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

> Pearson Edexcel A-Level Biology A (9BI0) — 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 water potential of −0.9 kPa when in normal body fluids.

## Mark scheme (5 marks)

1. Osmosis is the 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 water potential of the solution equals that of the cell cytoplasm, so there is no net movement of water and the cell stays the same size
3. In solution B, the solution has a higher water potential than the cell cytoplasm, so water moves into the cell by osmosis
4. The influx of water causes the cell to swell and may cause it to burst (lyse) because red blood cells have no cell wall to resist the pressure
5. In solution C, the solution has a lower water potential than the cell cytoplasm, so water moves out of the cell by osmosis, causing the cell to shrink / become crenated

## 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 Pearson Edexcel A-Level Biology A (9BI0)](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-403da7f8) · Published by Druglandscape Ltd.