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

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 8 Grey Matter · Explain · 5 marks

## Mark scheme (5 marks)

1. Single-celled organisms have a large surface area to volume ratio
2. So diffusion alone is sufficient to supply all cells with oxygen/nutrients and remove waste products
3. Multicellular organisms have a small surface area to volume ratio
4. So diffusion alone is too slow / insufficient to supply all cells with the required oxygen and nutrients and to remove all waste products
5. Therefore multicellular organisms require a specialised exchange surface (e.g. alveoli) AND a mass transport system (e.g. the circulatory system) to transport 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 Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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-aca4ba73) · Published by Druglandscape Ltd.