# Explain why multicellular organisms need both an exchange surface and a mass transport system, whereas single-celled organisms do not.

> Pearson Edexcel A-Level Biology A (9BI0) — Topic 4 Exchange and Transport · Explain · 5 marks

## Mark scheme (5 marks)

1. Single-celled organisms have a large surface area to volume ratio
2. So single-celled organisms can rely on diffusion alone to exchange all substances / oxygen, nutrients and waste
3. Multicellular organisms have a small surface area to volume ratio
4. So diffusion alone is insufficient / too slow to supply all cells with oxygen and nutrients and remove waste products
5. A specialised exchange surface increases the surface area / rate of diffusion, and a mass transport system (e.g. circulatory system) rapidly moves substances to and from all cells

## Key terms

- [exchange surface](https://www.gradenine.co.uk/glossary/exchange-surface)
- [mass transport system](https://www.gradenine.co.uk/glossary/mass-transport-system)
- [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 Pearson Edexcel A-Level Biology A (9BI0)](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-multicellular-organisms-need-both-4f60035e) · Published by Druglandscape Ltd.