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GCSE Biology · key term
A specialised boundary across which substances move between an organism and its surroundings, or between different parts of an organism.
Used by AQA, Edexcel and OCR
Specification 4.1.3.1 · Paper 1
Revise: Surface Area to Volume Ratio and Exchange SystemsThe organism therefore has specialised exchange surfaces, rather than relying only on its outer surface.
A surface across which substances move between an organism and its surroundings, or between different parts of an organism.
Link each problem to its solution: a small surface area:volume ratio limits exchange at the exterior, while a long diffusion distance makes diffusion to internal cells too slow. Exchange surfaces and transport systems solve these related problems.
Do not say that a transport system replaces diffusion. Blood carries substances over long distances; substances still cross exchange surfaces and move between capillaries and cells.
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Specification 8.2 · Paper 2
A region where substances pass between an organism and its surroundings.
Revise: Surface Area to Volume RatioAn exchange surface is where substances enter or leave an organism. Its area limits how much surface is available for exchange, while the organism's volume represents how much living material may need supplying. As an organism gets larger, its volume grows faster than its surface area.
Specification B2.2a · Paper 1 (Foundation), Paper 3 (Higher)
A specialised boundary across which substances pass between an organism and its environment.
Revise: Exchange surfacesA lower surface area:volume ratio alone does not mean that diffusion has stopped. It means there is less exchange surface relative to the tissue being supplied. The longer routes to internal cells also mean that relying on diffusion alone would not move enough material quickly enough to support normal cell activity.
Paper 1 (Foundation), Paper 2 (Foundation), Paper 3 (Higher), Paper 4 (Higher)
A specialised boundary where substances move between an organism and its surroundings, or between body systems.
Revise: Exchange SurfacesMulticellular organisms use specialised exchange surfaces to provide a large area where substances can enter or leave. In humans, the many alveoli in the lungs provide a large surface for oxygen to pass from air into blood and carbon dioxide to pass in the opposite direction. Villi in the small intestine increase the area available for absorbing digested food molecules into the blood.