# Explain why a single-celled organism does not need a specialised exchange surface or a circulatory system, but a multicellular organism does.

> Edexcel GCSE Biology (1BI0) — 8.1 Gas exchange · Explain · 4 marks

## Mark scheme (4 marks)

1. A single-celled organism has a large surface area to volume ratio
2. So diffusion alone is sufficient / fast enough to supply all parts of the cell with oxygen and nutrients and remove waste products
3. A multicellular organism has a small surface area to volume ratio
4. So diffusion alone is insufficient to supply all cells with required oxygen/nutrients and remove all waste products, requiring a specialised exchange surface and mass transport system

## Key terms

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

## Related

- [Revision notes for Edexcel GCSE Biology (1BI0)](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-does-e51efcca) · Published by Druglandscape Ltd.