# Explain how the cohesive and adhesive properties of water, both arising from hydrogen bonding, allow water to move upward through the xylem of tall plants against gravity.

> IB DP Biology Higher Level (2023 syllabus) — A1.1 Water · Explain · 4 marks

## Mark scheme (4 marks)

1. Hydrogen bonds form between adjacent water molecules, producing cohesion that keeps water molecules joined in a continuous column.
2. Adhesion occurs because water molecules form hydrogen bonds with the cellulose/hydrophilic walls of xylem vessels, allowing water to cling to the vessel walls.
3. Transpiration at the leaf surface creates tension / a negative pressure that pulls the water column upward, with cohesion transmitting this tension throughout the entire column from leaves to roots.
4. Adhesion to xylem walls prevents the water column from pulling away from the sides under tension, so the column remains intact and continuous movement against gravity is sustained (capillary action contributes).

## Key terms

- [hydrogen bonding](https://www.gradenine.co.uk/glossary/hydrogen-bonding)
- [xylem](https://www.gradenine.co.uk/glossary/xylem)

## 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-the-cohesive-and-adhesive-65686166) · Published by Druglandscape Ltd.