# A student compares a small unicellular organism living in pond water with a large multicellular organism such as an earthworm. Describe how the surface area to volume ratio differs between the two organisms and explain how this difference affects the way each organism obtains oxygen.

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

> Unicellular organisms such as Amoeba live in pond water and obtain all the oxygen they need directly through their outer surface. Earthworms are much larger multicellular organisms that also live in damp environments.

## Mark scheme (4 marks)

1. The unicellular organism has a large/high surface area to volume ratio (compared to the multicellular organism / earthworm)
2. The unicellular organism can obtain sufficient oxygen by diffusion directly through its outer surface / cell membrane
3. The earthworm / large multicellular organism has a low surface area to volume ratio, so diffusion through the outer surface alone is not sufficient to supply all cells with oxygen
4. The earthworm / large multicellular organism therefore requires a transport system (and/or a specialised exchange surface) to distribute oxygen to all cells

## Key terms

- [surface area to volume ratio](https://www.gradenine.co.uk/glossary/surface-area-to-volume-ratio)
- [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 "Describe" 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/a-student-compares-a-small-unicellular-f9035975) · Published by Druglandscape Ltd.