# 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.

> AQA A-Level Biology (7402) — 3.3.1 Surface Area to Volume Ratio · Explain · 5 marks

## Mark scheme (5 marks)

1. A mouse has a small surface area to volume ratio (compared to a single-celled 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. 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

- [specialised gas exchange surface](https://www.gradenine.co.uk/glossary/specialised-gas-exchange-surface)
- [circulatory system](https://www.gradenine.co.uk/glossary/circulatory-system)
- [multicellular](https://www.gradenine.co.uk/glossary/multicellular)

## Related

- [Revision notes for AQA A-Level Biology (7402)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-multicellular-animal-such-c28905d4) · Published by Druglandscape Ltd.