# Explain why a large multicellular organism cannot rely on diffusion alone to supply all of its cells with oxygen.

> AQA A-Level Biology (7402) — 3.3.1 Surface Area to Volume Ratio · Explain · 5 marks

## Mark scheme (5 marks)

1. As an organism gets larger, its volume increases more than its surface area
2. This means the surface area to volume ratio is smaller in a large organism
3. There is not enough surface area to absorb sufficient oxygen by diffusion for all the cells
4. Cells deep inside the organism are too far from the exchange surface for oxygen to reach them by diffusion alone
5. Large organisms require specialised exchange surfaces and/or transport systems to supply all cells with oxygen

## Key terms

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

## Related

- [Revision notes for AQA A-Level Biology (7402)](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-ea83fd6e) · Published by Druglandscape Ltd.