A-level biology definitions

A

A band

The dark band, the full length of the thick filaments, including any overlap with thin filaments. Muscles and movement: the sliding-filament model

Abiotic factor

A non-living influence: temperature, light, pH, water, oxygen, wind, soil depth. Populations and their limits

Abscisic acid

A plant hormone produced under water stress that binds a receptor on the guard cell membrane and brings about stomatal closure. Transpiration and stomatal control

Absorption

The movement of the products of digestion from the lumen of the gut, across the epithelium, into the blood or the lymph. Digestion and absorption in the human gut

Accurate

Close to the true value. Variables and controls: what an experiment can show

Acetyl coenzyme A

The two-carbon acetyl group attached to coenzyme A, which carries it into the Krebs cycle. Glycolysis, ATP and the link reaction

Acetylcholinesterase

The enzyme in the synaptic cleft that hydrolyses acetylcholine into choline and ethanoic acid. The synapse: transmission, and why it runs one way

ACFOR scale

A subjective five-point scale of abundance: Abundant, Common, Frequent, Occasional, Rare. Succession and environmental change

Acrosome

A vesicle at the front of a sperm head containing hydrolytic enzymes, which are released to digest a path through the layers around the egg. Gametes, fertilisation and early development

Activation energy

The minimum energy that colliding particles must have for a reaction to take place. Enzyme action and specificity

Active site

The region of an enzyme, formed by its tertiary structure, to which the substrate binds. Enzyme action and specificity

Active transport

The movement of a substance across a membrane against its concentration gradient, using a carrier protein and energy from the hydrolysis of ATP. Active transport, co-transport and bulk transport

Adaptation

A characteristic that increases an organism's chance of surviving and reproducing in its environment. Adaptations may be anatomical, physiological or behavioural. Natural selection: variation, selection and allele frequency

Adenosine deaminase

The enzyme missing in ADA-deficient severe combined immunodeficiency, supplied to patients as a recombinant protein. Pharming, recombinant medicines and synthetic biology

Affected

Showing the characteristic a pedigree follows; drawn as a shaded symbol. Monohybrid inheritance and genetic diagrams

Aleurone layer

The layer of living cells around the starchy endosperm of a cereal grain, which makes amylase when gibberellin reaches it during germination. Plant responses: tropisms, auxins and the classic experiments

Alkaloid

A bitter, nitrogen-containing compound made by a plant, toxic to or deterrent to herbivores; caffeine, nicotine and morphine are alkaloids. Plant disease and plant defences

All-or-nothing principle

Once threshold is reached an action potential of full size is produced; below threshold none is produced at all. The action potential and its propagation

Allele frequency

The proportion of all the alleles of a gene in a gene pool that are of one particular kind. Population genetics and Hardy-Weinberg equilibrium

Allosteric site

A binding site on an enzyme away from the active site, at which a regulatory molecule binds. Enzyme inhibition and metabolic control

Amino acid

A molecule containing an amine group, a carboxyl group and a variable side chain attached to the same carbon. Protein structure and function

Amyloplast

A colourless plastid with a double-membrane envelope that synthesises and stores starch, found in storage tissues such as potato tubers and in root cap cells. Eukaryotic cell structure and organelles

Anabolic steroid

A drug related to testosterone that enters muscle cells, binds a receptor and switches on genes for muscle protein, increasing muscle mass and strength. Exercise, sports medicine and performance-enhancing drugs

Anaerobic respiration

The release of energy from an organic molecule without oxygen, in which glycolysis continues because reduced NAD is oxidised by a reaction other than the electron transport chain. Anaerobic respiration and the respiratory quotient

Analogous structures

Structures with the same function and different origins, produced by convergent evolution. Classification, phylogeny and molecular evidence

Aneuploidy

A chromosome number that differs from the normal by less than a whole set. Meiosis and the sources of genetic variation

Aneurysm

A localised bulge in an artery wall that has been weakened, which may rupture and cause severe internal bleeding. Cholesterol, atheroma and what a risk factor means

Animal model

A species studied in place of humans because part of its biology is similar enough for the results to apply to humans. Brain imaging, development and learning

Antagonistic

Describes two effectors or hormones whose effects on a variable are opposite. Hormonal communication and negative feedback

Anterior pituitary

The lobe that makes its own hormones, under the control of releasing factors from the hypothalamus. Hormonal communication and negative feedback

Antibiotic

A substance produced by a microorganism, or a synthetic derivative of one, that kills or inhibits the growth of bacteria. Evidence for evolution, resistance and genetic drift

Antibiotic disc

A small disc of absorbent paper carrying a fixed amount of one antibiotic, laid on a lawn of bacteria to test whether that antibiotic stops them growing. Evidence for evolution, resistance and genetic drift

Antibiotic resistance

The ability of a bacterium to survive and reproduce in a concentration of an antibiotic that would kill or inhibit others of its species. Evidence for evolution, resistance and genetic drift

Antibody

A protein with a quaternary structure of four polypeptide chains, produced by plasma cells, with variable regions forming two binding sites complementary to one antigen. Antibodies, vaccination and the four kinds of immunity

Anticodon

A triplet of bases on tRNA, complementary to a codon on mRNA. Translation and protein synthesis

Antigen

A molecule, usually a surface protein or glycoprotein, that is recognised as self or non-self and can trigger an immune response. Pathogens and non-specific defences

Antigen-antibody complex

The structure formed when an antigen binds to the complementary binding site of an antibody. Antibodies, vaccination and the four kinds of immunity

Antigen-presenting cell

A cell displaying antigens from a pathogen or other non-self material on its own cell-surface membrane, such as a phagocyte after phagocytosis, an infected body cell or a B cell that has taken in the antigen. Pathogens and non-specific defences

Antigenic variation

Change in the antigens on a pathogen's surface, so that memory cells and antibodies produced against the previous form are no longer complementary to it. Antibodies, vaccination and the four kinds of immunity

Antiparallel

The two strands of a DNA molecule run in opposite directions: the 5′ end of one lies alongside the 3′ end of the other. DNA and RNA structure

Antiviral

A drug that inhibits a stage of virus replication, such as entry, copying of the genome, assembly or release, ideally by targeting a molecule coded for by the virus. Prokaryotic cells and viruses

Apical dominance

The inhibition of lateral bud growth by the apical bud. Plant responses: tropisms, auxins and the classic experiments

Apoplast pathway

Movement of water through the cellulose cell walls and the spaces between them, without entering the cytoplasm of any cell. Xylem and the transpiration stream

Apoptosis

Programmed cell death, in which a cell is broken down in an ordered sequence by its own enzymes and the fragments are removed by phagocytosis, without the release of the cell's contents. Homeobox genes, Hox genes and apoptosis

Apoptotic body

A membrane-bound fragment of a cell undergoing apoptosis, engulfed whole by a phagocyte. Homeobox genes, Hox genes and apoptosis

Artefact

A structure seen in a prepared specimen that was not present in the living cell, produced by the preparation or imaging process. Microscopy, magnification and resolution

Artificial selection

The breeding of organisms chosen by people for characteristics they want, so that the alleles for those characteristics become more common in the next generation. Also called selective breeding. Artificial selection and selective breeding

Aseptic conditions

Conditions from which unwanted microorganisms are excluded, achieved in a fermenter by steam-sterilising the vessel and medium and filtering the incoming air. Industrial biotechnology and fermentation

Atheroma

A plaque in the wall of an artery, consisting of lipid, foam cells and cell debris beneath a fibrous cap, which narrows the lumen and stiffens the wall. Cholesterol, atheroma and what a risk factor means

ATP

Adenosine triphosphate: a phosphorylated nucleotide made of adenine, ribose and three phosphate groups, used as the immediate source of energy in every cell. Glycolysis, ATP and the link reaction

Atrioventricular node

The only electrical connection between atria and ventricles; it delays the impulse so the atria empty first. The heart and cardiac cycle

Atrioventricular valves

The valves between each atrium and its ventricle; tricuspid on the right, bicuspid or mitral on the left. The heart and cardiac cycle

Attachment protein

A protein on a virus's surface with a shape complementary to a receptor on its host cell, which determines which cells it can infect. Prokaryotic cells and viruses

Autolysis

The digestion of cells by their own enzymes, released from lysosomes after death. Succession and environmental change

Autonomic nervous system

The part of the peripheral system carrying impulses to glands, smooth muscle and cardiac muscle, not under conscious control. Organisation of the nervous system

Autosomal

Carried on an autosome, any chromosome other than the sex chromosomes, so inherited in the same way by sons and daughters. Monohybrid inheritance and genetic diagrams

Autosomal linkage

The tendency of alleles of genes on the same autosome to be inherited together, unless crossing over separates them. Dihybrid inheritance, linkage, epistasis and the chi-squared test

Autotroph

An organism that makes its own organic molecules from inorganic ones, in almost all cases by photosynthesis. The five kingdoms, species concepts and viruses

Auxin

A plant growth substance, in practice indole-3-acetic acid, that promotes cell elongation in shoots. Plant responses: tropisms, auxins and the classic experiments

Auxin response factor

A transcription factor that activates auxin-responsive genes once IAA has caused the repressor proteins bound to it to be broken down. Plant responses: tropisms, auxins and the classic experiments

Axon

A fibre of a neurone that carries impulses away from the cell body. Organisation of the nervous system

B

Bacterial lawn

An even layer of bacteria grown over the whole surface of an agar plate from a spread liquid culture. Industrial biotechnology and fermentation

Bactericidal antibiotic

An antibiotic that kills bacteria, so the number of living cells falls while it is present. Penicillin is an example. Evidence for evolution, resistance and genetic drift

Bacteriophage

A virus that infects bacteria, often called a phage. Prokaryotic cells and viruses

Bacteriostatic antibiotic

An antibiotic that stops bacteria growing and dividing without killing them, leaving the immune system to remove them. Tetracycline is an example. Evidence for evolution, resistance and genetic drift

Balanced translocation carrier

A person with a reciprocal translocation who has lost and gained no genes, is usually healthy, but can produce unbalanced gametes. Mutation, gene expression and cancer

Bilayer

Two layers of phospholipid with the hydrophobic tails facing inwards and the hydrophilic heads facing the watery solutions on each side. Membrane structure and the fluid mosaic model

Binomial

The two-part international name of a species: genus then specific name, italicised, genus capitalised. Classification, phylogeny and molecular evidence

Biodegradable

Able to be broken down by microorganisms into simple substances such as carbon dioxide and water. Carbohydrate structure and function

Biofuel

A fuel made from recently grown biomass, such as bioethanol from fermented crop sugars or biodiesel from plant oils. Managing ecosystems sustainably

Biosensor

A device that uses a biological molecule, usually an immobilised enzyme, to detect a specific substance and convert its concentration into an electrical signal. Regulation of blood glucose and diabetes mellitus

Biotechnology

The use of living organisms, usually microorganisms, or of their components such as enzymes, in industrial processes that make useful products. Industrial biotechnology and fermentation

Biotic factor

A living influence: competitors, predators, prey, parasites, pathogens, mates. Populations and their limits

Bipolar neurone

A neurone in the retina that carries the signal from rods or cones to a ganglion cell. Receptors: transduction and the reflex arc

Bivalent

The structure formed when a homologous pair comes together in prophase I: two chromosomes, four chromatids. Meiosis and the sources of genetic variation

Blebbing

The bulging of the cell surface membrane during apoptosis, which changes the shape of the cell without the membrane breaking. Homeobox genes, Hox genes and apoptosis

Blind spot

The point where the optic nerve leaves the retina; it has no rods or cones, so light falling there is not detected. Receptors: transduction and the reflex arc

Blood smear

A single-cell-thick film of blood drawn across a slide and stained, used to examine and identify blood cells. Plan diagrams and tissue sections

Bohr shift

The rightward shift of the oxygen dissociation curve caused by an increased partial pressure of carbon dioxide and the fall in pH that goes with it, so that oxygen dissociates more readily at any given partial pressure. Haemoglobin and oxygen dissociation curves

Broth

A liquid culture medium, used to grow large numbers of microorganisms and to follow their growth over time. Industrial biotechnology and fermentation

C

Callose

A polysaccharide of beta-1,3 linked glucose, deposited within minutes of infection at plasmodesmata, sieve plates and wounds, sealing the routes a pathogen would spread by. Plant disease and plant defences

Callus

The mass of undifferentiated dividing cells produced from an explant on a nutrient medium, from which whole plantlets are regenerated. Cloning in plants and animals

Capsid

The protein coat surrounding a virus's genetic material. Prokaryotic cells and viruses

Capsule

A layer of slime outside the cell wall which retains water, aids attachment and gives some protection from phagocytosis. Prokaryotic cells and viruses

Carbon neutral

Describes a process that adds no net carbon dioxide to the atmosphere, because the carbon it releases is balanced by carbon taken up. Managing ecosystems sustainably

Carbon store

A place where carbon is held, such as the atmosphere, living organisms, soil, the oceans, sedimentary rock or fossil fuels. The carbon cycle and climate change

Carbon transfer

A process that moves carbon from one store to another, such as photosynthesis, respiration, decomposition or combustion. The carbon cycle and climate change

Carbonic anhydrase

The enzyme in red blood cells catalysing the reversible reaction between carbon dioxide and water to form carbonic acid. Haemoglobin and oxygen dissociation curves

Carpel

The female part of a flower: a stigma, a style and an ovary containing one or more ovules. Sexual reproduction in flowering plants

Carrier

An individual heterozygous for a recessive allele, so not showing the characteristic but able to pass the allele on. Monohybrid inheritance and genetic diagrams

Carrier (of infection)

An infected person with no symptoms who still sheds the pathogen and can pass it on. Infectious disease: transmission and control

Carrier protein

An intrinsic protein with a binding site specific to one substance, which changes shape to move it across the membrane. Active transport, co-transport and bulk transport

Carrying capacity

The population size that a particular environment can support over a sustained period, given the resources available in it. Populations and their limits

Cartilage

A connective tissue in which chondrocytes, lying in lacunae, are surrounded by a firm extracellular matrix of collagen that they secrete; it has no blood vessels and supports structures such as the trachea, joints, ear and nose. Cell specialisation and biological organisation

Case-control study

People who already have the disease are compared with matched people who do not, and both groups are asked about past exposure. Fast and efficient for rare diseases; yields an odds ratio, not a risk. Diet, energy and how to read a health claim

Casparian strip

A band of suberin in the walls of the endodermal cells that is impermeable to water, so the apoplast route is blocked and water must cross a plasma membrane. Xylem and the transpiration stream

Catalyst

A substance that increases the rate of a reaction by lowering its activation energy, and which is not used up in the process. Enzyme action and specificity

Causal relationship

A relationship in which a change in one variable brings about the change in the other, rather than both simply varying together. Exercise, sports medicine and performance-enhancing drugs

Causation

A relationship in which changing one variable changes the other. It cannot be read off a scatter graph. Variables and controls: what an experiment can show

Cell plate

The line of vesicles that fuse across the equator of a dividing plant cell, forming the new cell surface membranes and the new cell wall between the daughter cells. Meiosis and the sources of genetic variation

Cell signalling

Communication between cells in which one cell secretes a molecule that is transported to a target cell and binds to a receptor there. Membrane structure and the fluid mosaic model

Cell theory

The unifying concept that all living organisms are made of one or more cells, that the cell is the fundamental unit of structure, function and organisation, and that new cells arise only from existing ones. Cell specialisation and biological organisation

Central canal

The narrow channel through the centre of the grey matter of the spinal cord, containing cerebrospinal fluid. Organisation of the nervous system

Central nervous system

The brain and the spinal cord: the coordinating part of the nervous system, containing the relay neurones. Organisation of the nervous system

Cerebellum

The region below and behind the cerebrum that coordinates balance and the timing of movement, without conscious involvement. Organisation of the nervous system

Cerebrum

The paired, folded hemispheres responsible for conscious thought, learning, memory and the initiation of voluntary movement. Organisation of the nervous system

CFTR protein

A chloride ion channel in the cell-surface membrane of epithelial cells; when it is faulty, too little chloride and water are secreted and mucus becomes thick and sticky. Mutation, gene expression and cancer

Chemiosmosis

The synthesis of ATP driven by the movement of protons down an electrochemical gradient through ATP synthase. The Krebs cycle and oxidative phosphorylation

Chemoreceptor

A receptor that detects a chemical change; those controlling heart rate detect the fall in blood pH caused by dissolved carbon dioxide. The heart and cardiac cycle

Chiasma

The visible point at which two non-sister chromatids cross during a bivalent's crossover. Meiosis and the sources of genetic variation

Chloride shift

The movement of chloride ions into a red blood cell as hydrogencarbonate ions move out, maintaining electrical neutrality. Haemoglobin and oxygen dissociation curves

Chlorosis

Yellowing of leaves because too little chlorophyll is made, as when nitrate or magnesium ions are in short supply. Xylem and the transpiration stream

Cholinergic synapse

A synapse that uses acetylcholine as its neurotransmitter. The synapse: transmission, and why it runs one way

Chromatid

One of the two identical copies of a chromosome, joined to its sister at the centromere after DNA replication. The cell cycle and mitosis

Chromogen

A colourless substance that is converted into a coloured product, so that a reaction can be followed by eye. Regulation of blood glucose and diabetes mellitus

Chromosome mutation

A change in the structure of a chromosome, or in the number of chromosomes, that can affect many genes at once. Mutation, gene expression and cancer

Ciliated epithelium

An epithelium whose cells carry cilia that beat in a co-ordinated wave, sweeping mucus and trapped particles up towards the throat. Gas exchange in mammals: lungs and ventilation

Circadian rhythm

A biological rhythm with a period of about twenty-four hours that continues in constant conditions, showing that it is generated by an internal clock rather than by the daily change in the environment. Transpiration and stomatal control

Circular muscles

Rings of smooth muscle round the pupil in the iris; their contraction constricts the pupil. Organisation of the nervous system

Climate model

A computer simulation of the atmosphere, oceans, land and ice built from physical laws, used to project future climate under a given set of inputs. The carbon cycle and climate change

Climax community

The relatively stable community a succession reaches under the local climate, which persists until something disturbs it. Succession and environmental change

Clonal expansion

The repeated division by mitosis of the selected lymphocyte, producing a large clone of genetically identical cells. The specific immune response

Clonal selection

The binding of an antigen to the lymphocytes whose receptors are complementary to it, activating those cells and no others. The specific immune response

Clone

A group of genetically identical organisms, or a cell genetically identical to the cell it was produced from. Cloning in plants and animals

Closed circulatory system

A circulation in which blood remains inside vessels at all times and is returned to the heart through them. Blood vessels, tissue fluid and lymph

Cluster-randomised trial

A trial in which whole groups, such as villages, are allocated at random to the new method or the comparison, used where a method acts on a community. Infectious disease: transmission and control

Co-transport

The transport of two substances across a membrane by the same carrier protein at the same time, in which one moves down its concentration gradient and provides the means for the other to move against its own. Active transport, co-transport and bulk transport

Codon

A sequence of three bases in mRNA that specifies one amino acid or a stop signal. The genetic code and transcription

Coenzyme

An organic cofactor. In the usual A-level convention it binds to an enzyme, carries chemical groups or hydrogen between reactions and is released again; many are made from vitamins. Cofactors, coenzymes and where enzymes work

Cofactor

A non-protein substance that an enzyme requires in order to function, such as the chloride ion required by amylase. Cofactors, coenzymes and where enzymes work

Cohort study

A group of people free of the disease is recruited, their exposure is recorded, and they are followed forward in time to see who develops it. Gives incidence and relative risk directly, and gets the time order right by construction. Diet, energy and how to read a health claim

Colony

A visible mass of microorganisms on a solid medium, all descended from a single cell or a small clump of cells. Industrial biotechnology and fermentation

Community

All the populations of all the species living in the same place at the same time. Populations and their limits

Companion cell

A cell alongside each sieve tube element with dense cytoplasm, a nucleus and many mitochondria, connected to it by numerous plasmodesmata and responsible for loading it and keeping it alive. Phloem translocation and the mass-flow hypothesis

Companion diagnostic

A test carried out to decide whether a particular drug should be given to a particular patient, such as testing a tumour for HER2 before prescribing trastuzumab. Genome analysis, genetic screening and gene therapy

Compensation period

The time taken from first light for a plant to reach its compensation point. None of the A-level specifications this site follows uses the term. Limiting factors and primary productivity

Compensation point

The light intensity at which the rate of photosynthesis exactly equals the rate of respiration, so there is no net exchange of carbon dioxide or oxygen. Limiting factors and primary productivity

Competitive exclusion principle

Two species cannot occupy the same niche in the same place indefinitely; the better competitor eliminates the other. Populations and their limits

Competitive inhibitor

A molecule with a shape similar to the substrate that binds to the active site, preventing the substrate from binding. Enzyme inhibition and metabolic control

Complementary base pairing

Adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine by three, so the sequence of one strand fixes the sequence of the other. DNA and RNA structure

Complementary DNA (cDNA)

DNA made from an mRNA template by reverse transcriptase, and therefore containing no introns. Recombinant DNA technology

Computed tomography (CT)

A scan that builds cross-sectional images of the body from the absorption of X-rays passed through it from many angles. Brain imaging, development and learning

Concentration gradient

The difference in concentration between two points, divided by the distance between them. Surface area to volume ratio: why size limits diffusion

Condensation

A reaction that joins two molecules with a covalent bond and releases a molecule of water. Carbohydrate structure and function

Conflict of interest

A financial or personal interest that could influence how a study is designed, analysed or reported. Journals require authors to declare them. Diet, energy and how to read a health claim

Confounding variable

A variable that changes with the independent variable and could itself explain the change in the dependent variable, so the two effects cannot be separated by the data collected. Variables and controls: what an experiment can show

Conjugated protein

A protein with a non-protein prosthetic group permanently associated with it. Haemoglobin and oxygen dissociation curves

Conserved sequence

A sequence of bases that has changed very little between species over long periods, because nearly every mutation in it is harmful and is not passed on. Homeobox genes, Hox genes and apoptosis

Constant region

The part of an antibody that is the same in all antibodies of a class, and that binds to receptors on phagocytes. Antibodies, vaccination and the four kinds of immunity

Continuous variation

Variation in which the individuals of a population differ by small amounts over an unbroken range, with any value between the extremes possible; height is the standard example. Continuous and discontinuous variation

Control experiment

An extra run of the method, differing in one deliberate way, done to rule out an explanation the main runs cannot rule out on their own. Variables and controls: what an experiment can show

Control sequence

A region of DNA that is not translated but determines whether, when and how strongly a gene is transcribed. Mutation, gene expression and cancer

Control variable

A quantity held the same across every treatment, so that it cannot be the reason the dependent variable changed. Variables and controls: what an experiment can show

Core temperature

The temperature of the blood and of the deep organs, as distinct from that of the skin and extremities. Thermoregulation in endotherms and ectotherms

Coronary arteries

The vessels branching from the base of the aorta that supply the heart muscle itself with oxygenated blood. The heart and cardiac cycle

Correlation

A relationship in which two variables tend to change together. It may be positive or negative, strong or weak, and it makes no claim about which variable moved first. Variables and controls: what an experiment can show

Cortical granules

Vesicles lying just inside the egg's cell surface membrane, whose contents are released at fertilisation and change the zona pellucida. Gametes, fertilisation and early development

Counter-current flow

An arrangement in which two fluids move past each other in opposite directions, so a concentration gradient is maintained along the whole length of the exchange surface. Gas exchange in insects, fish and plants

Crenation

The shrinking and puckering of an animal cell when water leaves it by osmosis. Diffusion and osmosis

Crista

A fold of the inner mitochondrial membrane, giving a large surface area for the electron transport chain. Eukaryotic cell structure and organelles

Critical night length

The length of uninterrupted darkness that decides whether a photoperiodic plant flowers. Plant responses: tropisms, auxins and the classic experiments

Critical period

A window of time early in development when the nervous system needs a particular stimulus, such as light entering both eyes, for its connections to develop fully. Brain imaging, development and learning

Cross-cultural study

A study comparing people brought up in different cultures, to see whether an ability depends on upbringing. Brain imaging, development and learning

Crossing over

The exchange of equivalent sections of DNA between non-sister chromatids of a homologous pair during prophase I. Meiosis and the sources of genetic variation

Culture solution

A solution of mineral ions in distilled water in which plants are grown without soil, so that the ions available can be controlled. Xylem and the transpiration stream

Cytokine

A small signalling protein secreted by one cell that binds to receptors on another and changes its behaviour; T-helper cells release cytokines to activate B cells, T-killer cells and phagocytes. The specific immune response

Cytokinesis

The division of the cytoplasm that follows nuclear division: by a cleavage furrow in an animal cell and by a cell plate in a plant cell. Meiosis and the sources of genetic variation

Cytoplasm

Everything inside the cell-surface membrane except the nucleus: the cytosol and the organelles suspended in it. Eukaryotic cell structure and organelles

Cytoskeleton

The network of protein fibres (microfilaments, intermediate filaments and microtubules) that gives a cell its shape, anchors its organelles and provides tracks for movement. Eukaryotic cell structure and organelles

D

DCPIP

Dichlorophenolindophenol, a blue dye that becomes colourless when it is reduced. Vitamin C reduces it, so the volume of a sample needed to decolourise a fixed volume of DCPIP measures the sample's vitamin C content. Diet, energy and how to read a health claim

Deamination

The removal of the amino group from an amino acid, producing ammonia and a keto acid. The liver: detoxification, deamination and the making of urea

Decarboxylase

An enzyme that catalyses the removal of a carbon atom from a substrate, released as carbon dioxide. The Krebs cycle and oxidative phosphorylation

Decarboxylation

Removal of a carbon atom from a molecule, released as carbon dioxide. Glycolysis, ATP and the link reaction

Deflected succession

Succession that has been diverted from its natural course by an ongoing external factor. Succession and environmental change

Degenerate

Describing a code in which most amino acids are specified by more than one triplet. The genetic code and transcription

Degrees of freedom

A number, worked out from the number of categories for chi-squared or from the sample sizes for a t-test, that decides which critical value you read from the table. Averages, spread and the null hypothesis

Dehydrogenase

An enzyme that catalyses the removal of hydrogen from a substrate, passing it to a coenzyme such as NAD or FAD. The Krebs cycle and oxidative phosphorylation

Dehydrogenation

Removal of hydrogen from a molecule, catalysed by a dehydrogenase, with the hydrogen taken up by a coenzyme. Glycolysis, ATP and the link reaction

Deletion

A mutation in which a base is lost; within a coding region, losing a number of bases not divisible by three shifts the reading of every triplet that follows. DNA replication

Denaturation

The loss of a protein's specific three-dimensional shape through disruption of the hydrogen bonds, ionic bonds and hydrophobic interactions holding it, so the function is lost, while the primary structure is unchanged. Protein structure and function

Dendrochronology

Dating wood by counting and matching its annual growth rings, whose widths also record the growing conditions of each year. The carbon cycle and climate change

Dendron

A fibre of a neurone that carries impulses towards the cell body. Organisation of the nervous system

Dependent variable

The quantity you measure, whose value depends on the independent variable. It goes on the vertical axis. Variables and controls: what an experiment can show

Depolarisation

A change in membrane potential that makes the inside of the cell less negative. The action potential and its propagation

Detoxification

The conversion by the liver of a toxic substance into one that is less toxic and can be excreted. The liver: detoxification, deamination and the making of urea

Development rate

How fast an organism passes through a stage of its development, measured as 1 ÷ the time the stage takes, for example 1 ÷ the time for half a batch of eggs to hatch. The carbon cycle and climate change

Differentiation

The process by which a cell develops the structures and characteristics needed to carry out a particular function, through some genes being expressed and others not. Cell specialisation and biological organisation

Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random motion. Diffusion and osmosis

Diffusion distance

The thickness of the barrier a substance has to cross, sometimes called the length of the diffusion pathway. Surface area to volume ratio: why size limits diffusion

Digestion

The hydrolysis of large biological molecules into small soluble molecules that can be absorbed across cell membranes. Digestion and absorption in the human gut

Dipeptidase

An enzyme, bound to the membrane of the ileum epithelium, that hydrolyses a dipeptide into two amino acids. Digestion and absorption in the human gut

Diploid

Having chromosomes in homologous pairs, written 2n. Meiosis and the sources of genetic variation

Disaccharide

Two monosaccharides joined by a glycosidic bond. Carbohydrate structure and function

Discontinuous variation

Variation in which the individuals of a population fall into a small number of distinct categories with no intermediates; ABO blood group is the standard example. Continuous and discontinuous variation

DNA ladder

A mixture of DNA fragments of known lengths, run in one lane of a gel so that the lengths of fragments in the other lanes can be read against it. Recombinant DNA technology

DNA methylation

The addition of methyl groups to cytosine bases; heavy methylation of a promoter reduces or prevents transcription of that gene. Control of gene expression

DNA microarray

A plate carrying many different DNA probes at known positions, allowing thousands of sequences to be tested for at once. Genome analysis, genetic screening and gene therapy

DNA precipitation

The recovery of DNA from a filtered tissue extract as visible fibres, by layering ice-cold ethanol on top; the DNA, insoluble in cold ethanol, forms at the boundary and is spooled onto a rod. DNA replication

DNA probe

A short single-stranded length of DNA, labelled radioactively or fluorescently, with a base sequence complementary to a target sequence. Genome analysis, genetic screening and gene therapy

Dominant

An allele whose effect appears in the phenotype even when only one copy is present. Monohybrid inheritance and genetic diagrams

Dopamine

A neurotransmitter in the brain; too little where it controls movement contributes to Parkinson's disease. The synapse: transmission, and why it runs one way

Dorsal

The back or upper surface of an animal, opposite the ventral surface. Gas exchange in insects, fish and plants

Dorsal root

The root by which sensory neurones enter the spinal cord, at the back, bearing the dorsal root ganglion. Organisation of the nervous system

Double fertilisation

The fusion, in the embryo sac of a flowering plant, of one male nucleus with the egg nucleus to form a zygote and of the other with the two polar nuclei to form a triploid endosperm nucleus. Sexual reproduction in flowering plants

Double helix

Two antiparallel polynucleotide strands wound into a spiral, bases inside and backbones outside. DNA and RNA structure

Double-blind trial

A trial in which neither the patients nor the doctors and researchers assessing them know who is receiving the drug and who the placebo or comparison treatment. Antibodies, vaccination and the four kinds of immunity

Down's syndrome

The condition of a person with three copies of chromosome 21: a trisomy, and so a polysomy. Meiosis and the sources of genetic variation

E

Ecological species concept

The definition of a species by the niche its members occupy, useful for organisms that reproduce asexually. The five kingdoms, species concepts and viruses

Ecosystem

A community together with the non-living surroundings it interacts with, treated as one working unit. Populations and their limits

Ectotherm

An animal whose body temperature depends mainly on heat gained from its surroundings, and which controls it chiefly by behaviour. Thermoregulation in endotherms and ectotherms

Effector

A muscle or gland that brings about a response. Receptors: transduction and the reflex arc

Egestion

The removal of undigested food and other material that has never taken part in the body's metabolism. The liver: detoxification, deamination and the making of urea

Elastic artery

A large artery near the heart, such as the aorta, whose wall is dominated by elastic laminae and which stretches and recoils to smooth out the pulse. Blood vessels, tissue fluid and lymph

Elastic fibres

Fibres that stretch as the lungs inflate and recoil as the muscles relax, providing much of the force for quiet expiration. Gas exchange in mammals: lungs and ventilation

Electrocardiogram

A recording of the potential difference measured at the skin against time, produced by the wave of depolarisation spreading through the heart muscle. The heart and cardiac cycle

Electron transport chain

A series of carrier proteins in the inner mitochondrial membrane that pass electrons from one to the next, releasing energy used to pump protons into the intermembrane space. The Krebs cycle and oxidative phosphorylation

Embryo sac

The structure inside an ovule, formed from one megaspore by three mitotic divisions, holding the egg cell, two synergids, two polar nuclei and three antipodal cells. Sexual reproduction in flowering plants

Emissions scenario

A stated assumption about future emissions of greenhouse gases, based on population, energy use and policy, which a climate model is run on. The carbon cycle and climate change

Empty magnification

Enlarging an image beyond the detail its resolution contains, so nothing further becomes visible. Microscopy, magnification and resolution

End point

The point in a titration at which the reaction is just complete, seen as a change that does not reverse, such as the last of the blue of DCPIP disappearing. Diet, energy and how to read a health claim

End-product inhibition

The inhibition of an enzyme early in a metabolic pathway by the final product of that pathway, so that the rate of the pathway matches demand. Enzyme inhibition and metabolic control

Endangered species

A species at very high risk of extinction in the wild, as assessed against criteria such as the IUCN Red List's. Conservation in situ and ex situ

Endemic

Describes a disease permanently established in a region or population, as malaria is across much of the tropics. Infectious disease: transmission and control

Endemism

Being found naturally in only one place, such as a single island or region, and nowhere else; a species like this is endemic to that place. Measuring biodiversity

Endocrine gland

A ductless gland that secretes hormones directly into the blood. Hormonal communication and negative feedback

Endocytosis

The bulk transport of material into a cell by invagination of the cell surface membrane to form a vesicle; requires ATP. Active transport, co-transport and bulk transport

Endopeptidase

An enzyme that hydrolyses peptide bonds within a polypeptide chain, producing shorter chains and so more ends. Pepsin, trypsin and chymotrypsin. Digestion and absorption in the human gut

Endosperm

The triploid food-storing tissue of a seed, formed by mitosis from the nucleus produced when a male nucleus fuses with the two polar nuclei. Sexual reproduction in flowering plants

Endotherm

An animal that maintains its body temperature chiefly by heat released from its own metabolism, and controls it physiologically. Thermoregulation in endotherms and ectotherms

Endotoxin

The lipopolysaccharide of the outer membrane of a Gram-negative bacterium, released mainly when the bacterium dies; it causes fever and inflammation. Salmonella species are the standard example. Pathogens and non-specific defences

Enucleation

The removal of the nucleus from a cell, usually an egg cell, leaving its cytoplasm intact. Cloning in plants and animals

Environmental variation

Differences in phenotype between organisms caused by the conditions they grew in rather than by their alleles, and therefore not passed to offspring. Natural selection: variation, selection and allele frequency

Enzyme

A globular protein that acts as a biological catalyst, with an active site complementary to its substrate. Enzyme action and specificity

Enzyme-substrate complex

The temporary structure formed when a substrate binds to the active site. Enzyme action and specificity

Epigenetics

Heritable changes in gene expression caused by chemical modifications to DNA and to the histones associated with it, without any change to the base sequence. Control of gene expression

Epigenome

The full set of chemical marks on a cell's DNA and histones at a given time. Control of gene expression

Epistasis

Interaction between genes at different loci, in which one gene affects or masks the expression of another. Dihybrid inheritance, linkage, epistasis and the chi-squared test

EPO

Erythropoietin, a hormone made by the kidneys that stimulates the bone marrow to produce red blood cells. Exercise, sports medicine and performance-enhancing drugs

Ester bond

The covalent bond formed by condensation between a hydroxyl group of glycerol and the carboxyl group of a fatty acid. Lipids: triglycerides, phospholipids and ester bonds

Etiolation

The growth of a plant in darkness, with long, thin internodes and small, folded leaves that lack chlorophyll. Plant responses: tropisms, auxins and the classic experiments

Evasion mechanism

A feature of a pathogen that lets it avoid or survive the host's immune response, such as antigenic variation or surviving inside a phagocyte. Infectious disease: transmission and control

Evolutionary race

The continuing coevolution of a pathogen and its host, in which better host defences select for better pathogen evasion, and the reverse. Infectious disease: transmission and control

Ex situ conservation

Conserving a species outside its natural habitat, in a zoo, a botanic garden, a seed bank or a captive breeding centre. Conservation in situ and ex situ

Exchange surface

A surface across which substances move between an organism and its environment, or between two compartments within it. Surface area to volume ratio: why size limits diffusion

Excitatory postsynaptic potential

A depolarisation of the postsynaptic membrane that makes an action potential more likely. The synapse: transmission, and why it runs one way

Excretion

The removal from the body of the waste products of metabolism, such as carbon dioxide and urea. The liver: detoxification, deamination and the making of urea

Exocrine gland

A gland that secretes its product into a duct, which carries it to a surface. Hormonal communication and negative feedback

Exocytosis

The bulk transport of material out of a cell by fusion of a vesicle with the cell surface membrane; requires ATP. Active transport, co-transport and bulk transport

Exon

A section of a gene that is represented in the mature mRNA. The genetic code and transcription

Exopeptidase

An enzyme that hydrolyses the peptide bond at an end of a chain, removing a single amino acid. Carboxypeptidases work from the end carrying the free carboxyl group, aminopeptidases from the amine end. Digestion and absorption in the human gut

Exotoxin

A toxic protein secreted by a living bacterium, acting on a specific target in the host; Staphylococcus species are the standard example. Pathogens and non-specific defences

Explant

The small piece of tissue cut from a parent plant to start a tissue culture, usually a shoot tip or a node. Cloning in plants and animals

Exponential growth

Growth in which the population multiplies by a constant factor per unit time, so the number added per unit time keeps increasing. Populations and their limits

Exponential growth rate constant

The number of generations per unit time in the log phase: k = (log₁₀ N − log₁₀ N₀) ÷ 0.301t, where N₀ grows to N in time t. Industrial biotechnology and fermentation

Extinct in the wild

A species surviving only in cultivation, captivity or as a population well outside its past range. Conservation in situ and ex situ

Extracellular enzyme

An enzyme secreted from the cell that made it and catalysing reactions outside it, such as amylase and trypsin in digestion. Cofactors, coenzymes and where enzymes work

Extrinsic protein

A protein on one surface of the bilayer that does not enter the hydrophobic core. Membrane structure and the fluid mosaic model

Eyepiece graticule

A ruled scale in the eyepiece whose divisions have no fixed length until it is calibrated. Microscopy, magnification and resolution

F

F1 and F2

The first filial generation, the offspring of the original parental cross, and the second, the offspring of F1 individuals crossed with each other. Monohybrid inheritance and genetic diagrams

Factor VIII

The clotting protein deficient in haemophilia A, produced recombinantly in mammalian cell culture since the 1990s. Pharming, recombinant medicines and synthetic biology

Facultative anaerobe

An organism that respires aerobically when oxygen is available and anaerobically when it is not, as yeast does. Anaerobic respiration and the respiratory quotient

FAD

A hydrogen-carrying coenzyme like NAD, but one that hands its electrons to the transport chain further along, so it yields less ATP per molecule. The Krebs cycle and oxidative phosphorylation

Fermentation

The anaerobic conversion of pyruvate to lactate or to ethanol and carbon dioxide, regenerating oxidised NAD. Anaerobic respiration and the respiratory quotient

Fermentation (industrial)

The large-scale culture of microorganisms in a fermenter to make a product. The industrial word covers aerobic processes as well as the anaerobic pathway the respiration unit gave the same name. Industrial biotechnology and fermentation

Fermenter

A vessel in which microorganisms are cultured at large scale under controlled conditions of temperature, pH, oxygen and nutrient supply. Industrial biotechnology and fermentation

FEV₁ ÷ FVC ratio

The fraction of the forced vital capacity breathed out in the first second. In a healthy adult it is usually above 0.7, often around 0.8; a value below 0.7 is the threshold commonly used to indicate airway obstruction. Gas exchange in mammals: lungs and ventilation

Fibrous capsule

The tough outer layer enclosing a kidney, which protects it and holds its shape. Osmoregulation and kidney function

Final electron acceptor

The molecule that takes electrons from the end of the chain. In aerobic respiration it is oxygen, which combines with protons to form water. The Krebs cycle and oxidative phosphorylation

Fitness

The contribution an individual makes to the gene pool of the next generation, relative to other individuals in the population, usually counted as offspring that themselves survive to reproduce. Natural selection: variation, selection and allele frequency

Forced expiratory volume in one second (FEV₁)

The volume of air breathed out in the first second of a forced breath out. It falls when the airways are narrowed, because air cannot leave them quickly. Gas exchange in mammals: lungs and ventilation

Forced vital capacity (FVC)

The total volume of air breathed out in one forced breath after the deepest possible breath in. In healthy lungs it is close to the vital capacity. Gas exchange in mammals: lungs and ventilation

Forensic entomology

The use of insects found on a body, their species and stage of development, to estimate the time since death. Succession and environmental change

Fossil fuel

Coal, oil or natural gas, formed over millions of years from the remains of organisms that were buried in low-oxygen conditions before decomposers could break them down fully. The carbon cycle and climate change

Founder effect

The reduced and unrepresentative genetic diversity of a population established by a small number of individuals separated from a larger population. Evidence for evolution, resistance and genetic drift

Fovea

The small region of the retina directly behind the lens, packed with cones and containing no rods, where acuity is greatest. Receptors: transduction and the reflex arc

Frame shift

The displacement of the reading frame caused by insertion or deletion of a number of bases that is not a multiple of three. Mutation, gene expression and cancer

Functional magnetic resonance imaging (fMRI)

A series of MRI scans that detects increases in oxygenated blood flow, showing which brain regions are active during a task. Brain imaging, development and learning

G

Ganglion

A swelling containing the cell bodies of many neurones, outside the central nervous system. Organisation of the nervous system

Ganglion cell

A neurone in the retina whose axon forms part of the optic nerve, carrying action potentials to the brain. Receptors: transduction and the reflex arc

Gene

A sequence of DNA bases coding for a polypeptide or a functional RNA. The genetic code and transcription

Gene expression

The process by which the information in a gene is used to produce a functional product, usually a polypeptide. Control of gene expression

Gene flow

The movement of alleles between populations, by migration of individuals or of gametes, which keeps their gene pools similar. Speciation and reproductive isolation

Gene gun

A device that fires microscopic gold or tungsten particles coated with DNA into cells, used chiefly to transform plant tissue. Recombinant DNA technology

Gene pool

All the alleles of all the genes present in a population at a given time. Population genetics and Hardy-Weinberg equilibrium

Generative nucleus

The smaller nucleus of a pollen grain, which divides by mitosis to give the two male nuclei. Sexual reproduction in flowering plants

Generator potential

The graded depolarisation produced in a receptor by a stimulus, whose size depends on the strength of that stimulus. Receptors: transduction and the reflex arc

Genetic bottleneck

A sharp reduction in population size that leaves the survivors carrying only a fraction of the original genetic diversity. Evidence for evolution, resistance and genetic drift

Genetic diversity

The number of different alleles present in the gene pool of a population or species. Measuring biodiversity

Genetic drift

A change in allele frequencies from one generation to the next caused by chance sampling of gametes, rather than by selection. Its effect is greatest in small populations. Evidence for evolution, resistance and genetic drift

Genetic variation

Differences between individuals in a population that are caused by differences in their alleles, and can therefore be inherited. Natural selection: variation, selection and allele frequency

Genome

The complete set of DNA in a cell, including the DNA that does not code for polypeptides. The genetic code and transcription

Genotype

The alleles an organism carries, for the gene or genes under discussion. Monohybrid inheritance and genetic diagrams

Gill filament

One of the many thin projections from a gill arch, sometimes called a primary lamella. Gas exchange in insects, fish and plants

Gluconeogenesis

The production of glucose from non-carbohydrate sources such as amino acids and glycerol, in the liver. Promoted by glucagon. Regulation of blood glucose and diabetes mellitus

Glycogenesis

The synthesis of glycogen from glucose, in liver and muscle cells. Promoted by insulin. Regulation of blood glucose and diabetes mellitus

Glycogenolysis

The breakdown of glycogen to glucose. Promoted by glucagon and by adrenaline. Regulation of blood glucose and diabetes mellitus

Glycolipid

A lipid with a carbohydrate chain attached, acting in cell recognition and adhesion. Membrane structure and the fluid mosaic model

Glycolysis

The splitting of glucose into two molecules of pyruvate in the cytoplasm, producing a net gain of two ATP and two reduced NAD, without oxygen. Glycolysis, ATP and the link reaction

Glycoprotein

A protein with a carbohydrate chain attached. In the cell surface membrane, glycoproteins act in cell recognition, adhesion or as receptors. Membrane structure and the fluid mosaic model

Glycosidic bond

The covalent bond formed by condensation between two monosaccharides, joining them through an oxygen atom. Carbohydrate structure and function

Goblet cell

A cell in the airway epithelium that secretes mucus, which traps dust, pollen and bacteria. Gas exchange in mammals: lungs and ventilation

GP

Glycerate 3-phosphate, a three-carbon compound; the first stable product of fixation. The Calvin cycle

Granum

A stack of thylakoids, giving a large area of membrane within a small volume. Chloroplast structure and the light-dependent reactions

Grey matter

The central region of the spinal cord, containing cell bodies, dendrites, relay neurones and synapses. Organisation of the nervous system

Gridded quadrat

A frame quadrat divided into a grid of small squares, usually 100, used to estimate percentage cover or frequency by counting squares. Succession and environmental change

Guard cell

One of the pair of cells around a stoma. The pair bow apart and open the pore when they become turgid, and close it when they become flaccid. Gas exchange in insects, fish and plants

H

H zone

The lighter region at the centre of the A band, containing thick filaments only. Muscles and movement: the sliding-filament model

Habitat

The place where an organism lives: a rock pool, a hedgerow, the gut of a cow. Populations and their limits

Habituation

A reduced response to a repeated stimulus that is neither harmful nor rewarding; a simple form of learning. Brain imaging, development and learning

Haemocoel

The body cavity of an insect or other arthropod, in which haemolymph collects and washes over the organs. Blood vessels, tissue fluid and lymph

Haemoglobinic acid

Haemoglobin that has taken up hydrogen ions, buffering the contents of the red blood cell. Haemoglobin and oxygen dissociation curves

Haemolymph

The fluid of an open circulatory system, acting as blood and tissue fluid at the same time. Blood vessels, tissue fluid and lymph

Haemolysis

The bursting of a red blood cell when water enters it by osmosis. Diffusion and osmosis

Haploid

Having one chromosome from each homologous pair, written n. Meiosis and the sources of genetic variation

Haustorium

A branch of a fungal hypha that pushes into a living host plant cell, inside a fold of the host's cell-surface membrane, and absorbs nutrients from it. Pathogens and non-specific defences

Hazard

Anything with the potential to cause harm, such as a chemical, a sharp edge or a pathogen. Variables and controls: what an experiment can show

Hepatocyte

A liver cell, carrying out detoxification, deamination, storage and the secretion of bile. The liver: detoxification, deamination and the making of urea

Herd immunity

The protection of unvaccinated individuals that results when a large enough proportion of a population is immune for transmission not to be sustained. Antibodies, vaccination and the four kinds of immunity

Heterotroph

An organism that obtains organic molecules by consuming other organisms or their remains. The five kingdoms, species concepts and viruses

Heterozygous

Carrying two different alleles of a gene. Monohybrid inheritance and genetic diagrams

High-oleic soya

Soya genetically modified so that the enzyme converting oleic acid to linoleic acid is silenced, giving an oil with more monounsaturated fat that resists oxidation. Recombinant DNA technology

Histone acetylation

The addition of acetyl groups to histone tails, which reduces their positive charge, loosens the chromatin and increases transcription. Control of gene expression

Homeobox

A sequence of 180 base pairs found within many genes that control development, coding for the part of the protein that binds to DNA. Homeobox genes, Hox genes and apoptosis

Homeodomain

The 60 amino acid section of a protein coded for by a homeobox, which folds into a shape that binds to a specific DNA sequence. Homeobox genes, Hox genes and apoptosis

Homeostasis

The maintenance of a stable internal environment within narrow limits, despite changes in the external environment. Hormonal communication and negative feedback

Homologous pair

Two chromosomes of the same size carrying the same genes at the same loci, one inherited from each parent. Not the same thing as a pair of chromatids. The cell cycle and mitosis

Homologous structures

Structures with the same underlying plan, inherited from a common ancestor, whatever they are now used for. Classification, phylogeny and molecular evidence

Homozygous

Carrying two identical alleles of a gene. Monohybrid inheritance and genetic diagrams

Horizontal gene transfer

Passing genes between bacteria that are not parent and offspring, most often on a plasmid transferred by conjugation. Evidence for evolution, resistance and genetic drift

Hormone

A chemical messenger, secreted by an endocrine gland into the blood, that acts on target cells carrying a complementary receptor. Hormonal communication and negative feedback

Hospital-acquired infection

An infection that a patient acquires while in hospital, which was not present or incubating on admission. MRSA and Clostridioides difficile are examples. Evidence for evolution, resistance and genetic drift

Hybrid vigour

The greater size, yield or vigour of the offspring of a cross between two inbred lines, compared with either parent line. Artificial selection and selective breeding

Hybridisation

The binding of a probe to a complementary single-stranded target sequence by hydrogen bonds between complementary bases. Genome analysis, genetic screening and gene therapy

Hybridoma

A cell made by fusing an antibody-producing B lymphocyte from the spleen of an immunised mouse with a myeloma cell, so that it both makes the wanted antibody and divides indefinitely in culture. Antibodies, vaccination and the four kinds of immunity

Hydrogen bond

A weak attraction between a slightly positive hydrogen atom and a slightly negative atom such as oxygen or nitrogen, in another molecule or another part of the same molecule. Water and inorganic ions

Hydrogencarbonate indicator

A pH indicator that shows carbon dioxide concentration: yellow when it is raised, orange-red in equilibrium with air, purple when it is lowered. Limiting factors and primary productivity

Hydrolysis

A reaction that breaks a covalent bond between two molecules using a molecule of water. Carbohydrate structure and function

Hydrolysis of ATP

The breaking of the bond to the terminal phosphate by addition of water, catalysed by ATP hydrolase, giving ADP and inorganic phosphate and releasing about 30.5 kJ per mole. Glycolysis, ATP and the link reaction

Hydrophilic

Attracted to water: charged or polar, so it interacts with water molecules. Lipids: triglycerides, phospholipids and ester bonds

Hydrophobic

Not attracted to water: non-polar, so it is excluded from water rather than dissolving in it. Lipids: triglycerides, phospholipids and ester bonds

Hydrostatic pressure

The physical pressure the blood exerts against the capillary wall, pushing fluid out through the gaps between endothelial cells. Blood vessels, tissue fluid and lymph

Hyperpolarisation

A change in membrane potential that makes the inside more negative than the resting potential. The action potential and its propagation

Hypothalamus

The region beneath the thalamus that monitors the blood, controls thermoregulation and osmoregulation, and directs the pituitary. Organisation of the nervous system

I

I band

The light band, containing thin filaments only, spanning a Z line. Muscles and movement: the sliding-filament model

Ice core

A cylinder of ice drilled from an ice sheet, in which bubbles of trapped air preserve the composition of the atmosphere when each layer formed. The carbon cycle and climate change

Immobilised algae

Algal cells trapped in beads of sodium alginate, so equal numbers can be put into each tube of an experiment. Limiting factors and primary productivity

In situ conservation

Conserving a species in its natural habitat, so that the community and the conditions it is adapted to are conserved with it. Conservation in situ and ex situ

Inbreeding depression

The reduction in vigour, fertility and disease resistance that follows repeated breeding between close relatives, caused by harmful recessive alleles becoming homozygous. Artificial selection and selective breeding

Incidence

The number of new cases of a disease in a population over a stated period, usually given per 100 000 people per year so that populations of different sizes can be compared. Gas exchange in mammals: lungs and ventilation

Incipient plasmolysis

The point at which the pressure potential of a plant cell has just fallen to zero, so its water potential equals its solute potential. Diffusion and osmosis

Independent variable

The quantity you change on purpose, and the only one that should differ between your treatments. It goes on the horizontal axis. Variables and controls: what an experiment can show

Indirect active transport

Transport that is driven by a gradient which was itself established using ATP elsewhere in the cell. Glucose uptake in the ileum is the standard example. Digestion and absorption in the human gut

Inducible enzyme

An enzyme synthesised only when its substrate is present, because the substrate or a derivative of it inactivates the repressor of its gene, as allolactose does for β-galactosidase. Control of gene expression

Infection prevention and control

The hospital practices that stop pathogens spreading between patients, such as hand hygiene, screening, isolation and cleaning. Evidence for evolution, resistance and genetic drift

Inhibitory synapse

A synapse whose transmitter hyperpolarises the postsynaptic membrane, making an action potential less likely. The synapse: transmission, and why it runs one way

Initial rate

The rate of reaction at time zero, found from the gradient of the tangent to a progress curve at the origin. Factors affecting enzyme rate: temperature, pH and concentration

Inoculum

The small measured volume of culture added to a sterile medium to start a new culture. Industrial biotechnology and fermentation

Inorganic ion

An ion containing no carbon, apart from carbonate and hydrogencarbonate, present in solution in cytoplasm and body fluids, often in very small amounts, and each with a specific role. Water and inorganic ions

Instar

One of the larval stages of an insect, separated from the next by a moult. Succession and environmental change

Interferon

A protein secreted by a virus-infected cell that binds to receptors on neighbouring cells and makes them resistant to viral replication, by inhibiting protein synthesis and destroying viral RNA. Pathogens and non-specific defences

Interleukin

A cytokine released by one white blood cell to act on others, such as the interleukins an activated T-helper cell releases to stimulate B cells and T-killer cells to divide. The specific immune response

Intracellular enzyme

An enzyme that catalyses reactions inside the cell that made it, such as catalase in peroxisomes. Cofactors, coenzymes and where enzymes work

Intrinsic protein

A protein embedded within the bilayer, often spanning it completely. Membrane structure and the fluid mosaic model

Intron

A non-coding section of a gene, transcribed into pre-mRNA and then removed before the mRNA leaves the nucleus. The genetic code and transcription

Inverse square law

Light intensity from a small source is proportional to 1/d², where d is the distance from the source. Limiting factors and primary productivity

Iodopsin

The light-sensitive pigment in cone cells, in three types, broken down only by higher light intensities. Receptors: transduction and the reflex arc

K

Keyhole surgery

Surgery carried out through small incisions using a camera on a narrow tube and fine instruments; on a joint it is called arthroscopy. Exercise, sports medicine and performance-enhancing drugs

Kinesis

A non-directional response in which the speed of movement or the rate of turning changes with the intensity of a stimulus. Receptors: transduction and the reflex arc

Kingdom

The largest of the taxonomic groups within a domain, and the level at which organisms are separated by features such as the presence of a nucleus, the material of the cell wall and the method of nutrition. The five kingdoms, species concepts and viruses

Knockout mouse

A mouse in which both alleles of a chosen gene have been deliberately inactivated, so that the gene's function can be inferred from what changes. Recombinant DNA technology

Krebs cycle

A closed series of reactions in the mitochondrial matrix in which an acetyl group is combined with a four-carbon acceptor and completely oxidised, releasing carbon dioxide and reducing coenzymes, and regenerating the acceptor. The Krebs cycle and oxidative phosphorylation

Kupffer cell

A macrophage fixed in a sinusoid, which engulfs bacteria and breaks down old red blood cells. The liver: detoxification, deamination and the making of urea

L

L-Dopa

The precursor of dopamine, which crosses the blood-brain barrier and is converted to dopamine in the brain; used to treat Parkinson's disease. The synapse: transmission, and why it runs one way

Lamella

One of the thin plates set at right angles along a gill filament, covered by an epithelium one cell thick with blood flowing just beneath it; the actual exchange surface. Gas exchange in insects, fish and plants

Latency

A state in which a viral genome remains in a host cell without new virus particles being made, until a trigger reactivates it. Prokaryotic cells and viruses

Latent heat of vaporisation

The energy needed to change one kilogram of a liquid into a gas without a change in temperature. Water and inorganic ions

Leading zeros

Never significant. 0.0043 has two significant figures, because the zeros only locate the decimal point. Handling data: units, scales, rates and uncertainty

Lidocaine

A local anaesthetic that blocks voltage-gated sodium channels, so no action potentials are produced in the neurones it reaches. The action potential and its propagation

Ligand

A molecule that binds specifically to a receptor, such as a hormone or a neurotransmitter. Membrane structure and the fluid mosaic model

Lignin

The tough, waterproof polymer of woody tissue, added to walls around an infection site over days to make the seal permanent. Plant disease and plant defences

Limiting factor

The factor that, of all those affecting a rate, is the one restricting it at that moment. Factors affecting enzyme rate: temperature, pH and concentration

Link reaction

The oxidative decarboxylation of pyruvate in the mitochondrial matrix, producing carbon dioxide, reduced NAD and a two-carbon acetyl group carried by coenzyme A. Glycolysis, ATP and the link reaction

Lipid

A biological molecule that is largely non-polar, insoluble in water and soluble in organic solvents such as ethanol. Lipids: triglycerides, phospholipids and ester bonds

Loading

Association of oxygen with haemoglobin, at the gas exchange surface. Haemoglobin and oxygen dissociation curves

Loading dye

A dense, coloured solution mixed with DNA samples so that they sink into the wells of a gel and their progress during the run can be seen. Recombinant DNA technology

Lobule

The repeating structural unit of the liver: a roughly hexagonal block of hepatocytes with a central vein at its axis. The liver: detoxification, deamination and the making of urea

Local currents

The movement of ions along the inside of an axon from a depolarised region to the resting membrane next to it. The action potential and its propagation

Long-day plant

A plant that flowers only when the night is shorter than its critical night length. Plant responses: tropisms, auxins and the classic experiments

Longitudinal section

A section cut along an organ, parallel to its long axis. Plan diagrams and tissue sections

Lymph

Tissue fluid that has drained into the lymphatic system; similar to tissue fluid but containing lymphocytes and, after a meal, more lipid, carried in chylomicrons. Blood vessels, tissue fluid and lymph

Lymph node

A swelling in a lymph vessel where lymphocytes collect and pathogens in the lymph are removed. Blood vessels, tissue fluid and lymph

Lymphocyte

A white blood cell about 9 µm across whose round nucleus almost fills the cell, responsible for the specific immune response. Plan diagrams and tissue sections

Lysosome

A vesicle containing hydrolytic enzymes, bounded by a single membrane, which digests worn-out organelles and engulfed material. Eukaryotic cell structure and organelles

Lytic cycle

Viral replication in which the virus attaches, injects or releases its nucleic acid, replicates it, assembles new particles and then bursts the host cell to release them. Prokaryotic cells and viruses

M

Macromolecule

A very large molecule with a high relative molecular mass, such as a polysaccharide, a protein, a nucleic acid or a triglyceride. Carbohydrate structure and function

Magnetic resonance imaging (MRI)

A scan that uses a strong magnetic field and radio waves to image soft tissue from the signals given out by hydrogen nuclei. Brain imaging, development and learning

Magnification

How many times larger the image is than the actual object. A ratio, with no units. Microscopy, magnification and resolution

Mammary-specific promoter

The promoter of a milk protein gene, placed in front of an introduced gene so that the gene is transcribed only in mammary gland cells during lactation. Pharming, recombinant medicines and synthetic biology

Mass drug administration

Giving a drug to everyone in an area, whether or not they are ill, to reduce transmission. Infectious disease: transmission and control

Mass transport system

A system in which substances are moved in bulk, in a fluid driven by a pump, over distances too great for diffusion to serve. The heart and cardiac cycle

Maximum sustainable yield

The largest sustainable yield available from a population. In the simple logistic model it occurs at about half the carrying capacity, where net growth is greatest; that position depends on the model's assumptions. Managing ecosystems sustainably

MDMA

The active substance in Ecstasy, which blocks serotonin reuptake and causes serotonin to be released into the cleft. The synapse: transmission, and why it runs one way

Mean

The total divided by the number of readings. Uses every value, and is what almost every biological calculation wants. Averages, spread and the null hypothesis

Mean generation time

The time a population takes to double during the log phase, equal to 1 ÷ k. Industrial biotechnology and fermentation

Median

The middle value when the readings are put in order. Unmoved by a single extreme value, which is why it is used for skewed data such as incomes, reaction times or the number of parasites per host. Averages, spread and the null hypothesis

Medulla oblongata

The region of the brain that receives impulses from the chemoreceptors and pressure receptors and adjusts heart rate through the autonomic nervous system. The heart and cardiac cycle

Memory cell

A long-lived B or T lymphocyte produced by clonal expansion. It secretes no antibody, but divides rapidly if the same antigen is met again: a memory B cell into plasma cells, a memory T cell into T-helper or T-killer cells of its own type. The specific immune response

Meristem

A plant tissue of unspecialised cells that continue to divide, from which specialised cells such as xylem vessels and sieve tube elements differentiate. Cell specialisation and biological organisation

Messenger RNA (mRNA)

A single-stranded copy of one gene, made in the nucleus and carrying codons to a ribosome in the cytoplasm. DNA and RNA structure

Meta-analysis

A statistical combination of the results of several studies of the same question into one estimate, with larger and more precise studies given more weight. Diet, energy and how to read a health claim

Metabolic rate

The rate at which an organism uses energy, and so the rate at which it must be supplied with oxygen and relieved of carbon dioxide. The heart and cardiac cycle

Metastasis

The spread of cells from a malignant tumour through the blood or lymph to form secondary tumours. Mutation, gene expression and cancer

Micropropagation

The production of very large numbers of genetically identical plants from a small piece of tissue, grown on sterile nutrient medium containing plant hormones. Cloning in plants and animals

Micropyle

The small gap in the integuments at one end of an ovule, through which a pollen tube usually enters. Sexual reproduction in flowering plants

Microtubule

A hollow cylinder of tubulin about 25 nm wide, along which motor proteins move vesicles and organelles, and which builds the spindle, cilia and flagella. Eukaryotic cell structure and organelles

Middle lamella

The layer, mainly calcium pectate, between the walls of neighbouring plant cells, which cements the cells together. Eukaryotic cell structure and organelles

Mitotic index

The proportion of cells in a sample that are visibly in mitosis. The cell cycle and mitosis

Mode

The most common value. The only one of the three that works for categories such as blood group or flower colour. Averages, spread and the null hypothesis

Monoclonal antibody

A population of identical antibody molecules secreted by a clone of identical cells, and therefore all specific to the same antigen. Antibodies, vaccination and the four kinds of immunity

Monocyte

The largest white blood cell of a smear, up to about 18 µm, with a kidney-shaped nucleus, which matures into a macrophage. Plan diagrams and tissue sections

Monomer

A small molecule from which a larger one is built by joining many of them together. Carbohydrate structure and function

Monosaccharide

A single sugar unit, the monomer from which disaccharides and polysaccharides are built. Carbohydrate structure and function

Monosomy

The presence of only one copy of a chromosome where a diploid cell should have a pair, as in 45,X. Meiosis and the sources of genetic variation

Morphological species concept

The definition of a species by shared observable structural features, used where interbreeding cannot be tested, as with fossils. The five kingdoms, species concepts and viruses

Motor protein

A protein such as kinesin or dynein that uses ATP to walk along a cytoskeletal fibre, carrying a vesicle or organelle with it. Eukaryotic cell structure and organelles

Multipotent

Able to develop into a limited range of cell types. Found in adult stem cells, such as those in bone marrow which produce the blood cells. Cell specialisation and biological organisation

Muscular artery

An artery further from the heart, such as the femoral or renal artery, whose wall holds more smooth muscle and which distributes blood to organs by vasoconstriction and vasodilation. Blood vessels, tissue fluid and lymph

Mutation

A change in the base sequence of DNA. DNA replication

Mycoprotein

Single-cell protein made from the hyphae of the fungus Fusarium venenatum, grown in continuous culture and textured to resemble meat. Industrial biotechnology and fermentation

Mycorrhiza

A mutualistic association between a fungus and a plant root, in which the fungus increases the surface area for absorbing water and mineral ions and receives organic compounds in return. Nutrient cycles: nitrogen, phosphorus and eutrophication

Myelin sheath

Layers of Schwann cell membrane wrapped round an axon, insulating it so that ions cannot cross the membrane beneath. The action potential and its propagation

Myeloma cell

A cancer cell of the bone marrow that divides indefinitely in culture; the lines used for fusion make no antibody of their own. Antibodies, vaccination and the four kinds of immunity

Myogenic

Contracting from within the muscle itself, without needing a nerve impulse to initiate the beat. The heart and cardiac cycle

N

Natural selection

The process by which individuals with phenotypes better suited to the environment reproduce more successfully, so the alleles responsible become more frequent. Natural selection: variation, selection and allele frequency

Nature

The contribution of an organism's genes to a characteristic, such as an ability of the brain. Brain imaging, development and learning

Necrosis

Death of a cell caused by injury or by conditions it cannot survive, in which the cell swells, the membrane breaks and the contents escape and cause inflammation. Homeobox genes, Hox genes and apoptosis

Negative feedback

A control mechanism in which a deviation from the norm triggers a response that reverses the deviation. Hormonal communication and negative feedback

Nephron

The functional unit of the kidney: a renal capsule and its tubule, running from the cortex into the medulla and back. Osmoregulation and kidney function

Neuromuscular junction

The synapse between a motor neurone and a skeletal muscle fibre. The synapse: transmission, and why it runs one way

Neurotransmitter

A chemical released from a presynaptic neurone that diffuses across the cleft and binds to receptors on the postsynaptic membrane. The synapse: transmission, and why it runs one way

Neutrophil

A phagocytic white blood cell about 12 µm across with a lobed nucleus and granular cytoplasm. Plan diagrams and tissue sections

Niche

The role a species plays in its community: all the conditions it can tolerate and all the resources it uses, including where it lives, what it eats, what eats it and when it is active. Populations and their limits

Node of Ranvier

A gap in the myelin sheath between adjacent Schwann cells, where the axon membrane can depolarise. The action potential and its propagation

Non-competitive inhibitor

A molecule that binds to a site other than the active site, altering the enzyme's tertiary structure so that the active site is no longer complementary to the substrate. Enzyme inhibition and metabolic control

Non-disjunction

The failure of a homologous pair to separate at anaphase I, or of sister chromatids to separate at anaphase II, giving gametes with one chromosome too many or too few. Meiosis and the sources of genetic variation

Non-overlapping

Describing a code in which each base is part of one triplet only and triplets are read in sequence. The genetic code and transcription

Non-self

Antigens not present on the organism's own cells: pathogens, transplanted tissue, cancerous cells with altered surface proteins, and toxins. The specific immune response

Non-specific defence

A defence that acts against any pathogen, without recognising which one it is, and acts at the same speed every time. Pathogens and non-specific defences

Nuclear envelope

The double membrane surrounding the nucleus, continuous with the endoplasmic reticulum and perforated by nuclear pores. Eukaryotic cell structure and organelles

Nucleoid

The region of a prokaryotic cell occupied by its circular DNA, with no surrounding membrane. Prokaryotic cells and viruses

Nucleolus

A dense region within the nucleus where ribosomal RNA is made and ribosomes are assembled. Eukaryotic cell structure and organelles

Nucleotide

A phosphate group, a pentose sugar and a nitrogenous base joined together. DNA and RNA structure

Null hypothesis

A statement that there is no significant difference, no significant association or no significant correlation between the variables, and that any difference observed is due to chance. Averages, spread and the null hypothesis

Nurture

The contribution of the environment and experience, including conditions in the womb, to a characteristic. Brain imaging, development and learning

O

Obstructive lung disease

A disease that narrows the airways and raises the resistance to air flow, so FEV₁ falls more than FVC and the ratio drops. Asthma and emphysema are examples. Gas exchange in mammals: lungs and ventilation

Ocular dominance columns

Alternating bands of the visual cortex, each responding mainly to the left eye or mainly to the right eye. Brain imaging, development and learning

Oestrogen response element

A specific base sequence near a target gene to which an oestrogen-receptor complex binds, increasing the rate of transcription. Control of gene expression

Oncogene

A mutated or over-expressed proto-oncogene whose product stimulates division continuously. Mutation, gene expression and cancer

Oncotic pressure

The inward pull created by plasma proteins, which stay in the capillary and lower the water potential of the blood relative to the tissue fluid. Also called colloid osmotic pressure. Blood vessels, tissue fluid and lymph

Open circulatory system

A circulation in which blood leaves the vessels and bathes the organs directly in a body cavity before draining back to the heart. Blood vessels, tissue fluid and lymph

Operculum

The bony flap covering the gills of a bony fish, acting as a valve and part of the pump that maintains the flow of water. Gas exchange in insects, fish and plants

Optimum

The value of a variable at which an enzyme's rate is highest. Factors affecting enzyme rate: temperature, pH and concentration

Organ

A structure made of several different tissues working together to perform a particular function. Cell specialisation and biological organisation

Organ system

A group of organs working together to perform an overall life function. Cell specialisation and biological organisation

Organelle

A structure inside a cell with a distinct function, usually but not always surrounded by its own membrane. Eukaryotic cell structure and organelles

Osmoregulation

The control of the water potential of the blood and body fluids. Osmoregulation and kidney function

Osmosis

The net movement of water molecules from a solution of higher water potential to a solution of lower water potential, through a partially permeable membrane. Diffusion and osmosis

Osteoarthritis

A joint disease in which the articular cartilage is worn away, so that bone surfaces rub, causing pain, swelling and stiffness. Exercise, sports medicine and performance-enhancing drugs

Overtraining

A fall in performance, with persistent fatigue and frequent infections, caused by heavy training without enough recovery. Exercise, sports medicine and performance-enhancing drugs

Oxidation

In respiration, most easily read as the loss of hydrogen. A substrate that hands hydrogen to NAD has been oxidised; the NAD has been reduced. Glycolysis, ATP and the link reaction

Oxidative phosphorylation

The formation of ATP from ADP and inorganic phosphate using energy from the electron transport chain, with oxygen as the final electron acceptor. The Krebs cycle and oxidative phosphorylation

Oxygen consumption

The volume of oxygen the body takes up per minute. On a closed-circuit spirometer trace with soda lime present, it is the fall per minute of a line through the troughs. Gas exchange in mammals: lungs and ventilation

P

P wave

The deflection produced by depolarisation of the atria, which atrial contraction follows. The heart and cardiac cycle

Partial pressure of oxygen

The contribution oxygen makes to the total pressure of a gas mixture, measured in kPa; a measure of how much oxygen is present. Haemoglobin and oxygen dissociation curves

Passive

Requiring no energy from ATP; driven by the kinetic energy of the particles themselves. Diffusion and osmosis

Pathogen

A biological agent that causes disease in a host, including cellular organisms such as bacteria, fungi and protoctists and acellular agents such as viruses. Pathogens and non-specific defences

Pedigree diagram

A family tree showing how one characteristic is inherited through several generations, with males as squares, females as circles and affected people shaded. Monohybrid inheritance and genetic diagrams

Peer review

The checking of a scientific paper's methods, results and conclusions by other scientists in the same field before it is published. Infectious disease: transmission and control

Pentose

A sugar whose molecules contain five carbon atoms. Ribose, in RNA, and deoxyribose, in DNA, are the pentoses of nucleotides. The Calvin cycle

Percentage saturation

The proportion of the oxygen-binding sites on haemoglobin that are actually occupied. Haemoglobin and oxygen dissociation curves

Peripheral nervous system

The neurones outside the brain and spinal cord, carrying impulses into and out of the CNS. Organisation of the nervous system

Permeability

How readily a membrane allows a substance to cross it. Membrane structure and the fluid mosaic model

Personalised medicine

Choosing which drug to give a patient, and at what dose, using their genotype or the genotype of their tumour, rather than treating everyone with the same diagnosis alike; also called stratified medicine. Genome analysis, genetic screening and gene therapy

Pfr

The active form of phytochrome, which absorbs far-red light, about 730 nm, and is converted by it, and slowly in darkness, into Pr. Plant responses: tropisms, auxins and the classic experiments

Phagocytosis

The engulfing and hydrolysis of a pathogen or other foreign material by a phagocyte, forming a phagosome that fuses with a lysosome. Pathogens and non-specific defences

Phagosome

The vesicle formed when a phagocyte's cell-surface membrane closes around an engulfed pathogen. Pathogens and non-specific defences

Pharmacogenomics

The study of how a person's genotype affects the way they respond to a drug, including how quickly they break it down. Genome analysis, genetic screening and gene therapy

Pharming

The use of genetically modified organisms, including plants and animals, to produce pharmaceutical substances; in livestock, classically in milk. Pharming, recombinant medicines and synthetic biology

Phase I trial

The first test of a drug in humans, on a small number of healthy volunteers, to find whether it is safe, what dose is tolerated and how the body handles it. Antibodies, vaccination and the four kinds of immunity

Phase II trial

A test of a drug on a few hundred patients with the condition, to find whether it works, at what dose, and its side effects. Antibodies, vaccination and the four kinds of immunity

Phase III trial

A large randomised, usually double-blind trial on thousands of patients, comparing the drug with a placebo or the best existing treatment. Antibodies, vaccination and the four kinds of immunity

Phenetic classification

Grouping organisms by observable similarity, without regard to how they are related. Classification, phylogeny and molecular evidence

Phenotype

The observable characteristics of an organism, produced by its genotype interacting with its environment. Monohybrid inheritance and genetic diagrams

Pheromone

A chemical signal released by an organism that changes the behaviour of another; used loosely in plant biology for the volatile signals plants release when attacked. Plant disease and plant defences

Phosphodiester bond

The covalent bond joining the phosphate group of one nucleotide to the sugar of the next, formed by condensation. DNA and RNA structure

Phospholipid

A lipid in which one of the three fatty acids of a triglyceride is replaced by a phosphate group, giving a hydrophilic head and two hydrophobic tails. Membrane structure and the fluid mosaic model

Phosphorylation

The addition of a phosphate group to a molecule. The phosphate released from ATP is often added to another molecule, making it more reactive. Glycolysis, ATP and the link reaction

Photoionisation

The loss of an electron from a chlorophyll molecule after it absorbs light energy, leaving the molecule positively charged. Cambridge International calls the step photoactivation. Chloroplast structure and the light-dependent reactions

Photolysis

The splitting of water using light energy, giving electrons, protons and oxygen. Chloroplast structure and the light-dependent reactions

Photomorphogenesis

The control of a plant's growth and development by light, such as the de-etiolation of a seedling that reaches light. Plant responses: tropisms, auxins and the classic experiments

Photoperiodism

The response of a plant to the relative lengths of day and night, such as the timing of flowering. Plant responses: tropisms, auxins and the classic experiments

Photophosphorylation

The synthesis of ATP from ADP and inorganic phosphate using energy that originally came from light. Chloroplast structure and the light-dependent reactions

Photosynthometer

Apparatus that collects the gas given off by an aquatic plant at the mouth of a capillary tube, so its volume can be measured against a scale. Limiting factors and primary productivity

Photosystem

A cluster of pigment molecules held in a thylakoid membrane, funnelling absorbed energy to the chlorophyll a at its reaction centre. Chloroplast structure and the light-dependent reactions

Phylogenetic classification

Grouping organisms so that each group contains a common ancestor and all of its descendants. Classification, phylogeny and molecular evidence

Phylogeny

The evolutionary history of a group: which lineages split from which, and in what order. Classification, phylogeny and molecular evidence

Physical digestion

The mechanical breakdown of food into smaller pieces, increasing the surface area available to digestive enzymes without hydrolysing anything. Digestion and absorption in the human gut

Phytochrome

A photoreceptor protein in plants that exists in two interconvertible forms, Pr and Pfr, and controls responses to the presence, duration and colour of light. Plant responses: tropisms, auxins and the classic experiments

Pioneer species

The first species to colonise a bare surface, adapted to survive there when almost nothing else can. Succession and environmental change

Pit

A thin region of a plant cell wall where no secondary wall has been laid down, usually matched by a pit in the neighbouring cell, allowing water and solutes to pass between them. Eukaryotic cell structure and organelles

Pituitary gland

The endocrine gland hanging beneath the hypothalamus, whose two lobes release hormones under its control. Organisation of the nervous system

Placebo

An inactive substance made to look and be taken exactly like the drug being tested, given to the control group of a trial. Antibodies, vaccination and the four kinds of immunity

Plagioclimax

A community held at a stage short of its climax by a repeated interference, usually human or from grazing animals. Succession and environmental change

Plan diagram

A low-power drawing of a section that shows the positions and boundaries of tissues, in correct proportion, without drawing any individual cells. Plan diagrams and tissue sections

Plant-made pharmaceutical

A medicine, usually a protein, produced by genetically modified plants or plant cells carrying the gene for it. Pharming, recombinant medicines and synthetic biology

Plasma cell

A differentiated B cell that secretes large quantities of a single antibody into blood and lymph. Most survive only a few days; a minority settle in the bone marrow and secrete for years. The specific immune response

Plasmid

A small circular molecule of DNA, separate from the main chromosome, which replicates independently and often carries genes such as antibiotic resistance. Prokaryotic cells and viruses

Plasmolysis

The pulling away of the cell surface membrane from the cell wall as water leaves a plant cell. Diffusion and osmosis

Plastid

Any of the family of plant organelles, including chloroplasts and amyloplasts, that have a double-membrane envelope and their own DNA and develop from proplastids. Eukaryotic cell structure and organelles

Plateau

The region of a rate graph where increasing the factor on the horizontal axis no longer increases the rate, because something else has become limiting. Limiting factors and primary productivity

Pluripotent

Able to develop into any cell type in the body, but not into the placenta. Found in embryonic stem cells in the blastocyst. Cell specialisation and biological organisation

Point quadrat

A frame holding a row of pins lowered vertically through vegetation; each species a pin touches is recorded, and hits per pin give percentage cover. Succession and environmental change

Polar molecule

A molecule with an uneven distribution of charge, so that one part is slightly negative and another slightly positive. Water and inorganic ions

Polarised

Describes a membrane with a potential difference across it; at rest the axon membrane is polarised with the inside negative. The action potential and its propagation

Pollination

The transfer of pollen grains from an anther to a stigma. Sexual reproduction in flowering plants

Polygenes

A group of different genes, each with a small additive effect, that together control one characteristic and give it a continuous distribution. Continuous and discontinuous variation

Polylactic acid

A plastic made by fermenting glucose from plant starch into lactic acid and polymerising it; abbreviated to PLA. Carbohydrate structure and function

Polymer

A large molecule made of many similar monomers joined together. Carbohydrate structure and function

Polynucleotide

A chain of nucleotides joined by phosphodiester bonds between the sugar of one and the phosphate of the next. DNA and RNA structure

Polypeptide

A chain of amino acids joined by peptide bonds. Protein structure and function

Polysaccharide

Many monosaccharides joined by glycosidic bonds into one large molecule. Carbohydrate structure and function

Polysomy

The presence of one or more extra copies of a chromosome in a cell, as in trisomy 21. Meiosis and the sources of genetic variation

Polyspermy

Fusion of more than one sperm with a single egg, which gives a cell with too many chromosome sets to develop. Gametes, fertilisation and early development

Population

A group of organisms of the same species living in the same place at the same time and able to interbreed. Population genetics and Hardy-Weinberg equilibrium

Positive feedback

A mechanism in which a deviation from the norm triggers a response that increases the deviation. Hormonal communication and negative feedback

Positron emission tomography (PET)

A scan that locates a radioactive tracer, usually labelled glucose, showing where cells are most metabolically active. Brain imaging, development and learning

Post-mortem interval

The time that has passed since death. Succession and environmental change

Post-translational modification

Changes made to a polypeptide after translation, such as cutting out sections or adding carbohydrate or lipid groups, carried out in the endoplasmic reticulum and the Golgi apparatus. Folding is not one of them: it begins on the ribosome and is finished in the endoplasmic reticulum. Translation and protein synthesis

Posterior pituitary

The lobe that stores and releases ADH and oxytocin, both made in the hypothalamus. Hormonal communication and negative feedback

Potency

The range of cell types a stem cell is still able to become. Stem cells and cell potency

Pr

The form of phytochrome that absorbs red light, about 660 nm, and is converted by it into Pfr. Plant responses: tropisms, auxins and the classic experiments

Pre-mRNA

The immediate product of transcription in a eukaryote, containing both exons and introns. The genetic code and transcription

Precise

Repeat readings close to one another. Precision says nothing at all about being right. Variables and controls: what an experiment can show

Pressure receptor

A receptor in an artery wall that detects a change in blood pressure; also called a baroreceptor. The heart and cardiac cycle

Primary structure

The sequence of amino acids in a polypeptide, held by peptide bonds. Protein structure and function

Progeny testing

Judging a breeding animal by measuring the performance of its offspring, used where the characteristic cannot be measured in the animal itself. Artificial selection and selective breeding

Prokaryotic cell

A cell with no nucleus and no membrane-bound organelles, whose DNA is a single circular molecule free in the cytoplasm. Prokaryotic cells and viruses

Promoter

A region of DNA at the start of a gene to which RNA polymerase and transcription factors bind. Control of gene expression

Prophage

The DNA of a phage such as lambda inserted into the chromosome of its host bacterium and copied with it at each division. Prokaryotic cells and viruses

Prosthesis

An artificial device that replaces a missing or damaged part of the body, such as a joint replacement or an artificial limb. Exercise, sports medicine and performance-enhancing drugs

Prosthetic group

A cofactor tightly bound to an enzyme or other protein, and which may be organic, such as the haem group of catalase, or inorganic, such as the zinc ion of carbonic anhydrase. Cofactors, coenzymes and where enzymes work

Protein

One or more polypeptides folded into a specific three-dimensional shape. Protein structure and function

Proto-oncogene

A normal gene whose product stimulates cell division in response to a growth signal. Mutation, gene expression and cancer

Proxy record

Evidence preserved from the past, such as pollen in peat or tree rings, from which a condition that was not measured directly, such as temperature, is inferred. The carbon cycle and climate change

Publication bias

The tendency for studies with positive or striking results to be published more often than studies that found nothing, so the published literature overstates effects. Diet, energy and how to read a health claim

Pupil reflex

The automatic change in pupil diameter with light intensity: constriction in bright light, dilation in dim light. Organisation of the nervous system

Pure culture

A culture containing only one species, grown from a single isolated colony. Industrial biotechnology and fermentation

Purine

A base built from two fused rings: adenine and guanine. DNA and RNA structure

Purkyne fibres

Conducting fibres running from the bundle of His up through the ventricle walls, so contraction begins at the apex. The heart and cardiac cycle

Pyramid of biomass

A diagram of the dry mass of organisms at each trophic level per unit area, at one time. Limiting factors and primary productivity

Pyramid of energy

A diagram of the energy passing through each trophic level per unit area per year; it is never inverted. Limiting factors and primary productivity

Pyramid of numbers

A diagram of the number of individual organisms at each trophic level in an area. Limiting factors and primary productivity

Pyrimidine

A base built from a single ring: cytosine, thymine and, in RNA, uracil. DNA and RNA structure

Q

QRS complex

The large deflection produced by depolarisation of the ventricles, large because the ventricle walls hold most of the heart's muscle. The heart and cardiac cycle

Quaternary structure

The arrangement of two or more polypeptides, and any non-protein groups, in a functioning protein. Protein structure and function

R

Radial muscles

Smooth muscle fibres running outwards from the pupil in the iris; their contraction dilates the pupil. Organisation of the nervous system

Randomisation

Assigning subjects or sample positions by chance rather than by choice, so that variables not identified in advance tend to balance out between the groups on average. Variables and controls: what an experiment can show

Randomised controlled trial

Participants are allocated to an intervention group or a control group, often given a placebo, and the outcomes are compared. Allocation is by chance, so the groups tend to be balanced on every variable, including the ones nobody thought to measure. Diet, energy and how to read a health claim

Receptor

A protein, usually a glycoprotein in the cell surface membrane, whose binding site is complementary to one ligand; only cells carrying the receptor respond to the signal. Membrane structure and the fluid mosaic model

Recessive

An allele whose effect appears only when no dominant allele is present: in a homozygote, or in a male for a gene on the X chromosome. Monohybrid inheritance and genetic diagrams

Recombinant

A chromatid, or an offspring, carrying a combination of alleles that neither parental chromosome carried. Meiosis and the sources of genetic variation

Recombinant DNA

DNA made by joining together sequences from two different organisms. Recombinant DNA technology

Recombinant medicine

A protein drug produced by cells carrying an introduced copy of the human gene, rather than extracted from blood or tissue. Pharming, recombinant medicines and synthetic biology

Recombination frequency

The percentage of offspring of a test cross that are recombinant. It rises with the distance between linked loci and does not exceed 50%. Dihybrid inheritance, linkage, epistasis and the chi-squared test

Redox indicator

A dye that changes colour when it is reduced or oxidised, such as methylene blue, which is blue when oxidised and colourless when reduced. Anaerobic respiration and the respiratory quotient

Reduction division

A division that halves the chromosome number. Meiosis I is one; meiosis II is not. Meiosis and the sources of genetic variation

Reflex

A rapid, automatic response to a stimulus, involving a fixed pathway of neurones and no conscious decision. Receptors: transduction and the reflex arc

Reflex arc

The pathway of neurones taken by a reflex. In the examined withdrawal reflex: receptor, sensory neurone, relay neurone, motor neurone, effector. Receptors: transduction and the reflex arc

Reforestation

Replanting or allowing the regrowth of trees on land where forest has recently been cleared. Managing ecosystems sustainably

Refractory period

The short time after an action potential during which the membrane cannot produce another, or can do so only with a larger stimulus. The action potential and its propagation

Relative risk

The risk in the exposed group divided by the risk in the unexposed group; 1 means no association. Diet, energy and how to read a health claim

Renal pelvis

The funnel-shaped space at the centre of a kidney, into which the collecting ducts drain and which narrows into the ureter. Osmoregulation and kidney function

Repeatable

The same person, using the same method and the same apparatus, gets the same results again. Variables and controls: what an experiment can show

Repolarisation

The return of the membrane potential towards the resting value after depolarisation, as potassium ions leave. The action potential and its propagation

Repressible enzyme

An enzyme synthesised continuously until the product of its pathway accumulates and activates a repressor, switching transcription of its gene off. Control of gene expression

Reproducible

Somebody else, or the same person using different apparatus or a different method, gets the same results. Variables and controls: what an experiment can show

Reproductive cloning

Cloning in which the embryo produced is implanted into a surrogate and develops into a whole new individual. Cloning in plants and animals

Reproductive isolation

The state in which two populations cannot exchange alleles because of features of the organisms themselves, such as courtship, breeding time or place, gamete compatibility or hybrid fertility, so they stay separate even where they meet. Speciation and reproductive isolation

Residual volume

The volume left in the lungs after the hardest possible breath out, about 1.2 dm³. It cannot be measured with a spirometer, because it never leaves the lungs. Vital capacity plus residual volume is the total lung capacity. Gas exchange in mammals: lungs and ventilation

Resolution

The minimum distance between two points at which they can still be distinguished as separate. A length, and smaller is better. Microscopy, magnification and resolution

Resolution (of an instrument)

The smallest change an instrument can register: 0.01 g on a two-decimal-place balance, 1 mm on a metre rule. Variables and controls: what an experiment can show

Respiratory minute ventilation

The volume of air breathed in or out per minute: tidal volume × breathing rate, in dm³ min⁻¹. Also called pulmonary ventilation rate. Gas exchange in mammals: lungs and ventilation

Resting potential

The potential difference across the membrane of a neurone that is not conducting an impulse, about −70 mV inside relative to outside. The action potential and its propagation

Restrictive lung disease

A disease that stops the lungs expanding fully, so FVC and FEV₁ fall together and the ratio stays normal or rises. Pulmonary fibrosis is an example. Gas exchange in mammals: lungs and ventilation

Retinal convergence

Several rods connecting to one bipolar neurone, so their generator potentials are summed. Receptors: transduction and the reflex arc

Retting

Soaking or leaving plant stems in damp conditions so that microorganisms digest the pectins binding the fibres to the tissue around them, and the fibres can be separated. Carbohydrate structure and function

Reverse transcriptase

An enzyme carried by retroviruses such as HIV which catalyses the synthesis of DNA from an RNA template. Prokaryotic cells and viruses

Rhodopsin

The light-sensitive pigment in rod cells, broken down by low light intensities into retinal and opsin. Receptors: transduction and the reflex arc

Ribosomal RNA (rRNA)

RNA which, with proteins, forms the two subunits of a ribosome and catalyses the formation of peptide bonds. DNA and RNA structure

Ribosome

A structure of ribosomal RNA and protein, in two subunits, with no membrane, which is the site of translation. 80S in eukaryotic cytoplasm, 70S in prokaryotes and in mitochondria and chloroplasts. Eukaryotic cell structure and organelles

Rights-based ethics

The ethical standpoint that some actions are wrong whatever their consequences, because they violate a right or break a duty. Brain imaging, development and learning

Rigor mortis

The stiffening of muscles after death, caused by myosin heads staying bound to actin once ATP has run out. Succession and environmental change

Risk

How likely it is that a hazard causes harm, and how serious the harm would be, under the conditions in which the work is done. Control measures reduce the risk; the hazard stays the same. Variables and controls: what an experiment can show

Risk factor

Anything that increases the probability that a person will develop a disease, such as smoking for lung cancer and emphysema. Gas exchange in mammals: lungs and ventilation

RNA interference

The inhibition of gene expression by small RNA molecules binding to complementary mRNA. Control of gene expression

Root hair cell

An epidermal cell of the root extended into a long narrow projection, giving a large surface area for the absorption of water and mineral ions. Xylem and the transpiration stream

Rough endoplasmic reticulum

Flattened membrane sacs with ribosomes attached, which make and transport proteins for export or for membranes. Eukaryotic cell structure and organelles

Rubisco

RuBP carboxylase-oxygenase, the enzyme that joins carbon dioxide to RuBP. Slow, able to bind oxygen in place of carbon dioxide, and probably the most abundant protein on Earth. The Calvin cycle

RuBP

Ribulose bisphosphate, a five-carbon compound; the molecule that accepts carbon dioxide. The Calvin cycle

S

Saltatory conduction

Conduction along a myelinated axon in which depolarisation occurs only at the nodes of Ranvier, so the impulse jumps from node to node. The action potential and its propagation

Saprobiont

An organism that obtains its energy from dead material by secreting enzymes onto it and absorbing the soluble products. Nutrient cycles: nitrogen, phosphorus and eutrophication

Saprotrophic nutrition

Feeding by secreting enzymes onto dead organic material outside the body and absorbing the soluble products, which is how fungi and many bacteria feed. The five kingdoms, species concepts and viruses

Sarcomere

The functional unit of a myofibril, running from one Z line to the next. Muscles and movement: the sliding-filament model

Sarcoplasmic reticulum

The membrane system inside a muscle fibre that stores and releases calcium ions. Muscles and movement: the sliding-filament model

Scientific conference

A meeting at which researchers present and discuss their work, often before publication, and have it questioned by others in the field. The carbon cycle and climate change

Scientific journal

A publication in which researchers report their methods, data and conclusions, usually after peer review, so that others can check and repeat the work. The carbon cycle and climate change

Sclerenchyma fibre

A long, narrow, dead plant cell with tapering ends and a thick lignified secondary wall around a narrow lumen, whose function is support. Xylem and the transpiration stream

Second messenger

A substance inside a cell whose concentration changes when a hormone binds a receptor on the outside, carrying the signal onwards; calcium ions do this job in the guard cell. Transpiration and stomatal control

Secondary structure

The regular coiling or pleating of the backbone into an α-helix or β-pleated sheet, held by hydrogen bonds between the C=O of one peptide group and the N–H of another. Protein structure and function

Secondary thickening

Extra layers of cell wall laid down inside the primary wall once a cell has stopped growing, made of cellulose microfibrils at different angles in successive layers and usually impregnated with lignin. Xylem and the transpiration stream

Selection pressure

An environmental factor that causes individuals with certain phenotypes to survive and reproduce more successfully than others. Natural selection: variation, selection and allele frequency

Selective medium

A culture medium containing a substance, or lacking a nutrient, so that only certain kinds of microorganism can grow on it. Industrial biotechnology and fermentation

Selective reabsorption

The return of useful substances from the filtrate to the blood, by transport proteins in the tubule wall. Osmoregulation and kidney function

Self

Antigens present on an organism's own cells, which its lymphocytes do not respond to. The specific immune response

Self-renewal

Division that produces at least one daughter cell identical to the parent stem cell, so the population of stem cells is maintained. Stem cells and cell potency

Semi-conservative replication

Replication in which the two strands of the parent molecule separate and each acts as a template, so every daughter molecule contains one original strand and one newly synthesised strand. DNA replication

Semilunar valves

The valves at the base of the aorta and the pulmonary artery, preventing backflow into the ventricles. The heart and cardiac cycle

Sensory adaptation

A fall in the response of a receptor to a constant stimulus, so that fewer impulses are sent along the sensory neurone. Brain imaging, development and learning

Sensory receptor

A cell or structure that detects one kind of stimulus and converts it into a change in membrane potential. Receptors: transduction and the reflex arc

Sentience

The capacity to have experiences that feel good or bad to the animal itself, such as pain, distress or comfort. Variables and controls: what an experiment can show

Seral stage

One identifiable community in the sequence of a succession; the whole sequence is a sere. Succession and environmental change

Serial dilution

A series of dilutions in which each step dilutes the previous one by the same factor, usually tenfold. Industrial biotechnology and fermentation

Serotonin

A neurotransmitter in the brain involved in mood, sleep and appetite; an imbalance of it contributes to depression. The synapse: transmission, and why it runs one way

Short-day plant

A plant that flowers only when the night is longer than its critical night length. Plant responses: tropisms, auxins and the classic experiments

Sieve plate

The perforated end wall between two sieve tube elements, through which the sap passes. Phloem translocation and the mass-flow hypothesis

Sieve tube element

A living cell of the phloem, joined end to end with others into a tube; at maturity it has no nucleus, few organelles and only a thin layer of cytoplasm around the edge. Phloem translocation and the mass-flow hypothesis

Significance level

The probability threshold, conventionally 0.05, below which the null hypothesis is rejected. Averages, spread and the null hypothesis

Significant figures

The digits in a measurement that carry information about its size, counted from the first non-zero digit. Handling data: units, scales, rates and uncertainty

Silent mutation

A base substitution that produces a triplet coding for the same amino acid, so the primary structure of the polypeptide is unchanged. Mutation, gene expression and cancer

Simple diffusion

Diffusion directly through the phospholipid bilayer, available to small non-polar molecules such as oxygen and carbon dioxide. Diffusion and osmosis

Single-cell protein

Protein-rich food produced from cultured microorganisms, eaten by humans or fed to livestock. Industrial biotechnology and fermentation

Sinoatrial node

The patch of tissue in the right atrium wall that sets the rhythm of the heart. The heart and cardiac cycle

Sinusoid

A wide, leaky capillary running from the edge of a lobule to its central vein, carrying mixed blood from the hepatic artery and hepatic portal vein. The liver: detoxification, deamination and the making of urea

siRNA

Small interfering RNA: short double-stranded RNA fragments, one strand of which guides a protein complex to a complementary mRNA so that the mRNA is broken down and cannot be translated. Control of gene expression

Smooth endoplasmic reticulum

Membrane sacs without ribosomes, which synthesise lipids and steroids and store calcium ions. Eukaryotic cell structure and organelles

Smooth muscle

Involuntary muscle in the walls of hollow organs and vessels. In the airway wall its contraction narrows the lumen, and it is the tissue that contracts inappropriately during an asthma attack. Gas exchange in mammals: lungs and ventilation

Sodium-potassium pump

A carrier protein in the axon membrane that uses ATP to move three sodium ions out for every two potassium ions in. The action potential and its propagation

Somatic cell

Any body cell of a multicellular organism other than a gamete or the cells that produce gametes. Cloning in plants and animals

Somatic cell nuclear transfer

The transfer of a nucleus from a body cell into an enucleated egg cell, which is then stimulated to divide as an embryo genetically identical to the nucleus donor. Cloning in plants and animals

Somatic nervous system

The part of the peripheral system carrying impulses to skeletal muscle, under voluntary control. Organisation of the nervous system

Spatial summation

Summation of inputs arriving at the same time from several presynaptic neurones. The synapse: transmission, and why it runs one way

Specialised cell

A cell whose structure is adapted to one particular function. Cell specialisation and biological organisation

Speciation

The formation of a new species from an existing one, in which gene flow between populations is reduced or stopped, their gene pools diverge, and they become reproductively isolated. Speciation and reproductive isolation

Species

A group of organisms with similar characteristics that can interbreed to produce fertile offspring, and which are reproductively isolated from other such groups. Speciation and reproductive isolation

Species diversity

Richness and evenness together, usually expressed as an index. Measuring biodiversity

Species evenness

How equally the individuals are shared out between the species in a community. Measuring biodiversity

Species richness

The number of different species in a community. Measuring biodiversity

Specific heat capacity

The energy needed to raise the temperature of one kilogram of a substance by one kelvin. Water and inorganic ions

Specificity

The property of an enzyme that it catalyses only one reaction, or one class of reaction, because only a substrate complementary to its active site can bind. Enzyme action and specificity

Spindle

The framework of protein microtubules built across the cell before the chromosomes are moved, running from pole to pole and attaching to every centromere; shortening the fibres is what separates the chromatids. The cell cycle and mitosis

Spiracle

A pore in the body wall of an insect, guarded by valves, through which air enters and leaves the tracheal system. Gas exchange in insects, fish and plants

Splicing

The removal of introns from pre-mRNA and the joining of exons to produce mature mRNA. The genetic code and transcription

Squamous epithelium

An epithelium of very flattened cells, giving a wall a fraction of a micrometre thick; the lining of an alveolus. Gas exchange in mammals: lungs and ventilation

SSRI

Selective serotonin reuptake inhibitor: an antidepressant that blocks the reuptake of serotonin from the synaptic cleft. The synapse: transmission, and why it runs one way

Stage micrometer

A slide bearing a scale of known length, used to calibrate an eyepiece graticule for one particular objective lens. Microscopy, magnification and resolution

Stamen

The male part of a flower: a filament carrying an anther, in which pollen grains are made. Sexual reproduction in flowering plants

Standard part

A DNA sequence with a defined function, catalogued and designed to be assembled with any other standard part. Pharming, recombinant medicines and synthetic biology

Starch agar assay

A test for amylase in which tissue is placed on agar containing starch; after iodine is added, the size of the clear zone where the starch has been hydrolysed measures the amylase produced. Plant responses: tropisms, auxins and the classic experiments

Statolith

A dense, starch-filled organelle that settles to the lower side of a cell and so detects gravity. Plant responses: tropisms, auxins and the classic experiments

Stem cell

An unspecialised cell that can divide repeatedly to produce more cells like itself, and can differentiate into one or more specialised types. Stem cells and cell potency

Stem rust

A disease of wheat and other cereals caused by the fungus Puccinia graminis, spread by wind-borne spores and entering through stomata. Pathogens and non-specific defences

Steroid hormone

A lipid-soluble hormone that crosses the cell-surface membrane by simple diffusion and acts on an intracellular receptor. Control of gene expression

Stimulus

A detectable change in the internal or external environment. Receptors: transduction and the reflex arc

Stoma

A pore in the epidermis, bounded by two guard cells, through which gases and water vapour pass. Transpiration and stomatal control

Streak plate

An agar plate on which a culture has been spread in a series of areas, each with fewer cells than the last, so that single colonies grow in the final areas. Industrial biotechnology and fermentation

Stroma

The fluid surrounding the thylakoids, containing the enzymes of the light-independent stage, along with DNA and 70S ribosomes. Chloroplast structure and the light-dependent reactions

Substitution

A mutation in which one base is replaced by another. DNA replication

Substrate

The molecule on which an enzyme acts. Enzyme action and specificity

Substrate-level phosphorylation

The direct transfer of a phosphate group from a substrate molecule onto ADP, as opposed to ATP made using a proton gradient. Glycolysis, ATP and the link reaction

Succession

The change over time in the species occupying an area, in which each community alters the environment and makes it more suitable for the community that follows. Succession and environmental change

Surface area to volume ratio

The exchange surface available per unit of volume; it falls as an object gets larger. The heart and cardiac cycle

Sustainability

Using resources to meet present needs in a way that does not reduce the ability of future generations to meet theirs. Carbohydrate structure and function

Sustainable management

Using an ecosystem so that its resources are still available to future generations, and the ecosystem itself is not degraded. Managing ecosystems sustainably

Sustainable resource

A resource used no faster than it is replaced, so that it remains available to future generations. Managing ecosystems sustainably

Sustainable yield

The amount that can be harvested from a population in a given period without reducing the size of that population, so the same harvest can be repeated indefinitely. Managing ecosystems sustainably

Symplast pathway

Movement of water through the cytoplasm of the cells, passing from one to the next through plasmodesmata. Xylem and the transpiration stream

Synapse

The junction between the end of one neurone and another neurone or an effector cell, across which chemical transmission occurs. The synapse: transmission, and why it runs one way

Synthetic biology

The design and construction of new biological parts, devices and pathways, and the redesign of existing organisms, treating DNA as a specifiable engineering material. Pharming, recombinant medicines and synthetic biology

T

T wave

The deflection produced by repolarisation of the ventricles. The heart and cardiac cycle

Tannin

A phenolic compound in leaves and bark that binds proteins, inactivating digestive enzymes and making the tissue bitter and hard to digest. Plant disease and plant defences

Target cell

A cell with receptors complementary to a particular hormone, and therefore the only kind of cell that responds to it. Hormonal communication and negative feedback

Taxis

A directional movement in response to a directional stimulus, towards it (positive) or away from it (negative). Receptors: transduction and the reflex arc

Taxon

Any one group in the taxonomic hierarchy: Mammalia, Primates and Homo are all taxa. Classification, phylogeny and molecular evidence

Taxonomy

The naming and classifying of organisms into a hierarchy of groups. Classification, phylogeny and molecular evidence

Telomerase

An enzyme carrying its own RNA template, which adds telomere repeats back onto the end of a chromosome; it is active in germ line cells, stem cells and most cancer cells. The cell cycle and mitosis

Telomere

A cap of short non-coding sequences repeated many times at each end of a chromosome, which is eroded instead of the genes when replication fails to copy the end, and which stops the end being treated as a break in the DNA. The cell cycle and mitosis

Template strand

A strand of DNA whose base sequence determines the sequence of the new strand built against it. DNA replication

Temporal summation

Summation of impulses arriving in quick succession from one presynaptic neurone. The synapse: transmission, and why it runs one way

Tendinous cords

Inelastic strands running from the atrioventricular valve flaps to papillary muscles projecting from the ventricle wall. The muscles tense as the ventricle contracts, so ventricular pressure cannot turn the flaps inside out; the pressure does the opening and shutting, the cords only stop the flaps going too far. The heart and cardiac cycle

Tensile strength

The greatest pulling force a material can withstand before it breaks, divided by its cross-sectional area. Carbohydrate structure and function

Tertiary structure

The overall three-dimensional fold of a single polypeptide, held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between side chains. Protein structure and function

Test cross

A cross between an organism of dominant phenotype and a homozygous recessive, to find whether the first is homozygous or heterozygous. Monohybrid inheritance and genetic diagrams

The 3Rs

Replacement, Reduction and Refinement: replace animals with other methods where possible, reduce the number used, and refine procedures so that the animals used suffer as little as possible. Set out by Russell and Burch in 1959. Variables and controls: what an experiment can show

Therapeutic cloning

Cloning in which the embryo produced is used as a source of stem cells and is never implanted. Cloning in plants and animals

Therapeutic use exemption

Permission for an athlete with a diagnosed condition to take a prohibited medicine, provided it only restores normal health. Exercise, sports medicine and performance-enhancing drugs

Thermoregulation

The control of body temperature within narrow limits, despite changes in the temperature of the surroundings. Thermoregulation in endotherms and ectotherms

Three Rs

Replacement, reduction and refinement: the principles for minimising the use and suffering of animals in research. Brain imaging, development and learning

Threshold

The membrane potential, around −55 mV, that must be reached before an action potential is triggered. Receptors: transduction and the reflex arc

Thrombosis

The formation of a blood clot inside a blood vessel, most often on a ruptured atheroma, which may block the vessel completely. Cholesterol, atheroma and what a risk factor means

Thylakoid

A flattened, membrane-bound sac inside a chloroplast; the membrane holds the pigments, the electron carriers and ATP synthase. Chloroplast structure and the light-dependent reactions

Tidal volume

The volume of air moved in or out in one normal breath at rest, about 0.5 dm³ in an adult. Gas exchange in mammals: lungs and ventilation

Tissue

A group of similar cells working together to perform a particular function. Cell specialisation and biological organisation

Tissue fluid

The fluid surrounding cells, formed by ultrafiltration from capillaries; it is very poor in plasma protein, though not literally protein-free, and has no red blood cells and no platelets. Blood vessels, tissue fluid and lymph

Total count

A count of all the cells in a sample, living and dead, as made with a haemocytometer. Industrial biotechnology and fermentation

Totipotent

Able to develop into any cell type in the organism and into the extra-embryonic tissues such as the placenta. Found in the zygote and the cells of the first few divisions. Cell specialisation and biological organisation

Toxoid

An exotoxin treated so that it is no longer toxic but keeps its antigens, used as a vaccine against diseases such as tetanus and diphtheria. Pathogens and non-specific defences

TP

Triose phosphate, a three-carbon sugar phosphate; the product of reducing GP, and the molecule that either leaves the cycle or rebuilds RuBP. Pearson Edexcel calls it GALP, glyceraldehyde 3-phosphate. The Calvin cycle

Tracheole

One of the finest branches of an insect's tracheal system, under a micrometre across, running between and into the cells and delivering oxygen to them directly. Gas exchange in insects, fish and plants

Trailing zeros after a decimal point

Always significant. 4.20 has three, and says the hundredths digit was measured. Handling data: units, scales, rates and uncertainty

Transcription factor

A protein that binds to DNA near a gene and controls whether that gene is transcribed into mRNA. Homeobox genes, Hox genes and apoptosis

Transducer

Something that converts energy from one form into another; a receptor converts the energy of a stimulus into a change in membrane potential. Receptors: transduction and the reflex arc

Transfer RNA (tRNA)

A small folded RNA of about 80 nucleotides with an anticodon at one end and a binding site for a specific amino acid at the other. DNA and RNA structure

Transgenic organism

An organism containing DNA transferred into it from another species. Recombinant DNA technology

Transient expression

Production of a protein from a gene delivered into cells for a few days without being inserted into their chromosomes. Pharming, recombinant medicines and synthetic biology

Translocation

The transport of assimilates, mainly sucrose, through the phloem from a source to a sink. Phloem translocation and the mass-flow hypothesis

Translocation (chromosome)

A chromosome mutation in which a section of one chromosome breaks off and joins a non-homologous chromosome. Mutation, gene expression and cancer

Transmission

The passage of a pathogen from an infected host to an uninfected one, directly or through an intermediate such as water, food or a vector. Infectious disease: transmission and control

Transpiration

The loss of water vapour from the aerial parts of a plant, mainly through the stomata, by evaporation from the mesophyll cell walls followed by diffusion out of the leaf. Transpiration and stomatal control

Transpiration stream

The continuous movement of water from the soil, through the root and up the xylem, to replace the water lost from the leaves. Transpiration and stomatal control

Transverse section

A section cut across an organ, at right angles to its long axis. Plan diagrams and tissue sections

Triglyceride

One glycerol molecule joined to three fatty acids by three ester bonds. Lipids: triglycerides, phospholipids and ester bonds

Tropism

A growth response of a plant in which the direction of growth is determined by the direction of the stimulus. Plant responses: tropisms, auxins and the classic experiments

Tube nucleus

The larger of the two nuclei formed when a microspore's nucleus divides by mitosis; it controls the growth of the pollen tube. Sexual reproduction in flowering plants

Tumour suppressor gene

A gene whose product slows the cell cycle or triggers apoptosis; loss of both alleles removes that control. Mutation, gene expression and cancer

Turbidity

The cloudiness of a suspension, measured as absorbance in a colorimeter, which rises as a culture grows. Industrial biotechnology and fermentation

Turgid

A plant cell whose contents press against the wall, giving a positive pressure potential. Diffusion and osmosis

Turner's syndrome

The condition of a female with a single X chromosome and no second sex chromosome, 45,X: a monosomy. Meiosis and the sources of genetic variation

U

Ultrafiltration

Filtration under pressure through a partially permeable barrier, separating small molecules from large ones. Osmoregulation and kidney function

Ultrastructure

The detail of a cell's internal structure as revealed by an electron microscope. Eukaryotic cell structure and organelles

Unloading

Dissociation of oxygen from haemoglobin, at respiring tissue. Haemoglobin and oxygen dissociation curves

Utilitarianism

The ethical position that the right action is the one producing the greatest good for the greatest number of people. Exercise, sports medicine and performance-enhancing drugs

V

Vaccine

A preparation containing antigens from a pathogen, given to produce a primary immune response and memory cells without causing the disease. Antibodies, vaccination and the four kinds of immunity

Valid

A measurement is valid if it measures what it claims to measure; an investigation is valid if its design allows its conclusion to follow. A confounding variable destroys validity however careful the measuring was. Variables and controls: what an experiment can show

Variable region

The part of an antibody whose amino acid sequence differs between antibodies, forming the antigen-binding site. Antibodies, vaccination and the four kinds of immunity

Vascular bundle

A strand of xylem and phloem running along a stem, root or leaf; in a young stem the bundles form a ring, each with the xylem on the inner side and a cap of sclerenchyma fibres outside the phloem. Xylem and the transpiration stream

Vascular cambium

The meristem layer lying between the xylem and the phloem of a vascular bundle, whose divisions add secondary xylem on its inner side and secondary phloem on its outer side. Cell specialisation and biological organisation

Vector

An organism that carries a pathogen between hosts without suffering the disease itself; the female Anopheles mosquito is the vector of malaria. Infectious disease: transmission and control

Vegetative propagation

Asexual reproduction in plants, in which a new plant develops from a stem, root or leaf of the parent and is genetically identical to it. Cloning in plants and animals

Ventilation rate

The number of breaths taken per minute, about 12 to 15 at rest. Also called breathing rate. Gas exchange in mammals: lungs and ventilation

Ventral root

The root by which motor neurones leave the spinal cord, at the front. Organisation of the nervous system

Vertical gene transfer

Passing genes from a parent cell to its daughter cells by binary fission. Evidence for evolution, resistance and genetic drift

Viable count

A count of the living cells in a sample, as made by dilution plating, where each living cell grows into one colony. Industrial biotechnology and fermentation

Viral vector

A virus with its replication and disease-causing genes removed and a chosen gene inserted, used to carry that gene into host cells. Recombinant DNA technology

Virus

An acellular infectious particle consisting of nucleic acid within a protein capsid, sometimes with a lipid envelope, which can only replicate inside a host cell. Prokaryotic cells and viruses

Visking tubing

A partially permeable cellulose membrane, with pores that pass water and small solutes but not large molecules, used as a non-living model of a membrane. Diffusion and osmosis

Visual acuity

The ability to distinguish two points close together as separate. Receptors: transduction and the reflex arc

Visual cortex

The region at the back of the cerebrum that receives and processes impulses from the eyes. Brain imaging, development and learning

Vital capacity

The largest volume that can be breathed out after the deepest possible breath in; roughly 4.6 dm³ in a young adult man and 3.1 dm³ in a young adult woman. Gas exchange in mammals: lungs and ventilation

Voltage-gated channel

A channel protein that opens or closes in response to a change in the potential difference across the membrane. The action potential and its propagation

W

Water potential (ψ)

The tendency of water molecules to move out of a solution, measured in kPa. Pure water at atmospheric pressure has a water potential of 0 kPa; adding solute makes the solute potential negative. Diffusion and osmosis

White matter

The outer region of the spinal cord, made of myelinated axons carrying impulses up and down the cord. Organisation of the nervous system

X

Xerophyte

A plant whose leaves and stems have features that reduce water loss by transpiration, found where water is scarce. Gas exchange in insects, fish and plants

Z

Zona pellucida

The glycoprotein coat outside the egg's cell surface membrane, carrying the receptors a sperm binds to. Gametes, fertilisation and early development

Zone of inhibition

The clear area round a disc on a lawn of bacteria, where a substance diffusing from the disc has stopped the bacteria growing. Industrial biotechnology and fermentation