# Explain how osmosis causes a plant cell to become turgid when placed in a hypotonic solution.

> IB DP Biology Standard Level (2023 syllabus) — B2.1 Membranes and membrane transport · Explain · 4 marks

## Mark scheme (4 marks)

1. The hypotonic solution has a lower solute concentration (higher water potential) than the cell contents, so a water potential gradient exists across the membrane.
2. Water moves by osmosis — by passive/diffusion movement through the partially permeable (phospholipid bilayer / aquaporins) membrane — down the water potential gradient into the cell.
3. The vacuole (and cytoplasm) takes in water, causing the cell to swell / increase in volume.
4. The cell wall exerts an inward pressure (wall pressure / pressure potential) on the swollen protoplast, creating turgor pressure; this pressure opposes further water entry, resulting in a turgid cell.

## Key terms

- [osmosis](https://www.gradenine.co.uk/glossary/osmosis)
- [hypotonic](https://www.gradenine.co.uk/glossary/hypotonic)

## 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-osmosis-causes-a-plant-2c1e7108) · Published by Druglandscape Ltd.