# Explain why a large multicellular organism, such as a mammal, requires a specialised exchange surface, whereas a single-celled organism such as Amoeba does not.

> OCR A-Level Biology A (H420) — 3.1 Exchange surfaces · Explain · 4 marks

## Mark scheme (4 marks)

1. Large organisms have a small surface area to volume ratio
2. The outer body surface alone is insufficient to supply the metabolic demands of all cells by diffusion alone
3. Specialised exchange surfaces increase the surface area available for diffusion / have features that increase the rate of diffusion
4. Single-celled organisms have a high surface area to volume ratio so the cell surface membrane provides sufficient area for exchange by diffusion to meet the cell's needs

## Key terms

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

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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-large-multicellular-organism-c06a6c95) · Published by Druglandscape Ltd.