# Explain how auxins cause a shoot to grow towards a light source.

> Edexcel GCSE Biology (1BI0) — 6.3 Plant hormones · Explain · 4 marks

> A seedling is placed on a windowsill so that light reaches it from one side only.

## Mark scheme (4 marks)

1. Auxins accumulate / move to the shaded side of the shoot
2. The higher concentration of auxins on the shaded side stimulates (greater) cell elongation on that side
3. The lit side has a lower concentration of auxins so cells elongate less / grow more slowly on that side
4. Unequal elongation causes the shoot to bend / curve towards the light (positive phototropism)

## Key terms

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

## Related

- [Revision notes for Edexcel GCSE Biology (1BI0)](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-auxins-cause-a-shoot-3a229e44) · Published by Druglandscape Ltd.