Explain why a large multicellular animal, such as a fish, requires a specialised exchange surface for gas exchange, whereas a single-celled organism such as Amoeba does not.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A single‑cell organism has a large surface‑area‑to‑volume ratio, so diffusion across the cell membrane is sufficient to supply all parts of the cell with oxygen and to remove carbon dioxide.
A large multicellular animal has a small surface‑area‑to‑volume ratio; diffusion across the body surface is too slow to meet the oxygen demand of all cells. Therefore it requires a specialised exchange surface (e.g. gills) to increase the rate of gas exchange.
A large multicellular animal has a small surface‑area‑to‑volume ratio; diffusion across the body surface is too slow to meet the oxygen demand of all cells. Therefore it requires a specialised exchange surface (e.g. gills) to increase the rate of gas exchange.
Examiner tips
- Use the terms ‘surface‑area‑to‑volume ratio’ and ‘diffusion’ explicitly.
- Explain the consequence of a small ratio for a multicellular organism.
- Show the link between the need for a specialised surface and increased gas‑exchange rate.
- Keep the answer concise – 4 marks can be earned with two short points for each organism type.
Common mistakes
- Confusing ‘surface area’ with ‘volume’ or vice versa.
- Failing to mention diffusion as the transport mechanism.
- Using vague phrases like ‘more efficient’ without explaining why.
Mark scheme (4 marks)
- A single-celled organism has a large surface area to volume ratio
- So diffusion alone / across the cell surface membrane is sufficient to supply all parts of the cell with oxygen (and remove carbon dioxide)
- A large multicellular organism has a small surface area to volume ratio
- So diffusion across the body surface is too slow / insufficient to supply all cells with oxygen, requiring a specialised exchange surface to increase the rate of gas exchange
Key terms in this question
single-celled organism · exchange surface
Related
- All Edexcel GCSE Biology (1BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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