# Explain how water potential gradients drive the movement of water from the soil into the xylem of a plant root, referring to the apoplast and symplast pathways.

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

## Mark scheme (4 marks)

1. Water moves by osmosis from a region of higher (less negative) water potential to a region of lower (more negative) water potential.
2. Soil water has a higher (less negative) water potential than root hair cell cytoplasm / root cortex cells, so water enters by osmosis.
3. In the apoplast pathway, water moves through cell walls and intercellular spaces without crossing membranes, until blocked at the Casparian strip in the endodermis.
4. In the symplast pathway, water moves through the cytoplasm and plasmodesmata of connected cells; active transport of ions into the xylem lowers the water potential there, maintaining the gradient that drives water into the xylem.

## Key terms

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

## 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)
- [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-water-potential-gradients-drive-a2399f6f) · Published by Druglandscape Ltd.