# Explain why a fish uses specialised gill structures to obtain oxygen from water, rather than relying on diffusion across its general body surface.

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

## Mark scheme (5 marks)

1. A fish is a large/multicellular organism, so its volume is large relative to its surface area
2. The body surface alone does not provide enough surface area to supply all the cells with sufficient oxygen by diffusion
3. Gills provide a large surface area for diffusion of oxygen
4. Gills have a rich blood supply / thin walls, maintaining a steep concentration gradient for diffusion
5. Diffusion alone across the general body surface would be too slow to meet the metabolic demands of all the cells

## 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-fish-uses-specialised-46331e2a) · Published by Druglandscape Ltd.