IGCSE Biology Key Terms Glossary¶

This glossary aggregates the ## Key Terms sections from the maintained topic pages. Each definition stays grouped under the topic where it is taught in full so revision can move quickly from term recall back to fuller explanation.

How To Use This Glossary¶

Use the linked topic page when a definition needs examples, diagrams or process detail.
Compare repeated ideas across modules rather than assuming the same term is always being used in exactly the same context.
Treat the topic pages as the main study layer; this glossary is the fast lookup layer.

1 The Nature and Variety of Living Organisms¶

1a Characteristics of Living Organisms ¶

Excretion: removal of waste products of metabolism and other unwanted materials.
Homeostasis: maintenance of a stable internal environment.
Nutrition: taking in molecules needed for energy, growth and repair.
Respiration: the chemical reactions in cells that release energy from food.
Stimulus: a change in the internal or external environment that can be detected.

1b Variety of Living Organisms ¶

Eukaryote: an organism whose cells contain genetic material enclosed in a nucleus.
Hypha: a thread-like fungal filament.
Mycelium: the network of hyphae that forms the body of many fungi.
Pathogen: an organism or agent that causes disease.
Prokaryote: an organism made of cells that do not contain a nucleus.
Protoctist: a mostly single-celled eukaryotic organism that does not fit into the plant, animal or fungal groups.
Saprotrophic nutrition: feeding by extracellular digestion followed by absorption of the soluble products.
Virus: a non-cellular infectious particle made of nucleic acid inside a protein coat.

2 Structure and Functions in Living Organisms¶

2a Level of Organisation ¶

Cell: the basic structural and functional unit of a living organism.
Organ: a structure made of different tissues working together.
Organ system: a group of organs that work together to carry out a major body function.
Organelle: a specialised structure inside a cell that carries out a particular job.
Tissue: a group of similar cells working together to perform a particular function.

2b Cell Structure ¶

Cell membrane: a partially permeable membrane surrounding the cell that controls what enters and leaves the cell.
Chloroplast: the organelle that contains chlorophyll and carries out photosynthesis.
Differentiation: the process by which a cell becomes specialised for a particular function.
Mitochondrion: the organelle where most aerobic respiration occurs.
Nucleus: the structure that contains genetic material.
Ribosome: the site of protein synthesis.
Stem cell: a cell that can divide and then differentiate into specialised cell types.
Vacuole: a fluid-filled space in plant cells that contains cell sap.

2c Biological Molecules ¶

Active site: the part of an enzyme where the substrate fits and the reaction is catalysed.
Carbohydrate: a biological molecule made from carbon, hydrogen and oxygen; used in energy storage and release.
Denature: to change shape in a way that makes an enzyme lose function.
Enzyme: a protein that acts as a biological catalyst.
Lipid: a biological molecule made from fatty acids and glycerol.
Protein: a biological molecule made from amino acids and containing nitrogen.

2d Movement of Substances into and out of Cells ¶

Active transport: movement of substances against a concentration gradient using energy from respiration.
Concentration gradient: the difference in concentration between two regions.
Diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration.
Osmosis: the net movement of water through a partially permeable membrane from a dilute solution to a more concentrated solution, or from high water potential to low water potential.
Partially permeable membrane: a membrane that lets some substances pass through more easily than others.
Surface area to volume ratio: a measure comparing exchange surface with internal volume.

2e Nutrition ¶

Amylase: a digestive enzyme that breaks starch into maltose.
Bile: an alkaline fluid made by the liver, stored in the gallbladder, that neutralises stomach acid and emulsifies fats.
Chlorophyll: the green pigment that absorbs light energy for photosynthesis.
Dietary fibre: indigestible plant material that helps food move through the gut.
Lacteal: a lymph vessel inside a villus that absorbs fatty acids and glycerol.
Limiting factor: the factor in least supply that controls the rate of photosynthesis.
Lipase: a digestive enzyme that breaks lipids into fatty acids and glycerol.
Peristalsis: wave-like contractions of muscles that move food through the gut.
Photosynthesis: the process by which plants make glucose from carbon dioxide and water using light energy.
Villus: a finger-like projection in the small intestine that increases surface area for absorption.

2f Respiration ¶

Aerobic respiration: respiration that uses oxygen and releases a large amount of energy.
Anaerobic respiration: respiration that occurs without oxygen and releases less energy.
ATP: an energy-carrying molecule used to power cell activities.
Fermentation: anaerobic respiration in yeast or plant cells producing ethanol and carbon dioxide.
Hydrogen-carbonate indicator: a solution that changes colour with carbon dioxide concentration, used to detect respiration.
Lactic acid: the product of anaerobic respiration in animal cells.
Oxygen debt: the extra oxygen needed after anaerobic exercise to remove lactic acid from the body.
Respiration: the chemical process in cells that releases energy from food.

2g Gas Exchange ¶

Alveolus: a tiny air sac in the lungs where gas exchange occurs.
Bronchi: the two airways that branch from the trachea and enter the lungs.
Bronchioles: small airways branching from the bronchi that lead to the alveoli.
Diaphragm: a sheet of muscle beneath the lungs that changes shape during breathing.
Guard cell: a cell that changes shape to open or close a stoma.
Intercostal muscles: muscles between the ribs that help move the chest during breathing.
Pleural membranes: membranes that lubricate the surface of the lungs and the chest wall.
Pressure gradient: a difference in air pressure that causes air to move from a region of higher pressure to a region of lower pressure.
Stoma: a pore in the leaf surface through which gases and water vapour move.
Ventilation: the movement of air into and out of the lungs.

2h Transport ¶

Antibody: a specific protein produced by lymphocytes in response to an antigen.
Antigen: a molecule on the surface of a pathogen that triggers an immune response.
Aorta: the main artery leaving the left ventricle, carrying oxygenated blood to the body.
Atherosclerosis: the build-up of fatty plaques in artery walls, narrowing the lumen.
Capillary: a tiny blood vessel with a thin wall for exchange.
Coronary heart disease (CHD): disease caused by narrowing of the coronary arteries, reducing blood flow to the heart muscle.
Double circulatory system: a circulatory system with two separate circuits, one to the lungs and one to the body.
Fibrin: the insoluble protein that forms the mesh of a blood clot.
Haemoglobin: the red pigment in red blood cells that carries oxygen.
Immunity: protection against a disease because memory cells can trigger a rapid secondary response.
Lymphocyte: a white blood cell that produces antibodies.
Memory cell: a long-lived immune cell that enables rapid antibody production on re-exposure to a pathogen.
Phagocyte: a white blood cell that engulfs and digests pathogens.
Phloem: plant tissue that transports sucrose and amino acids around the plant.
Plasma: the liquid part of blood that transports dissolved substances and cells.
Platelet: a blood fragment involved in clotting.
Translocation: the movement of dissolved sugars and amino acids through the phloem.
Transpiration: the evaporation of water from the surface of a plant.
Transpiration stream: the continuous movement of water from roots through xylem to leaves, driven by transpiration.
Vaccination: the introduction of dead or inactive pathogen material to stimulate immunity without causing disease.
Vena cava: the main vein returning deoxygenated blood from the body to the right atrium.
Xylem: plant tissue that transports water and mineral ions from the roots.

2i Excretion ¶

ADH (antidiuretic hormone): a hormone that regulates how much water is reabsorbed by the kidneys.
Bowman's capsule: the cup-shaped structure at the start of the nephron that collects the filtrate.
Collecting duct: the final section of the nephron where water reabsorption is regulated by ADH.
Excretion: removal of waste products of metabolism and other unwanted materials.
Glomerulus: the microscopic knot of capillaries where ultrafiltration occurs.
Nephron: the microscopic unit in the kidney that filters blood and forms urine.
Osmoregulation: control of the water content of the blood.
Selective reabsorption: the process of taking useful substances back into the blood from the filtrate.
Ureter: the tube that carries urine from the kidney to the bladder.
Urethra: the tube that carries urine from the bladder to the outside of the body.
Ultrafiltration: pressure filtration of small molecules from blood into Bowman's capsule.
Urea: a nitrogen-containing waste product made from excess amino acids.

2j Co-ordination and Response ¶

Adrenaline: a hormone that prepares the body for increased activity.
Tropism: a directional growth response of a plant to a stimulus.
Auxin: a plant hormone involved in growth responses such as phototropism.
Effector: a muscle or gland that brings about a response.
Geotropism (gravitropism): a directional growth response of a plant to gravity.
Homeostasis: maintenance of a stable internal environment.
Insulin: a hormone that lowers blood glucose concentration.
Neurotransmitter: a chemical that transmits a signal across a synapse.
Phototropism: a directional growth response of a plant to light.
Receptor: a cell or organ that detects a stimulus.
Reflex arc: the neural pathway involved in a rapid automatic response.

3 Reproduction and Inheritance¶

3a Reproduction ¶

Amniotic fluid: fluid that protects the developing embryo.
Fertilisation: fusion of male and female gametes to form a zygote.
FSH (follicle-stimulating hormone): a hormone involved in follicle development in the ovary.
Germination: the start of growth of a seed into a seedling.
LH (luteinising hormone): a hormone that triggers ovulation.
Placenta: the organ that allows exchange between mother and embryo.
Pollen tube: the tube that grows from a pollen grain to the ovule.
Pollination: transfer of pollen from anther to stigma.
Zygote: the first cell formed after fertilisation.

3b Inheritance ¶

Allele: an alternative form of a gene.
Codominance: a pattern in which both alleles contribute to the phenotype.
Dominant: describing an allele that is expressed when it is present in the genotype.
Evolution: the gradual change in the inherited characteristics of a population over generations.
Gene: a section of DNA that codes for a specific protein.
Genome: the entire DNA of an organism.
Genotype: the combination of alleles an organism has.
Heterozygous: having two different alleles of a gene.
Homozygous: having two identical alleles of a gene.
Meiosis: cell division producing four genetically different haploid cells.
Mitosis: cell division producing two genetically identical daughter cells.
Mutation: a rare random change in genetic material.
Natural selection: the mechanism by which evolution occurs; a process by which advantageous alleles become more frequent in a population over time.
Phenotype: the observable characteristics of an organism.
Recessive: describing an allele expressed only when both alleles are recessive.

4 Ecology and the Environment¶

4a The Organism in the Environment ¶

Abiotic factor: a non-living factor affecting organisms in a habitat.
Biodiversity: the range of species living in an ecosystem.
Biotic factor: a living factor affecting organisms in a habitat.
Community: all the populations of different species living together in an area.
Ecosystem: the interaction of a community with the non-living environment.
Habitat: the place where an organism lives.
Population: all the organisms of one species in a particular area.
Quadrat: a square frame of known area used for ecological sampling.
Transect: a line across a habitat along which samples are taken.

4b Feeding Relationships ¶

Consumer: an organism that gets food by eating other organisms.
Decomposer: an organism that breaks down dead material and waste.
Food web: a network of interconnected food chains.
Producer: an organism that makes its own food, usually by photosynthesis.
Pyramid of biomass: a diagram showing the biomass at each trophic level.
Pyramid of energy transfer: a diagram showing the amount of energy transferred at each trophic level.
Trophic level: a feeding position in a food chain or food web.

4c Cycles within Ecosystems ¶

Combustion: burning of fuel, releasing carbon dioxide.
Decomposer: an organism that breaks down dead organic matter and waste.
Decomposition: breakdown of dead material by decomposers.
Denitrifying bacteria: bacteria that return nitrogen to the atmosphere.
Nitrifying bacteria: bacteria that convert nitrogen compounds in the soil into nitrates.
Nitrogen fixation: conversion of atmospheric nitrogen into usable nitrogen compounds.

4d Human Influences on the Environment ¶

Eutrophication: nutrient enrichment of water leading to excessive algal growth and oxygen depletion.
Evapotranspiration: combined water loss from evaporation and plant transpiration.
Global warming: long-term increase in average global temperature caused by an enhanced greenhouse effect.
Greenhouse gas: a gas that traps thermal energy in the atmosphere.
Leaching: washing of dissolved substances out of the soil by water.
Sewage pollution: water pollution caused by waste rich in organic matter.
Soil erosion: wearing away and removal of topsoil.

5 Use of Biological Resources¶

5a Food Production ¶

Agitation: stirring or mixing that keeps fermenter contents uniform.
Anaerobic respiration: respiration without oxygen that releases less energy than aerobic respiration.
Aseptic precautions: steps taken to prevent contamination by unwanted microorganisms.
Biological control: use of living organisms to control pests.
Fermenter: a vessel used to grow microorganisms under controlled conditions.
Fertiliser: a substance added to soil to provide mineral ions needed for plant growth.
Fish farming: raising large numbers of fish under controlled conditions as a food source.
Lactobacillus: a bacterium used in yoghurt production.

5b Selective Breeding ¶

Desired characteristic: a trait that breeders want to increase in a population.
Gene pool: the full range of alleles present in a population.
Inbreeding: breeding between closely related individuals.
Selective breeding: choosing parents with desired characteristics and breeding them over generations.

5c Genetic Modification (Genetic Engineering)¶

Ligase: an enzyme that joins pieces of DNA together.
Plasmid: a small circular piece of bacterial DNA that can be used as a vector.
Recombinant DNA: DNA formed by joining genetic material from different sources.
Restriction enzyme: an enzyme that cuts DNA at specific base sequences.
Transgenic: describing an organism that contains genetic material transferred from another species.
Vector: a carrier such as a plasmid or virus used to transfer DNA into cells.

5d Cloning ¶

Cloning: producing genetically identical copies of cells or organisms.
Diploid nucleus: a nucleus containing the full set of paired chromosomes.
Enucleated egg cell: an egg cell from which the nucleus has been removed.
Explant: a small piece of plant tissue used to start tissue culture.
In vitro: grown or carried out outside the organism in controlled conditions.
Micropropagation: cloning plants by growing explants in vitro on nutrient medium.