# A gardener notices that the roots of seedlings always grow downwards into the soil, even when the seeds were planted at different angles. Explain how auxin controls this response in roots.

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

> Auxin is a plant hormone that is produced in the tips of shoots and roots. It moves to the lower side of roots in response to gravity.

## Mark scheme (5 marks)

1. Auxin moves to the lower side of the root (due to gravity / unequal distribution of auxin)
2. The higher concentration of auxin on the lower side inhibits / slows cell elongation on that side
3. The upper side of the root has a lower concentration of auxin, so cells elongate more on the upper side
4. The difference in growth / elongation between the two sides causes the root to bend / curve downwards
5. This is a gravitropic / geotropic response (positive gravitropism in roots)

## Key terms

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

## 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/a-gardener-notices-that-the-roots-37042aeb) · Published by Druglandscape Ltd.