# Explain why a large multicellular organism, such as a mammal, requires a specialised exchange surface to absorb oxygen, 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, meaning the body surface alone cannot supply enough oxygen for all cells
2. In large organisms the diffusion distance from the surface to the innermost cells is too great for diffusion alone to meet metabolic demands in time
3. A specialised exchange surface increases the total surface area available for diffusion, maintaining a steeper concentration gradient
4. Amoeba has a large surface area to volume ratio so oxygen can diffuse directly across the cell surface membrane quickly enough to meet the cell's needs

## Key terms

- [specialised exchange surface](https://www.gradenine.co.uk/glossary/specialised-exchange-surface)
- [single-celled organism](https://www.gradenine.co.uk/glossary/single-celled-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-fd19b1cb) · Published by Druglandscape Ltd.