A level Biology B · Edexcel 9BI0 · Topic 4A
Surface area and exchange
Key points
- Surface area to volume ratio (SA:V) falls as an organism gets bigger, because volume increases faster than surface area.
- Small organisms can rely on diffusion across their body surface; large organisms need specialised exchange surfaces and transport systems.
- Fick's law: rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of the exchange surface.
- Good exchange surfaces have a large surface area, a short diffusion distance and a steep concentration gradient (kept up by ventilation and blood flow).
- High metabolic rate (e.g. in mammals and birds) increases the need for efficient exchange.
- You may be asked to calculate SA:V for a cube: SA = , V = .
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All organisms exchange substances such as oxygen, carbon dioxide, nutrients and waste with their surroundings. How easily they do this depends on their size and shape, which is why large, active organisms have specialised exchange surfaces…
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