Explain why a multicellular animal such as a mouse needs both a specialised gas exchange surface and a circulatory system to deliver oxygen to its cells, whereas a single-celled organism such as Amoeba does not need either.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. A mouse has a small surface area to volume ratio compared with a single‑cell organism.
2. The outer surface of a mouse is not large enough to supply sufficient oxygen by diffusion alone for all its cells.
3. A specialised gas‑exchange surface (lungs) has a large surface area, increasing the rate of diffusion of oxygen into the blood.
4. A circulatory system is needed to transport oxygen quickly from the gas‑exchange surface to all body cells, because diffusion alone is too slow over large distances.
5. An amoeba has a large surface area to volume ratio so all parts of the cell are close to the surface, meaning diffusion alone is fast enough to meet its oxygen needs.
2. The outer surface of a mouse is not large enough to supply sufficient oxygen by diffusion alone for all its cells.
3. A specialised gas‑exchange surface (lungs) has a large surface area, increasing the rate of diffusion of oxygen into the blood.
4. A circulatory system is needed to transport oxygen quickly from the gas‑exchange surface to all body cells, because diffusion alone is too slow over large distances.
5. An amoeba has a large surface area to volume ratio so all parts of the cell are close to the surface, meaning diffusion alone is fast enough to meet its oxygen needs.
Examiner tips
- Use the exact wording from the mark scheme; each point counts for one mark.
- Show the logical sequence: size → diffusion limitation → need for lungs and blood.
- Keep each point concise and separate with numbers or bullets.
Common mistakes
- Confusing surface area to volume ratio with absolute surface area; students may say ‘mouse has a large surface area’ instead of ‘small ratio’.
- Failing to explain why diffusion is too slow over large distances; simply stating ‘diffusion is slow’ is insufficient.
Mark scheme (5 marks)
- A mouse has a small surface area to volume ratio (compared to a single-celled organism)
- The outer surface of a mouse is not large enough to supply sufficient oxygen by diffusion alone for all its cells
- A specialised gas exchange surface (lungs) has a large surface area, increasing the rate of diffusion of oxygen into the blood
- A circulatory system is needed to transport oxygen quickly from the gas exchange surface to all body cells, because diffusion alone is too slow over large distances
- Amoeba has a large surface area to volume ratio so all parts of the cell are close to the surface, meaning diffusion alone is fast enough to meet its oxygen needs
Key terms in this question
specialised gas exchange surface · circulatory system · multicellular
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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