# Explain why large multicellular organisms need a mass transport system, but single-celled organisms do not.

> AQA A-Level Biology (7402) — 3.3.4 Mass Transport · Explain · 5 marks

## Mark scheme (5 marks)

1. Single-celled organisms have a large surface area to volume ratio
2. So substances can diffuse directly in/out of the cell quickly / efficiently over the whole surface
3. Large multicellular organisms have a small surface area to volume ratio
4. Diffusion alone would be too slow / insufficient to supply all cells with enough substances (e.g. oxygen/glucose) or remove waste
5. A mass transport system (e.g. the circulatory system/blood) carries substances rapidly to all cells / over long distances throughout the body

## Key terms

- [mass transport system](https://www.gradenine.co.uk/glossary/mass-transport-system)
- [multicellular](https://www.gradenine.co.uk/glossary/multicellular)

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-large-multicellular-organisms-need-9bdef53e) · Published by Druglandscape Ltd.