Biology terms in Interaction and interdependence

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Short-term protection from antibodies made by another organism, such as antibodies passed from mother to baby or given by injection; no memory cells are formed.

An organism or virus that causes disease in another organism (its host); most human pathogens are viruses, bacteria, fungi or protists.

Partly decomposed plant matter that accumulates in waterlogged, acidic, oxygen-poor conditions where decomposition is very slow; peatlands are long-term carbon sinks.

The change in a quantity between two times expressed as a percentage of its original value: (new value − original value) ÷ original value × 100.

The difference between two comparable values measured at the same time, expressed as a percentage of their mean: |a − b| ÷ ((a + b) ÷ 2) × 100.

Waves of contraction of the circular and longitudinal muscle in the wall of the gut that push food along in one direction.

A white blood cell, such as a neutrophil or macrophage, that engulfs pathogens by endocytosis and digests them with enzymes from lysosomes.

The addition of a phosphate group to a molecule, which usually makes it less stable and more reactive, as when glucose is phosphorylated at the start of glycolysis.

An autotroph that uses light as its external energy source to make carbon compounds by photosynthesis, such as plants, algae and cyanobacteria.

The splitting of water molecules using energy from light, which releases electrons, protons (H⁺) and oxygen; in photosynthesis the oxygen released comes from this reaction.

The production of carbon compounds in cells using light energy, with carbon dioxide and water as the raw materials and oxygen released as a by-product.

A coloured molecule, such as chlorophyll or a carotenoid, that absorbs specific wavelengths of light and uses the energy in photosynthesis.

A complex of proteins and an array of pigment molecules in a thylakoid membrane that absorbs light and passes the energy to a reaction centre, which emits an excited electron.

The photosystem (PSI, reaction centre P700) that re-excites electrons arriving from the electron transport chain and passes them to NADP reductase, or back to the chain in cyclic flow.

The photosystem (PSII, reaction centre P680) that emits excited electrons to the electron transport chain and replaces them by the photolysis of water, releasing oxygen.

A directional growth response to light from one side; shoots show positive phototropism, growing towards the light.

A signalling chemical produced by plant cells that controls growth, development or responses to stimuli, often in tissues away from where it is made.

A small endocrine gland near the centre of the brain that secretes melatonin, mainly at night, under the control of the suprachiasmatic nucleus.

A small endocrine gland below the hypothalamus whose anterior and posterior lobes secrete hormones, many of which control other endocrine glands.

An activated B-lymphocyte that has differentiated to secrete large amounts of one specific antibody.

A lipid-soluble electron carrier in the thylakoid membrane that accepts electrons from photosystem II and carries protons from the stroma into the thylakoid space.

A small cell fragment without a nucleus, found in blood, that sticks to damaged vessel walls and releases clotting factors.

A group of organisms of the same species that live in the same area at the same time and normally interbreed.

A control mechanism in which a change in a variable triggers a response that increases the change still further, so that a process speeds up until it is completed.

The neuron (or effector cell) on the receiving side of a synapse, whose membrane carries receptors for the neurotransmitter.

The neuron that releases neurotransmitter into the synaptic cleft at a synapse.

A consumer that feeds on producers; it occupies the second trophic level.

The barriers that prevent pathogens from entering the body, mainly the skin and mucous membranes and their secretions.

The first response of the adaptive immune system to an antigen; it is slow to start and produces a relatively small amount of antibody, but forms memory cells.

The region of the cerebral cortex, in a strip across the top of each cerebral hemisphere, that initiates voluntary contractions of skeletal muscles.

The accumulation of carbon compounds in biomass by autotrophs, measured as mass of carbon (or dry biomass) per unit area per unit time, usually g m⁻² yr⁻¹.

An autotroph that makes carbon compounds from inorganic substances using light or chemical energy; producers form the first trophic level.

An enzyme that adds phosphate groups from ATP to specific proteins, changing their activity; chains of protein kinases form the cascades of signal transduction pathways.

A difference in proton (H⁺) concentration across a membrane, built up by electron transport, which stores potential energy used by ATP synthase.

The three-carbon end product of glycolysis; it enters the link reaction when oxygen is available or is converted to lactate or ethanol when it is not.

A square frame of known area used to mark out a sample area so that the organisms inside it can be counted or their percentage cover estimated.

A form of cell signalling in which bacteria release autoinducer molecules and change their gene expression when the autoinducer concentration, and therefore the population density, rises above a threshold.

A method of selecting samples in which every individual or every position in the study area has an equal chance of being chosen, so that the samples are representative and unbiased.

The change in the amount of substrate or product per unit time; for an enzyme it can also be estimated as 1/time for a fixed amount of change.

The special pair of chlorophyll a molecules at the centre of a photosystem that receives energy from the surrounding pigments and emits an excited electron.

A reaction in which one substance is oxidised (loses electrons) and another is reduced (gains electrons) at the same time.

The gain of electrons by a substance, often by the addition of hydrogen or the removal of oxygen; it always occurs together with an oxidation.

A rapid, automatic response to a stimulus that does not involve conscious thought, such as withdrawing the hand from a hot object.

The pathway of neurons that carries impulses in a reflex: from a receptor, along a sensory neuron to the CNS, through one or more interneurons, and along a motor neuron to an effector.

The short time after an action potential, about 1–2 ms, during which a region of membrane cannot produce another action potential, because its voltage-gated sodium channels are inactivated.

The return of the membrane potential towards the negative resting potential after depolarisation, caused by voltage-gated potassium channels opening and K⁺ diffusing out of the neuron.

The region of the medulla oblongata that sends rhythmic impulses to the diaphragm and intercostal muscles and changes ventilation rate in response to blood pH.

An organic compound that is broken down in cell respiration to release energy for ATP production, such as glucose, fatty acids or amino acids.

An apparatus that measures the rate of cell respiration, usually as the volume of oxygen taken up by living organisms per unit time, with an alkali absorbing the carbon dioxide released.

The potential difference across the plasma membrane of a neuron that is not conducting an impulse, about −70 mV, with the inside negative compared with the outside.

The removal of a neurotransmitter from the synaptic cleft by transporter proteins that carry it back into the presynaptic neuron for reuse.

The distance moved by a substance from the origin divided by the distance moved by the solvent front; a ratio between 0 and 1, with no units, used to identify substances.

A swelling on the root of a legume containing nitrogen-fixing bacteria (Rhizobium) that convert nitrogen gas into ammonium, which the plant uses.

Ribulose bisphosphate carboxylase, the enzyme in the stroma that catalyses carbon fixation by combining CO₂ with RuBP; it is slow and is the most abundant protein on Earth.

Ribulose bisphosphate, the five-carbon compound that accepts CO₂ in carbon fixation; it is regenerated from triose phosphate using ATP.

The conduction of a nerve impulse along a myelinated axon in which action potentials occur only at the nodes of Ranvier, so the impulse jumps from node to node.

The difference between an estimate obtained from a sample and the true value for the whole population; it arises by chance and is unavoidable when only part of a population is measured.

A small molecule or ion, such as cyclic AMP or Ca²⁺, released inside a cell when a first messenger binds a receptor, which diffuses through the cytoplasm and activates the response.

A consumer that feeds on primary consumers; it occupies the third trophic level.

The response to a second exposure to the same antigen; memory cells make it faster, larger and longer-lasting than the primary response.

The accumulation of carbon compounds in biomass by heterotrophs; it is lower than primary production because carbon compounds are lost in cell respiration at each stage.

A neuron that carries impulses from a sensory receptor to the central nervous system (the spinal cord or the brain).

A cell or nerve ending that detects a particular kind of stimulus, such as light, chemicals, pressure or temperature, and converts it into nerve impulses.

Describes an organism that stays fixed in one place, such as a plant or a barnacle, so that it can be sampled with quadrats.

An S-shaped curve of population size against time, with an exponential phase, a transitional phase in which growth slows, and a plateau phase at the carrying capacity.

The sequence of molecular events inside a cell, started by a ligand binding to its receptor, that carries the signal to the parts of the cell that respond.

The part of the central nervous system inside the vertebral column; it carries impulses to and from the brain and acts as the integrating centre for many unconscious reflexes.

A measure of the spread of a set of values around their mean; in a normal distribution about 68% of values lie within one standard deviation of the mean.

A medicine that lowers blood cholesterol by competitively inhibiting HMG-CoA reductase, an enzyme of cholesterol synthesis in the liver.

The conclusion that a difference or association is unlikely to have arisen by chance alone, usually because the probability of it occurring by chance is less than 5% (p < 0.05).

A lipid hormone made from cholesterol that diffuses through the plasma membrane and binds an intracellular receptor, forming a complex that regulates transcription of specific genes.

The volume of blood pumped out of the left ventricle in one heartbeat; it increases when the heart contracts more forcefully.

The molecule on which an enzyme acts, which binds to the active site and is converted into products.

The combining of excitatory and inhibitory postsynaptic potentials arriving at a postsynaptic neuron, which produces an action potential only if their combined effect reaches the threshold potential.

A small group of neurons in the hypothalamus that acts as the master circadian clock; it receives light information from the retina and controls melatonin secretion by the pineal gland.

A junction between a neuron and another neuron or an effector cell, across which a signal is passed by a neurotransmitter diffusing across a narrow gap, the synaptic cleft.

The narrow gap, about 20 nm wide, between the presynaptic and postsynaptic membranes at a synapse, across which neurotransmitter diffuses.

The molecule that accepts electrons at the end of an electron transport chain; in aerobic respiration it is oxygen, which combines with electrons and protons to form water.

A consumer that feeds on secondary consumers; it occupies the fourth trophic level.

The membrane potential, about −50 mV, that must be reached for enough voltage-gated sodium channels to open to start an action potential.

An enzyme formed from the inactive plasma protein prothrombin during clotting, which converts soluble fibrinogen into insoluble fibrin.

A group of similar cells, often with the material between them, that work together to perform a particular function.

Regulation of the populations in a community by consumers at higher trophic levels, such as predators limiting the numbers of herbivores.

A receptor protein that spans the plasma membrane, with a binding site for a hydrophilic ligand outside the cell, a hydrophobic region in the membrane and a region inside that passes on the signal.

A bacterial enzyme that forms peptide cross-links between peptidoglycan chains in the cell wall; the target of penicillin.

A three-carbon sugar phosphate; formed by the lysis of hexose bisphosphate in glycolysis, and the product of the Calvin cycle in photosynthesis.

A series of indirect effects passing down a food chain when a change in the population of a consumer, such as a top predator, alters the populations at lower trophic levels.

The position of an organism in a food chain, defined by the number of feeding steps between it and the producers.

A directional growth response of a plant to a directional stimulus; growth towards the stimulus is positive and growth away from it is negative.

An enzyme that phosphorylates tyrosine residues in proteins; the insulin receptor has tyrosine kinase activity in its region inside the cell.

The deliberate giving of a vaccine to stimulate a primary immune response and the formation of memory cells, so that the person becomes immune.

A preparation containing antigens of a pathogen, or nucleic acid coding for them, that stimulates immunity to that pathogen without causing the disease.

A cranial nerve of the parasympathetic system; impulses along it from the medulla to the sinoatrial node slow the heart rate.

An infectious disease that can be transmitted from other animal species to humans.

Single-celled photosynthetic algae (dinoflagellates) that live inside the cells of hard corals in a mutualistic relationship, supplying the coral with products of photosynthesis.