# A gardener notices that a plant kept in a dark cupboard has grown tall and spindly, with pale yellow leaves, compared to a plant of the same species kept on a sunny windowsill, which is short and has dark green leaves. Explain how the unequal distribution of auxin causes the shoot of the plant in the dark cupboard to grow tall and spindly.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 2.4 Plant structure and function · Explain · 5 marks

> Auxin is a plant hormone produced at the shoot tip. It promotes cell elongation in shoot tissue. When light hits a shoot from one direction, auxin moves away from the light. In a plant kept in darkness, auxin is distributed evenly throughout the shoot tip.

## Mark scheme (5 marks)

1. Auxin is produced at the (shoot) tip / apex
2. In darkness, auxin is distributed evenly / equally on all sides of the shoot
3. Auxin stimulates / promotes cell elongation
4. Because auxin is evenly distributed, cell elongation occurs equally on all sides of the shoot, causing the shoot to grow straight upwards / vertically
5. The shoot grows taller / more elongated overall because auxin continuously promotes cell elongation throughout the shoot with no competing directional stimulus to limit elongation / the plant is etiolated

## 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-a-plant-e26561c8) · Published by Druglandscape Ltd.