# A farmer grows a crop of tomato plants in a large greenhouse. The farmer notices that the tomato plants near the edges of the greenhouse grow with their stems bent towards the glass walls, while plants in the centre grow upright. Explain why the edge plants grow bent towards the glass walls.

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

> Tomato plants are flowering plants that show tropisms in response to environmental stimuli. The hormone auxin controls the direction of growth in plant shoots.

## Mark scheme (5 marks)

1. Light is coming from one direction (through the glass walls / unequally / more light from the side)
2. Auxin moves away from / is redistributed away from the light source, to the shaded side of the shoot
3. The higher concentration of auxin on the shaded side causes cells on that side to elongate more / grow faster
4. Because cells on the shaded side are longer / grow faster than cells on the light side, the shoot bends towards the light
5. This response is called phototropism / the shoot is positively phototropic

## 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-farmer-grows-a-crop-of-c00e2a0c) · Published by Druglandscape Ltd.