# Explain how the addition of solutes to a plant cell's vacuole affects the water potential of that cell and the direction of water movement across its tonoplast.

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

## Mark scheme (4 marks)

1. Adding solutes to the vacuole lowers (makes more negative) the solute potential (ψs) of the vacuolar solution.
2. This lowers the overall water potential (ψ) of the vacuole relative to the cytoplasm / surrounding solution.
3. Water moves by osmosis across the tonoplast from the higher water potential (cytoplasm) to the lower water potential (vacuole).
4. As water enters the vacuole, pressure potential (turgor) increases, which raises the water potential of the vacuole, until equilibrium (equal water potentials) is reached and net water movement stops.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](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-how-the-addition-of-solutes-364c1bad) · Published by Druglandscape Ltd.