# Explain how the unequal distribution of auxin in a shoot tip causes the shoot to grow towards a light source coming from one direction only.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.7 Plant responses · Explain · 5 marks

> Plants produce a hormone called auxin in their shoot tips. When light hits a shoot from one side, auxin moves away from the light source towards the shaded side of the shoot.

## Mark scheme (5 marks)

1. Auxin moves (laterally) from the illuminated side to the shaded side of the shoot tip
2. This results in a higher concentration of auxin on the shaded side than the illuminated side
3. Auxin promotes / stimulates cell elongation
4. Cells on the shaded side elongate more than cells on the illuminated side
5. This unequal elongation causes the shoot to bend / curve towards the light source (phototropism)

## Key terms

- [auxin](https://www.gradenine.co.uk/glossary/auxin)
- [shoot tip](https://www.gradenine.co.uk/glossary/shoot-tip)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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-unequal-distribution-of-649946de) · Published by Druglandscape Ltd.