IGCSE Biology Definitions Sheet: Every Key Term You Need to Know

Studying for IGCSE Biology means mastering a huge number of technical terms. Examiners frequently award marks for precise definitions, so knowing the exact wording can make the difference between a grade 7 and a grade 9.

This comprehensive IGCSE Biology definitions sheet organises every essential term by topic, matching the Cambridge IGCSE Biology (0610/0970) syllabus. Bookmark this page or print it out for quick revision.

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1. Characteristics and Classification of Living Organisms

Term	Definition
Movement	An action by an organism or part of an organism causing a change of position or place.
Respiration	The chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
Sensitivity	The ability to detect and respond to changes in the internal or external environment.
Growth	A permanent increase in size and dry mass.
Reproduction	The processes that make more of the same kind of organism.
Excretion	The removal of toxic materials and substances in excess of requirements from the body.
Nutrition	The taking in of materials for energy, growth and development.
Species	A group of organisms that can reproduce to produce fertile offspring.
Binomial nomenclature	The system of naming organisms using two names — the genus and species.

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2. Cells

Term	Definition
Cell	The basic structural and functional unit of all living organisms.
Cell membrane	A partially permeable membrane that controls the movement of substances into and out of the cell.
Cytoplasm	A jelly-like substance where most chemical reactions take place.
Nucleus	The organelle that contains genetic material (DNA) and controls the activities of the cell.
Cell wall	A rigid outer covering made of cellulose that provides support and protection in plant cells.
Chloroplast	An organelle containing chlorophyll for photosynthesis, found in green plant cells.
Vacuole	A fluid-filled space in plant cells that stores cell sap and helps maintain turgor pressure.
Mitochondria	Organelles where aerobic respiration takes place, releasing energy for the cell.
Diffusion	The net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.
Osmosis	The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
Active transport	The movement of particles against a concentration gradient using energy from respiration.

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3. Enzymes

Term	Definition
Enzyme	A biological catalyst — a protein that speeds up the rate of a chemical reaction without being used up.
Active site	The region of an enzyme molecule where substrate molecules bind and undergo a chemical reaction.
Lock-and-key hypothesis	A model explaining enzyme specificity — the substrate fits exactly into the active site like a key into a lock.
Denaturation	A permanent change in the shape of the active site of an enzyme, caused by high temperature or extreme pH, so that the substrate can no longer fit.
Optimum temperature	The temperature at which an enzyme works at its maximum rate.
Optimum pH	The pH at which an enzyme works at its maximum rate.

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4. Nutrition in Plants

Term	Definition
Photosynthesis	The process by which plants manufacture glucose from carbon dioxide and water using light energy, releasing oxygen as a by-product.
Chlorophyll	The green pigment found in chloroplasts that absorbs light energy for photosynthesis.
Limiting factor	A factor that directly controls a process when it is in short supply (e.g. light intensity, CO₂ concentration, temperature).
Xylem	Tissue that transports water and dissolved mineral ions from roots to leaves.
Phloem	Tissue that transports dissolved sugars (sucrose) from leaves to other parts of the plant.
Transpiration	The loss of water vapour from the leaves of a plant through the stomata.
Stomata	Small pores on the surface of leaves that allow gas exchange and water vapour loss.

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5. Nutrition in Humans

Term	Definition
Balanced diet	A diet that contains all the essential nutrients in the correct proportions for healthy functioning.
Ingestion	The taking of food into the body through the mouth.
Digestion	The breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed.
Absorption	The movement of digested food molecules through the wall of the intestine into the blood.
Assimilation	The uptake and use of nutrients by cells for energy, growth and repair.
Egestion	The removal of undigested food from the body as faeces.
Peristalsis	Waves of muscular contraction that push food along the alimentary canal.
Villi	Finger-like projections in the small intestine that increase the surface area for absorption.

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6. Transport in Plants and Animals

Term	Definition
Circulatory system	The organ system that transports blood, nutrients, gases and waste products around the body.
Artery	A blood vessel that carries blood away from the heart under high pressure with thick, muscular walls.
Vein	A blood vessel that carries blood towards the heart under low pressure, containing valves to prevent backflow.
Capillary	A tiny blood vessel with walls one cell thick that allows exchange of substances between blood and tissues.
Haemoglobin	The red pigment in red blood cells that combines reversibly with oxygen to form oxyhaemoglobin.
Plasma	The liquid component of blood that carries dissolved substances such as glucose, urea and hormones.
Platelet	A cell fragment involved in blood clotting.
Translocation	The movement of sucrose and amino acids in phloem from sources (leaves) to sinks (roots, fruits).

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7. Respiration

Term	Definition
Aerobic respiration	The release of a large amount of energy from glucose in the presence of oxygen.
Anaerobic respiration	The release of a small amount of energy from glucose without oxygen.
Lactic acid	The waste product of anaerobic respiration in animals.
Ethanol	The waste product of anaerobic respiration in yeast (along with carbon dioxide).
Oxygen debt	The extra oxygen needed after anaerobic respiration to break down the accumulated lactic acid.

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8. Gas Exchange

Term	Definition
Gas exchange	The movement of oxygen into the blood and carbon dioxide out of the blood in the lungs.
Alveoli	Tiny air sacs in the lungs with thin walls and a large surface area where gas exchange occurs.
Diaphragm	A sheet of muscle beneath the lungs that contracts and flattens during inhalation.
Trachea	The windpipe — a tube reinforced with C-shaped cartilage rings that carries air to the lungs.
Bronchi	Two branches of the trachea that lead into the left and right lungs.

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9. Excretion and Homeostasis

Term	Definition
Homeostasis	The maintenance of a constant internal environment within narrow limits.
Excretion	The removal from the body of toxic waste products of metabolism.
Kidney	An organ that filters blood, removes urea and excess water and ions, and produces urine.
Nephron	The functional unit of the kidney where filtration and reabsorption occur.
Urea	A nitrogenous waste product formed in the liver from the deamination of excess amino acids.
Deamination	The removal of the amino group from excess amino acids in the liver, forming urea.
Negative feedback	A mechanism where a change in a condition triggers a response that reverses the change, restoring the condition to normal.

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10. Coordination and Response

Term	Definition
Nervous system	The organ system that detects stimuli and coordinates responses using electrical impulses.
Stimulus	A change in the environment that is detected by a receptor.
Receptor	A cell or organ that detects a stimulus.
Effector	A muscle or gland that carries out a response.
Synapse	The junction between two neurones where transmission occurs via a chemical neurotransmitter.
Reflex arc	The pathway of a reflex action: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response.
Hormone	A chemical substance produced by an endocrine gland, carried in the blood, that alters the activity of one or more target organs.
Adrenaline	A hormone secreted by the adrenal glands in response to stress or danger, increasing heart rate and blood glucose.
Insulin	A hormone secreted by the pancreas that lowers blood glucose concentration by promoting conversion of glucose to glycogen.

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11. Reproduction

Term	Definition
Asexual reproduction	A process resulting in genetically identical offspring from one parent, without fusion of gametes.
Sexual reproduction	A process involving the fusion of two gametes to form a zygote, resulting in genetic variation.
Gamete	A sex cell (sperm or egg) that is haploid — containing half the chromosome number.
Fertilisation	The fusion of the nuclei of a male and female gamete to form a zygote.
Zygote	The cell formed by the fusion of two gametes; it is diploid.
Placenta	The organ that provides the foetus with oxygen and nutrients and removes waste via the mother’s blood.
Pollination	The transfer of pollen from the anther to the stigma of a flower.

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12. Inheritance

Term	Definition
Gene	A length of DNA that codes for a specific protein.
Allele	A version of a gene — different alleles code for different variations of a characteristic.
Dominant allele	An allele that is expressed in the phenotype when at least one copy is present.
Recessive allele	An allele that is only expressed in the phenotype when two copies are present (homozygous recessive).
Genotype	The genetic makeup of an organism — the alleles it possesses for a particular gene.
Phenotype	The observable characteristics of an organism, resulting from the interaction of genotype and environment.
Homozygous	Having two identical alleles for a particular gene.
Heterozygous	Having two different alleles for a particular gene.
Mutation	A random change in the base sequence of DNA that may alter a protein and therefore a characteristic.
Natural selection	The process whereby organisms with favourable traits are more likely to survive and reproduce, passing on those traits.

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13. Ecology

Term	Definition
Ecosystem	A community of living organisms interacting with each other and their non-living environment.
Habitat	The place where an organism lives.
Population	A group of organisms of the same species living in the same area at the same time.
Community	All the populations of different species living and interacting in the same area.
Food chain	A diagram showing the transfer of energy from one organism to the next, starting with a producer.
Producer	An organism that makes its own food by photosynthesis (autotroph).
Consumer	An organism that obtains energy by feeding on other organisms.
Decomposer	An organism that breaks down dead organic matter, returning nutrients to the soil.
Trophic level	The position of an organism in a food chain.
Biodiversity	The variety of species in an ecosystem.

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Exam Tips for IGCSE Biology Definitions

1. Use the exact Cambridge wording — examiners look for specific keywords like “net movement,” “concentration gradient,” and “partially permeable membrane.”
2. Don’t confuse excretion with egestion — excretion removes metabolic waste; egestion removes undigested food.
3. Learn the difference between osmosis and diffusion — osmosis specifically involves water through a partially permeable membrane.
4. Practice definition recall — write out definitions from memory, then check against this sheet.
5. Use flashcards — put the term on one side and the definition on the other for active recall practice.

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Last updated: March 2026. Based on the Cambridge IGCSE Biology (0610/0970) syllabus.

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