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