Explain how auxins cause a shoot to grow towards a light source.

Edexcel GCSE Biology (1BI0) — 6.3 Plant hormones · Explain · 4 marks · View as Markdown

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

A seedling is placed on a windowsill so that light reaches it from one side only.

Model answer (4 marks)

Auxins are produced in the shoot tip and are transported downwards. When a seedling is exposed to unilateral light, auxins are redistributed and accumulate on the shaded side of the shoot. The higher concentration of auxins on the shaded side stimulates greater cell elongation there. On the lit side the auxin concentration is lower, so cells elongate less. The unequal elongation causes the shoot to bend or curve towards the light, giving positive phototropism.

Examiner tips

Mention auxin production in the tip and downward transport. State that auxins accumulate on the shaded side. Explain that higher auxin causes greater cell elongation. Show that the shoot bends towards the light because of unequal elongation.

Common mistakes

Confusing auxin accumulation with light absorption. Forgetting to link higher auxin to increased cell elongation. Using vague terms like "growth" instead of "cell elongation".

Mark scheme (4 marks)

Auxins accumulate / move to the shaded side of the shoot
The higher concentration of auxins on the shaded side stimulates (greater) cell elongation on that side
The lit side has a lower concentration of auxins so cells elongate less / grow more slowly on that side
Unequal elongation causes the shoot to bend / curve towards the light (positive phototropism)

Key terms in this question

auxins