IGCSE Biology Key Definitions: The Ultimate Revision Sheet

Studying for your IGCSE Biology exam requires a solid understanding of key terminology. Whether you are preparing for Cambridge IGCSE (0610) or Edexcel International GCSE Biology, knowing precise definitions can make the difference between a good grade and a great one. This comprehensive definition sheet covers every major topic in the IGCSE Biology syllabus so you can revise efficiently and confidently.

Bookmark this page or print it out — it is designed to be your go-to reference throughout your Biology revision journey.

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1. Cells and Organisation

Cell — The basic structural and functional unit of all living organisms.

Cell membrane — A thin, partially permeable barrier surrounding the cell that controls the movement of substances in and out of the cell.

Cell wall — A rigid outer layer found in plant cells, made of cellulose, that provides structural support and protection.

Cytoplasm — The jelly-like substance inside the cell where most chemical reactions take place.

Nucleus — The organelle that contains genetic material (DNA) and controls the activities of the cell.

Mitochondria — Organelles where aerobic respiration occurs, releasing energy for the cell’s processes.

Chloroplast — An organelle found in plant cells and algae that contains chlorophyll and is the site of photosynthesis.

Vacuole — A fluid-filled space in a cell. In plant cells, the large central vacuole stores cell sap and helps maintain turgor pressure.

Ribosome — A small organelle responsible for protein synthesis.

Tissue — A group of similar cells that work together to perform a specific function.

Organ — A structure made up of different tissues working together to carry out a particular function.

Organ system — A group of organs that work together to perform a major bodily function.

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2. Biological Molecules

Carbohydrate — An organic molecule made of carbon, hydrogen, and oxygen, used as an energy source. Examples include glucose, starch, and glycogen.

Protein — A large biological molecule made of amino acids, essential for growth, repair, and enzyme function.

Lipid — A biological molecule (fat or oil) that stores energy, provides insulation, and forms cell membranes.

Enzyme — A biological catalyst, usually a protein, that speeds up a specific chemical reaction without being used up.

Active site — The specific region on an enzyme where the substrate binds and the reaction is catalysed.

Substrate — The molecule upon which an enzyme acts.

Denaturation — A permanent change in the shape of an enzyme’s active site, usually caused by high temperature or extreme pH, so that the substrate can no longer fit.

Lock and key model — A model describing enzyme action in which the enzyme’s active site has a specific shape complementary to the substrate, like a lock and key.

Catalyst — A substance that increases the rate of a chemical reaction without being consumed in the process.

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3. Movement of Substances

Diffusion — The net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.

Osmosis — The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.

Active transport — The movement of substances against a concentration gradient, from low to high concentration, using energy from respiration.

Partially permeable membrane — A membrane that allows some molecules to pass through but not others, based on size or other properties.

Turgor pressure — The pressure exerted by water inside a plant cell pushing against the cell wall, keeping the cell firm.

Plasmolysis — The shrinkage of the cytoplasm away from the cell wall when a plant cell loses water in a hypertonic solution.

Concentration gradient — The difference in the concentration of a substance between two areas.

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4. Photosynthesis and Nutrition

Photosynthesis — The process by which green plants and algae use light energy to convert carbon dioxide and water into glucose and oxygen.

Photosynthesis equation — 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light and chlorophyll).

Chlorophyll — The green pigment found in chloroplasts that absorbs light energy for photosynthesis.

Limiting factor — A factor that, when in short supply, restricts the rate of a process such as photosynthesis (e.g., light intensity, CO₂ concentration, temperature).

Balanced diet — A diet that contains the correct amounts of all the necessary nutrients (carbohydrates, proteins, fats, vitamins, minerals, water, and fibre) for healthy living.

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 process by which digested food molecules pass through the wall of the small intestine into the blood.

Assimilation — The process by which absorbed food molecules are used 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 lining the small intestine that increase the surface area for efficient absorption of nutrients.

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5. Respiration

Aerobic respiration — The release of energy from glucose in the presence of oxygen. The equation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.

Anaerobic respiration — The release of energy from glucose without the use of oxygen. In animals it produces lactic acid; in yeast it produces ethanol and carbon dioxide.

Fermentation — Anaerobic respiration in yeast and some bacteria, producing ethanol and carbon dioxide.

Oxygen debt — The amount of extra oxygen needed after anaerobic respiration to break down the accumulated lactic acid.

ATP — Adenosine triphosphate, the molecule that stores and provides energy for cellular processes.

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6. Gas Exchange and Breathing

Gas exchange — The process by which oxygen moves into the blood and carbon dioxide moves out, occurring at the alveoli in the lungs.

Alveoli — Tiny air sacs in the lungs where gas exchange takes place. They have a large surface area, thin walls, and a rich blood supply.

Trachea — The windpipe; a tube reinforced with cartilage rings that carries air from the throat to the bronchi.

Bronchi — Two tubes that branch from the trachea and lead into the lungs.

Bronchioles — Smaller tubes that branch from the bronchi within the lungs, leading to the alveoli.

Diaphragm — A sheet of muscle beneath the lungs that contracts and flattens during inhalation, helping to draw air into the lungs.

Intercostal muscles — Muscles between the ribs that assist in breathing by moving the rib cage up and out (external) or down and in (internal).

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7. Transport in Plants and Animals

Xylem — Plant tissue that transports water and dissolved minerals from the roots to the leaves. Xylem vessels are dead, hollow tubes with lignified walls.

Phloem — Plant tissue that transports dissolved sugars (sucrose) and amino acids from the leaves to other parts of the plant (translocation).

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. Their opening and closing is controlled by guard cells.

Guard cells — Specialised cells surrounding stomata that control their opening and closing.

Translocation — The movement of dissolved sugars and other organic substances through the phloem from the leaves to the rest of the plant.

Artery — A blood vessel that carries blood away from the heart, with thick muscular walls and a narrow lumen.

Vein — A blood vessel that carries blood towards the heart, with thinner walls, a wider lumen, and valves to prevent backflow.

Capillary — A tiny blood vessel with walls one cell thick, allowing the exchange of substances between blood and tissues.

Plasma — The liquid component of blood that carries dissolved substances such as glucose, hormones, and carbon dioxide.

Red blood cells (erythrocytes) — Cells containing haemoglobin that transport oxygen around the body. They are biconcave in shape and lack a nucleus.

White blood cells (leucocytes) — Cells that are part of the immune system. Phagocytes engulf pathogens; lymphocytes produce antibodies.

Platelets — Cell fragments that play a key role in blood clotting at wound sites.

Haemoglobin — The iron-containing protein in red blood cells that binds to oxygen to form oxyhaemoglobin for transport.

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8. Coordination and Response

Nervous system — The organ system consisting of the brain, spinal cord, and nerves that detects stimuli and coordinates responses.

Stimulus — A change in the environment that is detected by a receptor.

Receptor — A cell or organ that detects a stimulus (e.g., light receptors in the eye).

Effector — A muscle or gland that carries out a response to a stimulus.

Neurone — A nerve cell that transmits electrical impulses.

Sensory neurone — A neurone that carries impulses from a receptor to the central nervous system.

Motor neurone — A neurone that carries impulses from the central nervous system to an effector.

Relay neurone — A neurone found in the central nervous system that connects sensory and motor neurones.

Synapse — The junction between two neurones where a chemical neurotransmitter is released to transmit the impulse across the gap.

Reflex arc — The pathway taken by a nerve impulse in a reflex action: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response.

Hormone — A chemical substance produced by an endocrine gland and carried in the blood to a target organ where it produces an effect.

Homeostasis — The maintenance of a constant internal environment within the body.

Negative feedback — A control mechanism where a change in a condition triggers a response that reverses the change, returning conditions to normal.

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9. Reproduction

Asexual reproduction — Reproduction involving only one parent, producing genetically identical offspring (clones) with no fusion of gametes.

Sexual reproduction — Reproduction involving the fusion of male and female gametes to produce offspring with genetic variation.

Gamete — A sex cell (sperm or egg) that is haploid, containing half the normal number of chromosomes.

Fertilisation — The fusion of a male gamete with a female gamete to form a zygote.

Zygote — The cell formed by the fusion of two gametes; the first cell of a new organism.

Pollination — The transfer of pollen from an anther to a stigma.

Germination — The process by which a seed begins to grow and develop into a new plant when conditions are favourable.

Placenta — The organ in pregnant mammals that allows exchange of nutrients, oxygen, and waste between the mother’s blood and the foetus’s blood.

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10. Inheritance and Genetics

DNA (deoxyribonucleic acid) — The molecule that carries genetic information in all living organisms, found in the nucleus.

Gene — A short section of DNA on a chromosome that codes for a specific protein or characteristic.

Chromosome — A thread-like structure of DNA and protein found in the nucleus. Humans have 46 chromosomes (23 pairs).

Allele — An alternative form of a gene that can occupy the same position (locus) on a chromosome.

Dominant allele — An allele that is expressed in the phenotype even when only one copy is present (heterozygous condition).

Recessive allele — An allele that is only expressed in the phenotype when two copies are present (homozygous condition).

Genotype — The genetic makeup of an organism, represented by letters (e.g., Bb).

Phenotype — The observable physical characteristics of an organism, determined by its genotype and environment.

Homozygous — Having two identical alleles for a particular gene (e.g., BB or bb).

Heterozygous — Having two different alleles for a particular gene (e.g., Bb).

Mutation — A random change in the base sequence of DNA that may alter the protein produced and lead to a new characteristic.

Natural selection — The process by which organisms with advantageous traits are more likely to survive, reproduce, and pass on their alleles to the next generation.

Evolution — The gradual change in the inherited characteristics of a population over many generations through natural selection.

Variation — Differences between individuals of the same species, caused by genetic and/or environmental factors.

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11. Ecology and the Environment

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.

Biodiversity — The variety of different species of organisms in an ecosystem.

Food chain — A diagram showing the flow of energy from one organism to the next, starting with a producer.

Food web — A network of interconnected food chains in an ecosystem.

Producer — An organism that makes its own food by photosynthesis (e.g., green plants).

Consumer — An organism that obtains energy by eating other organisms.

Decomposer — An organism (usually bacteria or fungi) that breaks down dead organic matter, recycling nutrients back into the ecosystem.

Trophic level — A feeding level in a food chain or food web.

Carbon cycle — The biogeochemical cycle by which carbon is exchanged among the atmosphere, oceans, soil, and living organisms.

Deforestation — The large-scale cutting down of forests, leading to habitat loss, increased CO₂ levels, and soil erosion.

Eutrophication — The enrichment of water with excess nutrients (often from fertiliser run-off), leading to excessive algal growth, oxygen depletion, and death of aquatic organisms.

Global warming — The gradual increase in Earth’s average surface temperature due to the enhanced greenhouse effect, primarily caused by increased levels of CO₂ and methane.

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12. Human Influences on Ecosystems

Pollution — The addition of harmful substances to the environment, such as chemicals, noise, or waste.

Acid rain — Rain with a lower pH than normal, caused by sulfur dioxide and nitrogen oxides dissolving in rainwater. It damages forests, lakes, and buildings.

Greenhouse effect — The trapping of heat by greenhouse gases (CO₂, methane, water vapour) in the Earth’s atmosphere, keeping the planet warm enough to support life.

Sustainable development — Development that meets the needs of the present without compromising the ability of future generations to meet their own needs.

Conservation — The protection and management of natural resources and ecosystems to maintain biodiversity.

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How to Use This Definition Sheet for Revision

1. Read through each section aligned with your syllabus topics.
2. Cover the definitions and try to recall them from memory.
3. Use flashcards — write the term on one side and the definition on the other.
4. Practice past paper questions and check whether you are using the correct key terms.
5. Group study — quiz each other using these definitions.

Knowing precise definitions is essential because examiners look for specific scientific terminology in your answers. A well-defined term can earn you full marks on definition-based questions.

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This definition sheet covers the major terms in the Cambridge IGCSE Biology (0610) syllabus. Always cross-reference with your specific exam board’s syllabus for the most accurate and up-to-date requirements.

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