# A student compares gas exchange in a single-celled organism, such as an amoeba, with gas exchange in a large multicellular organism, such as a mammal. Explain why a large multicellular organism needs a specialised gas exchange surface, whereas a single-celled organism does not.

> OCR A-Level Biology B — Advancing Biology (H422) — 2.4 Gas exchange surfaces · Explain · 5 marks

> Single-celled organisms exchange gases directly across their outer cell membrane. Large multicellular organisms, such as mammals, have specialised gas exchange surfaces such as alveoli in the lungs.

## Mark scheme (5 marks)

1. A single-celled organism has a large surface area to volume ratio, so gases can diffuse across the cell membrane quickly enough to meet the cell's needs.
2. A large multicellular organism has a small surface area to volume ratio, so its outer surface is insufficient to supply all its cells with enough oxygen by diffusion.
3. Specialised gas exchange surfaces, such as alveoli, provide a large surface area to increase the rate of diffusion.
4. Gas exchange surfaces are thin / have a short diffusion pathway, so gases diffuse across quickly down a concentration gradient.
5. A good blood supply / capillary network maintains the concentration gradient at the gas exchange surface, ensuring efficient diffusion continues.

## Key terms

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

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](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/a-student-compares-gas-exchange-in-d82a1089) · Published by Druglandscape Ltd.