# Explain how the surface tension of water arises from hydrogen bonding and outline one biological example in which surface tension is important.

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

## Mark scheme (4 marks)

1. Water molecules form hydrogen bonds with neighbouring water molecules in all directions within the bulk of the liquid.
2. Molecules at the surface lack hydrogen-bonding partners above them, so they experience a net inward cohesive force, creating surface tension.
3. This high surface tension makes water resistant to breaking at its surface / gives water a high surface energy compared with most liquids.
4. A named biological example explained correctly — e.g. small arthropods such as pond skaters (Gerridae) can walk on water because their body weight is insufficient to break the surface tension; or pulmonary surfactant reduces surface tension in alveoli to prevent their collapse during exhalation; or the water surface in a xylem vessel resists breakage, maintaining the water column under tension during transpiration.

## Key terms

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

## 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-surface-tension-of-7c51a0e7) · Published by Druglandscape Ltd.