Biology terms in Form and function

The dark band of a sarcomere, equal to the length of the thick myosin filaments; its length does not change during contraction.

Movement of a limb or body part away from the midline of the body, such as raising the leg sideways at the hip.

A non-living physical or chemical feature of the environment that affects organisms, such as temperature, light intensity, water availability, salinity or pH.

A classification of human blood into groups A, B, AB and O according to the A and B carbohydrate antigens on the glycoproteins and glycolipids of red blood cells.

In nutrition, the movement of the small molecules produced by digestion across the wall of the gut into the blood or lymph.

A vesicle over the front of a sperm head containing hydrolytic enzymes, released when the sperm meets the egg so that it can digest a path through the zona pellucida.

A globular protein that polymerises into the thin filaments of a sarcomere; myosin heads bind to sites on actin to form cross-bridges.

The movement of particles across a membrane against their concentration gradient, from a lower to a higher concentration, by pump proteins using energy from ATP.

A heritable characteristic that increases the chance of an organism surviving and reproducing in its environment; adaptations arise by natural selection over many generations.

Movement of a limb or body part towards the midline of the body, such as bringing a raised leg back to the side.

Animal tissue made of cells that store large droplets of triglyceride; it is used for long-term energy storage, thermal insulation and protection of organs.

A secondary structure in which a polypeptide coils into a spiral held by hydrogen bonds between the C=O of one peptide bond and the N–H four amino acids along.

The ring form of glucose in which the hydroxyl group on carbon 1 lies below the plane of the ring; the monomer of starch and glycogen.

A tiny air sac at the end of a bronchiole in the lungs, with a wall one cell thick and a dense capillary network, where gas exchange takes place.

The –NH₂ group of an amino acid, which forms part of a peptide bond; it can gain a hydrogen ion to become –NH₃⁺.

A molecule with a central carbon atom bonded to an amine group, a carboxyl group, a hydrogen atom and a variable R-group; the monomer of polypeptides.

Describes a molecule with both a hydrophilic region and a hydrophobic region, such as a phospholipid with its hydrophilic phosphate head and hydrophobic fatty acid tails.

The branched component of starch: chains of α-glucose joined by 1,4 glycosidic bonds, with 1,6 glycosidic bonds at the branch points.

The unbranched component of starch: a chain of α-glucose units joined by 1,4 glycosidic bonds that coils into a helix.

A pair of muscles that produce opposite movements at a joint or structure: when one contracts, the other relaxes and is stretched.

An integral channel protein forming a narrow pore through which water molecules pass in single file, greatly increasing the permeability of a membrane to water.

One of the three domains of life: single-celled prokaryotes that differ from bacteria in their membrane lipids and other molecules, and that obtain energy from light, inorganic chemicals or carbon compounds.

A small branch of an artery leading into a capillary bed, whose smooth muscle can narrow or widen the lumen to control blood flow to a tissue.

A blood vessel that carries blood away from the heart, at high pressure, with a thick wall of smooth muscle and elastic fibres.

The uptake and use of absorbed food molecules by body cells, either to build new cell components or as respiratory substrates.

The build-up of fatty plaques containing cholesterol in the walls of arteries, which narrows the lumen, reduces elasticity and restricts blood flow.

A valve between an atrium and a ventricle (tricuspid on the right, bicuspid or mitral on the left) that closes during ventricular systole to prevent backflow into the atrium.

One of the two thin-walled upper chambers of the mammalian heart, which receive blood from veins and pump it into the ventricles.

A synovial joint in which a rounded bone end fits into a cup-shaped socket, allowing movement in three planes, as at the hip and shoulder.

The ring form of glucose in which the hydroxyl group on carbon 1 lies above the plane of the ring; the monomer of cellulose.

A secondary structure in which sections of polypeptide lie side by side in a pleated sheet, held by hydrogen bonds between backbone C=O and N–H groups of neighbouring sections.

A group of ecosystems with similar communities of organisms, resulting from similar abiotic conditions, especially temperature and rainfall, and from convergent evolution.

An effect of other living organisms on an organism or population, such as competition, predation, parasitism, disease or food supply.

The force per unit area exerted by blood on the walls of blood vessels, measured in mmHg and recorded as systolic over diastolic pressure, such as 120/80 mmHg.

The movement of the oxygen dissociation curve to the right when carbon dioxide concentration rises, because haemoglobin's affinity for oxygen falls, so more oxygen is released.

An 80S ribosome attached to the rough endoplasmic reticulum, synthesising a protein that enters the ER lumen for secretion, the plasma membrane or lysosomes.

At constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume, so increasing the volume lowers the pressure.

One of the narrow, branching airways in the lungs that carry air from the bronchi to the alveoli.

The upper layer of a forest formed by the crowns of the tallest trees, which absorbs most of the light.

The narrowest type of blood vessel, with a wall made of a single layer of thin endothelial cells, in which materials are exchanged between blood and the surrounding tissue or environment.

A compound of carbon, hydrogen and oxygen, usually with hydrogen and oxygen in the ratio 2 : 1; carbohydrates include monosaccharides, disaccharides and polysaccharides.

The –COOH group of an amino acid or fatty acid; in amino acids it forms part of a peptide bond, and it can lose a hydrogen ion to become –COO⁻.

The sequence of contraction (systole) and relaxation (diastole) of the atria and ventricles that makes up one heartbeat, together with the opening and closing of the valves.

Striated muscle of the heart, made of short, branched cells with usually one nucleus each, joined end to end by intercalated discs.

An animal that feeds on other animals; carnivorous mammals typically have long pointed canines and blade-like cheek teeth.

A smooth, tough, flexible connective tissue; at synovial joints it covers the ends of bones, reducing friction and absorbing shock.

A relationship in which a change in one variable directly produces a change in another; correlation alone does not demonstrate it.

The breaking open of cells and separation of their organelles into different fractions, usually by centrifugation at increasing speeds, so that each type can be studied.

A protein in the plasma membrane that binds to a matching molecule on a neighbouring cell, attaching cells to each other to form tissues; different CAMs form different junctions.

The ability of cells to identify other cells, or to distinguish self from non-self, by binding to specific molecules such as the carbohydrate chains of glycoproteins on their surfaces.

A structural polysaccharide of plant cell walls made of β-glucose joined by 1,4 glycosidic bonds, with alternate units inverted, forming straight chains bundled into microfibrils.

An integral membrane protein with a hydrophilic pore through which specific ions or polar molecules diffuse across the membrane; many channels are gated and can open or close.

A steroid lipid found among the phospholipids of animal cell membranes, which regulates membrane fluidity by reducing it at high temperatures and preventing stiffening at low temperatures.

One of the flattened, membrane-bound sacs that make up the Golgi apparatus or the endoplasmic reticulum.

A protein that assembles into a curved, cage-like coat on the cytoplasmic side of a membrane, bending it into a pit that buds off as a vesicle.

A graph showing climate: either the monthly temperature and precipitation of one place, or the mean annual temperature and precipitation at which each biome occurs.

The explanation that water is pulled up the xylem as a continuous column under tension, created by evaporation from leaf cell walls and transmitted down the column by cohesion between water molecules.

A fibrous structural protein made of three polypeptides wound into a triple helix; it gives tendons, skin and bone high tensile strength and is the most abundant protein in mammals.

The division of a cell into separate membrane-bound compartments, each with its own conditions, enzymes and substrates, which concentrates reactants and keeps incompatible processes apart.

The principle that two species with identical niches cannot coexist indefinitely in the same place, because the better competitor will eventually eliminate the other.

A difference in the concentration of a substance between two regions; particles diffuse down it from higher to lower concentration, and moving them against it requires energy.

A reaction in which two molecules are joined by a covalent bond and a molecule of water is released; used to build polysaccharides, polypeptides, nucleic acids and lipids.

The precise three-dimensional shape of a protein, produced by the folding of its polypeptide chain or chains.

A change in the three-dimensional shape of a protein, for example when a substrate, signalling molecule or phosphate group binds to it.

A protein that contains a non-polypeptide component, a prosthetic group, in addition to its polypeptides; haemoglobin, with its haem groups, is an example.

Binding in which each oxygen molecule that binds to haemoglobin changes its conformation and increases the affinity of the remaining haem groups for oxygen (and the reverse on release).

A marine ecosystem built on a calcium carbonate framework deposited by reef-building corals, found in warm, clear, shallow tropical seas of normal salinity.

One of the arteries that branch from the aorta and supply the heart muscle itself with oxygenated blood.

Disease caused by narrowing or blockage of the coronary arteries, which reduces the blood supply to heart muscle; also called ischaemic heart disease (CHD).

A relationship between two variables in which a change in one is associated with a change in the other; it may be positive or negative.

A number from −1 to +1 (r) that measures the strength and direction of a linear relationship between two variables; values near 0 show little or no correlation.

In a plant stem or root, the region of mainly parenchyma cells between the epidermis and the vascular tissue, used for support and storage.

A vesicle just beneath the plasma membrane of an egg whose contents, released after fertilisation, harden the zona pellucida and prevent other sperm entering.

A membrane protein that moves two different particles across a membrane together in the same direction, using the movement of one down its concentration gradient to move the other against its gradient.

A form of photosynthesis in which stomata open at night and CO₂ is stored as organic acids, then released for photosynthesis during the day while the stomata are closed, reducing water loss.

The link formed when a myosin head binds to a binding site on an actin filament during muscle contraction.

An electronic device that records measurements from one or more sensors at set intervals over a period of time.

An early model of membrane structure, proposed in 1935, in which a phospholipid bilayer is sandwiched between two continuous layers of protein; falsified by later evidence.

An organism that breaks down dead organic matter and releases carbon dioxide and mineral ions that can be reused; mainly saprotrophic fungi and bacteria.

A change in the three-dimensional conformation of a protein, caused by heat or pH breaking the bonds that hold its shape, so that it loses its function; usually permanent.

The number, type, shape and arrangement of the teeth of an animal, which reflect its diet.

An animal that ingests dead organic matter (detritus) and digests it internally in its gut, such as an earthworm or woodlouse.

A dome-shaped sheet of muscle below the lungs; it contracts and flattens during inspiration, increasing the volume of the thorax.

The phase of the cardiac cycle in which the heart muscle relaxes and the chambers fill with blood.

A flowering plant whose seeds have two seed leaves (cotyledons), typically with net-veined leaves and vascular bundles arranged in a ring in the stem.

The process by which an unspecialised cell becomes specialised for a particular function, by expressing some of its genes and not others.

The breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed, by mechanical action and by enzyme-catalysed hydrolysis.

A molecule made of two amino acids joined by a peptide bond in a condensation reaction.

A sugar made of two monosaccharides joined by a glycosidic bond in a condensation reaction, such as maltose, sucrose or lactose.

A covalent bond between the sulfur atoms of two cysteine R-groups, which stabilises the tertiary or quaternary structure of a protein.

A circulatory system in which blood passes through the heart twice in each complete circuit, through separate pulmonary (lungs) and systemic (body) circulations, as in mammals.

The role of a species in an ecosystem, including all its interactions with biotic and abiotic factors that affect its growth, survival and reproduction, including how it obtains food.

A community of interacting organisms together with the abiotic (non-living) environment with which they interact.

The removal from the gut of undigested food that was never absorbed into body cells, as faeces; different from excretion of metabolic waste.

The return of stretched elastic fibres in an artery wall to their original length, which pushes blood onward and maintains blood pressure between heartbeats.

An organism in the early stages of development after the zygote begins to divide; in humans, up to about eight weeks after fertilisation.

The uptake of material into a cell by the plasma membrane folding inwards around it and pinching off to form a vesicle; requires ATP.

An internal skeleton of bone and cartilage, as in vertebrates, with muscles attached to the outside of the bones by tendons.

An animal that maintains a high, constant body temperature using heat generated by its own metabolism, such as a mammal or bird.

The study of the incidence, distribution and causes of diseases in populations, used to identify risk factors and evaluate ways of preventing disease.

The outer layer of cells covering a leaf, stem or root; in leaves it is transparent, secretes the cuticle and contains stomata.

A plant that grows on another plant, such as on a tree branch, for support and access to light, without taking nutrients from it; for example many orchids and bromeliads.

An amino acid that an organism cannot synthesise, so it must be obtained from the diet; adult humans have nine.

The covalent bond formed by a condensation reaction between the carboxyl group of a fatty acid and a hydroxyl group of an alcohol such as glycerol.

A membrane transport protein that moves one type of particle into the cell and another type out of the cell in the same cycle, such as the sodium–potassium pump.

The release of material from a cell by a vesicle fusing with the plasma membrane and emptying its contents outside the cell; requires ATP.

A hard external skeleton, such as the chitin cuticle of arthropods, that supports and protects the body and provides inner surfaces for muscle attachment.

Breathing out: the volume of the thorax decreases, raising the pressure in the lungs so that air flows out; it is mostly passive at rest.

The extra volume of air that can be breathed out with maximum effort after a normal expiration.

Straightening movement at a joint that increases the angle between the two bones, such as straightening the knee.

The passive movement of particles down a concentration gradient across a membrane through a channel protein (or with a carrier protein), without the use of energy from ATP.

An organism that respires aerobically when oxygen is present and anaerobically when it is absent, so it can survive with or without oxygen, for example yeast.

A molecule with a carboxyl group at one end of a long hydrocarbon chain; fatty acids combine with glycerol to form triglycerides and phospholipids.

A capillary whose endothelial cells have pores (fenestrations) that allow especially rapid exchange of water and solutes, as in kidney glomeruli, intestinal villi and endocrine glands.

The haemoglobin of the fetus, with two γ chains instead of β chains; it has a higher affinity for oxygen than adult haemoglobin, so it takes up oxygen from maternal blood at the placenta.

A protein with a long, thin, strand-like shape, usually insoluble in water and with a structural function, such as collagen or keratin.

Bending movement at a joint that decreases the angle between the two bones, such as bending the knee.

The model of membrane structure in which proteins are embedded in or attached to a fluid phospholipid bilayer, with phospholipids and many proteins free to move sideways.

An 80S ribosome in the cytosol that is not attached to a membrane; it synthesises proteins for use within the cytoplasm.

The potential range of conditions and resources in which a species could survive and reproduce, given its adaptations and tolerance limits, in the absence of competition.

A haploid sex cell (a sperm or an egg) that fuses with another gamete at fertilisation to form a zygote.

The uptake of one gas and release of another across a surface, by diffusion, such as the uptake of oxygen and loss of carbon dioxide in respiration.

A surface across which gases diffuse between an organism and its surroundings; it is permeable, thin, moist and has a large surface area.

The gas exchange organ of a fish, made of filaments with thin lamellae and many capillaries, ventilated by a one-way flow of water.

A protein folded into a compact, rounded shape, usually soluble in water, such as an enzyme, insulin or haemoglobin.

A small three-carbon molecule with a hydroxyl group on each carbon, which forms ester bonds with fatty acids to make triglycerides and phospholipids.

The energy storage polysaccharide of animals and fungi, made of α-glucose with 1,4 and 1,6 glycosidic bonds; more highly branched than amylopectin, stored in liver and muscle.

A lipid with a carbohydrate chain attached, found in the outer layer of the plasma membrane with the carbohydrate facing outside the cell; involved in cell recognition and adhesion.

A protein with one or more carbohydrate chains covalently attached; in the plasma membrane the carbohydrate faces outwards and functions in cell recognition and adhesion.

The covalent bond formed by a condensation reaction between two monosaccharides, linking them through an oxygen atom.

The addition of carbohydrate chains to a protein (or lipid), which takes place in the rough ER and Golgi apparatus and produces glycoproteins.

An instrument with two hinged arms and a protractor scale, used to measure the angle and range of motion of a joint.

A biome with low to moderate rainfall and a dry season, dominated by grasses, with few trees because of drought, fire and grazing; includes temperate grassland and savanna.

One of a pair of cells surrounding a stoma that change shape to open or close it, controlling gas exchange and water loss.

The central region of the A band that contains myosin filaments but no actin; it shortens and may disappear during contraction.

The place in which a community, species, population or organism lives, described by its geographical location, physical location and type of ecosystem.

A ring-shaped prosthetic group with an iron ion (Fe²⁺) at its centre; each haem group in haemoglobin binds one oxygen molecule.

A conjugated globular protein in red blood cells made of four polypeptides, each with a haem group containing iron, which binds oxygen for transport.

A plant adapted to grow in soil or water with a high salt concentration, for example by excluding or excreting salt.

The number of times the heart beats per minute (bpm), equal to the pulse rate.

An animal that feeds on plants or parts of plants; herbivorous mammals typically have large, flat molars for grinding.

The eating of plants or parts of plants by animals.

A monosaccharide with six carbon atoms, formula C₆H₁₂O₆, such as glucose, fructose or galactose; usually found as a ring of five carbons and one oxygen.

A synovial joint that allows movement in one plane only (flexion and extension), as at the knee and elbow.

A mode of heterotrophic nutrition in which food is ingested, digested internally, absorbed and assimilated, and undigested material is egested; used by animals.

A member of the family Hominidae, the great apes: humans, chimpanzees, gorillas, orangutans and their extinct relatives such as Paranthropus.

A biome with very low, unpredictable rainfall (under about 250 mm a year), very high daytime temperatures and sparse, drought-adapted vegetation.

A reaction in which a covalent bond is broken by the addition of a water molecule, whose –H and –OH become part of the two products; used to digest polymers into monomers.

The clustering of non-polar R-groups away from water in a protein, caused by water molecules hydrogen bonding with each other and excluding the non-polar groups.

The light band of a sarcomere, containing thin actin filaments only and divided by a Z-line; it shortens during contraction.

Movement of a substance against its concentration gradient using energy from another substance moving down a concentration gradient that a pump has set up using ATP.

The taking of food into the body, for example through the mouth into the gut, or into a food vacuole by phagocytosis.

Breathing in: contraction of the diaphragm and external intercostal muscles increases the volume of the thorax, lowering pressure so that air flows into the lungs.

The extra volume of air that can be breathed in with maximum effort after a normal inspiration.

A globular protein hormone secreted by β cells of the pancreas that lowers blood glucose; it has two polypeptides (21 and 30 amino acids) linked by disulfide bonds.

A membrane protein embedded in one or both layers of the phospholipid bilayer, held there by a hydrophobic region in the core; many span the whole membrane.

A specialised junction joining cardiac muscle cells end to end, containing gap junctions that allow electrical impulses to pass between cells and strong junctions that hold them together.

Muscles between the ribs: the external intercostals raise the ribs in inspiration and the internal intercostals lower them in forced expiration; they form an antagonistic pair.

The narrow space between the outer and inner membranes of a mitochondrion (or chloroplast) into which protons are pumped, building up a proton gradient.

Competition between individuals of different species for the same limited resource, such as food, light or space.

An inward folding of a membrane or layer of cells, which increases surface area; in some cells the plasma membrane folds deeply into the cytoplasm.

An attraction between oppositely charged ions or groups; in proteins, between positively charged (–NH₃⁺) and negatively charged (–COO⁻) R-groups.

An inadequate blood supply to a tissue, which deprives its cells of oxygen and nutrients.

A rigid bar that turns about a fixed point (the fulcrum) so that an effort applied at one point moves a load at another; in animals, bones act as levers at joints.

A woody climbing plant rooted in the soil that uses trees for support to reach light in the canopy.

A tough band of fibrous connective tissue that joins bone to bone, holding a joint together and limiting its movement.

An ion channel that opens when a specific signalling molecule (ligand), such as a neurotransmitter, binds to it.

A hard, waterproof polymer deposited in the cell walls of xylem vessels and other plant tissues, which strengthens them and stops them collapsing under tension.

The environmental factor that is furthest from its optimum value and so restricts the growth, survival, distribution or abundance of a species, or the rate of a process.

A member of a diverse group of carbon compounds that dissolve in non-polar solvents but are only sparingly soluble in aqueous solvents; includes fats, oils, waxes and steroids.

The movement of a whole organism from one place to another, for example by walking, swimming, flying or crawling.

The space inside a tubular structure such as a blood vessel, xylem vessel or gut, through which material flows.

Excess tissue fluid that has entered the lymphatic vessels, which carry it through lymph nodes and return it to the blood in the subclavian veins.

A thin-walled vessel of the lymphatic system with gaps that let tissue fluid in and valves that keep lymph flowing towards the subclavian veins.

A very large molecule made of thousands or more atoms, such as a polysaccharide, protein or nucleic acid; most macromolecules are polymers.

The ease with which phospholipids and proteins move within a membrane; it depends on temperature, the proportion of unsaturated fatty acids and, in animal cells, cholesterol.

Water produced inside an organism by aerobic cell respiration of food, an important water source for desert animals such as kangaroo rats.

A bundle of parallel cellulose molecules held together by hydrogen bonds; microfibrils give plant cell walls their high tensile strength.

The regular, usually seasonal, movement of animals from one region to another and back, for example between breeding and feeding areas.

An organism that combines autotrophic nutrition (photosynthesis) with heterotrophic nutrition, such as Euglena; mixotrophs may be obligate or facultative.

The way in which an organism obtains its energy and carbon compounds, such as photosynthesis, holozoic, mixotrophic or saprotrophic nutrition.

A small molecule that can be joined to other similar molecules by condensation reactions to form a polymer, for example glucose, an amino acid or a nucleotide.

A single sugar unit, such as glucose, fructose or ribose; the monomer from which disaccharides and polysaccharides are built by condensation.

A fatty acid with one carbon–carbon double bond in its hydrocarbon chain, which puts a kink in the chain and lowers its melting point.

A signalling chemical that forms a concentration gradient across an embryo and causes cells to develop differently according to the concentration they receive.

A single motor neuron together with all the skeletal muscle fibres it stimulates through neuromuscular junctions; all the fibres of the unit contract together.

Describes a stem cell that can differentiate into a limited range of cell types, usually within one tissue, such as blood stem cells in bone marrow.

The death of part of the heart muscle because its blood supply has been blocked, usually by a thrombus in a coronary artery; commonly called a heart attack.

A long cylindrical bundle of actin and myosin filaments inside a muscle fibre, divided along its length into repeating sarcomeres.

A motor protein forming the thick filaments of a sarcomere; its heads bind to actin, hydrolyse ATP and perform the power stroke that slides the filaments.

The division of resources between species with overlapping niches, so that each uses a different part of the resource and both can coexist.

A ligand-gated ion channel in postsynaptic membranes that opens when acetylcholine binds to it, allowing positive ions, mainly Na⁺, to diffuse into the cell.

An amino acid that an organism can synthesise from other amino acids or compounds, so it need not be obtained from the diet.

An organism that needs oxygen gas to survive because it can obtain enough energy only by aerobic respiration, for example Mycobacterium tuberculosis.

An organism that cannot survive in the presence of oxygen gas and respires only anaerobically, for example Clostridium botulinum.

The main female sex hormone, a steroid made mainly in the ovaries; being non-polar, it diffuses through plasma membranes to receptors inside target cells.

An animal that feeds on both plants and animals, such as Homo sapiens.

A discrete subunit of a cell that has a distinct structure and is adapted to perform one or more specific functions, for example the nucleus, ribosomes, vesicles and the plasma membrane.

A compound containing carbon atoms covalently bonded to other atoms, usually including hydrogen, of the kind made by living organisms; carbon dioxide and carbonates are not usually counted as organic.

The net movement of water across a partially permeable membrane from a region of lower solute concentration to a region of higher solute concentration.

The large, non-motile female gamete, with a haploid nucleus and cytoplasm containing nutrient stores and organelles for the early embryo.

A graph of the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen; it is S-shaped because of cooperative binding.

The layer of tall, closely packed cells below the upper epidermis of a leaf, containing many chloroplasts; the main site of photosynthesis.

The pressure exerted by one gas in a mixture of gases, proportional to its concentration; pO₂ is used as a measure of oxygen concentration, in kilopascals (kPa).

A membrane that allows some particles to pass through it but not others; in cells, small non-polar molecules cross the bilayer while ions and large polar molecules need membrane proteins.

Movement of particles across a membrane down their concentration gradient using only their own kinetic energy, without energy from ATP; includes simple diffusion, facilitated diffusion and osmosis.

The covalent C–N bond between the carboxyl carbon of one amino acid and the amine nitrogen of the next, formed by a condensation reaction.

A membrane protein attached to one surface of the membrane, to phospholipid heads or to an integral protein, without entering the hydrophobic core of the bilayer.

A membrane-bound vacuole formed around a particle, such as a bacterium, taken into a cell by phagocytosis; it fuses with lysosomes so that the particle is digested.

A type of endocytosis in which a cell takes in large solid particles, such as bacteria, by enclosing them in a vesicle formed from the plasma membrane.

The vascular tissue that transports organic compounds, mainly sucrose, in solution from where they are made or stored to where they are used, through living sieve tubes.

A lipid made of glycerol linked to two fatty acids and a phosphate group; it is amphipathic, with a hydrophilic phosphate head and two hydrophobic tails.

A double layer of phospholipids in which the hydrophilic heads face the water on each side and the hydrophobic tails form the core; the basis of all cell membranes.

A type of endocytosis in which a cell takes in small droplets of the surrounding fluid, with the solutes dissolved in it, in small vesicles.

A diagram showing the outlines and positions of the tissues in an organ, without individual cells, drawn with clear, continuous lines and no shading.

A narrow channel of cytoplasm through the cell walls of adjacent plant cells, allowing substances to pass directly between them, for example between companion cells and sieve tube elements.

Describes a stem cell that can differentiate into any type of cell in the body, but not extra-embryonic cells, so cannot form a whole organism alone.

A specialised root that grows vertically upwards out of waterlogged mud, allowing gas exchange with the air through lenticels, as in Avicennia mangroves.

A large molecule made of many similar subunits (monomers) linked together by covalent bonds, such as a polysaccharide, polypeptide or nucleic acid.

An unbranched chain of many amino acids joined by peptide bonds, made on ribosomes; one or more polypeptides fold to form a protein.

A polymer of many monosaccharides joined by glycosidic bonds, such as starch, glycogen or cellulose.

A fatty acid with two or more carbon–carbon double bonds in its hydrocarbon chain, giving several kinks and a low melting point.

The tilting of a myosin head bound to actin, releasing ADP and phosphate, which pulls the actin filament towards the centre of the sarcomere.

An interaction in which one animal, the predator, kills and eats another animal, the prey.

An animal that kills and eats other animals (its prey).

The movement of water and small solutes out of a capillary through gaps in its wall, driven by the high hydrostatic pressure of the blood, while cells and large proteins are held back.

The sequence of amino acids in a polypeptide, linked by peptide bonds; it determines the higher levels of structure and therefore the conformation of the protein.

A non-polypeptide component permanently bound to a protein and needed for its function, such as the haem group of haemoglobin.

A large molecule made of one or more polypeptides folded into a specific three-dimensional shape that determines its function.

The rhythmic expansion and recoil of an artery wall caused by the pressure wave that each heartbeat sends through the arteries.

An integral membrane protein that uses energy from ATP to move specific particles across a membrane against their concentration gradient by changing shape.

The arrangement of two or more polypeptide subunits, and of any non-polypeptide prosthetic groups, in a protein.

The variable side chain attached to the central carbon of an amino acid; the 20 amino acids differ only in their R-groups, which give each its properties.

The full extent of movement possible at a joint in a particular direction, measured as the angle between the limits of movement.

The range of values of an abiotic factor between the upper and lower limits within which a species can survive; abundance is highest at the optimum.

The actual range of conditions and resources a species occupies in nature, restricted by competition with other species and other biotic interactions.

A protein that binds a specific signalling molecule, such as a hormone or neurotransmitter, and responds by triggering a change in the cell.

The extent to which repeated measurements give consistent results; it is improved by taking repeated readings or replicate samples and calculating a mean.

Any characteristic or behaviour that increases the probability of developing a particular disease, such as smoking or high blood pressure for coronary heart disease.

A positive pressure in the xylem of roots, generated when mineral ions are actively transported into the xylem and water follows by osmosis, which pushes water up the stem.

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 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.

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.

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.

The ability of a stem cell to divide repeatedly by mitosis while keeping a population of unspecialised stem cells.

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.

The perforated end wall between two sieve tube elements in phloem, whose pores allow sap to flow from one element to the next.

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 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.

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.

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.

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.

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 small, motile male gamete, with a haploid nucleus in its head, mitochondria in its midpiece and a flagellum for swimming.

An instrument that measures the volume of air breathed in and out, recording lung volumes against time.

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.

The energy storage polysaccharide of plants, a mixture of amylose and amylopectin, both made of α-glucose; it is compact and insoluble.

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 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.

A smooth, tapering body shape that reduces drag, allowing an animal to move through water or air using less energy.

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 surface area of a cell or organism divided by its volume; it decreases as size increases, because volume increases faster than surface area.

A mixture of phospholipids and proteins secreted onto the moist lining of the alveoli that reduces surface tension, preventing the alveoli from collapsing.

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.

The phase of the cardiac cycle in which the muscle of a heart chamber contracts, pumping blood out of it.

A biome with long, very cold winters, short cool summers and low to moderate precipitation, dominated by evergreen coniferous trees.

A biome with warm summers, cool or cold winters and moderate rainfall through the year, dominated by deciduous broadleaved trees.

A tough, inelastic cord of fibrous connective tissue that joins a muscle to a bone and transmits the force of contraction to it.

The overall three-dimensional folding of a single polypeptide, held by hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions between R-groups.

The main male sex hormone, a steroid made mainly in the testes; being non-polar, it diffuses through plasma membranes to receptors inside target cells.

The formation of a blood clot (thrombus) inside an intact blood vessel, which may block it.

The volume of air breathed in or out in one normal breath at rest, about 0.5 dm³ in an adult.

The fluid that surrounds the cells of a tissue, formed from blood plasma forced out of capillaries; it has a similar composition to plasma but contains very few proteins and no red blood cells.

A very large elastic protein running from the Z-line to the M-line of a sarcomere; it holds myosin in place, prevents overstretching and recoils to help the sarcomere return to its resting length.

Describes a stem cell that can differentiate into any type of cell, including extra-embryonic cells such as the placenta, and so can form a whole organism.

A line across a habitat along which organisms are sampled at regular intervals, used to study how their distribution changes along an environmental gradient.

The transport of organic compounds, mainly sucrose and amino acids, in solution through the sieve tubes of the phloem from sources to sinks.

The loss of water vapour from the leaves and stems of a plant, by evaporation from mesophyll cell walls and diffusion out through the stomata.

A lipid formed from one glycerol molecule and three fatty acids joined by ester bonds in three condensation reactions; fats and oils are triglycerides.

A biome with high temperatures (about 25–28 °C) and heavy rainfall (usually over 2,000 mm) all year, dominated by tall, broadleaved evergreen trees.

A long regulatory protein lying along the actin filament that covers the myosin-binding sites in relaxed muscle and is moved aside when Ca²⁺ binds to troponin.

A regulatory protein complex on the thin filament; when calcium ions bind to it, it moves tropomyosin so that the myosin-binding sites on actin are exposed.

A treeless biome with very low temperatures for most of the year, low precipitation and permanently frozen subsoil (permafrost), with mosses, lichens and dwarf shrubs.

An extremely thin, flattened cell that forms most of the alveolar wall, keeping the diffusion distance for gas exchange very short.

A rounded cell in the alveolar epithelium that secretes surfactant from lamellar bodies, reducing surface tension, and can divide to replace type I pneumocytes.

A centrifuge that spins samples fast enough to produce forces of more than 100,000 times gravity, used to separate small organelles and large molecules.

A strand of transport tissue containing xylem and phloem; in a leaf it forms a vein, with xylem towards the upper surface.

A blood vessel that carries blood towards the heart at low pressure, with a thin, flexible wall, a wide lumen and valves to prevent backflow.

The movement of air or water into and out of a gas exchange organ, which maintains concentration gradients for oxygen and carbon dioxide at the exchange surface.

The number of breaths taken per minute; multiplied by tidal volume it gives the volume of air breathed per minute.

One of the two thick-walled lower chambers of the mammalian heart, which pump blood into the arteries at high pressure.

The maximum volume of air that can be breathed out after a maximum inspiration; the sum of tidal volume, inspiratory reserve volume and expiratory reserve volume.

An ion channel that opens or closes in response to a change in the potential difference (voltage) across the membrane, such as the Na⁺ and K⁺ channels of axons.

Bright, conspicuous colours or patterns of a toxic or dangerous animal that predators learn to recognise and avoid; also called aposematism.

A layer of wax secreted by the epidermis of a leaf that is almost impermeable to water, reducing water loss by evaporation.

A plant adapted to survive in dry habitats where water is scarce, by reducing water loss, storing water or obtaining water efficiently.

The vascular tissue that transports water and dissolved mineral ions from the roots to the rest of the plant, through dead, hollow, lignified vessels.

The line at each end of a sarcomere to which the thin actin filaments are anchored; the distance between two Z-lines is one sarcomere.

A thick layer of glycoproteins surrounding the plasma membrane of a mammalian egg, to which sperm bind before fertilisation.

The diploid cell formed when the nuclei of a male and a female gamete fuse at fertilisation; the first cell of a new organism.