# Explain why auxin causes differential growth on the shaded side of a shoot tip when the shoot is exposed to unilateral light.

> OCR A-Level Biology A (H420) — 5.5 Plant and animal responses · Explain · 4 marks

> When a plant shoot is exposed to light from one side only, auxin (IAA) redistributes laterally across the shoot tip. The shaded side elongates faster than the illuminated side, causing the shoot to bend towards the light source.

## Mark scheme (4 marks)

1. Auxin moves laterally from the illuminated side to the shaded side of the shoot tip
2. The shaded side therefore has a higher concentration of auxin than the illuminated side
3. Auxin promotes cell elongation by stimulating loosening of cell walls, allowing cells to take up more water and expand
4. Greater elongation on the shaded side compared to the illuminated side causes the shoot to bend towards the light (positive phototropism)

## Key terms

- [auxin](https://www.gradenine.co.uk/glossary/auxin)
- [unilateral light](https://www.gradenine.co.uk/glossary/unilateral-light)
- [differential growth](https://www.gradenine.co.uk/glossary/differential-growth)
- [shoot tip](https://www.gradenine.co.uk/glossary/shoot-tip)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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/explain-why-auxin-causes-differential-growth-7c583ddd) · Published by Druglandscape Ltd.