seed leaves (cotyledons), typically with net-veined leaves and vascular bundles arranged in a ring in the stem.

Natural selection acting in different directions on two populations because they experience different selection pressures, so that they diverge.

The process by which an unspecialised cell becomes specialised for a particular function, by expressing some of its genes and not others.

The breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed, by mechanical action and by enzyme-catalysed hydrolysis.

A genetic cross that follows the inheritance of two different genes at the same time.

A molecule made of two amino acids joined by a peptide bond in a condensation reaction.

Describes a nucleus or cell that has two sets of chromosomes, so that each chromosome has a homologous partner (2n).

Natural selection in which individuals at one extreme of the range of a trait have the highest fitness, so the mean value of the trait in the population shifts in that direction over generations.

The difference between the two ends of a nucleic acid strand, the 5′ end with a phosphate and the 3′ end with a free –OH; strands are built 5′ to 3′.

A sugar made of two monosaccharides joined by a glycosidic bond in a condensation reaction, such as maltose, sucrose or lactose.

Variation in which individuals fall into a small number of distinct categories with no intermediates, such as the four ABO blood groups; also called discontinuous variation.

Natural selection in which individuals at both extremes of the range of a trait have higher fitness than intermediates, so the distribution may split into two groups.

A wrong option in a multiple-choice question, written to be attractive to a candidate who holds a common misconception or makes a common error.

A covalent bond between the sulfur atoms of two cysteine R-groups, which stabilises the tertiary or quaternary structure of a protein.

The process by which species descended from a common ancestor become increasingly different as they adapt to different environments, producing homologous structures with different functions.

Deoxyribonucleic acid: a double-stranded nucleic acid with deoxyribose and the bases A, C, G and T, which is the genetic material of all living organisms.

The identification of species using the base sequence of a short, standardised region of DNA, such as part of the COI gene in animals, compared with a reference database.

A mixture of DNA fragments of known lengths run in one lane of a gel, used as a scale to estimate the sizes of unknown fragments.

An enzyme that joins DNA fragments, such as Okazaki fragments, by forming a phosphodiester bond between the 3′ end of one and the 5′ end of the next.

The addition of methyl groups to cytosine bases in DNA; methylation of a promoter represses transcription of the gene that follows it.

An enzyme that links free nucleotides into a new DNA strand complementary to a template strand, forming covalent phosphodiester bonds between them.

A prokaryotic enzyme that removes RNA primers from new DNA strands and replaces them with DNA nucleotides.

The main replicating enzyme in prokaryotes: it extends primers by adding nucleotides to the 3′ end of the new strand (5′ to 3′) and proofreads it.

An enzyme that builds short RNA primers complementary to the template, giving DNA polymerase III a 3′ end to extend.

Analysis of an individual’s DNA, usually the lengths of several short tandem repeat loci, to give a pattern of fragments that is almost unique to that individual.

The production of exact copies of a DNA molecule, with base sequences identical to the original, using each existing strand as a template.

The highest rank in the classification of organisms; all life is divided into three domains, Bacteria, Archaea and Eukarya.

An allele that has the same effect on the phenotype whether it is present in the homozygous or the heterozygous state.

A circulatory system in which blood passes through the heart twice in each complete circuit, through separate pulmonary (lungs) and systemic (body) circulations, as in mammals.

The shape of a DNA molecule: two antiparallel strands of nucleotides coiled around each other, with the backbones outside and paired bases inside.

A condition caused by trisomy 21, three copies of chromosome 21, usually resulting from non-disjunction during meiosis.

A state in which movement or change continues in opposite directions at equal rates, so that there is no net change; for example water entering and leaving a cell in an isotonic solution.

The exit site of a ribosome, where a tRNA that has given up its polypeptide is held briefly before it leaves.

The role of a species in an ecosystem, including all its interactions with biotic and abiotic factors that affect its growth, survival and reproduction, including how it obtains food.

The sequence of changes in the species composition of a community over time, in which one group of species replaces another, often ending in a relatively stable climax community.

A community of interacting organisms together with the abiotic (non-living) environment with which they interact.

The variety of habitats, communities and ecosystems within a region.

The capacity of an ecosystem to persist over time, keeping its structure and functioning despite disturbances, either by resisting change or by returning to its previous state.

A species that is both Evolutionarily Distinct and Globally Endangered, and so is given priority for conservation by the EDGE of Existence programme.

A muscle or gland that carries out a response when it is stimulated by a motor neuron or a hormone.

The removal from the gut of undigested food that was never absorbed into body cells, as faeces; different from excretion of metabolic waste.

The return of stretched elastic fibres in an artery wall to their original length, which pushes blood onward and maintains blood pressure between heartbeats.

A microscope that uses a beam of electrons focused by electromagnets to form an image; its very short wavelength gives a resolution below 1 nm, but specimens must be dead and in a vacuum.

A series of electron carrier proteins in the inner mitochondrial membrane (or thylakoid membrane) that pass electrons along, using the energy released to pump protons across the membrane.

The tendency of an atom to attract the shared electrons in a covalent bond towards itself; oxygen is much more electronegative than hydrogen.

An organism in the early stages of development after the zygote begins to divide; in humans, up to about eight weeks after fertilisation.

A property of a whole system that arises from the interactions between its parts and is not shown by any of the parts on their own.

A species that is native to a particular area, having evolved there or arrived without human help (in some contexts, found naturally only in that area).

The system of glands that secrete hormones directly into the blood, which carries them to target cells throughout the body.

The uptake of material into a cell by the plasma membrane folding inwards around it and pinching off to form a vesicle; requires ATP.

The inner lining of the uterus, rich in blood vessels, which thickens during each menstrual cycle ready for the implantation of an embryo.

An internal skeleton of bone and cartilage, as in vertebrates, with muscles attached to the outside of the bones by tendons.

The process by which a prokaryote cell taken into a larger cell survived and became a permanent part of it; mitochondria and chloroplasts are thought to have originated in this way.

An animal that maintains a high, constant body temperature using heat generated by its own metabolism, such as a mammal or bird.

A diagram in which each trophic level is drawn as a horizontal bar whose width is proportional to the energy flowing through that level per unit area per unit time, for example in kJ m⁻² yr⁻¹.

The increase in the greenhouse effect caused by human activities that raise atmospheric concentrations of greenhouse gases such as carbon dioxide and methane, resulting in global warming.

A non-coding DNA sequence, often far from the gene, that binds activator proteins and increases the rate of transcription of a gene.

The network of neurons in the wall of the gut that controls peristalsis and other gut activities involuntarily, without needing instructions from the brain.

DNA that has been shed by organisms into their surroundings, such as water, soil or air, and can be collected and analysed to identify the species present; abbreviated eDNA.

A biological catalyst, usually a globular protein, that speeds up a specific chemical reaction in a living organism without being used up.

A substance that binds to an enzyme and reduces the rate of the reaction it catalyses; inhibitors may be competitive, non-competitive or irreversible.

The ability of an enzyme to catalyse only one reaction or a small group of similar reactions, because only a substrate with a complementary shape and chemistry can bind to its active site.

The temporary structure formed when a substrate binds to the active site of an enzyme, before it is converted into products.

A rapid increase in the number of cases of an infectious disease in a population, above the level normally expected.

The study of the incidence, distribution and causes of diseases in populations, used to identify risk factors and evaluate ways of preventing disease.

The outer layer of cells covering a leaf, stem or root; in leaves it is transparent, secretes the cuticle and contains stomata.

The development of patterns of differentiation in the cells of a multicellular organism, as the organism develops from a single unspecialised cell.

The passing on of patterns of gene expression to daughter cells or offspring by epigenetic tags, without any change in the base sequence of DNA.

A chemical group, such as a methyl group on cytosine or a methyl or acetyl group on a histone, that alters gene expression without changing the base sequence of DNA.

The study of heritable changes in gene expression that do not involve changes to the base sequence of DNA.

A hormone (also called adrenaline) secreted by the adrenal glands that prepares the body for vigorous activity, for example by increasing heart rate, ventilation and the supply of glucose and oxygen to muscles.

A plant that grows on another plant, such as on a tree branch, for support and access to light, without taking nutrients from it; for example many orchids and bromeliads.

A line drawn through a plotted mean on a graph, extending above and below it, that shows the variability or uncertainty of the data, usually ±1 standard error or ±1 standard deviation.

An amino acid that an organism cannot synthesise, so it must be obtained from the diet; adult humans have nine.

The covalent bond formed by a condensation reaction between the carboxyl group of a fatty acid and a hydroxyl group of an alcohol such as glycerol.

A gaseous phytohormone (ethene, C₂H₄) that stimulates fruit ripening; ripening fruit releases more ethylene, giving positive feedback.

The domain containing all organisms whose cells have a nucleus and membrane-bound organelles: animals, plants, fungi and protists.

An organism whose cells have a nucleus enclosed in a double membrane and membrane-bound organelles; animals, plants, fungi and protists are eukaryotes.

The enrichment of a body of water with mineral nutrients, especially nitrate and phosphate, causing excessive growth of algae and other producers and leading to depletion of dissolved oxygen.

Change in the heritable characteristics of a population over many generations.

A measure of how much unique evolutionary history a species represents, based on how few close relatives it has and how long ago its lineage separated from others.

The conservation of species outside their natural habitat, for example in zoos, botanic gardens or seed banks.

The number of deaths from all causes during a period above the number expected from previous years; it estimates the total effect of an event such as a pandemic.

A membrane transport protein that moves one type of particle into the cell and another type out of the cell in the same cycle, such as the sodium–potassium pump.

A local depolarisation of the postsynaptic membrane, caused by a neurotransmitter opening channels that let positive ions such as Na⁺ into the cell, which makes an action potential more likely.

An electron raised to a higher energy level by absorbed light energy; when emitted by a reaction centre, it carries that energy to an electron acceptor.

The removal from an organism of waste products of its metabolism, such as carbon dioxide, urea or excess water; the removal of undigested food (egestion) is not excretion.

The release of material from a cell by a vesicle fusing with the plasma membrane and emptying its contents outside the cell; requires ATP.

A section of a eukaryotic gene that remains in the mature mRNA after splicing and is usually translated.

A hard external skeleton, such as the chitin cuticle of arthropods, that supports and protects the body and provides inner surfaces for muscle attachment.

The frequency predicted for a category if the null hypothesis is true; in a contingency table it equals row total × column total ÷ grand total.

Breathing out: the volume of the thorax decreases, raising the pressure in the lungs so that air flows out; it is mostly passive at rest.

The extra volume of air that can be breathed out with maximum effort after a normal expiration.

Growth in which the population increases by a constant proportion in each unit of time, so that it doubles in a fixed time; it occurs when resources are unlimited and there are no limiting factors.

Straightening movement at a joint that increases the angle between the two bones, such as straightening the knee.

The permanent loss of a species, when its last individual dies.

An enzyme that is secreted from the cell that made it and catalyses reactions outside that cell, such as digestive enzymes in the gut.

A transparent scale with arbitrary divisions placed in the eyepiece of a microscope; once calibrated with a stage micrometer it is used to measure specimens.

The first generation of offspring produced by crossing two parents (the P generation).

The second generation of offspring, produced by crossing F1 individuals with each other or by self-pollination of the F1.

The passive movement of particles down a concentration gradient across a membrane through a channel protein (or with a carrier protein), without the use of energy from ATP.

An organism that respires aerobically when oxygen is present and anaerobically when it is absent, so it can survive with or without oxygen, for example yeast.

Flavin adenine dinucleotide: a hydrogen carrier reduced in the Krebs cycle; reduced FAD passes its electrons to the electron transport chain at a later carrier than reduced NAD.

The rejection of a hypothesis or theory when evidence contradicts a prediction made from it; a scientific idea must be testable in a way that could show it to be false.

A molecule with a carboxyl group at one end of a long hydrocarbon chain; fatty acids combine with glycerol to form triglycerides and phospholipids.

Regulation of a metabolic pathway in which the end product inhibits an enzyme of an early step, usually the first, by binding to its allosteric site.

A capillary whose endothelial cells have pores (fenestrations) that allow especially rapid exchange of water and solutes, as in kidney glomeruli, intestinal villi and endocrine glands.

The fusion of a male gamete with a female gamete to form a diploid zygote.

The haemoglobin of the fetus, with two γ chains instead of β chains; it has a higher affinity for oxygen than adult haemoglobin, so it takes up oxygen from maternal blood at the placenta.

An insoluble protein formed from fibrinogen by thrombin; its fibres make a mesh that traps blood cells to form a clot.

A protein with a long, thin, strand-like shape, usually insoluble in water and with a structural function, such as collagen or keratin.

The circular area of a specimen that is visible through the eyepiece at one time; its diameter decreases as the magnification increases.

The fluid formed in Bowman's capsule by ultrafiltration, containing water, glucose, amino acids, ions and urea at about plasma concentrations but no blood cells or plasma proteins.

The survival and reproductive potential of a genotype, measured by the number of offspring individuals with that genotype contribute to the next generation, relative to others.

Describes a plant cell whose contents no longer press against the cell wall because it has lost water, so that it has no turgor pressure; flaccid tissue is soft and limp.

A long whip-like projection from a cell that moves it through liquid; eukaryote flagella have a microtubule core and are usually one or two per cell, while prokaryote flagella are simpler protein fibres.

Bending movement at a joint that decreases the angle between the two bones, such as bending the knee.

The model of membrane structure in which proteins are embedded in or attached to a fluid phospholipid bilayer, with phospholipids and many proteins free to move sideways.

A dye that absorbs light of one wavelength and emits light of a longer wavelength, so that the structures it binds to glow against a dark background in a fluorescence microscope.

A fluid-filled sac in the ovary containing a developing oocyte surrounded by follicle cells, which secrete oestradiol.

A hormone (FSH) secreted by the anterior pituitary that stimulates the development of follicles in the ovary and their secretion of oestradiol.

A sequence of organisms in which each feeds on the one before it, showing the direction of transfer of energy and biomass from producers to consumers.

A network of interconnected food chains showing all the feeding relationships in a community.

The preserved remains or traces of an organism from the past, such as bones, shells, imprints or structures like stromatolites, usually found in sedimentary rock.

A fuel such as coal, oil or natural gas formed over millions of years from the buried remains of organisms in which decomposition was incomplete.

An insertion or deletion of a number of nucleotides that is not a multiple of three, which changes the reading frame so every codon after the mutation is altered.

The branched, unprotected ending of a sensory neuron in a tissue such as the skin, which detects stimuli such as heat, acid and tissue damage and acts as a pain receptor.

An 80S ribosome in the cytosol that is not attached to a membrane; it synthesises proteins for use within the cytoplasm.

A field experiment (FACE) in which CO₂ is released over a plot of vegetation in the open air to raise its CO₂ concentration while other conditions stay natural.

An electron microscopy technique in which a specimen is frozen rapidly and split so that the fracture runs through the centre of membranes, showing the proteins embedded in them.

The processes carried out by all living organisms: metabolism, nutrition, growth, response to stimuli, movement, excretion, homeostasis and reproduction.

The potential range of conditions and resources in which a species could survive and reproduce, given its adaptations and tolerance limits, in the absence of competition.

A protein of three subunits on the inner surface of the plasma membrane that is activated by a G protein-coupled receptor when GDP on its α subunit is replaced by GTP.

A transmembrane receptor made of one polypeptide crossing the membrane seven times, which transmits a signal into the cell by activating a G protein when its ligand binds.

The first growth phase of interphase, in which the cell grows, synthesises proteins and increases its number of organelles before DNA replication.

The second growth phase of interphase, after DNA replication, in which the cell grows further and prepares for mitosis.

A haploid sex cell (a sperm or an egg) that fuses with another gamete at fertilisation to form a zygote.

The uptake of one gas and release of another across a surface, by diffusion, such as the uptake of oxygen and loss of carbon dioxide in respiration.

A surface across which gases diffuse between an organism and its surroundings; it is permeable, thin, moist and has a large surface area.

A technique that separates DNA fragments (or proteins) by size by moving them through a gel in an electric field; smaller DNA fragments move further towards the positive electrode.

A section of DNA with a specific base sequence that codes for a polypeptide or a functional RNA molecule and influences a characteristic.

The deliberate alteration of the base sequence of DNA at a specific, chosen position in the genome, for example with CRISPR–Cas9.

The process by which the information in a gene is used to make a functional product, such as a polypeptide or RNA; it begins with transcription.

The transfer of alleles from one population to another when individuals or their gametes move between the populations and interbreed.

A technique for investigating the function of a gene by changing it so that it is inoperative, then observing the effect on the organism.

The tendency of alleles of genes on the same chromosome to be inherited together, so that they do not assort independently.

A change to the base sequence of a gene, by substitution, insertion or deletion of one or more nucleotides.

All the alleles of all the genes in an interbreeding population.

The set of rules by which the sequence of base triplets (codons) in mRNA specifies the sequence of amino acids in a polypeptide; it is almost universal.

A mating between two chosen parents, used to study the inheritance of characteristics or to produce offspring with a desired combination of alleles.

The variety of alleles and genes within a species, both within a population and between populations of that species.

Random change in allele frequencies in a population from one generation to the next, caused by chance differences in which individuals survive and reproduce; its effects are greatest in small populations.

Differences in the alleles and base sequences carried by individuals of a population or species; mutation is its original source.

The whole of the genetic information of an organism, consisting of the entire base sequence of its DNA (or RNA in some viruses).

The expression of only one allele of a gene, depending on whether it was inherited from the mother or the father, because epigenetic tags from one parent are not removed in the gametes.

The combination of alleles of a gene (or genes) that an organism has inherited.

A group of closely related species that share many features and a recent common ancestor; the first part of a binomial name.

Reproductive isolation of populations caused by a physical barrier, such as a mountain range, river or sea, that prevents individuals from meeting and interbreeding.

A cell that gives rise to gametes, or a gamete itself; mutations in germ cells can be inherited by offspring.

The theory that many diseases are caused by microorganisms (germs) that invade the body and can be passed from one person to another.

The development of a seed into a seedling, starting with the uptake of water and ending when the young plant can photosynthesise.

A phytohormone, made mainly in young leaves and seeds, that promotes stem elongation, seed germination and, in some species, flowering.

The gas exchange organ of a fish, made of filaments with thin lamellae and many capillaries, ventilated by a one-way flow of water.

A protein folded into a compact, rounded shape, usually soluble in water, such as an enzyme, insulin or haemoglobin.

A knot of capillaries inside Bowman's capsule, supplied by a wide afferent arteriole and drained by a narrower efferent arteriole, where blood plasma is filtered under high pressure.

A hormone secreted by the alpha cells of the islets of Langerhans when blood glucose concentration falls; it stimulates liver cells to break down glycogen to glucose and release it into the blood.

A glucose transporter protein of muscle and fat cells, stored in vesicles and moved to the plasma membrane in response to insulin, so that glucose enters by facilitated diffusion.

GP, the three-carbon compound formed when RuBP combines with CO₂; it is reduced to triose phosphate using ATP and reduced NADP.

A small three-carbon molecule with a hydroxyl group on each carbon, which forms ester bonds with fatty acids to make triglycerides and phospholipids.

The energy storage polysaccharide of animals and fungi, made of α-glucose with 1,4 and 1,6 glycosidic bonds; more highly branched than amylopectin, stored in liver and muscle.

The synthesis of glycogen from glucose, carried out in liver and muscle cells and stimulated by insulin.

The hydrolysis of glycogen to glucose, carried out in liver cells when stimulated by glucagon (and in muscle cells for their own use).

A lipid with a carbohydrate chain attached, found in the outer layer of the plasma membrane with the carbohydrate facing outside the cell; involved in cell recognition and adhesion.

The first stage of cell respiration, in the cytoplasm: glucose is converted by phosphorylation, lysis, oxidation and ATP formation into two pyruvate, with a net yield of 2 ATP and 2 reduced NAD.

A protein with one or more carbohydrate chains covalently attached; in the plasma membrane the carbohydrate faces outwards and functions in cell recognition and adhesion.

The covalent bond formed by a condensation reaction between two monosaccharides, linking them through an oxygen atom.

The addition of carbohydrate chains to a protein (or lipid), which takes place in the rough ER and Golgi apparatus and produces glycoproteins.

The range of distances from a star within which a planet's surface temperature allows water to exist as a liquid; also called the habitable zone.

A stack of flattened, curved membrane sacs (cisternae) that receives proteins from the rough ER in vesicles, modifies and sorts them, and sends them out in new vesicles.

A peptide hormone (GnRH) secreted by cells of the hypothalamus that stimulates the anterior pituitary gland to secrete FSH and LH.

An instrument with two hinged arms and a protractor scale, used to measure the angle and range of motion of a joint.

A bacterium with a thick cell wall of peptidoglycan and no outer membrane, which stains purple with the Gram stain; Bacillus and Staphylococcus are examples.

A stack of thylakoids in a chloroplast; grana are joined to each other by unstacked thylakoid membranes called intergranal lamellae.

A biome with low to moderate rainfall and a dry season, dominated by grasses, with few trees because of drought, fire and grazing; includes temperate grassland and savanna.

A directional growth response to gravity; roots grow downwards (positive gravitropism) and shoots grow upwards (negative gravitropism).

The rise of free oxygen in Earth's atmosphere about 2.4 billion years ago, caused by oxygen released by photolysis in cyanobacteria.

The warming of the Earth's surface and lower atmosphere because greenhouse gases absorb long-wave infrared radiation emitted by the surface and re-emit some of it back towards the surface.

A gas in the atmosphere, such as water vapour, carbon dioxide, methane or nitrous oxide, that absorbs and re-emits long-wave infrared radiation, contributing to the greenhouse effect.

Nervous tissue made mainly of the cell bodies, dendrites and synapses of neurons, where information is integrated; it forms the cerebral cortex and the central part of the spinal cord.

A purine base (G) found in DNA and RNA, which pairs with cytosine by three hydrogen bonds.

One of a pair of cells surrounding a stoma that change shape to open or close it, controlling gas exchange and water loss.

An RNA molecule with about 20 bases complementary to a target DNA sequence, which binds to Cas9 and directs it to cut at that sequence.

The central region of the A band that contains myosin filaments but no actin; it shortens and may disappear during contraction.

The place in which a community, species, population or organism lives, described by its geographical location, physical location and type of ecosystem.

The clearing, draining, burning or fragmentation of a habitat so that it can no longer support the species that lived there.

A ring-shaped prosthetic group with an iron ion (Fe²⁺) at its centre; each haem group in haemoglobin binds one oxygen molecule.

A conjugated globular protein in red blood cells made of four polypeptides, each with a haem group containing iron, which binds oxygen for transport.

A sex-linked genetic disease in which blood clots slowly or not at all, caused by a recessive allele of an X-linked gene for a clotting factor (factor VIII or IX).

A plant adapted to grow in soil or water with a high salt concentration, for example by excluding or excreting salt.

Describes a nucleus or cell that has one set of chromosomes (n), with one chromosome of each type.

The equations $p + q = 1$ and $p^2 + 2pq + q^2 = 1$, where $p$ and $q$ are the frequencies of two alleles, used to calculate allele and genotype frequencies in a population in genetic equilibrium.

The state of a population in which allele and genotype frequencies stay constant between generations, as the Hardy–Weinberg equation predicts, because there is no selection, mutation, migration or non-random mating and the population is very large.

The number of times the heart beats per minute (bpm), equal to the pulse rate.

An enzyme that unwinds the DNA double helix and separates the two strands by breaking the hydrogen bonds between complementary bases.

A lymphocyte that is activated by antigen presented by a macrophage and then activates B-lymphocytes that recognise the same antigen; it is the cell infected by HIV.

An animal that feeds on plants or parts of plants; herbivorous mammals typically have large, flat molars for grinding.

The eating of plants or parts of plants by animals.

The protection of a whole population, including non-immune individuals, from an infectious disease when a large enough proportion of the population is immune that transmission is greatly reduced.

Describes a characteristic that is passed from parents to offspring through the genes in their gametes.

An organism, or a flower, that has both male and female reproductive organs and so produces both male and female gametes.

The 1952 experiment that used T2 bacteriophages labelled with ³⁵S (protein) or ³²P (DNA) to show that DNA, not protein, enters bacteria during infection and is the genetic material.

An organism that obtains carbon compounds from other organisms and uses them to synthesise the carbon compounds it needs.

Having two different alleles of a gene, for example Rr.

A monosaccharide with six carbon atoms, formula C₆H₁₂O₆, such as glucose, fructose or galactose; usually found as a ring of five carbons and one oxygen.

A base or amino acid sequence that has remained identical or very similar over long periods of evolution, in distantly related species.

A synovial joint that allows movement in one plane only (flexion and extension), as at the knee and elbow.

A basic protein around which eukaryote DNA is wound to package it into chromosomes; prokaryote DNA is not associated with histones.

The addition or removal of chemical groups, such as methyl or acetyl groups, on amino acids in histone proteins, which changes how tightly DNA is packed and so affects transcription.

Human immunodeficiency virus: a retrovirus, transmitted in body fluids, that infects and destroys helper T-lymphocytes and leads to AIDS.

A mode of heterotrophic nutrition in which food is ingested, digested internally, absorbed and assimilated, and undigested material is egested; used by animals.

The maintenance of the internal environment of an organism within preset limits, despite fluctuations in the external environment.

A member of the family Hominidae, the great apes: humans, chimpanzees, gorillas, orangutans and their extinct relatives such as Paranthropus.

A pair of chromosomes, one from each parent, with the same length, centromere position and sequence of genes, though not necessarily the same alleles.

A structure that is similar in basic arrangement in different species because it was inherited from a common ancestor, even if it now has a different function.

Having two identical alleles of a gene, for example RR or rr.

The transfer of genetic material between organisms other than from parent to offspring, for example between different bacterial species by conjugation, transformation or transduction.

A chemical signal secreted by an endocrine gland directly into the blood, which carries it to target cells that have receptors for it.

Treatment with oestrogen, often combined with a progestogen, to replace the hormones that the ovaries stop secreting at the menopause.

A biome with very low, unpredictable rainfall (under about 250 mm a year), very high daytime temperatures and sparse, drought-adapted vegetation.

A glycoprotein hormone secreted by the embryo (trophoblast) and later the placenta, which maintains the corpus luteum in early pregnancy; its presence in urine is the basis of pregnancy tests.

The offspring of a cross between two different species (or two genetically different varieties); hybrids between species are often sterile.

The inability of a hybrid between two species to produce viable gametes, usually because its chromosomes from the two parent species cannot pair in meiosis.

The attraction between a partially positive hydrogen atom in one polar molecule and a partially negative atom, such as oxygen or nitrogen, in another polar molecule or another part of the same large molecule.

A reaction in which a covalent bond is broken by the addition of a water molecule, whose –H and –OH become part of the two products; used to digest polymers into monomers.

Attracted to water and able to dissolve in it; ions and polar molecules are hydrophilic because their charges attract water molecules.

Not attracted to water and insoluble or poorly soluble in it; non-polar substances such as fats and oils are hydrophobic.

The clustering of non-polar R-groups away from water in a protein, caused by water molecules hydrogen bonding with each other and excluding the non-polar groups.

An opening in the deep sea floor that releases hot, mineral-rich water; vent communities are based on chemoautotrophic bacteria rather than sunlight.

A change in membrane potential that makes the inside more negative than the resting potential, as after an action potential or during an inhibitory postsynaptic potential.

Describes a solution with a higher concentration of solutes than another solution, such as the cytoplasm of a cell; water moves by osmosis into it from the less concentrated solution.

One of the thin, branching threads that make up the body of most fungi; aseptate hyphae have no cross walls, so many nuclei share one continuous cytoplasm.

A region at the base of the brain that monitors the blood and receives nervous input, and controls the pituitary gland, linking the nervous system with the endocrine system.

A tentative, testable explanation of an observation, from which predictions can be made and checked by experiment or further observation.

Describes a solution with a lower concentration of solutes than another solution, such as the cytoplasm of a cell; water moves by osmosis out of it into the more concentrated solution.

The light band of a sarcomere, containing thin actin filaments only and divided by a Z-line; it shortens during contraction.

A cylinder of ice drilled from an ice sheet or glacier, containing trapped bubbles of ancient air and other evidence from which past atmospheric gas concentrations and temperatures are measured.

The ability to eliminate an infectious disease from the body, due to memory cells that can rapidly produce the specific antibody against its pathogen.

A light microscopy technique in which antibodies carrying a fluorescent marker bind to one specific protein, showing where that protein is located in a cell or tissue.

The embedding of the blastocyst in the endometrium of the uterus, about 6–7 days after fertilisation.

The conservation of species in their natural habitat, for example by protecting and managing ecosystems in nature reserves and national parks.

Fertilisation of eggs by sperm outside the body, in a laboratory dish, after which one or more embryos are transferred to the uterus.

Reproduction between closely related individuals, or self-fertilisation, which increases homozygosity and decreases genetic diversity and vigour.

The point at which the plasma membrane of a plant cell is just beginning to pull away from the cell wall as the cell loses water; turgor pressure has fallen to zero.

A pattern of inheritance in which a heterozygote has a phenotype intermediate between those of the two homozygotes, for example pink flowers from red and white parents.

The random and independent distribution of the alleles of different genes into gametes, caused by the random orientation of pairs of homologous chromosomes in meiosis I.

The variable that is deliberately changed by the experimenter in an investigation, such as temperature in an enzyme experiment.

Movement of a substance against its concentration gradient using energy from another substance moving down a concentration gradient that a pump has set up using ATP.

A model of enzyme action in which binding of the substrate changes the shape of both the active site and the substrate, so they fit closely and bonds in the substrate are stressed.

Reasoning from many specific observations to a general statement or theory, such as concluding that all organisms are made of cells after observing cells in many different organisms.

A disease caused by a pathogen that can be passed from one host to another, such as measles, tuberculosis or malaria.

The taking of food into the body, for example through the mouth into the gut, or into a food vacuole by phagocytosis.

A hyperpolarisation of the postsynaptic membrane, caused by an inhibitory neurotransmitter such as GABA opening Cl⁻ or K⁺ channels, which makes an action potential less likely.

The part of the immune system that responds in the same, non-specific way to broad categories of pathogen and does not change during an organism's life.

A gene mutation in which one or more nucleotides are added to the base sequence of DNA.

Breathing in: contraction of the diaphragm and external intercostal muscles increases the volume of the thorax, lowering pressure so that air flows into the lungs.

The extra volume of air that can be breathed in with maximum effort after a normal inspiration.

A globular protein hormone secreted by β cells of the pancreas that lowers blood glucose; it has two polypeptides (21 and 30 amino acids) linked by disulfide bonds.

Reduced sensitivity of target cells, such as liver, muscle and adipose cells, to insulin, so they take up less glucose; the physiological change underlying type 2 diabetes.

A membrane protein embedded in one or both layers of the phospholipid bilayer, held there by a hydrophobic region in the core; many span the whole membrane.

The coordination of the component parts of a system so that, together, they carry out the overall function of the system.

A specialised junction joining cardiac muscle cells end to end, containing gap junctions that allow electrical impulses to pass between cells and strong junctions that hold them together.

Muscles between the ribs: the external intercostals raise the ribs in inspiration and the internal intercostals lower them in forced expiration; they form an antagonistic pair.

The dependence of the parts of a biological system on one another, so that a change in one part, such as a molecule, cell, organ or species, affects the others.

The narrow space between the outer and inner membranes of a mitochondrion (or chloroplast) into which protons are pumped, building up a proton gradient.

An attraction between separate molecules, such as a hydrogen bond, which is much weaker than the covalent bonds holding atoms together within a molecule.

A neuron within the central nervous system that passes impulses from one neuron to another, for example from a sensory neuron to a motor neuron in a reflex arc.

The part of the cell cycle between divisions, in which the cell grows, synthesises proteins, replicates its DNA and carries out its normal functions.

The difference between the upper quartile and the lower quartile, which measures the spread of the middle 50% of the data.

Competition between individuals of different species for the same limited resource, such as food, light or space.

An enzyme that catalyses reactions inside the cell that produced it, in the cytoplasm or within an organelle, such as the enzymes of glycolysis.

A receptor protein in the cytoplasm or nucleus that binds a hydrophobic ligand, such as a steroid hormone, that has crossed the plasma membrane; the complex usually regulates transcription.

Competition between individuals of the same species for resources that are in limited supply, such as food, water, territory, light or mates.

A species moved by humans, deliberately or accidentally, to an area outside its natural range.

A non-coding sequence within a eukaryotic gene that is transcribed but removed from the pre-mRNA by splicing before translation.

An inward folding of a membrane or layer of cells, which increases surface area; in some cells the plasma membrane folds deeply into the cytoplasm.

An introduced species that spreads rapidly in its new range and harms endemic species or ecosystems, often by outcompeting them for resources.

An atom or group of atoms with an overall electrical charge because it has gained or lost electrons, such as Na⁺ (a cation) or Cl⁻ (an anion).

An attraction between oppositely charged ions or groups; in proteins, between positively charged (–NH₃⁺) and negatively charged (–COO⁻) R-groups.

An inadequate blood supply to a tissue, which deprives its cells of oxygen and nutrients.

Groups of endocrine cells in the pancreas; their beta cells secrete insulin and their alpha cells secrete glucagon in response to changes in blood glucose concentration.

Describes a solution with the same concentration of solutes as another solution, such as the cytoplasm of a cell, so that there is no net movement of water between them by osmosis.

The International Union for Conservation of Nature's assessment of species' risk of extinction, placing each in a category from Least Concern to Extinct using standard criteria.

An image of the chromosomes of a cell, arranged in homologous pairs in order of decreasing length, used to show the karyotype.

The number and type of chromosomes present in the nucleus of a cell of an organism.

The graph of atmospheric carbon dioxide concentration measured at Mauna Loa, Hawaii, since 1958, showing an annual fluctuation and a long-term rise.

A species that has a disproportionately large effect on the structure of its ecological community relative to its abundance; its removal causes major changes and risks ecosystem collapse.

One of a pair of organs that excrete metabolic waste such as urea and regulate the osmotic concentration of the blood by forming urine in about a million nephrons each.

A protein structure at the centromere of each chromatid to which spindle microtubules attach during nuclear division.

A cycle of reactions in the mitochondrial matrix in which acetyl groups are oxidised: each turn gives 2 CO₂, 3 reduced NAD, 1 reduced FAD and 1 ATP and regenerates oxaloacetate.

The three-carbon compound produced by anaerobic respiration of glucose in human cells such as muscle fibres; its formation releases no carbon dioxide, and at HL it is the step that regenerates NAD.

The new DNA strand built discontinuously, away from the replication fork, as a series of Okazaki fragments that are later joined.

The hypothesis, proposed by Lamarck, that characteristics acquired by an organism during its lifetime through use or disuse are inherited by its offspring.

Sea ice that is attached to the coast, the sea floor in shallow water or an ice shelf, so that it does not drift with winds and currents.

The most recent population of organisms from which all organisms living today are descended; it was already a cell with DNA, ribosomes and the genetic code.

The loss of soluble mineral ions, such as nitrate, from soil as water drains through it, carrying them below the reach of plant roots and into groundwater and rivers.

The new DNA strand built continuously in the same direction as the replication fork moves, from a single RNA primer.

Carbon that has accumulated in the soil and biomass of an ecosystem over many years, decades or centuries, which can be released rapidly by fire or decomposition.

One of the four scales at which biology is studied: molecules, cells, organisms and ecosystems; each level is built from the one before it.

A rigid bar that turns about a fixed point (the fulcrum) so that an effort applied at one point moves a load at another; in animals, bones act as levers at joints.

A woody climbing plant rooted in the soil that uses trees for support to reach light in the canopy.

A tough band of fibrous connective tissue that joins bone to bone, holding a joint together and limiting its movement.

A signalling chemical that binds specifically to a receptor protein, causing a conformational change in the receptor that passes on the signal.

An ion channel that opens when a specific signalling molecule (ligand), such as a neurotransmitter, binds to it.

A microscope that uses visible light focused by glass lenses to form a magnified image; its resolution is limited to about 0.2 µm, but it can be used to view living cells in colour.

The reactions of photosynthesis on the thylakoid membranes that use light energy to split water, release oxygen and produce ATP and reduced NADP.

The reactions of photosynthesis in the stroma, also called the Calvin cycle, in which CO₂ is fixed by Rubisco, GP is reduced to triose phosphate using ATP and reduced NADP, and RuBP is regenerated.

A hard, waterproof polymer deposited in the cell walls of xylem vessels and other plant tissues, which strengthens them and stops them collapsing under tension.

The environmental factor that is furthest from its optimum value and so restricts the growth, survival, distribution or abundance of a species, or the rate of a process.

An estimate of population size from capture–mark–release–recapture data: $N = M \times n / m$, where M is the number marked in the first sample, n the total in the second sample and m the number recaptured.

The oxidation and decarboxylation of pyruvate in the mitochondrial matrix, producing CO₂, reduced NAD and an acetyl group carried by coenzyme A as acetyl-CoA.

The short stretch of DNA that connects one nucleosome to the next.

A member of a diverse group of carbon compounds that dissolve in non-polar solvents but are only sparingly soluble in aqueous solvents; includes fats, oils, waxes and steroids.

The movement of ions, mainly Na⁺ diffusing along the inside of an axon, from a depolarised region to the adjacent resting region, which depolarises that region to the threshold potential.

An early model of enzyme action in which the active site is a rigid shape that exactly fits the substrate, as a key fits a lock; replaced by the induced-fit model.

The movement of a whole organism from one place to another, for example by walking, swimming, flying or crawling.

The particular position of a gene on a chromosome.

A scale on which each equal interval represents multiplication by the same factor (for example 1, 10, 100, 1000), so that exponential change appears as a straight line.

The hairpin part of the nephron in the medulla; its water-impermeable ascending limb actively transports sodium ions out, making the medulla concentrated so water can be reabsorbed from collecting ducts.

The space inside a tubular structure such as a blood vessel, xylem vessel or gut, through which material flows.

A hormone (LH) secreted by the anterior pituitary; a surge of LH triggers ovulation and the formation of the corpus luteum.

Excess tissue fluid that has entered the lymphatic vessels, which carry it through lymph nodes and return it to the blood in the subclavian veins.

A small organ along the lymphatic vessels that filters lymph and contains large numbers of lymphocytes, which multiply there during an infection.

A thin-walled vessel of the lymphatic system with gaps that let tissue fluid in and valves that keep lymph flowing towards the subclavian veins.

A type of white blood cell with a large nucleus that is part of the adaptive immune system; B-lymphocytes produce antibodies and T-lymphocytes help control the response.

The bursting of a cell when its plasma membrane breaks, for example when an animal cell takes in water by osmosis in a hypotonic solution; in red blood cells it is called haemolysis.

The cycle in which phage DNA is integrated into the host chromosome as a prophage and replicated with it at every cell division, without making new virions, until induction triggers the lytic cycle.

A small membrane-bound vesicle in animal cells containing hydrolytic (digestive) enzymes, which break down food, worn-out organelles and engulfed bacteria.

The cycle in which a virus enters a host cell, uses the cell's machinery to replicate its genome and make viral proteins, assembles new virions and releases them by lysis, killing the cell.

A very large molecule made of thousands or more atoms, such as a polysaccharide, protein or nucleic acid; most macromolecules are polymers.

A large phagocyte found in tissues that engulfs pathogens and presents their antigens on its surface to helper T-lymphocytes.

A large piece of plastic debris, larger than 5 mm, such as a bag, bottle, packaging or fishing net, which can entangle or be swallowed by animals.

The number of times larger an image is than the real object; magnification = image size ÷ actual size, with both sizes in the same units.

A cancerous tumour whose cells divide rapidly, invade neighbouring tissue and can spread to other parts of the body by metastasis.

An event in which a large proportion of the Earth's species, usually 75% or more, become extinct in a geologically short time.

The largest harvest that can be taken from a population year after year without reducing its size, obtained by keeping the population at about half its carrying capacity, where growth rate is greatest.

The average of a set of values, calculated by adding the values and dividing by the number of values.

The range within which the true value of a measurement is expected to lie, set by the precision of the instrument and how it is read; written as ± a value with the same unit.

Irreversible inhibition in which an inhibitor in the active site is chemically changed by the enzyme and forms a covalent bond that permanently inactivates it, as penicillin does.

The middle value of a set of data arranged in order; for an even number of values, the mean of the two middle values.

The lowest part of the brainstem, joining the brain to the spinal cord, containing the cardiovascular and respiratory centres that control heart rate and ventilation.

Division of a diploid nucleus into four haploid nuclei by two successive divisions, producing nuclei that are genetically different from one another.

A hormone secreted by the pineal gland, mainly during darkness, that helps to set the timing of the sleep–wake cycle and other circadian rhythms.

The ease with which phospholipids and proteins move within a membrane; it depends on temperature, the proportion of unsaturated fatty acids and, in animal cells, cholesterol.

The difference in electrical charge (potential difference) across a plasma membrane, measured in millivolts, with the inside usually negative compared with the outside.

A long-lived lymphocyte produced during an immune response that remains in the body and responds rapidly if the same antigen is met again.

The monthly cycle of changes in the ovaries (the ovarian cycle) and the endometrium (the uterine cycle), controlled by FSH, LH, oestradiol and progesterone.

A region of undifferentiated plant cells that divide repeatedly by mitosis, producing new cells for growth.

A 1958 experiment that grew E. coli on heavy nitrogen (¹⁵N) and then light nitrogen (¹⁴N) and separated their DNA by density, showing that DNA replication is semi-conservative.

A small experimental ecosystem, enclosed in a container such as a sealed glass vessel, set up to investigate the effects of variables on a community under controlled conditions.

A sequence of enzyme-catalysed reactions in which the product of each reaction is the substrate of the next.

Water produced inside an organism by aerobic cell respiration of food, an important water source for desert animals such as kangaroo rats.

The complex network of interdependent and interacting chemical reactions that take place in a living organism.

The phase of mitosis in which chromosomes, attached to spindle microtubules from both poles, line up on the equator of the cell.

The spread of cancer cells from a primary tumour to other parts of the body, through blood or lymph, where they form secondary tumours.

A greenhouse gas (CH₄) produced by anaerobic decomposition by methanogens, for example in ruminants, rice paddies, wetlands and landfill; per molecule it absorbs infrared radiation far more strongly than CO₂.

A bundle of parallel cellulose molecules held together by hydrogen bonds; microfibrils give plant cell walls their high tensile strength.

A thin solid filament made of the protein actin, part of the cytoskeleton; microfilaments change cell shape and are involved in cell movement and division of the cytoplasm.

A photograph or digital image taken through a microscope; an electron micrograph is taken with an electron microscope and a light micrograph with a light microscope.

A piece of plastic smaller than 5 mm, either manufactured at that size or formed by the breakdown of larger plastic items.

A hollow cylinder made of the protein tubulin, part of the cytoskeleton; microtubules form the spindle in cell division and the core of cilia and flagella.

A tiny finger-like fold of the plasma membrane on the surface of some animal cells, such as those lining the small intestine, that increases the surface area for absorption.

The regular, usually seasonal, movement of animals from one region to another and back, for example between breeding and feeding areas.

The 1953 experiment in which methane, ammonia, hydrogen and water vapour were exposed to electrical sparks in a closed apparatus, producing amino acids and showing that carbon compounds can form from inorganic molecules.

A base substitution that changes a codon into one for a different amino acid, so one amino acid in the polypeptide is replaced.

The fluid inside the inner membrane of a mitochondrion, containing enzymes of the Krebs cycle and link reaction, 70S ribosomes and naked circular DNA.

A double-membrane organelle in which aerobic cell respiration produces ATP; the inner membrane is folded into cristae around a fluid matrix, which holds 70S ribosomes and circular DNA.

Division of the nucleus into two genetically identical daughter nuclei, each with the same number of chromosomes as the parent nucleus.

The ratio of the number of cells in mitosis to the total number of cells in a sample of tissue, used as a measure of how fast cells are dividing.

An organism that combines autotrophic nutrition (photosynthesis) with heterotrophic nutrition, such as Euglena; mixotrophs may be obligate or facultative.

The most frequent value, or class, in a set of data.

The way in which an organism obtains its energy and carbon compounds, such as photosynthesis, holozoic, mixotrophic or saprotrophic nutrition.

A species studied in depth because it is easy to keep, breed and manipulate, so that discoveries about it can be applied to other organisms.

Using the number of differences between base or amino acid sequences of two species to estimate the time since they diverged, assuming differences accumulate at a roughly constant rate.

An antibody produced by a clone of identical cells derived from a single B-lymphocyte, so all its molecules bind to the same site on one specific antigen.

A genetic cross that follows the inheritance of a single gene.

A small molecule that can be joined to other similar molecules by condensation reactions to form a polymer, for example glucose, an amino acid or a nucleotide.

A single sugar unit, such as glucose, fructose or ribose; the monomer from which disaccharides and polysaccharides are built by condensation.

A fatty acid with one carbon–carbon double bond in its hydrocarbon chain, which puts a kink in the chain and lowers its melting point.

Twins that develop from a single fertilised egg that splits into two embryos, so they have identical genomes.

A signalling chemical that forms a concentration gradient across an embryo and causes cells to develop differently according to the concentration they receive.

The definition of a species as a group of organisms that share the same set of observable structural features (morphology).

Describes an organism that can move from place to place by itself, such as most animals; motile animals are sampled by capture–mark–release–recapture.

A neuron that carries impulses from the central nervous system to an effector, such as a muscle, stimulating it to contract.

A single motor neuron together with all the skeletal muscle fibres it stimulates through neuromuscular junctions; all the fibres of the unit contract together.

Messenger RNA: RNA transcribed from a gene that carries its base sequence, as codons, to the ribosomes for translation.

A thin layer of tissue lining the openings and internal passages of the body that secretes mucus, which traps pathogens and particles.

The condition of an organism made of many cells that stay together, communicate and are specialised for different functions; it has evolved independently many times.

The existence of more than two alleles of a gene in the gene pool of a population; each individual still has only two of them.

Describes a stem cell that can differentiate into a limited range of cell types, usually within one tissue, such as blood stem cells in bone marrow.

An agent that increases the rate of mutation, such as benzo[a]pyrene, nitrosamines, mustard gas, UV light, X-rays and gamma rays.

A close interspecific relationship between two species in which both species benefit.

A mutualistic association between a fungus and the roots of a plant, in which fungal hyphae supply water and mineral ions to the plant and receive carbon compounds in return.

An insulating layer around an axon, made of many layers of plasma membrane wrapped round by Schwann cells, with small gaps at intervals, that increases the speed of nerve impulses.

The death of part of the heart muscle because its blood supply has been blocked, usually by a thrombus in a coronary artery; commonly called a heart attack.

A long cylindrical bundle of actin and myosin filaments inside a muscle fibre, divided along its length into repeating sarcomeres.

Describes contraction that starts in the muscle tissue itself, without a nervous stimulus; the heartbeat is myogenic because each beat starts in the sinoatrial node.

A motor protein forming the thick filaments of a sarcomere; its heads bind to actin, hydrolyse ATP and perform the power stroke that slides the filaments.

Nicotinamide adenine dinucleotide: a coenzyme that carries hydrogen in respiration; it is reduced when it accepts two electrons and a proton, and oxidised when it passes them to the electron transport chain.

A hydrogen carrier in photosynthesis; it is reduced by two electrons from photosystem I and a proton to form reduced NADP, which supplies hydrogen to the Calvin cycle.

The process by which individuals with heritable traits that make them better adapted to their environment survive and reproduce more than others, so the alleles for those traits become more common in the population over generations.

An area of land or water that is protected and managed for the conservation of its wildlife, habitats and natural features.

A gland at the base of a flower that secretes nectar, a sugary liquid that attracts and rewards pollinating animals.

A control mechanism in which a change in a variable triggers a response that reverses the change, returning the variable towards its set value.

The theory that combines Darwin's natural selection with Mendelian and population genetics, so that evolution is described as a change in allele frequencies in a population's gene pool over generations.

An insecticide that binds to nicotinic acetylcholine receptors in insect synapses and is not broken down by acetylcholinesterase, so it blocks synaptic transmission.

The functional unit of the kidney: a tubule consisting of Bowman's capsule, proximal convoluted tubule, loop of Henle and distal convoluted tubule, draining into a collecting duct.

A bundle of nerve fibres from many neurons, surrounded by a protective sheath of connective tissue; a nerve may contain sensory fibres, motor fibres or both.

An elongated extension of a neuron, either an axon or a dendrite, along which electrical impulses are conducted.

An action potential propagated along a nerve fibre, carrying a signal from one end of a neuron to the other.

A synapse between a motor neuron and a skeletal muscle fibre, at which acetylcholine is released to stimulate contraction.

A cell of the nervous system, made of a cell body with a nucleus and elongated nerve fibres, that carries electrical impulses.

A neuron that secretes a hormone into the blood instead of releasing a neurotransmitter at a synapse, as in the hypothalamus.

A signalling chemical released by a presynaptic neuron that diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.

The division of resources between species with overlapping niches, so that each uses a different part of the resource and both can coexist.

A ligand-gated ion channel in postsynaptic membranes that opens when acetylcholine binds to it, allowing positive ions, mainly Na⁺, to diffuse into the cell.

A nitrogen-containing ring molecule attached to carbon 1′ of the pentose in a nucleotide; the five bases are adenine, cytosine, guanine, thymine and uracil.

A branch point on a cladogram, representing the last common ancestor of the groups that branch from it and the point at which its lineage split.

A short gap in the myelin sheath of an axon where the membrane is exposed and voltage-gated channels are concentrated, so action potentials can occur.

Describes a material that cannot be broken down into simpler substances by decomposers such as bacteria and fungi, so it persists in the environment.

A DNA sequence that is not translated into a polypeptide, such as promoters, enhancers, introns, telomeres and the genes for tRNA and rRNA.

Inhibition in which the inhibitor binds reversibly to an allosteric site, changing the active site so the reaction cannot be catalysed; increasing substrate concentration does not overcome it.

ATP production in which electrons flow from water through photosystem II, the electron transport chain and photosystem I to NADP, producing ATP, reduced NADP and oxygen.

The failure of homologous chromosomes (in anaphase I) or sister chromatids (in anaphase II) to separate during meiosis, producing gametes with an extra or a missing chromosome.

An amino acid that an organism can synthesise from other amino acids or compounds, so it need not be obtained from the diet.

A base substitution that changes a codon for an amino acid into a stop codon, so translation ends early and the polypeptide is shortened.

A sterile 0.9% w/v solution of sodium chloride in water, approximately isotonic with human blood plasma, used for intravenous fluids, washing wounds and storing tissues.

The double membrane that surrounds the nucleus of a eukaryote cell, perforated by nuclear pores.

A protein-lined opening through both membranes of the nuclear envelope that controls movement of molecules such as mRNA and proteins between the nucleus and cytoplasm.

An enzyme that hydrolyses nucleic acids (DNA or RNA) by breaking the bonds between their nucleotides.

A polymer of nucleotides that stores or transmits genetic information; the two nucleic acids are DNA and RNA.

The region of a prokaryote cell that contains its main DNA molecule, a naked, circular loop not enclosed by a membrane.

A dense region inside the nucleus of a eukaryote cell where ribosomal RNA is made and ribosome subunits are assembled.

The unit of DNA packing in eukaryotes: about 147 base pairs of DNA wrapped about 1.65 turns around an octamer of eight histones, held in place by histone H1.

The monomer of nucleic acids, consisting of a pentose sugar, a phosphate group and a nitrogenous base.

The organelle of eukaryote cells that contains the chromosomes, enclosed by a double membrane with pores; it controls the cell through the genes that are transcribed.

A statement that there is no difference, effect or association between the variables being studied, so that any observed difference is due to chance.

The continuous movement of a chemical element, such as carbon, nitrogen or phosphorus, between organisms and the abiotic environment, in which decomposers release it from dead matter in inorganic forms that producers reuse.

An organism that needs oxygen gas to survive because it can obtain enough energy only by aerobic respiration, for example Mycobacterium tuberculosis.

An organism that cannot survive in the presence of oxygen gas and respires only anaerobically, for example Clostridium botulinum.

An organism or agent that can reproduce only inside the living cells of a host, using the host cell's metabolism; all viruses are obligate intracellular parasites.

The decrease in the pH of seawater caused by absorption of atmospheric carbon dioxide, which forms carbonic acid and reduces the concentration of carbonate ions available for calcification.

The main female sex hormone, a steroid made mainly in the ovaries; being non-polar, it diffuses through plasma membranes to receptors inside target cells.

A short section of DNA built discontinuously on the lagging strand during replication, each starting from its own RNA primer.

An animal that feeds on both plants and animals, such as Homo sapiens.

A mutated form of a proto-oncogene whose product is permanently active or overproduced, stimulating uncontrolled cell division.

The production of egg cells in the ovary; its divisions are unequal, so one cell keeps most of the cytoplasm and the others form small polar bodies.

A system that exchanges both energy and matter with its surroundings; natural ecosystems are open systems.

A group of genes in a prokaryote that are transcribed together as one mRNA from a single promoter, under the control of one operator.

An infection by a pathogen that rarely causes disease in a healthy person but does so when the immune system is weakened, as in AIDS.

The value of a factor, such as temperature or pH, at which the rate of an enzyme-catalysed reaction is highest.

A structure made of two or more different tissues that work together to perform one or more functions, such as the heart or the brain.

A group of organs that work together to perform a major function in the body, such as the circulatory system or the nervous system.

A discrete subunit of a cell that has a distinct structure and is adapted to perform one or more specific functions, for example the nucleus, ribosomes, vesicles and the plasma membrane.

A compound containing carbon atoms covalently bonded to other atoms, usually including hydrogen, of the kind made by living organisms; carbon dioxide and carbonates are not usually counted as organic.

A specific base sequence at which DNA replication begins; prokaryotes usually have one per chromosome, eukaryotes many.

An instrument that displays changes in voltage against time; used to record resting and action potentials from electrodes inside and outside a neuron.

The total concentration of all solute particles (molecules and ions) in a solution, measured in osmoles or milliosmoles per litre (mOsm/L); osmolality is measured per kilogram of water.

A receptor cell in the hypothalamus that detects the osmotic concentration of the blood and causes ADH secretion to change.

The control of the osmotic concentration (water and solute content) of the blood and other body fluids; in mammals, mainly by the kidneys.

The net movement of water across a partially permeable membrane from a region of lower solute concentration to a region of higher solute concentration.

A value that lies unusually far from the rest of the data; in a box-and-whisker plot, more than 1.5 × IQR above the upper quartile or below the lower quartile.

The part of the menstrual cycle that takes place in the ovary: development of a follicle, ovulation, and formation and breakdown of the corpus luteum.

One of the two female gonads, containing follicles in which eggs develop; it releases eggs at ovulation and secretes oestradiol and progesterone.

Harvesting a species by hunting, fishing, logging or collecting faster than its population can replace the individuals removed.

The production by organisms of more offspring than the environment can support, so that many die before they reproduce.

The tube that collects the egg released from the ovary and carries it to the uterus; the usual site of fertilisation. Also called the fallopian tube.

The release of an egg (secondary oocyte) from a mature follicle in the ovary, triggered by a surge in LH.

The structure inside the ovary of a flower that contains the female gamete (egg cell) and develops into a seed after fertilisation.

The large, non-motile female gamete, with a haploid nucleus and cytoplasm containing nutrient stores and organelles for the early embryo.

The four-carbon compound of the Krebs cycle that accepts an acetyl group to form citrate and is regenerated at the end of each turn.

The loss of electrons from a substance, often by the removal of hydrogen (dehydrogenation) or the addition of oxygen; it releases energy from the substance oxidised.

The synthesis of ATP in mitochondria using energy released by the oxidation of reduced NAD and reduced FAD in the electron transport chain, through chemiosmosis.

A graph of the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen; it is S-shaped because of cooperative binding.

A hormone secreted by the posterior pituitary that stimulates contraction of the muscle of the uterus during childbirth; its secretion increases by positive feedback.

The parental generation in a series of genetic crosses: the two individuals (often pure-breeding) crossed first.

The peptidyl site of a ribosome, which holds the tRNA carrying the growing polypeptide chain.

The layer of tall, closely packed cells below the upper epidermis of a leaf, containing many chloroplasts; the main site of photosynthesis.

An epidemic that spreads across many countries or continents, affecting a large number of people.

A fundamental change in the accepted framework of ideas in a field of science, when evidence accumulates that the existing framework cannot explain.

A group that contains a common ancestor and some, but not all, of its descendants, such as the traditional reptiles, which exclude birds.

An interspecific relationship in which a parasite lives on or in a host and obtains food from it, harming the host but usually not killing it quickly.

The principle that, of all possible cladograms for a group of organisms, the one requiring the fewest evolutionary changes is the most likely to be correct.

A charge smaller than that of an electron or proton, written δ+ or δ−, found on atoms at the ends of a polar covalent bond because electrons are shared unequally.

The pressure exerted by one gas in a mixture of gases, proportional to its concentration; pO₂ is used as a measure of oxygen concentration, in kilopascals (kPa).

A membrane that allows some particles to pass through it but not others; in cells, small non-polar molecules cross the bilayer while ions and large polar molecules need membrane proteins.

Short-term protection from antibodies made by another organism, such as antibodies passed from mother to baby or given by injection; no memory cells are formed.

Movement of particles across a membrane down their concentration gradient using only their own kinetic energy, without energy from ATP; includes simple diffusion, facilitated diffusion and osmosis.

An organism or virus that causes disease in another organism (its host); most human pathogens are viruses, bacteria, fungi or protists.

Partly decomposed plant matter that accumulates in waterlogged, acidic, oxygen-poor conditions where decomposition is very slow; peatlands are long-term carbon sinks.

A diagram that shows the occurrence of a characteristic in the members of a family over several generations, used to deduce its pattern of inheritance.

A limb with one upper bone, two lower bones, a group of wrist or ankle bones and up to five digits, found in amphibians, reptiles, birds and mammals and inherited from a common ancestor.

A sugar with five carbon atoms; the pentose in DNA is deoxyribose and in RNA is ribose.

The covalent C–N bond between the carboxyl carbon of one amino acid and the amine nitrogen of the next, formed by a condensation reaction.

The polymer of sugars cross-linked by short chains of amino acids that forms the cell walls of bacteria; it is thick in Gram-positive bacteria.

The change in a quantity between two times expressed as a percentage of its original value: (new value − original value) ÷ original value × 100.

The difference between two comparable values measured at the same time, expressed as a percentage of their mean: |a − b| ÷ ((a + b) ÷ 2) × 100.

A membrane protein attached to one surface of the membrane, to phospholipid heads or to an integral protein, without entering the hydrophobic core of the bilayer.

Waves of contraction of the circular and longitudinal muscle in the wall of the gut that push food along in one direction.

Ground, including soil and rock, that has remained frozen for at least two consecutive years; in the Arctic it often contains large stores of undecomposed organic matter.

Medical care in which the prevention, diagnosis or treatment of disease is tailored to an individual, using information such as their genome sequence.

A white blood cell, such as a neutrophil or macrophage, that engulfs pathogens by endocytosis and digests them with enzymes from lysosomes.

A membrane-bound vacuole formed around a particle, such as a bacterium, taken into a cell by phagocytosis; it fuses with lysosomes so that the particle is digested.

A type of endocytosis in which a cell takes in large solid particles, such as bacteria, by enclosing them in a vesicle formed from the plasma membrane.

A loss of synchrony between the timing of life-cycle events in interacting species, such as a consumer's breeding and the peak of its food supply, often because they respond to different cues.

The study of the timing of recurring biological events, such as flowering, budburst, bud set, migration and breeding, and of how they are influenced by variables such as photoperiod and temperature.

The observable characteristics of an organism, resulting from its genotype and the effects of the environment.

The capacity of an organism to develop traits suited to its environment by changing patterns of gene expression, without any change in its genotype.

An inherited disease caused by a recessive allele of the gene for phenylalanine hydroxylase, so phenylalanine cannot be converted to tyrosine and builds up in the blood.

The vascular tissue that transports organic compounds, mainly sucrose, in solution from where they are made or stored to where they are used, through living sieve tubes.

The negatively charged group in a nucleotide, attached to carbon 5′ of the pentose, that links to the next sugar in the sugar–phosphate backbone.

The covalent bond, formed by a condensation reaction, linking the phosphate of one nucleotide to carbon 3′ of the pentose of the next in a nucleic acid strand.

A lipid made of glycerol linked to two fatty acids and a phosphate group; it is amphipathic, with a hydrophilic phosphate head and two hydrophobic tails.

A double layer of phospholipids in which the hydrophilic heads face the water on each side and the hydrophobic tails form the core; the basis of all cell membranes.

The addition of a phosphate group to a molecule, which usually makes it less stable and more reactive, as when glucose is phosphorylated at the start of glycolysis.

An autotroph that uses light as its external energy source to make carbon compounds by photosynthesis, such as plants, algae and cyanobacteria.

The splitting of water molecules using energy from light, which releases electrons, protons (H⁺) and oxygen; in photosynthesis the oxygen released comes from this reaction.

The length of the period of light in each 24-hour day, which organisms use as a cue to time seasonal events such as flowering, migration and breeding.

The production of carbon compounds in cells using light energy, with carbon dioxide and water as the raw materials and oxygen released as a by-product.

A coloured molecule, such as chlorophyll or a carotenoid, that absorbs specific wavelengths of light and uses the energy in photosynthesis.

A complex of proteins and an array of pigment molecules in a thylakoid membrane that absorbs light and passes the energy to a reaction centre, which emits an excited electron.

The photosystem (PSI, reaction centre P700) that re-excites electrons arriving from the electron transport chain and passes them to NADP reductase, or back to the chain in cyclic flow.

The photosystem (PSII, reaction centre P680) that emits excited electrons to the electron transport chain and replaces them by the photolysis of water, releasing oxygen.

A directional growth response to light from one side; shoots show positive phototropism, growing towards the light.

A branching diagram of the evolutionary relationships of a group of organisms; when drawn to scale, branch lengths represent time since divergence or amount of genetic change.

A signalling chemical produced by plant cells that controls growth, development or responses to stimuli, often in tissues away from where it is made.

A small endocrine gland near the centre of the brain that secretes melatonin, mainly at night, under the control of the suprachiasmatic nucleus.

A type of endocytosis in which a cell takes in small droplets of the surrounding fluid, with the solutes dissolved in it, in small vesicles.

The first organisms to colonise bare ground in primary succession; they tolerate harsh conditions and begin soil formation, for example lichens and mosses on bare rock.

A small endocrine gland below the hypothalamus whose anterior and posterior lobes secrete hormones, many of which control other endocrine glands.

The organ formed from fetal and maternal tissue in the uterus, through which substances are exchanged between the blood of the mother and the fetus without the two bloods mixing.

A diagram showing the outlines and positions of the tissues in an organ, without individual cells, drawn with clear, continuous lines and no shading.

An activated B-lymphocyte that has differentiated to secrete large amounts of one specific antibody.

The partially permeable membrane of phospholipids and proteins that surrounds the cytoplasm of every cell and controls what enters and leaves; also called the cell membrane.

A small, circular DNA molecule in many prokaryotes, separate from the main loop of DNA; plasmids carry a few genes, such as antibiotic resistance, and are not essential.

A narrow channel of cytoplasm through the cell walls of adjacent plant cells, allowing substances to pass directly between them, for example between companion cells and sieve tube elements.

The shrinking of the protoplast of a plant cell away from its cell wall when the cell loses water by osmosis in a hypertonic solution.

A lipid-soluble electron carrier in the thylakoid membrane that accepts electrons from photosystem II and carries protons from the stroma into the thylakoid space.

A small cell fragment without a nucleus, found in blood, that sticks to damaged vessel walls and releases clotting factors.

Describes a stem cell that can differentiate into any type of cell in the body, but not extra-embryonic cells, so cannot form a whole organism alone.

A specialised root that grows vertically upwards out of waterlogged mud, allowing gas exchange with the air through lenticels, as in Avicennia mangroves.

A change to a single base in the base sequence of DNA, such as the substitution of one base for another.

A small haploid cell, with very little cytoplasm, formed by the unequal division of the cytoplasm in meiosis I and meiosis II of oogenesis; it usually degenerates.

A covalent bond in which the electrons are shared unequally because one atom is more electronegative, giving one end a partial negative charge and the other a partial positive charge.

A molecule with an uneven distribution of electrical charge, so that one part carries a partial negative charge and another part a partial positive charge.

A small structure produced in the anther of a flower, with a tough outer wall, that contains the male gametes.

A tube that grows from a pollen grain on the stigma, through the style to an ovule, and carries the male gametes to the egg cell.

The transfer of pollen from an anther to a stigma.

A chain of adenine nucleotides added to the 3′ end of eukaryotic pre-mRNA, which protects the mRNA from breakdown.

The inheritance of a characteristic that is controlled by two or more genes, each with a small additive effect, which gives continuous variation.

A large molecule made of many similar subunits (monomers) linked together by covalent bonds, such as a polysaccharide, polypeptide or nucleic acid.

A technique that amplifies a chosen DNA sequence by repeated cycles of denaturation, primer annealing and extension, doubling the number of copies in each cycle.

An unbranched chain of many amino acids joined by peptide bonds, made on ribosomes; one or more polypeptides fold to form a protein.

Describes a group whose members evolved from more than one ancestral source, so they do not share a single most recent common ancestor; viruses are thought to be polyphyletic.

The condition of having more than two complete sets of chromosomes in each cell, such as three sets (triploid) or four sets (tetraploid).

A polymer of many monosaccharides joined by glycosidic bonds, such as starch, glycogen or cellulose.

The fertilisation of an egg by more than one sperm, which produces an embryo with too many sets of chromosomes.

A fatty acid with two or more carbon–carbon double bonds in its hydrocarbon chain, giving several kinks and a low melting point.

A group of organisms of the same species that live in the same area at the same time and normally interbreed.

A control mechanism in which a change in a variable triggers a response that increases the change still further, so that a process speeds up until it is completed.

Changes made to a eukaryotic pre-mRNA before translation: removal of introns and splicing of exons, and addition of a 5′ cap and a 3′ poly-A tail.

Changes made to a polypeptide after translation, such as removal of amino acids, cutting of the chain, glycosylation or forming disulfide bonds, that make it functional.

A mechanism acting after a hybrid zygote has formed, such as hybrid inviability, hybrid sterility or hybrid breakdown, so that alleles do not pass between species.

The neuron (or effector cell) on the receiving side of a synapse, whose membrane carries receptors for the neurotransmitter.

The tilting of a myosin head bound to actin, releasing ADP and phosphate, which pulls the actin filament towards the centre of the sarcomere.

A mechanism that prevents mating or fertilisation between individuals of different species, so that no hybrid zygote is formed.

How close repeated measurements of the same quantity are to each other; precise measurements show little spread, though they may still be inaccurate.

An interaction in which one animal, the predator, kills and eats another animal, the prey.

An animal that kills and eats other animals (its prey).

The movement of water and small solutes out of a capillary through gaps in its wall, driven by the high hydrostatic pressure of the blood, while cells and large proteins are held back.

The component of water potential due to hydrostatic pressure on water; positive under pressure, as in a turgid plant cell, and negative under tension, as in xylem (symbol Ψp).

The neuron that releases neurotransmitter into the synaptic cleft at a synapse.

A consumer that feeds on producers; it occupies the second trophic level.

The barriers that prevent pathogens from entering the body, mainly the skin and mucous membranes and their secretions.

The first response of the adaptive immune system to an antigen; it is slow to start and produces a relatively small amount of antibody, but forms memory cells.

The region of the cerebral cortex, in a strip across the top of each cerebral hemisphere, that initiates voluntary contractions of skeletal muscles.

The accumulation of carbon compounds in biomass by autotrophs, measured as mass of carbon (or dry biomass) per unit area per unit time, usually g m⁻² yr⁻¹.

The sequence of amino acids in a polypeptide, linked by peptide bonds; it determines the higher levels of structure and therefore the conformation of the protein.

Succession that begins in an area with no soil and no previous community, such as bare rock, a new volcanic island or ground exposed by a retreating glacier.

The tumour that forms first, at the site in the body where a cancer began.

A short single strand of nucleic acid that pairs with a complementary sequence on a template and provides a 3′ end from which DNA polymerase can start building a new strand.

An autotroph that makes carbon compounds from inorganic substances using light or chemical energy; producers form the first trophic level.

A steroid hormone secreted by the corpus luteum and later the placenta that maintains the endometrium and inhibits FSH and LH secretion.

The hypothesis that viruses arose from pieces of genetic material that escaped from cells and gained a protein capsid; also called the escape hypothesis.

An organism whose cells have no nucleus and no membrane-bound organelles; its DNA is a naked loop in the cytoplasm. Bacteria and archaea are prokaryotes.

A base sequence of DNA just upstream of a gene, where RNA polymerase binds, with transcription factors, to start transcription.

Checking of a new DNA strand by DNA polymerase III, which removes a nucleotide with a mismatched base from the 3′ end and replaces it with a correctly paired one.

Phage DNA that has been inserted into the chromosome of a host bacterium and is replicated with it, without producing new virions.

The first phase of mitosis, in which chromosomes condense, the spindle forms and the nuclear membrane breaks down.

A non-polypeptide component permanently bound to a protein and needed for its function, such as the haem group of haemoglobin.

A large protein complex that breaks down unneeded or damaged proteins, tagged with ubiquitin, into short peptides, allowing their amino acids to be reused.

A large molecule made of one or more polypeptides folded into a specific three-dimensional shape that determines its function.

An enzyme that adds phosphate groups from ATP to specific proteins, changing their activity; chains of protein kinases form the cascades of signal transduction pathways.

The complete set of proteins produced by a cell, tissue or organism at a particular time.

A normal gene whose product stimulates cell division, such as a growth factor receptor; a mutation can convert it into an oncogene.

A hypothetical early stage in the origin of cells: a vesicle enclosing self-replicating molecules such as RNA, able to grow and divide.

A difference in proton (H⁺) concentration across a membrane, built up by electron transport, which stores potential energy used by ATP synthase.

The coiled part of the nephron after Bowman's capsule, lined with cells with microvilli and many mitochondria, where all glucose and amino acids and most ions and water are reabsorbed.

The period of development during which the reproductive organs mature and secondary sexual characteristics appear, caused by rising secretion of GnRH, LH, FSH and the steroid sex hormones.

The rhythmic expansion and recoil of an artery wall caused by the pressure wave that each heartbeat sends through the arteries.

An integral membrane protein that uses energy from ATP to move specific particles across a membrane against their concentration gradient by changing shape.

A table that shows all the possible combinations of gametes from two parents, used to predict the genotypes and phenotypes of their offspring and their expected ratios.

A nitrogenous base with a double-ring structure; adenine and guanine are purines, and each pairs with a pyrimidine.

A nitrogenous base with a single-ring structure; cytosine, thymine and uracil are pyrimidines, and each pairs with a purine.

The three-carbon end product of glycolysis; it enters the link reaction when oxygen is available or is converted to lactate or ethanol when it is not.

A square frame of known area used to mark out a sample area so that the organisms inside it can be counted or their percentage cover estimated.

Observations described in words rather than measured, for example the colour, shape or texture of cells seen with a microscope.

Observations recorded as numbers with units, obtained by measuring or counting, for example the length of a cell in micrometres.

The arrangement of two or more polypeptide subunits, and of any non-polypeptide prosthetic groups, in a protein.

A form of cell signalling in which bacteria release autoinducer molecules and change their gene expression when the autoinducer concentration, and therefore the population density, rises above a threshold.

The variable side chain attached to the central carbon of an amino acid; the 20 amino acids differ only in their R-groups, which give each its properties.

A radioactive form of an element, such as ³²P or ³⁵S, whose radiation can be detected so that molecules containing it can be traced.

A method of finding the age of rocks or fossils from the known, constant rate of decay of radioactive isotopes into their decay products.

Unpredictable variation in measurements that scatters repeated values on either side of the true value; it reduces precision, and its effect on a mean is reduced by taking more repeats.

The independent and random alignment of each bivalent on the equator in metaphase I, which gives 2ⁿ possible combinations of chromosomes in the gametes.

A method of selecting samples in which every individual or every position in the study area has an equal chance of being chosen, so that the samples are representative and unbiased.

An experiment in which participants are assigned at random to a treatment group or a control (placebo) group, so that differences in outcome can be attributed to the treatment.

The full extent of movement possible at a joint in a particular direction, measured as the angle between the limits of movement.

The range of values of an abiotic factor between the upper and lower limits within which a species can survive; abundance is highest at the optimum.

A change in the geographical distribution of a species, for example towards the poles or to higher altitudes, as the area with suitable conditions moves because of climate change.

The change in the amount of substrate or product per unit time; for an enzyme it can also be estimated as 1/time for a fixed amount of change.

The measurements and observations exactly as they were recorded during an investigation, before any calculation or processing.

The special pair of chlorophyll a molecules at the centre of a photosystem that receives energy from the surrounding pigments and emits an excited electron.

The actual range of conditions and resources a species occupies in nature, restricted by competition with other species and other biotic interactions.

A protein that binds a specific signalling molecule, such as a hormone or neurotransmitter, and responds by triggering a change in the cell.

An allele that affects the phenotype only in the homozygous state, when no dominant allele is present.

A gamete or offspring with a new combination of alleles, not present in either of the combinations inherited by its parent, produced by independent assortment or by crossing over.

A biconcave blood cell packed with haemoglobin that carries oxygen; in mammals the mature cell has no nucleus and no mitochondria.

A reaction in which one substance is oxidised (loses electrons) and another is reduced (gains electrons) at the same time.

An atmosphere with little or no free oxygen, rich in gases such as hydrogen, methane and ammonia that can donate electrons or hydrogen, which favours the formation of carbon compounds.

The gain of electrons by a substance, often by the addition of hydrogen or the removal of oxygen; it always occurs together with an oxidation.

A rapid, automatic response to a stimulus that does not involve conscious thought, such as withdrawing the hand from a hot object.

The pathway of neurons that carries impulses in a reflex: from a receptor, along a sensory neuron to the CNS, through one or more interneurons, and along a motor neuron to an effector.

Re-establishing forest on land from which it was recently cleared, either by planting trees or by allowing natural regeneration from seeds and roots.

The short time after an action potential, about 1–2 ms, during which a region of membrane cannot produce another action potential, because its voltage-gated sodium channels are inactivated.

The hypothesis that viruses descend from small parasitic cells that lost most of their genes while living inside host cells; also called the reduction hypothesis.

The extent to which repeated measurements give consistent results; it is improved by taking repeated readings or replicate samples and calculating a mean.

The Y-shaped region where the DNA double helix is being unwound and separated by helicase, so that both strands can act as templates.

The return of the membrane potential towards the negative resting potential after depolarisation, caused by voltage-gated potassium channels opening and K⁺ diffusing out of the neuron.

A protein that binds to a specific base sequence in DNA, such as an operator or silencer, and prevents or reduces the transcription of genes.

The separation of two populations so that they no longer interbreed and no gene flow occurs between them.

A single, focused question that names the independent variable (or two variables being correlated), the dependent variable and how it is measured, and the organism or system studied.

The smallest distance between two points at which they can still be seen as separate points; it determines how much detail a microscope can show.

The region of the medulla oblongata that sends rhythmic impulses to the diaphragm and intercostal muscles and changes ventilation rate in response to blood pH.

An organic compound that is broken down in cell respiration to release energy for ATP production, such as glucose, fatty acids or amino acids.

An apparatus that measures the rate of cell respiration, usually as the volume of oxygen taken up by living organisms per unit time, with an alkali absorbing the carbon dioxide released.

The potential difference across the plasma membrane of a neuron that is not conducting an impulse, about −70 mV, with the inside negative compared with the outside.

An RNA virus that uses the enzyme reverse transcriptase to make a DNA copy of its RNA genome, which is then inserted into a chromosome of the host cell; HIV is a retrovirus.

The removal of a neurotransmitter from the synaptic cleft by transporter proteins that carry it back into the presynaptic neuron for reuse.

A viral enzyme that synthesises DNA using RNA as a template; it has no proofreading activity, so it makes frequent errors.

Ecological restoration that re-establishes natural processes in an ecosystem, for example by reintroducing apex predators and keystone species, reconnecting habitats and reducing human impact, so the ecosystem becomes self-sustaining.

The distance moved by a substance from the origin divided by the distance moved by the solvent front; a ratio between 0 and 1, with no units, used to identify substances.

The pentose sugar in RNA nucleotides, which has an –OH group on carbon 2′.

A small structure of RNA and protein, not bounded by a membrane, that synthesises polypeptides; prokaryotes, mitochondria and chloroplasts have 70S ribosomes, while the eukaryote cytoplasm has larger 80S ribosomes.

An RNA molecule that acts as a catalyst, such as the RNA in the ribosome that catalyses the formation of peptide bonds.

An evaluation of the hazards in a method, the likelihood and severity of harm from each, and the precautions taken to reduce the risk to people, other organisms and the environment.

Any characteristic or behaviour that increases the probability of developing a particular disease, such as smoking or high blood pressure for coronary heart disease.

Ribonucleic acid: a usually single-stranded nucleic acid with ribose and the bases A, C, G and U, involved in expressing genetic information.

The enzyme that unwinds DNA, separates the strands and links RNA nucleotides into a strand complementary to the template strand during transcription.

The hypothesis that early life used RNA both to store genetic information and to catalyse reactions, before DNA and proteins took over these roles.

The base of a cladogram, representing the common ancestor of all the organisms shown on it.

A swelling on the root of a legume containing nitrogen-fixing bacteria (Rhizobium) that convert nitrogen gas into ammonium, which the plant uses.

A positive pressure in the xylem of roots, generated when mineral ions are actively transported into the xylem and water follows by osmosis, which pushes water up the stem.

A network of flattened membrane sacs (cisternae) with 80S ribosomes attached to the outer surface, which synthesise proteins that are then transported in vesicles, often for secretion.

Ribosomal RNA: RNA that, with proteins, forms the two subunits of ribosomes and catalyses the formation of peptide bonds.

Ribulose bisphosphate carboxylase, the enzyme in the stroma that catalyses carbon fixation by combining CO₂ with RuBP; it is slow and is the most abundant protein on Earth.

Ribulose bisphosphate, the five-carbon compound that accepts CO₂ in carbon fixation; it is regenerated from triose phosphate using ATP.

The synthesis phase of interphase, in which DNA is replicated so that each chromosome comes to consist of two sister chromatids.

The conduction of a nerve impulse along a myelinated axon in which action potentials occur only at the nodes of Ranvier, so the impulse jumps from node to node.

A base substitution that changes a codon into another codon for the same amino acid, so the polypeptide is unchanged; also called a silent mutation.

The difference between an estimate obtained from a sample and the true value for the whole population; it arises by chance and is unavoidable when only part of a population is measured.

An organism, such as many fungi and bacteria, that secretes digestive enzymes onto dead organic matter, digesting it externally, and then absorbs the products of digestion.

The repeating unit of a myofibril between two Z-lines, made of overlapping actin and myosin filaments; it is the unit that shortens when skeletal muscle contracts.

The specialised endoplasmic reticulum of a muscle fibre, which stores calcium ions and releases them to trigger contraction when the fibre is stimulated.

A fatty acid with only single bonds between the carbon atoms of its hydrocarbon chain, so the chain is straight and the melting point is relatively high.

A line drawn on a micrograph or drawing and labelled with the actual length it represents, so that sizes can be worked out whatever the image is enlarged to.

Ice that forms when seawater freezes, including drifting pack ice and landfast ice; it is a habitat for breeding, resting and feeding in polar regions.

A small molecule or ion, such as cyclic AMP or Ca²⁺, released inside a cell when a first messenger binds a receptor, which diffuses through the cytoplasm and activates the response.

A substance made by a plant that is not needed for its basic metabolism but defends it, for example by making its tissues toxic or distasteful to herbivores.

A consumer that feeds on primary consumers; it occupies the third trophic level.

The response to a second exposure to the same antigen; memory cells make it faster, larger and longer-lasting than the primary response.

The accumulation of carbon compounds in biomass by heterotrophs; it is lower than primary production because carbon compounds are lost in cell respiration at each stage.

Regular coiling or folding of the polypeptide backbone into alpha helices or beta-pleated sheets, stabilised by hydrogen bonds between C=O and N–H groups of the backbone.

Succession that occurs where an existing community has been disturbed or removed but the soil remains, for example after a fire or on abandoned farmland.

A tumour formed in another part of the body by cancer cells that have spread from a primary tumour.

The release from a cell of useful substances that the cell has made, such as enzymes, hormones or mucus, usually by exocytosis.

A membrane-bound vesicle that carries substances made in the cell, usually proteins from the Golgi apparatus, to the plasma membrane and releases them by exocytosis.