# Explain why multicellular organisms require a dedicated communication system to maintain homeostasis, whereas single-celled organisms do not.

> OCR A-Level Biology A (H420) — 5.1 Communication and homeostasis · Explain · 4 marks

## Mark scheme (4 marks)

1. In multicellular organisms, cells are not all in direct contact with the external environment, so conditions at internal cells cannot be regulated by simple diffusion alone.
2. Multicellular organisms have specialised cells and tissues with different metabolic demands, so conditions must be coordinated between different parts of the body.
3. A communication system allows signals (nervous or hormonal) to be sent between receptor cells detecting a change and effector cells that bring about a corrective response.
4. Single-celled organisms are in direct contact with their external environment, so any change in conditions is detected and responded to by the whole cell without the need for intercellular communication.

## Key terms

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

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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-require-a-b07149e0) · Published by Druglandscape Ltd.