# Explain why multicellular organisms need specialised exchange surfaces and transport systems, whereas single-celled organisms do not.

> OCR GCSE Biology A: Gateway Science (J247) — B2.2 The challenges of size · Explain · 4 marks

> As organisms increase in size, the ratio of their surface area to their volume decreases. A single-celled organism such as an amoeba can exchange substances directly across its outer membrane, but a large multicellular organism such as a mammal cannot rely on simple diffusion across its outer surface to supply all of its cells.

## Mark scheme (4 marks)

1. In a single-celled organism, the surface area is large enough relative to its volume for diffusion to supply all parts of the cell
2. Multicellular organisms have a small surface area relative to their volume, so diffusion across the outer surface is too slow / insufficient to meet the needs of all cells
3. Specialised exchange surfaces (e.g. alveoli / root hair cells) increase the surface area to allow faster exchange of substances
4. Transport systems (e.g. the blood / circulatory system / phloem) are needed to move substances quickly between exchange surfaces and all cells in the body

## Key terms

- [multicellular organism](https://www.gradenine.co.uk/glossary/multicellular-organism)

## Related

- [Revision notes for OCR GCSE Biology A: Gateway Science (J247)](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-specialised-299b37ad) · Published by Druglandscape Ltd.