# Explain why a bacterium can obtain all the oxygen it needs by diffusion across its outer surface, but a mammal cannot rely on diffusion alone to supply oxygen to its cells.

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

> A bacterium is a single-celled organism roughly 1–2 micrometres in diameter. A mammal is a large, multicellular organism with cells deep inside its body.

## Mark scheme (5 marks)

1. A bacterium has a large surface area to volume ratio
2. So the surface area is large enough relative to its volume / number of cells to supply sufficient oxygen by diffusion alone
3. A mammal has a small surface area to volume ratio
4. The surface area is not large enough relative to volume / the number of cells to supply all cells with sufficient oxygen by diffusion
5. Many mammal cells are far from the outer surface / too far for oxygen to reach by diffusion in sufficient quantities / in time

## Key terms

- [diffusion](https://www.gradenine.co.uk/glossary/diffusion)

## 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-bacterium-can-obtain-ef9bddf2) · Published by Druglandscape Ltd.