IGCSE Biology Key Definitions
Biology awards more marks for precise wording than any other science. These definitions are marked against a fixed set of phrases, so learning them word for word converts directly into marks.
Every definition below appears regularly as a two or three mark question, and each mark corresponds to a specific phrase. Approximating the wording loses marks even when the underlying understanding is correct. Learn these exactly, then use the topic pages to practise applying them.
Transport and cell processes
| Term | Definition | Phrases that carry the marks |
|---|---|---|
| Diffusion | The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of random movement | net movement, 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 | water molecules, water potential, partially permeable membrane |
| Active transport | The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration | against the gradient, energy from respiration |
| Enzyme | A protein that functions as a biological catalyst | protein, biological catalyst |
| Denatured | The enzyme's shape has changed permanently so the active site is no longer complementary to the substrate | active site, no longer complementary |
Energy and gas exchange
| Term | Definition | Phrases that carry the marks |
|---|---|---|
| Photosynthesis | The process by which plants make carbohydrates from raw materials using energy from light | carbohydrates, energy from light |
| Respiration | The chemical reactions in cells that break down nutrient molecules and release energy | in cells, release energy |
| Aerobic respiration | The release of energy from glucose using oxygen | using oxygen |
| Anaerobic respiration | The release of a relatively small amount of energy from the breakdown of glucose in the absence of oxygen | small amount of energy, absence of oxygen |
| Transpiration | The loss of water vapour from leaves, by evaporation at the mesophyll cell surfaces followed by diffusion through the stomata | water vapour, evaporation, diffusion, stomata |
Organisms and inheritance
| Term | Definition | Phrases that carry the marks |
|---|---|---|
| Excretion | The removal of the toxic waste products of metabolism and substances in excess of requirements | waste products of metabolism, in excess of requirements |
| Species | A group of organisms that can reproduce to produce fertile offspring | fertile offspring |
| Gene | A length of DNA that codes for a protein | length of DNA, codes for a protein |
| Allele | An alternative form of a gene | alternative form |
| Mutation | A change in the base sequence of DNA | base sequence |
| Natural selection | Organisms better adapted to their environment are more likely to survive and reproduce, passing on their alleles | more likely to survive, passing on alleles |
| Pathogen | A disease causing organism | disease causing |
How to use this sheet
- Learn the exact wording, not the gist. Each underlined phrase is worth a mark.
- The word net in diffusion and osmosis is not optional. Particles move both ways, and the definition describes the overall direction.
- Osmosis is about water molecules specifically. Writing particles turns it into a definition of diffusion.
- Fertile is the word that makes the species definition correct. Without it the definition is wrong.
- Test yourself by writing definitions from memory, then compare phrase by phrase rather than reading them over.
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Frequently asked questions
Why does IGCSE Biology mark definitions so strictly?
Because each definition contains specific technical ideas, and the mark scheme allocates one mark per idea. A definition that captures the general sense but omits a key phrase, such as partially permeable membrane, has missed one of the ideas being tested.
What is the difference between excretion and egestion?
Excretion removes the toxic waste products of metabolism and substances in excess of requirements, such as urea and carbon dioxide. Egestion removes undigested food as faeces, which has never been absorbed into cells and so is not a product of metabolism.
How should I revise these definitions?
Write them from memory rather than reading them, then compare phrase by phrase against the correct version. Rereading creates a feeling of familiarity without building recall, which is why students who feel prepared still lose these marks in the exam.
Do I need the exact Cambridge wording?
You need every scoring idea present, expressed accurately. Alternative wording is accepted when it conveys the same idea precisely, but the standard phrasing is the safest route because it maps directly onto the mark scheme.
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Other reference sheets
Written to the published Cambridge IGCSE syllabuses. Always check the formula list and data sheet issued with your own paper, since the material provided differs between subjects and syllabus versions. Last reviewed 2026-08-12.