# A student placed cress seedlings on a windowsill so that light came from one direction only. After several days, the shoots had bent towards the light. Explain why the shoots bent towards the light.

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

> Cress seedlings were kept in a box with a hole cut in one side to allow light to enter from one direction only. After five days, the shoots had grown towards the hole.

## Mark scheme (5 marks)

1. The stimulus (light) is detected and auxin is produced at the shoot tip
2. Auxin moves to / accumulates on the shaded (darker) side of the shoot
3. Auxin causes cells to elongate / grow longer
4. Cells on the shaded side elongate more than cells on the light side
5. This unequal growth causes the shoot to bend towards the light (phototropism)

## 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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