# Explain why a single-celled organism, such as an amoeba, can survive without a specialised gas exchange surface, but a mammal cannot.

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

## Mark scheme (5 marks)

1. A single-celled organism has a large surface area to volume ratio
2. So sufficient oxygen / glucose can diffuse across the cell surface membrane to meet the cell's needs / reach the inside of the cell quickly enough
3. A mammal has a small surface area to volume ratio (as body size / volume increases, SA:V ratio decreases)
4. The body surface area is not large enough to allow sufficient oxygen to diffuse in to supply all the cells / meet the organism's metabolic demands
5. So a mammal needs a specialised gas exchange surface (e.g. lungs / alveoli) with a large surface area to increase the rate of diffusion / supply enough oxygen

## Key terms

- [specialised gas exchange surface](https://www.gradenine.co.uk/glossary/specialised-gas-exchange-surface)
- [single-celled organism](https://www.gradenine.co.uk/glossary/single-celled-organism)

## 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-single-celled-organism-such-e367a839) · Published by Druglandscape Ltd.