# 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

> Homeostasis is the maintenance of a stable internal environment despite changes in the external environment. In mammals, specialised systems coordinate responses across the body to keep conditions within narrow limits.

## Mark scheme (4 marks)

1. Multicellular organisms have cells that are not in direct contact with the external environment, so cannot respond independently to environmental changes
2. A communication system is needed to coordinate responses between specialised cells, tissues and organs across the body
3. Single-celled organisms have a large surface area to volume ratio, so the entire cell surface is exposed to the environment and can detect and respond to changes directly
4. In multicellular organisms, receptors detect changes and pass information via chemical or electrical signals so that effectors elsewhere in the body can produce an appropriate response to restore optimum conditions

## Key terms

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

## 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-7f8da75e) · Published by Druglandscape Ltd.