# Red blood cells are placed into three different solutions. In solution A, the cells swell and burst. In solution B, the cells remain the same size and shape. In solution C, the cells shrink and become crenated. Explain these three observations in terms of water potential and osmosis.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 1.6 Membranes and transport · Explain · 5 marks

> Red blood cells have a partially permeable membrane. The cytoplasm of a red blood cell has a specific water potential.

## Mark scheme (5 marks)

1. Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential
2. In solution A, the solution has a higher water potential than the cell cytoplasm, so water enters the cell by osmosis, causing it to swell and burst
3. In solution B, the water potential of the solution is equal to the water potential of the cytoplasm, so there is no net movement of water
4. In solution C, the solution has a lower water potential than the cell cytoplasm, so water leaves the cell by osmosis
5. Loss of water from the cell in solution C causes the cell to shrink / become crenated because the cytoplasm / cell membrane contracts

## Key terms

- [osmosis](https://www.gradenine.co.uk/glossary/osmosis)
- [water potential](https://www.gradenine.co.uk/glossary/water-potential)
- [crenated](https://www.gradenine.co.uk/glossary/crenated)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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/red-blood-cells-are-placed-into-a2547cae) · Published by Druglandscape Ltd.