# A student investigating plant responses placed a young seedling horizontally in a dark box for 48 hours. After 48 hours, the shoot had curved upward and the root had curved downward. Explain how the unequal distribution of auxin caused these two different growth responses.

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

> Auxin is a plant hormone produced at the tip of shoots and roots. When a plant is placed on its side, gravity causes auxin to move to the lower side of both the shoot and the root.

## Mark scheme (5 marks)

1. Auxin moves to / accumulates on the lower side of both the shoot and root
2. In the shoot, the higher concentration of auxin on the lower side causes greater cell elongation on that side
3. Because the lower side of the shoot grows more than the upper side, the shoot bends/curves upward (positive gravitropism)
4. In the root, auxin inhibits/reduces cell elongation at the concentration present on the lower side
5. Because the upper side of the root elongates more than the lower side, the root bends/curves downward (positive gravitropism)

## 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-student-investigating-plant-responses-placed-aefe1f39) · Published by Druglandscape Ltd.