# A farmer grows a crop of sunflowers in a large open field. During the growing season, the farmer observes that the young sunflower shoots consistently face east in the morning and west in the evening, tracking the movement of the Sun across the sky. Explain how auxin brings about this directional growth response in the young sunflower shoots.

> Pearson Edexcel A-Level Biology A (9BI0) — 4.7 Plant responses · Explain · 5 marks

> Auxin is a plant hormone produced at the tip of a shoot. It moves away from regions of high light intensity towards regions of lower light intensity, causing unequal distribution across the shoot.

## Mark scheme (5 marks)

1. Auxin is produced at the tip of the shoot
2. Auxin moves away from the light / towards the shaded side of the shoot
3. The higher concentration of auxin on the shaded side causes cells on that side to elongate more
4. Unequal cell elongation causes the shoot to bend / curve towards the light
5. As the Sun moves across the sky throughout the day, the position of the light source changes, so auxin redistribution changes continuously, causing the shoot to track the Sun

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](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/a-farmer-grows-a-crop-of-8ba35e99) · Published by Druglandscape Ltd.