149 of the Biology glossary’s terms, each with the part of the syllabus where you first meet it.
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.