# Explain why a large multicellular organism cannot rely solely on diffusion across its outer body surface to meet its oxygen demands, whereas a single-celled organism such as Amoeba can.

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

## Mark scheme (4 marks)

1. Large organisms have a smaller surface area to volume ratio compared to small organisms
2. The volume of respiring tissue (metabolic demand) is too great relative to the surface area available for diffusion
3. Diffusion distance from the outer surface to inner cells is too great in a large organism, so oxygen cannot reach all cells quickly enough by diffusion alone
4. In Amoeba, all parts of the cell are close to the external surface so diffusion distance is short, and the high surface area to volume ratio means enough oxygen can diffuse in to meet demand

## Key terms

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

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-large-multicellular-organism-15722b73) · Published by Druglandscape Ltd.