# Explain how the water potential of a plant cell changes as it undergoes plasmolysis, and describe the state of the cell at the point of incipient plasmolysis.

> IB DP Biology Higher Level (2023 syllabus) — D2.3 Water potential · Explain · 4 marks

> When plant cells are placed in solutions of increasing solute concentration, they lose water by osmosis and may undergo plasmolysis.

## Mark scheme (4 marks)

1. As the cell loses water by osmosis, the solute concentration inside the cell increases, causing the solute potential (ψs) to become more negative.
2. As the cell loses water, the cell wall no longer pushes inward on the contents, so pressure potential (ψp / turgor pressure) decreases towards zero.
3. Overall water potential of the cell decreases (becomes more negative) as water is lost, because ψ = ψs + ψp and both components decrease.
4. At incipient plasmolysis, the cell membrane has just begun to pull away from the cell wall, turgor pressure equals zero, so water potential equals solute potential (ψ = ψs); the cell is flaccid but the protoplast still fills the cell wall.

## Key terms

- [plasmolysis](https://www.gradenine.co.uk/glossary/plasmolysis)
- [incipient plasmolysis](https://www.gradenine.co.uk/glossary/incipient-plasmolysis)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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