# Explain why a single-celled organism such as Amoeba can rely solely on diffusion to meet its respiratory needs, whereas a mammal cannot.

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

> Amoeba is a unicellular organism approximately 0.5 mm in diameter. Mammals are multicellular organisms with body volumes many orders of magnitude greater, composed of metabolically active tissues throughout.

## Mark scheme (4 marks)

1. Amoeba has a large surface area to volume ratio, so the surface area available for diffusion is sufficient relative to the volume of respiring cytoplasm
2. In a mammal the surface area to volume ratio is small/low, so the body surface alone cannot supply sufficient oxygen to all cells
3. The diffusion distance from the exchange surface to respiring cells deep within a mammal is too large for diffusion alone to supply oxygen quickly enough
4. Mammals require specialised exchange surfaces and a mass transport system to deliver oxygen efficiently to all cells

## Key terms

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

## 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-single-celled-organism-such-f8ba2be9) · Published by Druglandscape Ltd.