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