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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Auxins are produced at the tip of the shoot and are transported down the stem.
When light comes from one side, auxins are carried away from the light source and accumulate on the shaded side.
The higher auxin concentration on the shaded side stimulates greater cell elongation there.
Because the shaded side grows longer/faster than the lit side, the shoot bends towards the light – a positive phototropism.

Examiner tips

Use the word ‘accumulate’ to show auxin distribution. Mention ‘greater cell elongation’ to link auxin to growth. State the result as ‘shoot bends towards light’. Include the term ‘positive phototropism’ to show understanding of the response type.

Common mistakes

Failing to specify that auxins accumulate on the shaded side. Describing growth on the lit side instead of the shaded side. Using vague terms like ‘grow more’ without linking to cell elongation.

Mark scheme (4 marks)

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

Key terms in this question

auxins