# Describe how auxins cause a shoot to grow towards a light source when light is directed from one side only.

> Cambridge International IGCSE Biology (0610) — 14.5 Tropic responses · Describe · 4 marks

## Mark scheme (4 marks)

1. Auxins are produced at the tip of the shoot and move away from the light source (accumulate on the shaded side).
2. The higher concentration of auxins on the shaded side stimulates greater cell elongation on that side.
3. The shaded side of the shoot grows longer / faster than the lit side, causing the shoot to bend.
4. The shoot therefore bends / grows towards the light source, which is a positive phototropism.

## Key terms

- [auxins](https://www.gradenine.co.uk/glossary/auxins)

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Source: [GradeNine](https://www.gradenine.co.uk/q/describe-how-auxins-cause-a-shoot-32127fc4) · Published by Druglandscape Ltd.