128 of the Biology glossary’s terms, each with the part of the syllabus where you first meet it.
The synthesis phase of interphase, in which DNA is replicated so that each chromosome comes to consist of two sister chromatids.
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.
A base substitution that changes a codon into another codon for the same amino acid, so the polypeptide is unchanged; also called a silent mutation.
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.
An organism, such as many fungi and bacteria, that secretes digestive enzymes onto dead organic matter, digesting it externally, and then absorbs the products of digestion.
The repeating unit of a myofibril between two Z-lines, made of overlapping actin and myosin filaments; it is the unit that shortens when skeletal muscle contracts.
The specialised endoplasmic reticulum of a muscle fibre, which stores calcium ions and releases them to trigger contraction when the fibre is stimulated.
A fatty acid with only single bonds between the carbon atoms of its hydrocarbon chain, so the chain is straight and the melting point is relatively high.
A line drawn on a micrograph or drawing and labelled with the actual length it represents, so that sizes can be worked out whatever the image is enlarged to.
Ice that forms when seawater freezes, including drifting pack ice and landfast ice; it is a habitat for breeding, resting and feeding in polar regions.
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 substance made by a plant that is not needed for its basic metabolism but defends it, for example by making its tissues toxic or distasteful to herbivores.
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.
Regular coiling or folding of the polypeptide backbone into alpha helices or beta-pleated sheets, stabilised by hydrogen bonds between C=O and N–H groups of the backbone.
Succession that occurs where an existing community has been disturbed or removed but the soil remains, for example after a fire or on abandoned farmland.
A tumour formed in another part of the body by cancer cells that have spread from a primary tumour.
The release from a cell of useful substances that the cell has made, such as enzymes, hormones or mucus, usually by exocytosis.
A membrane-bound vesicle that carries substances made in the cell, usually proteins from the Golgi apparatus, to the plasma membrane and releases them by exocytosis.
A structure that develops from a fertilised ovule, containing an embryo plant and a food store inside a protective seed coat (testa).
A facility that stores dried seeds at low temperature so that they remain viable for many years, conserving the genetic diversity of plant species and crop varieties.
The spread of seeds away from the parent plant by wind, water, animals or explosive release, which reduces competition and allows colonisation of new areas.
The separation of the two alleles of a gene into different gametes, caused by the separation of homologous chromosomes in meiosis I.
A factor in the environment, biotic or abiotic, that affects the survival or reproduction of individuals differently according to their traits, and so causes natural selection.
Deliberate choice by humans of individuals with desired heritable characteristics as parents, so those characteristics become more common in a domesticated population over generations; also called artificial selection.
The reabsorption of useful substances, such as glucose, amino acids, ions and water, from the filtrate back into the blood, mainly in the proximal convoluted tubule, while waste remains in the filtrate.
The spontaneous organisation of molecules into larger ordered structures without outside direction, such as fatty acids forming bilayers and vesicles in water.
A genetic mechanism that prevents fertilisation by pollen from the same plant (or a plant with the same S-allele), so male and female gametes come from different plants.
The transfer of pollen from an anther to a stigma of the same flower or of another flower on the same plant.
The ability of a stem cell to divide repeatedly by mitosis while keeping a population of unspecialised stem cells.
The production of copies of a molecule using the molecule itself as a template, as RNA can do by complementary base pairing; needed for heredity and natural selection.
Replication in which each new DNA molecule consists of one strand from the original molecule and one newly built strand.
A pocket-shaped valve at the base of the aorta or pulmonary artery that closes when the ventricles relax, preventing backflow of blood into the ventricles.
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.
Arranging base sequences of a gene, or amino acid sequences of a protein, from different organisms in rows so that corresponding positions line up and differences can be identified.
A stepwise dilution in which each solution is made by diluting the one before it by the same factor, giving a series of concentrations such as 1, 0.1, 0.01 and 0.001 mol dm⁻³.
Describes an organism that stays fixed in one place, such as a plant or a barnacle, so that it can be sampled with quadrats.
The value of a regulated variable that homeostasis maintains, such as a core body temperature of about 37 °C; the variable fluctuates within a narrow range around it.
A chromosome that determines the sex of an organism; in humans the X and Y chromosomes, with females XX and males XY.
The pattern of inheritance of a gene carried on a sex chromosome (usually the X chromosome), which differs between males and females.
Reproduction in which two gametes, usually from two different parents, fuse to form a zygote, producing offspring with new combinations of alleles.
Selection for physical or behavioural traits that increase an individual's success in attracting or competing for mates, so that the alleles for those traits become more common.
Rapid, involuntary contraction and relaxation of skeletal muscles, stimulated by the hypothalamus when the body is cold; the respiration supplying ATP for contraction releases heat.
A region of DNA in which a short base sequence (typically 2–6 bases) is repeated many times in a row; the number of repeats varies between individuals.
An inherited disease caused by a base substitution in the β-globin gene that replaces glutamic acid with valine, so haemoglobin forms fibres and red blood cells become sickle-shaped.
The perforated end wall between two sieve tube elements in phloem, whose pores allow sap to flow from one element to the next.
A living phloem cell with no nucleus and very little cytoplasm, joined end to end with others through perforated sieve plates to form a sieve tube that transports sap.
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.
A short sequence of amino acids at the start of a polypeptide that directs the ribosome making it to the rough ER, so the polypeptide enters the ER lumen.
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.
A base sequence in DNA that reduces the transcription of a gene when a repressor protein binds to it.
The net movement of particles from a region of higher concentration to a region of lower concentration directly through the phospholipid bilayer, as a result of their random motion.
An index of species diversity, , where N is the total number of individuals and n the number of each species; higher values mean greater richness and evenness.
A circulatory system in which blood passes through the heart once in each complete circuit of the body, as in bony fish: heart → gills → body → heart.
A position in the genome where the base differs between individuals of the same species, for example C in some individuals and T in others; abbreviated SNP.
In translocation, a part of a plant that removes sugars from the phloem for growth, respiration or storage, such as roots, fruits and growing shoots.
A group of specialised cardiac muscle cells in the wall of the right atrium that acts as the pacemaker, starting each heartbeat with an electrical impulse.
The model of muscle contraction in which myosin heads pull actin filaments towards the centre of the sarcomere, so the sarcomere shortens while the filaments stay the same length.
A network of membrane tubules without ribosomes that synthesises lipids such as phospholipids and steroids and, in liver cells, detoxifies drugs.
Muscle tissue made of unstriated, spindle-shaped cells that contract slowly and involuntarily, found in the walls of arteries, the gut and other organs.
A cotransporter (SGLT) that moves glucose into a cell against its concentration gradient together with Na⁺ ions moving down their concentration gradient; found in the small intestine and nephron.
A pump protein in animal plasma membranes that uses one ATP per cycle to move three Na⁺ ions out of the cell and two K⁺ ions in, both against their concentration gradients.
The removal of soil, especially fertile topsoil, by wind or water faster than new soil forms, often after the vegetation that protects it has been cleared.
A substance that is dissolved in a solvent to form a solution, for example glucose or sodium ions dissolved in water.
The component of water potential due to dissolved solutes; zero for pure water and more negative as solute concentration increases (symbol Ψs).
The process in which solvent molecules surround and separate the particles of a solute; in water, a shell of water molecules forms around each ion or polar molecule.
A liquid in which other substances (solutes) dissolve; water is the solvent in which most reactions of life take place.
Any body cell other than a germ cell; mutations in somatic cells are not passed to offspring but can cause cancer.
In translocation, a part of a plant that releases sugars into the phloem, such as a photosynthesising leaf or a storage organ being used.
A cell whose structure and contents are adapted to carry out a particular function, as a result of differentiation.
The formation of new species by the splitting of an existing species into two or more populations that become reproductively isolated from each other.
A group of organisms with shared traits that can interbreed to produce fertile offspring; the basic unit of classification.
The variety of species in a community, depending on both the number of species (richness) and how evenly individuals are distributed among them (evenness).
A measure of how similar the abundances of the different species in a community are; evenness is high when all species have similar numbers of individuals.
The number of different species present in a community or area.
The amount of energy needed to raise the temperature of 1 g of a substance by 1 °C; for water it is 4.18 J g⁻¹ °C⁻¹.
The small, motile male gamete, with a haploid nucleus in its head, mitochondria in its midpiece and a flagellum for swimming.
The production of sperm in the seminiferous tubules of the testes by mitosis, cell growth, two divisions of meiosis and differentiation.
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.
An array of microtubules that forms during nuclear division, attaches to chromosomes at their centromeres and moves chromosomes or chromatids to opposite poles.
An instrument that measures the volume of air breathed in and out, recording lung volumes against time.
The removal of introns from pre-mRNA and the joining of exons to form mature mRNA.
The layer of loosely packed, rounded cells with large air spaces in the lower part of a leaf; its moist cell surfaces are the site of gas exchange.
Natural selection in which individuals with intermediate values of a trait have the highest fitness and both extremes are selected against, so variation decreases while the mean stays the same.
A microscope slide engraved with a scale of known length, usually 1 mm divided into 100 divisions of 0.01 mm (10 µm), used to calibrate an eyepiece graticule.
A coloured chemical that binds to particular cell structures and increases contrast, such as iodine for starch in plant cells or methylene blue for nuclei in animal cells.
The male part of a flower, made of an anther held on a stalk called the filament.
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.
The standard error of the mean: an estimate of how far a sample mean is likely to be from the true mean, equal to the standard deviation divided by the square root of the number of measurements.
The energy storage polysaccharide of plants, a mixture of amylose and amylopectin, both made of α-glucose; it is compact and insoluble.
The codon AUG, which signals the start of translation and codes for methionine.
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).
An unspecialised cell that can divide repeatedly to make more stem cells and can differentiate into one or more types of specialised cell.
A microenvironment in a tissue that maintains stem cells and controls whether they remain unspecialised or divide and differentiate, for example in bone marrow or hair follicles.
A lipid whose molecule is based on four fused carbon rings (three of six carbons and one of five); examples are cholesterol, oestradiol and testosterone.
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 sticky tip of the carpel that receives pollen during pollination.
A pore in the epidermis of a leaf, between two guard cells that open and close it, through which carbon dioxide, oxygen and water vapour diffuse.
The number of stomata per unit area of leaf surface, usually expressed per mm².
One of the three codons UAA, UAG and UGA, which code for no amino acid and signal the end of translation.
A smooth, tapering body shape that reduces drag, allowing an animal to move through water or air using less energy.
A skeletal muscle cell: a very long, cylindrical, multinucleate fibre formed by the fusion of many cells, with a striped (striated) appearance due to its regularly arranged proteins.
The volume of blood pumped out of the left ventricle in one heartbeat; it increases when the heart contracts more forcefully.
The fluid inside the inner membrane of a chloroplast, surrounding the thylakoids, where the enzymes of the Calvin cycle are found.
A layered rock structure built up by mats of microorganisms, mainly cyanobacteria; fossil stromatolites about 3.48 billion years old are among the oldest evidence of life.
The link between the physical form of a biological structure and the role it performs, so that features of the structure can be explained by what they allow it to do.
The molecule on which an enzyme acts, which binds to the active site and is converted into products.
The chain of alternating pentose sugars and phosphate groups, linked by covalent bonds, that forms the structural core of a strand of DNA or RNA, with the bases projecting from it.
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.
The repeated coiling of DNA and its histones into more and more compact structures, which condenses chromosomes before nuclear division.
The development of many follicles at the same time, caused by artificially high doses of FSH, so that several eggs can be collected in one cycle.
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.
The surface area of a cell or organism divided by its volume; it decreases as size increases, because volume increases faster than surface area.
The tendency of a liquid surface to behave like a stretched elastic film, caused by cohesion pulling surface molecules inwards; it lets small animals such as pond skaters stand on water.
A mixture of phospholipids and proteins secreted onto the moist lining of the alveoli that reduces surface tension, preventing the alveoli from collapsing.
Removing organisms from a natural population at a rate no greater than the rate at which the population replaces them by reproduction and growth, so that harvesting can continue indefinitely.
The secretion of a watery salt solution by sweat glands onto the skin surface; evaporation of the water removes heat from the skin and cools the body.
The formation of new species from populations living in the same area, through reproductive isolation caused by behavioural or temporal differences or by polyploidy.
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 viscous fluid in the cavity of a synovial joint, secreted by the synovial membrane, which lubricates the joint and nourishes the cartilage.
A freely movable joint in which the cartilage-covered ends of bones are separated by a cavity of synovial fluid enclosed in a joint capsule.
An error that shifts every measurement in the same direction by a similar amount or proportion, caused by a fault in the instrument or method; it reduces accuracy and is not reduced by repeating measurements.
The phase of the cardiac cycle in which the muscle of a heart chamber contracts, pumping blood out of it.