# Explain why a cell with a higher surface area to volume ratio is able to rely on diffusion alone to meet its metabolic demands, whereas a larger cell cannot.

> OCR A-Level Biology A (H420) — 2.1 Cell structure · Explain · 4 marks

> Unicellular organisms such as Amoeba proteus can exchange substances with their environment entirely by diffusion across their cell surface membrane. Multicellular organisms require specialised exchange surfaces and transport systems to supply their cells.

## Mark scheme (4 marks)

1. A higher surface area to volume ratio means the surface area available for diffusion is large relative to the volume of cytoplasm that needs to be supplied.
2. Diffusion distance from the surface membrane to the centre of the cell is short in a small cell, so substances reach all parts of the cytoplasm quickly.
3. In a larger cell, the volume increases proportionally more than surface area, so the surface area to volume ratio decreases.
4. The rate of diffusion across the membrane becomes insufficient to supply the metabolic demands of the larger volume of cytoplasm, so diffusion alone cannot meet the cell's requirements.

## Key terms

- [surface area to volume ratio](https://www.gradenine.co.uk/glossary/surface-area-to-volume-ratio)
- [diffusion](https://www.gradenine.co.uk/glossary/diffusion)
- [metabolic demands](https://www.gradenine.co.uk/glossary/metabolic-demands)

## 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-a-cell-with-a-43d153a0) · Published by Druglandscape Ltd.