# A student claims that the tropistic response of a shoot to light is entirely explained by the presence of auxin on the shaded side of the shoot tip. Evaluate this claim.

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

> Auxin (indole-3-acetic acid, IAA) is a plant growth hormone produced at the shoot tip. When a shoot is illuminated from one side, unequal distribution of auxin is thought to drive differential cell elongation, resulting in bending towards the light source.

## Mark scheme (4 marks)

1. Auxin redistributes to the shaded side of the shoot tip, causing greater cell elongation on that side
2. This differential elongation causes the shoot to bend towards the light, which supports the claim
3. The claim is incomplete because other factors or mechanisms also contribute to phototropism
4. A named limitation or additional mechanism, such as the role of phototropins as the light receptors that initiate auxin redistribution, or evidence that cytokinin/other growth inhibitors on the light side also contribute to bending

## Key terms

- [auxin](https://www.gradenine.co.uk/glossary/auxin)
- [tropistic response](https://www.gradenine.co.uk/glossary/tropistic-response)
- [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 "Evaluate" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-claims-that-the-tropistic-03ca045e) · Published by Druglandscape Ltd.