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A Level Biology Key Terms Glossary

This glossary is built from the maintained topic pages across the full course. It is grouped by module and topic so terms stay tied to the places where they are properly explained.

Module 1: Development Of Practical Skills In Biology

1.1.1 Planning

  • Biological variable: a factor in a biological system that can change and be measured or controlled in an investigation. Main page: 1.1.1 Planning
  • Independent variable: the factor that is deliberately changed by the investigator. Main page: 1.1.1 Planning
  • Dependent variable: the measured outcome used to judge the effect of the independent variable. Main page: 1.1.1 Planning
  • Controlled variable: a factor kept the same so it does not distort the result. Main page: 1.1.1 Planning
  • Apparatus choice: selection of equipment that matches the scale, precision and type of measurement required. Main page: 1.1.1 Planning
  • Validity: the extent to which the method actually tests the biological question being asked. Main page: 1.1.1 Planning

1.1.2 Implementing

  • Raw data: the original measurements or observations recorded during the practical. Main page: 1.1.2 Implementing
  • Quantitative observation: an observation recorded as a number, often with a unit. Main page: 1.1.2 Implementing
  • Qualitative observation: a descriptive observation such as colour, appearance or presence of growth. Main page: 1.1.2 Implementing
  • Unit: the standard quantity used to express a measurement, such as seconds, centimetres or degrees Celsius. Main page: 1.1.2 Implementing
  • Precision: the level of detail or fineness of measurement an instrument can provide. Main page: 1.1.2 Implementing
  • Scientific drawing: a clear biological drawing that uses labels and scale carefully enough to communicate what was observed. Main page: 1.1.2 Implementing

1.1.3 Analysis

  • Trend: the overall pattern shown by the data. Main page: 1.1.3 Analysis
  • Gradient: the steepness of a graph line, often used to represent rate of change. Main page: 1.1.3 Analysis
  • Intercept: the point where a graph crosses an axis, which can carry biological meaning. Main page: 1.1.3 Analysis
  • Significant figures: the digits used to show a value to a sensible level of precision. Main page: 1.1.3 Analysis
  • Anomalous result: a result that does not fit the overall pattern shown by the rest of the data. Main page: 1.1.3 Analysis
  • Interpretation: explanation of what the processed data mean biologically. Main page: 1.1.3 Analysis

1.1.4 Evaluation

  • Accuracy: closeness of a result to the true value or to what was intended to be measured. Main page: 1.1.4 Evaluation
  • Precision: closeness of repeated measurements to one another. Main page: 1.1.4 Evaluation
  • Uncertainty: the doubt attached to a measurement because of apparatus limits or method design. Main page: 1.1.4 Evaluation
  • Reliability: the extent to which repeated measurements give consistent results. Main page: 1.1.4 Evaluation
  • Validity: the extent to which the investigation measured the biological factor it was supposed to measure. Main page: 1.1.4 Evaluation
  • Improvement: a specific change to method or apparatus that directly addresses an identified weakness. Main page: 1.1.4 Evaluation

Module 2: Foundations In Biology

2.1.1 Cell Structure

  • Magnification: how many times larger the image is than the real object; calculated as image size divided by object size. Main page: 2.1.1 Cell Structure
  • Resolution: the ability to distinguish two close points as separate; depends on the wavelength of radiation used. Main page: 2.1.1 Cell Structure
  • Ultrastructure: the fine internal detail of cell structure visible with an electron microscope. Main page: 2.1.1 Cell Structure
  • Eyepiece graticule: a numbered scale in the eyepiece used to measure specimen dimensions; must be calibrated against a stage micrometer. Main page: 2.1.1 Cell Structure
  • Stage micrometer: a glass slide engraved with a scale in µm, used to calibrate the eyepiece graticule. Main page: 2.1.1 Cell Structure
  • Artefact: a visible feature in a microscope image that is not genuinely part of the specimen, introduced during preparation. Main page: 2.1.1 Cell Structure
  • Eukaryotic cell: a cell with a nucleus and membrane-bound organelles (animals, plants, fungi, protists). Main page: 2.1.1 Cell Structure
  • Prokaryotic cell: a cell without a nucleus or membrane-bound organelles; DNA is circular and found in the nucleoid region. Main page: 2.1.1 Cell Structure
  • Nuclear envelope: the double membrane surrounding the eukaryotic nucleus, perforated by nuclear pores. Main page: 2.1.1 Cell Structure
  • Nucleolus: the region within the nucleus where ribosomal RNA is synthesised. Main page: 2.1.1 Cell Structure
  • Cristae: the folds of the inner mitochondrial membrane that increase the surface area available for aerobic respiration. Main page: 2.1.1 Cell Structure
  • Cisternae: flattened, fluid-filled membrane sacs found in the RER, SER, and Golgi apparatus. Main page: 2.1.1 Cell Structure
  • Plasmodesmata: channels through plant cell walls that connect neighbouring cells. Main page: 2.1.1 Cell Structure
  • Tonoplast: the selectively permeable membrane surrounding the plant cell vacuole. Main page: 2.1.1 Cell Structure
  • Thylakoids: membrane-bound sacs within chloroplasts, stacked into grana, where light-dependent reactions of photosynthesis occur. Main page: 2.1.1 Cell Structure
  • Murein (peptidoglycan): the polymer making up prokaryotic cell walls. Main page: 2.1.1 Cell Structure
  • Plasmid: a small, circular loop of DNA in prokaryotes, carrying additional genes separate from the main chromosome. Main page: 2.1.1 Cell Structure
  • Capsule: polysaccharide outer layer found in some prokaryotes, protecting against attack by white blood cells and antibiotics. Main page: 2.1.1 Cell Structure

2.1.2 Biological Molecules

  • Monomer: a small molecular subunit that joins with others to form a polymer. Main page: 2.1.2 Biological Molecules
  • Polymer: a large molecule built from many repeating monomers linked by covalent bonds. Main page: 2.1.2 Biological Molecules
  • Condensation reaction: formation of a covalent bond between two molecules with simultaneous release of water. Main page: 2.1.2 Biological Molecules
  • Hydrolysis: breakage of a covalent bond using water. Main page: 2.1.2 Biological Molecules
  • Dipolar molecule: a molecule with partial positive and partial negative charges, such as water. Main page: 2.1.2 Biological Molecules
  • Hydrogen bond: a weak electrostatic attraction between a δ+ hydrogen and a δ− atom on a neighbouring molecule. Main page: 2.1.2 Biological Molecules
  • Cohesion: the tendency of water molecules to stick together via hydrogen bonds. Main page: 2.1.2 Biological Molecules
  • Hexose sugar: a monosaccharide with six carbon atoms (e.g. glucose, fructose, galactose). Main page: 2.1.2 Biological Molecules
  • Pentose sugar: a monosaccharide with five carbon atoms (e.g. ribose, deoxyribose). Main page: 2.1.2 Biological Molecules
  • Alpha-glucose: an isomer of glucose in which the −OH on carbon 1 points downward; monomer of starch and glycogen. Main page: 2.1.2 Biological Molecules
  • Beta-glucose: an isomer of glucose in which the −OH on carbon 1 points upward; monomer of cellulose. Main page: 2.1.2 Biological Molecules
  • Glycosidic bond: the covalent bond linking monosaccharides in carbohydrates, formed by condensation. Main page: 2.1.2 Biological Molecules
  • Amylose: the unbranched, helical component of starch, formed from α-glucose via 1–4 glycosidic bonds. Main page: 2.1.2 Biological Molecules
  • Amylopectin: the branched component of starch, with 1–4 and 1–6 glycosidic bonds. Main page: 2.1.2 Biological Molecules
  • Microfibril: a bundle of parallel cellulose chains cross-linked by hydrogen bonds, contributing to cell wall strength. Main page: 2.1.2 Biological Molecules
  • Saturated fatty acid: a fatty acid with no carbon–carbon double bonds; solid at room temperature. Main page: 2.1.2 Biological Molecules
  • Unsaturated fatty acid: a fatty acid with one or more carbon–carbon double bonds; liquid at room temperature. Main page: 2.1.2 Biological Molecules
  • Ester bond: the covalent bond between glycerol and a fatty acid in a lipid, formed by condensation. Main page: 2.1.2 Biological Molecules
  • Triglyceride: a lipid made from one glycerol joined to three fatty acids by three ester bonds. Main page: 2.1.2 Biological Molecules
  • Phospholipid: a lipid with two fatty acid tails and a phosphate-containing head; amphipathic. Main page: 2.1.2 Biological Molecules
  • Amphipathic: having both hydrophilic and hydrophobic regions. Main page: 2.1.2 Biological Molecules
  • Cholesterol: a sterol lipid that intercalates between phospholipids in animal cell membranes, reducing fluidity. Main page: 2.1.2 Biological Molecules
  • Amino acid: the monomer of proteins; has a central carbon, amino group, carboxyl group, hydrogen, and variable R group. Main page: 2.1.2 Biological Molecules
  • Peptide bond: the covalent bond between amino acids in a polypeptide, formed by condensation between carboxyl and amino groups. Main page: 2.1.2 Biological Molecules
  • Primary structure: the specific sequence of amino acids in a polypeptide. Main page: 2.1.2 Biological Molecules
  • Secondary structure: regular, repeating local folding (alpha-helix or beta-pleated sheet) driven by hydrogen bonds between backbone groups. Main page: 2.1.2 Biological Molecules
  • Tertiary structure: the overall 3D shape of a polypeptide, stabilised by hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions. Main page: 2.1.2 Biological Molecules
  • Quaternary structure: the association of two or more polypeptide chains, with optional prosthetic groups. Main page: 2.1.2 Biological Molecules
  • Prosthetic group: a non-protein component permanently incorporated into a protein (e.g. haem in haemoglobin). Main page: 2.1.2 Biological Molecules
  • Globular protein: a compact, soluble protein with a metabolic function. Main page: 2.1.2 Biological Molecules
  • Fibrous protein: a long, insoluble, structurally specialised protein. Main page: 2.1.2 Biological Molecules
  • Disulfide bridge: a covalent bond between the sulphur atoms of two cysteine residues, contributing to tertiary and quaternary structure. Main page: 2.1.2 Biological Molecules
  • Reducing sugar: a sugar capable of donating electrons to Benedict's reagent, causing a colour change from blue to brick-red. Main page: 2.1.2 Biological Molecules

2.1.3 Nucleotides And Nucleic Acids

2.1.4 Enzymes

  • Enzyme: a globular protein that acts as a biological catalyst, speeding up chemical reactions by lowering their activation energy. Main page: 2.1.4 Enzymes
  • Active site: the region of an enzyme, determined by its tertiary structure, where the substrate binds to form an enzyme–substrate complex. Main page: 2.1.4 Enzymes
  • Substrate: the molecule upon which an enzyme acts. Main page: 2.1.4 Enzymes
  • Activation energy: the minimum energy required for a chemical reaction to proceed. Main page: 2.1.4 Enzymes
  • Enzyme–substrate complex: the temporary structure formed when a substrate binds to the active site of an enzyme. Main page: 2.1.4 Enzymes
  • Lock-and-key model: a model of enzyme action in which the substrate fits rigidly into a pre-formed, complementary active site. Main page: 2.1.4 Enzymes
  • Induced-fit model: a model of enzyme action in which the active site changes shape slightly as the substrate binds, placing strain on the substrate's bonds and contributing to catalysis. Main page: 2.1.4 Enzymes
  • Intracellular enzyme: an enzyme that acts within the cell that produced it (e.g. catalase). Main page: 2.1.4 Enzymes
  • Extracellular enzyme: an enzyme that is secreted and acts outside its producing cell (e.g. amylase, trypsin). Main page: 2.1.4 Enzymes
  • Denaturation: an irreversible change to a protein's tertiary structure, altering the active site so the substrate can no longer bind. Main page: 2.1.4 Enzymes
  • Optimum: the temperature or pH at which an enzyme's activity is greatest. Main page: 2.1.4 Enzymes
  • Q10: the factor by which reaction rate changes for a 10 °C increase in temperature; calculated as R2/R1. Main page: 2.1.4 Enzymes
  • Limiting factor: the factor in shortest supply that restricts the rate of a process. Main page: 2.1.4 Enzymes
  • Saturation point: the substrate concentration at which all enzyme active sites are occupied and rate plateaus. Main page: 2.1.4 Enzymes
  • Cofactor: a non-protein molecule or ion required for an enzyme to be active. Main page: 2.1.4 Enzymes
  • Coenzyme: an organic cofactor, often vitamin-derived, that temporarily associates with an enzyme during catalysis. Main page: 2.1.4 Enzymes
  • Prosthetic group: a cofactor permanently bound to an enzyme (e.g. Zn²⁺ in carbonic anhydrase). Main page: 2.1.4 Enzymes
  • Competitive inhibitor: a molecule that binds at the active site and competes directly with the substrate; its effect can be overcome by increasing substrate concentration. Main page: 2.1.4 Enzymes
  • Non-competitive inhibitor: a molecule that binds at the allosteric site, changing the active site shape; its effect cannot be overcome by increasing substrate concentration. Main page: 2.1.4 Enzymes
  • Allosteric site: a region of an enzyme away from the active site where non-competitive inhibitors (and some regulatory molecules) bind. Main page: 2.1.4 Enzymes
  • End-product inhibition: regulation in which the final product of a metabolic pathway inhibits an early enzyme in that pathway, acting as a negative feedback mechanism. Main page: 2.1.4 Enzymes
  • Reversible inhibition: inhibition in which the inhibitor forms weak bonds and can dissociate from the enzyme. Main page: 2.1.4 Enzymes
  • Irreversible inhibition: inhibition in which the inhibitor forms permanent covalent bonds, permanently disabling the enzyme. Main page: 2.1.4 Enzymes

2.1.5 Biological Membranes

  • Fluid mosaic model: the description of membranes as a dynamic bilayer of phospholipids with embedded proteins and other components, in which molecules can move laterally. Main page: 2.1.5 Biological Membranes
  • Phospholipid bilayer: the two-layered core structure of a membrane, with hydrophilic heads facing outward and hydrophobic tails facing inward. Main page: 2.1.5 Biological Membranes
  • Hydrophilic: water-attracting; describes the phosphate head of a phospholipid. Main page: 2.1.5 Biological Membranes
  • Hydrophobic: water-repelling; describes the fatty acid tails of a phospholipid. Main page: 2.1.5 Biological Membranes
  • Intrinsic (integral) protein: a membrane protein that spans the entire bilayer, including channel and carrier proteins. Main page: 2.1.5 Biological Membranes
  • Extrinsic (peripheral) protein: a membrane protein present on only one side of the bilayer; involved in support or signalling. Main page: 2.1.5 Biological Membranes
  • Channel protein: an intrinsic protein forming a hydrophilic pore through which specific ions pass by facilitated diffusion. Main page: 2.1.5 Biological Membranes
  • Carrier protein: an intrinsic protein that changes shape to transport specific molecules across the membrane, used in both facilitated diffusion and active transport. Main page: 2.1.5 Biological Membranes
  • Cholesterol: a lipid molecule interspersed in the bilayer that reduces fluidity and increases membrane stability. Main page: 2.1.5 Biological Membranes
  • Glycoprotein: an intrinsic protein with attached carbohydrate chains, involved in cell adhesion, recognition, and signalling. Main page: 2.1.5 Biological Membranes
  • Glycolipid: a lipid with attached carbohydrate, also involved in cell recognition and signalling. Main page: 2.1.5 Biological Membranes
  • Partially permeable: allowing some substances to pass through but not others; a property of all biological membranes. Main page: 2.1.5 Biological Membranes
  • Compartmentalisation: the division of the cell into distinct regions by organelle membranes, allowing different reactions to occur simultaneously. Main page: 2.1.5 Biological Membranes
  • Simple diffusion: passive movement of small, non-polar molecules directly through the phospholipid bilayer, down a concentration gradient. Main page: 2.1.5 Biological Membranes
  • Facilitated diffusion: passive movement of large or polar molecules through specific channel or carrier proteins, down a concentration gradient. Main page: 2.1.5 Biological Membranes
  • Active transport: movement of particles against a concentration gradient using carrier proteins and ATP. Main page: 2.1.5 Biological Membranes
  • Endocytosis: bulk uptake of material into a cell by the cell-surface membrane engulfing it to form a vesicle; includes phagocytosis (solids) and pinocytosis (liquids). Main page: 2.1.5 Biological Membranes
  • Exocytosis: bulk release of material from a cell when vesicles fuse with the cell-surface membrane. Main page: 2.1.5 Biological Membranes
  • Phagocytosis: endocytosis of solid material. Main page: 2.1.5 Biological Membranes
  • Pinocytosis: endocytosis of liquid material. Main page: 2.1.5 Biological Membranes
  • Water potential (Ψ): the pressure exerted by water molecules on their surrounding membrane or container; measured in kPa; pure water = 0 kPa, decreasing as solute concentration increases. Main page: 2.1.5 Biological Membranes
  • Osmosis: the diffusion of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential. Main page: 2.1.5 Biological Membranes
  • Hypotonic solution: a solution with a higher water potential than the cell; water enters the cell. Main page: 2.1.5 Biological Membranes
  • Isotonic solution: a solution with the same water potential as the cell; no net water movement. Main page: 2.1.5 Biological Membranes
  • Hypertonic solution: a solution with a lower water potential than the cell; water leaves the cell. Main page: 2.1.5 Biological Membranes
  • Turgid: the state of a plant cell that has taken up water by osmosis; the vacuole is full and the membrane presses against the cell wall. Main page: 2.1.5 Biological Membranes
  • Plasmolysis: the state of a plant cell that has lost water by osmosis; the membrane has pulled away from the cell wall. Main page: 2.1.5 Biological Membranes
  • Crenation: shrinkage of an animal cell due to water loss by osmosis in a hypertonic solution. Main page: 2.1.5 Biological Membranes

2.1.6 Cell Division, Cell Diversity And Cellular Organisation

Module 3: Exchange And Transport

3.1.1 Exchange Surfaces

  • Surface area to volume ratio (SA:V): the relationship between an organism's exchange surface and its internal volume; decreases as body size increases, driving the need for specialised exchange surfaces. Main page: 3.1.1 Exchange Surfaces
  • Diffusion gradient: the difference in concentration across a membrane that drives net diffusion. Main page: 3.1.1 Exchange Surfaces
  • Ventilation: the mechanical movement of air or water across an exchange surface to maintain a steep concentration gradient. Main page: 3.1.1 Exchange Surfaces
  • Gas exchange: diffusion of oxygen and carbon dioxide across an exchange surface. Main page: 3.1.1 Exchange Surfaces
  • Tidal volume: the volume of air moved in or out during a normal resting breath (typically ~0.5 dm³). Main page: 3.1.1 Exchange Surfaces
  • Vital capacity: the maximum volume of air that can be moved in a single breath, excluding residual volume. Main page: 3.1.1 Exchange Surfaces
  • Residual volume: the volume of air remaining in the lungs after maximum exhalation; cannot be measured by spirometry. Main page: 3.1.1 Exchange Surfaces
  • Ventilation rate: tidal volume multiplied by breathing rate; units dm³ min⁻¹. Main page: 3.1.1 Exchange Surfaces
  • Alveolus: a microscopic air sac in the lungs; the primary gas exchange surface in mammals. Main page: 3.1.1 Exchange Surfaces
  • Squamous epithelium: very thin, flat epithelial cells forming the alveolar and capillary walls to minimise diffusion distance. Main page: 3.1.1 Exchange Surfaces
  • Goblet cell: a mucus-secreting cell in the airways; traps particles and pathogens. Main page: 3.1.1 Exchange Surfaces
  • Ciliated epithelial cell: a cell with surface cilia that beat to move mucus up and out of the airways. Main page: 3.1.1 Exchange Surfaces
  • External intercostal muscles: muscles between the ribs that contract during inspiration to raise the ribcage. Main page: 3.1.1 Exchange Surfaces
  • Internal intercostal muscles: muscles between the ribs that contract during forced expiration to lower the ribcage. Main page: 3.1.1 Exchange Surfaces
  • Surfactant: a phospholipid-containing substance secreted by alveolar cells that reduces surface tension and prevents alveolar collapse. Main page: 3.1.1 Exchange Surfaces
  • Countercurrent flow: arrangement in fish gills where blood and water flow in opposite directions, maintaining a diffusion gradient across the full length of the exchange surface. Main page: 3.1.1 Exchange Surfaces
  • Gill lamella: a thin, plate-like projection from a gill filament providing a large surface area for gas exchange. Main page: 3.1.1 Exchange Surfaces
  • Operculum: the bony flap covering the gills in bony fish; its opening and closing contributes to gill ventilation. Main page: 3.1.1 Exchange Surfaces
  • Spiracle: an external opening in the insect exoskeleton through which air enters the tracheal system. Main page: 3.1.1 Exchange Surfaces
  • Trachea (insect): chitin-reinforced air tube in the insect body. Main page: 3.1.1 Exchange Surfaces
  • Tracheole: the finest terminal branches of the insect tracheal system; penetrate directly into tissues for gas exchange. Main page: 3.1.1 Exchange Surfaces
  • Tracheal fluid: liquid at the tips of tracheoles in which oxygen dissolves before diffusing into cells. Main page: 3.1.1 Exchange Surfaces
  • Buccal cavity: the mouth and throat region in fish; acts as a pump during gill ventilation. Main page: 3.1.1 Exchange Surfaces

3.1.2 Transport in Animals

  • Open circulatory system: a system in which blood is not fully enclosed in vessels and flows freely through the body cavity. Main page: 3.1.2 Transport in Animals
  • Closed circulatory system: a system in which blood is enclosed within vessels at all times. Main page: 3.1.2 Transport in Animals
  • Double circulation: a system in which blood passes through the heart twice per complete circuit, allowing separate pulmonary and systemic pressures. Main page: 3.1.2 Transport in Animals
  • Vasoconstriction: narrowing of a blood vessel by smooth muscle contraction, reducing blood flow. Main page: 3.1.2 Transport in Animals
  • Vasodilation: widening of a blood vessel by smooth muscle relaxation, increasing blood flow. Main page: 3.1.2 Transport in Animals
  • Plasma: the liquid component of blood; mostly water; transports dissolved substances. Main page: 3.1.2 Transport in Animals
  • Tissue fluid: fluid forced out of capillaries by hydrostatic pressure; surrounds cells and is the medium for local exchange. Main page: 3.1.2 Transport in Animals
  • Hydrostatic pressure: the outward pressure exerted by blood against capillary walls, driving fluid into tissue spaces. Main page: 3.1.2 Transport in Animals
  • Oncotic pressure: the osmotic pull exerted by plasma proteins that draws water back into capillaries at the venous end. Main page: 3.1.2 Transport in Animals
  • Lymph: fluid derived from excess tissue fluid; transported through lymphatic vessels; richer in lymphocytes and fatty acids than tissue fluid. Main page: 3.1.2 Transport in Animals
  • Haemoglobin: the iron-containing protein in red blood cells that reversibly binds oxygen. Main page: 3.1.2 Transport in Animals
  • Oxyhaemoglobin: haemoglobin with oxygen bound to its haem groups. Main page: 3.1.2 Transport in Animals
  • Oxygen dissociation curve: a sigmoidal graph showing the relationship between pO₂ and haemoglobin saturation; reflects cooperative binding. Main page: 3.1.2 Transport in Animals
  • Bohr effect: the reduction in haemoglobin's affinity for oxygen at higher pCO₂, caused by H⁺ ions from carbonic acid binding to haemoglobin. Main page: 3.1.2 Transport in Animals
  • Cooperative binding: the property of haemoglobin whereby binding one oxygen molecule increases the ease of binding subsequent oxygen molecules. Main page: 3.1.2 Transport in Animals
  • Fetal haemoglobin (HbF): haemoglobin with higher oxygen affinity than adult haemoglobin; allows oxygen uptake from the mother's blood across the placenta. Main page: 3.1.2 Transport in Animals
  • Carbonic anhydrase: enzyme in red blood cells that catalyses the interconversion of CO₂ and water with carbonic acid. Main page: 3.1.2 Transport in Animals
  • Chloride shift: movement of Cl⁻ ions into red blood cells as HCO₃⁻ exits, maintaining electrical neutrality. Main page: 3.1.2 Transport in Animals
  • Hydrogencarbonate ion (HCO₃⁻): the form in which ~70% of CO₂ is transported in blood plasma. Main page: 3.1.2 Transport in Animals
  • Carbaminohaemoglobin: haemoglobin with CO₂ bound directly to globin chains (~23% of CO₂ transport). Main page: 3.1.2 Transport in Animals
  • Aorta: the main artery leaving the left ventricle, carrying oxygenated blood to the systemic circuit. Main page: 3.1.2 Transport in Animals
  • Vena cava: the major vein returning deoxygenated blood from the body to the right atrium. Main page: 3.1.2 Transport in Animals
  • Atrium: an upper heart chamber that receives blood from veins. Main page: 3.1.2 Transport in Animals
  • Ventricle: a lower, thicker-walled heart chamber that pumps blood into arteries. Main page: 3.1.2 Transport in Animals
  • Septum: the muscular wall separating the left and right sides of the heart. Main page: 3.1.2 Transport in Animals
  • Atrioventricular valve: a valve (tricuspid or bicuspid/mitral) between an atrium and a ventricle; prevents backflow during ventricular systole. Main page: 3.1.2 Transport in Animals
  • Semilunar valve: a valve at the base of the aorta or pulmonary artery; prevents backflow into ventricles during diastole. Main page: 3.1.2 Transport in Animals
  • Cardiac output: heart rate × stroke volume; total volume of blood pumped per minute. Main page: 3.1.2 Transport in Animals
  • Myogenic: able to initiate contraction independently of nervous stimulation (property of cardiac muscle). Main page: 3.1.2 Transport in Animals
  • Sino-atrial node (SAN): the pacemaker of the heart; generates the initiating electrical impulse in the right atrium. Main page: 3.1.2 Transport in Animals
  • Atrio-ventricular node (AVN): conducts and delays the electrical impulse between atria and ventricles. Main page: 3.1.2 Transport in Animals
  • Bundle of His: conducting tissue conveying impulses from the AVN down the interventricular septum to the apex. Main page: 3.1.2 Transport in Animals
  • Purkyne fibres: branches from the bundle of His that rapidly conduct excitation through the ventricular walls from apex to base. Main page: 3.1.2 Transport in Animals
  • Diastole: the relaxation phase of the cardiac cycle when chambers fill with blood. Main page: 3.1.2 Transport in Animals
  • Systole: the contraction phase of the cardiac cycle when chambers pump blood. Main page: 3.1.2 Transport in Animals
  • ECG (electrocardiogram): a trace of the heart's electrical activity; P wave = atrial systole; QRS = ventricular systole; T wave = diastole. Main page: 3.1.2 Transport in Animals
  • Tachycardia: abnormally fast heart rate. Main page: 3.1.2 Transport in Animals
  • Bradycardia: abnormally slow heart rate. Main page: 3.1.2 Transport in Animals
  • Atrial fibrillation: irregular, disorganised atrial activity; QRS complexes irregularly spaced on ECG. Main page: 3.1.2 Transport in Animals

3.1.3 Transport in Plants

  • Xylem: vascular tissue transporting water and mineral ions upward through the plant; provides support through lignified cell walls. Main page: 3.1.3 Transport in Plants
  • Xylem vessel: an elongated, dead, hollow xylem cell with lignified, non-end-walled structure forming a continuous tube. Main page: 3.1.3 Transport in Plants
  • Lignin: a hard, waterproof polymer deposited in xylem cell walls, providing strength and waterproofing. Main page: 3.1.3 Transport in Plants
  • Pit: a non-lignified area in the xylem wall through which water and ions can move laterally. Main page: 3.1.3 Transport in Plants
  • Phloem: vascular tissue transporting assimilates (sucrose, amino acids) in both directions between sources and sinks. Main page: 3.1.3 Transport in Plants
  • Sieve tube element: a phloem cell connected end to end with others via perforated sieve plates; lacks nucleus and most organelles at maturity. Main page: 3.1.3 Transport in Plants
  • Sieve plate: a perforated end wall between sieve tube elements allowing flow of assimilates. Main page: 3.1.3 Transport in Plants
  • Companion cell: a metabolically active cell linked by plasmodesmata to a sieve tube element; provides energy and control for loading/unloading. Main page: 3.1.3 Transport in Plants
  • Plasmodesmata: cytoplasmic channels through plant cell walls connecting adjacent cells; key to the symplast pathway and to phloem loading. Main page: 3.1.3 Transport in Plants
  • Apoplast pathway: movement of water through cell walls and intercellular spaces, without crossing cell membranes. Main page: 3.1.3 Transport in Plants
  • Symplast pathway: movement of water through cytoplasm from cell to cell via plasmodesmata. Main page: 3.1.3 Transport in Plants
  • Casparian strip: a band of suberin in the walls of endodermal cells that blocks the apoplast pathway, forcing water into the symplast before it enters the xylem. Main page: 3.1.3 Transport in Plants
  • Suberin: the waterproof substance forming the Casparian strip. Main page: 3.1.3 Transport in Plants
  • Endodermis: the innermost layer of the root cortex, containing the Casparian strip. Main page: 3.1.3 Transport in Plants
  • Transpiration: evaporation and diffusion of water vapour from the aerial parts of a plant, mainly through stomata. Main page: 3.1.3 Transport in Plants
  • Transpiration stream: the continuous column of water pulled upward through the xylem by transpiration. Main page: 3.1.3 Transport in Plants
  • Cohesion: attraction between water molecules through hydrogen bonds; holds the water column together in xylem. Main page: 3.1.3 Transport in Plants
  • Adhesion: attraction between water molecules and the cellulose/lignin of xylem walls; helps maintain the water column. Main page: 3.1.3 Transport in Plants
  • Cohesion-tension theory: the explanation for upward xylem transport: transpiration creates tension which, transmitted through the cohesive water column, pulls water up from the roots. Main page: 3.1.3 Transport in Plants
  • Stomata: pores in the leaf epidermis, surrounded by guard cells, through which gas exchange and transpiration occur. Main page: 3.1.3 Transport in Plants
  • Guard cell: a cell that changes shape (swells or shrinks) to open or close the stoma, controlling gas exchange and water loss. Main page: 3.1.3 Transport in Plants
  • Potometer: apparatus for measuring water uptake by a cut plant shoot, used as a proxy for transpiration rate. Main page: 3.1.3 Transport in Plants
  • Translocation: the movement of assimilates in the phloem from source to sink. Main page: 3.1.3 Transport in Plants
  • Source: a region of a plant that produces or releases assimilates (e.g. photosynthesising leaf). Main page: 3.1.3 Transport in Plants
  • Sink: a region that consumes or stores assimilates (e.g. growing root, meristem, fruit). Main page: 3.1.3 Transport in Plants
  • Mass flow hypothesis: the explanation for translocation: active loading at the source creates high hydrostatic pressure in phloem; active unloading at the sink creates low pressure; the resulting gradient drives the flow of solution from source to sink. Main page: 3.1.3 Transport in Plants
  • Xerophyte: a plant adapted to dry environments, with structural features that reduce the rate of transpiration. Main page: 3.1.3 Transport in Plants
  • Hydrophyte: a plant adapted to aquatic or waterlogged environments. Main page: 3.1.3 Transport in Plants

Module 4: Biodiversity, Evolution And Disease

4.1.1 Communicable Diseases, Disease Prevention And The Immune System

4.2.1 Biodiversity

  • Biodiversity: the variety of life at habitat, species, and genetic levels. Main page: 4.2.1 Biodiversity
  • Habitat diversity: the range of different habitats in an area. Main page: 4.2.1 Biodiversity
  • Species richness: the total number of different species in a habitat. Main page: 4.2.1 Biodiversity
  • Species evenness: the relative abundance of each species; how evenly individuals are distributed across species. Main page: 4.2.1 Biodiversity
  • Genetic diversity: the variation in alleles within a population of a species. Main page: 4.2.1 Biodiversity
  • Polymorphic gene locus: a gene locus at which more than one allele exists in the population. Main page: 4.2.1 Biodiversity
  • Monomorphic gene locus: a gene locus at which only one allele exists. Main page: 4.2.1 Biodiversity
  • Proportion of polymorphic gene loci: number of polymorphic loci divided by total number of loci; a measure of genetic diversity. Main page: 4.2.1 Biodiversity
  • Gene flow: interbreeding between different populations that introduces new alleles, increasing genetic diversity. Main page: 4.2.1 Biodiversity
  • Genetic bottleneck: a sudden sharp reduction in population size that severely reduces genetic diversity. Main page: 4.2.1 Biodiversity
  • Founder effect: a reduction in genetic diversity when a small group establishes a new population. Main page: 4.2.1 Biodiversity
  • Genetic drift: random changes in allele frequency from generation to generation, which can reduce genetic diversity. Main page: 4.2.1 Biodiversity
  • Random sampling: sampling in which every location or organism has an equal probability of selection, using randomly generated coordinates. Main page: 4.2.1 Biodiversity
  • Systematic sampling: sampling at regular intervals, as in transect surveys. Main page: 4.2.1 Biodiversity
  • Stratified sampling: dividing the population into sub-groups (strata) and sampling each proportionally. Main page: 4.2.1 Biodiversity
  • Opportunistic sampling: sampling using conveniently available organisms; susceptible to bias. Main page: 4.2.1 Biodiversity
  • Quadrat: a square frame used to sample organisms in a defined area. Main page: 4.2.1 Biodiversity
  • Transect: a line along which samples are taken at regular intervals to study distribution changes across a gradient. Main page: 4.2.1 Biodiversity
  • Simpson's Index of Diversity (D): a measure combining species number and relative abundance, calculated as D = 1 − Σ(n/N)²; higher values indicate greater diversity. Main page: 4.2.1 Biodiversity
  • In situ conservation: conservation of species within their natural habitat. Main page: 4.2.1 Biodiversity
  • Ex situ conservation: conservation of species outside their natural habitat. Main page: 4.2.1 Biodiversity
  • CITES: Convention on International Trade in Endangered Species; makes trading endangered species internationally illegal. Main page: 4.2.1 Biodiversity
  • CBD: Convention on Biological Diversity; an international legal framework for biodiversity conservation and sustainable use. Main page: 4.2.1 Biodiversity
  • IUCN: International Union for Conservation of Nature; publishes the Red List of Threatened Species. Main page: 4.2.1 Biodiversity
  • Countryside Stewardship Scheme: a UK government scheme subsidising farmers to manage land sustainably. Main page: 4.2.1 Biodiversity
  • Monoculture: growing a single crop species over a large area, typically without rotation, which reduces biodiversity and depletes soil. Main page: 4.2.1 Biodiversity
  • Keystone species: a species with a disproportionately large effect on its ecosystem relative to its abundance. Main page: 4.2.1 Biodiversity

4.2.2 Classification And Evolution

Module 5: Communication, Homeostasis And Energy

5.1.1 Communication and Homeostasis

5.1.2 Excretion as an Example of Homeostatic Control

5.1.3 Neuronal Communication

  • Neurone: a specialised cell that transmits electrical impulses through the nervous system. Main page: 5.1.3 Neuronal Communication
  • Myelin sheath: lipid-rich insulating layer around many axons, formed by Schwann cells. Main page: 5.1.3 Neuronal Communication
  • Resting potential: the membrane potential of a neurone when it is not transmitting an impulse, usually about -70 mV. Main page: 5.1.3 Neuronal Communication
  • Threshold: the membrane potential that must be reached to trigger an action potential. Main page: 5.1.3 Neuronal Communication
  • Depolarisation: change in membrane potential in which the inside of the neurone becomes less negative. Main page: 5.1.3 Neuronal Communication
  • Repolarisation: return of the membrane potential towards its resting value after depolarisation. Main page: 5.1.3 Neuronal Communication
  • Action potential: rapid, temporary reversal of membrane potential that travels along an axon. Main page: 5.1.3 Neuronal Communication
  • Refractory period: short period after an action potential when another impulse cannot be generated normally. Main page: 5.1.3 Neuronal Communication
  • Saltatory conduction: transmission in a myelinated neurone where the action potential appears to jump between nodes of Ranvier. Main page: 5.1.3 Neuronal Communication
  • Synapse: junction between neurones, or between a neurone and an effector, across which signals are transmitted chemically. Main page: 5.1.3 Neuronal Communication
  • Neurotransmitter: chemical messenger released from a synaptic vesicle into the synaptic cleft. Main page: 5.1.3 Neuronal Communication
  • Summation: combining of excitatory and inhibitory inputs to determine whether threshold is reached. Main page: 5.1.3 Neuronal Communication
  • Neuromuscular junction: specialised synapse between a motor neurone and a skeletal muscle fibre. Main page: 5.1.3 Neuronal Communication

5.1.4 Hormonal Communication

5.1.5 Plant and Animal Responses

5.2.1 Photosynthesis

  • Chloroplast: organelle in plant and algal cells where photosynthesis takes place. Main page: 5.2.1 Photosynthesis
  • Photosystem II: pigment-protein complex that absorbs light and drives photolysis and electron excitation. Main page: 5.2.1 Photosynthesis
  • Photolysis: splitting of water by light to produce electrons, protons, and oxygen. Main page: 5.2.1 Photosynthesis
  • Photophosphorylation: synthesis of ATP using light energy during the light-dependent stage. Main page: 5.2.1 Photosynthesis
  • Chemiosmosis: ATP production driven by proton flow down an electrochemical gradient through ATP synthase. Main page: 5.2.1 Photosynthesis
  • Reduced NADP: electron-carrying molecule formed in the light-dependent stage and used in the Calvin cycle. Main page: 5.2.1 Photosynthesis
  • Photosystem I: pigment-protein complex that re-excites electrons so NADP can be reduced. Main page: 5.2.1 Photosynthesis
  • Cyclic photophosphorylation: pathway in which electrons cycle back to PSI and generate ATP but not reduced NADP. Main page: 5.2.1 Photosynthesis
  • RuBisCO: enzyme that catalyses fixation of CO2 to RuBP in the Calvin cycle. Main page: 5.2.1 Photosynthesis
  • GP: glycerate 3-phosphate, the first stable product of carbon fixation in the Calvin cycle. Main page: 5.2.1 Photosynthesis
  • TP: triose phosphate, the reduced three-carbon product used to make carbohydrates and regenerate RuBP. Main page: 5.2.1 Photosynthesis
  • RuBP: ribulose bisphosphate, the five-carbon carbon acceptor in the Calvin cycle. Main page: 5.2.1 Photosynthesis
  • Compensation point: light intensity at which photosynthesis and respiration occur at equal rates. Main page: 5.2.1 Photosynthesis

5.2.2 Respiration

  • Glycolysis: enzyme-controlled pathway in the cytoplasm that converts glucose to pyruvate. Main page: 5.2.2 Respiration
  • Link reaction: reaction in which pyruvate is decarboxylated and dehydrogenated to acetate before entering the Krebs cycle. Main page: 5.2.2 Respiration
  • Krebs cycle: cyclic pathway in the mitochondrial matrix that releases CO2 and reduces NAD and FAD. Main page: 5.2.2 Respiration
  • Oxidative phosphorylation: ATP production using electrons from reduced coenzymes and a proton gradient across the inner mitochondrial membrane. Main page: 5.2.2 Respiration
  • Substrate-level phosphorylation: ATP synthesis by direct transfer of phosphate from a phosphorylated intermediate to ADP. Main page: 5.2.2 Respiration
  • Dehydrogenation: removal of hydrogen from a molecule, usually reducing NAD or FAD. Main page: 5.2.2 Respiration
  • Decarboxylation: removal of carbon dioxide from a molecule. Main page: 5.2.2 Respiration
  • Reduced NAD: hydrogen-carrying coenzyme that donates electrons to the electron transport chain. Main page: 5.2.2 Respiration
  • Reduced FAD: coenzyme reduced during respiration that donates electrons to the electron transport chain. Main page: 5.2.2 Respiration
  • Chemiosmosis: ATP generation driven by proton flow through ATP synthase. Main page: 5.2.2 Respiration
  • Anaerobic respiration: release of energy without oxygen, regenerating NAD so glycolysis can continue. Main page: 5.2.2 Respiration
  • Respiratory quotient (RQ): ratio of carbon dioxide produced to oxygen consumed during respiration. Main page: 5.2.2 Respiration

Module 6: Genetics, Evolution And Ecosystems

6.1.1 Cellular Control

  • Gene mutation: change in the base sequence of DNA within a gene. Main page: 6.1.1 Cellular Control
  • Frameshift mutation: mutation caused by insertion or deletion that shifts the reading frame of downstream codons. Main page: 6.1.1 Cellular Control
  • Missense mutation: substitution mutation that changes one amino acid in the polypeptide. Main page: 6.1.1 Cellular Control
  • Nonsense mutation: substitution mutation that creates a stop codon and truncates translation. Main page: 6.1.1 Cellular Control
  • Silent mutation: substitution mutation that does not change the amino acid sequence because of code degeneracy. Main page: 6.1.1 Cellular Control
  • Lac operon: inducible gene-control system in E. coli that regulates lactose-metabolising enzymes. Main page: 6.1.1 Cellular Control
  • Repressor protein: regulatory protein that binds to an operator and blocks transcription. Main page: 6.1.1 Cellular Control
  • Transcription factor: protein that binds specific DNA sequences to increase or decrease transcription. Main page: 6.1.1 Cellular Control
  • Alternative splicing: production of different mature mRNA molecules from the same pre-mRNA by joining different exon combinations. Main page: 6.1.1 Cellular Control
  • Post-translational modification: chemical change to a polypeptide after translation that alters its activity or function. Main page: 6.1.1 Cellular Control
  • Homeobox gene: developmental control gene that codes for a transcription factor determining body plan. Main page: 6.1.1 Cellular Control
  • Apoptosis: programmed, controlled cell death. Main page: 6.1.1 Cellular Control
  • Caspase: protease enzyme that drives the ordered breakdown of the cell during apoptosis. Main page: 6.1.1 Cellular Control

6.1.2 Patterns of Inheritance

6.1.3 Manipulating Genomes

6.2.1 Cloning and Biotechnology

6.3.1 Ecosystems

  • Ecosystem: all the organisms in an area together with the abiotic components of their environment. Main page: 6.3.1 Ecosystems
  • Niche: the role of an organism in its environment, including how it uses resources and interacts with other organisms. Main page: 6.3.1 Ecosystems
  • Succession: directional change in community composition over time. Main page: 6.3.1 Ecosystems
  • Pioneer species: first species to colonise an area during succession. Main page: 6.3.1 Ecosystems
  • Climax community: relatively stable end-stage community in a succession sequence. Main page: 6.3.1 Ecosystems
  • Gross primary productivity (GPP): total chemical energy fixed by photosynthesis in an ecosystem. Main page: 6.3.1 Ecosystems
  • Net primary productivity (NPP): energy remaining in plant biomass after respiratory losses; NPP = GPP - R. Main page: 6.3.1 Ecosystems
  • Ammonification: conversion of organic nitrogen in dead material or waste into ammonia or ammonium compounds. Main page: 6.3.1 Ecosystems
  • Nitrification: oxidation of ammonium compounds to nitrites and then nitrates by nitrifying bacteria. Main page: 6.3.1 Ecosystems
  • Denitrification: reduction of nitrates to nitrogen gas by anaerobic bacteria. Main page: 6.3.1 Ecosystems
  • Quadrat: square frame used to sample plants or slow-moving organisms in a measured area. Main page: 6.3.1 Ecosystems
  • Line transect: sampling method in which organisms touching a line are recorded along a gradient. Main page: 6.3.1 Ecosystems

6.3.2 Populations and Sustainability