Describe how auxin is distributed in a shoot tip that is receiving light from one direction only, and explain how this distribution leads to a change in the direction of growth of the shoot.

Pearson Edexcel International GCSE Biology (4BI1) — 2.14 Coordination and response: flowering plants · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Auxin is produced at the tip of the shoot.
When the shoot receives light from one side, auxin is transported away from the light and accumulates on the shaded side.
The higher concentration of auxin on the shaded side stimulates greater cell elongation there.
The unequal elongation causes the shoot to bend towards the light (phototropism).

Examiner tips

  • Use the exact terms "auxin", "phototropism", "cell elongation". Show the direction of auxin transport (away from light).
  • Explain the link between higher auxin and faster elongation.

Mark scheme (4 marks)

  1. Auxin is produced at the tip of the shoot
  2. Auxin moves away from the light / accumulates on the shaded / darker side of the shoot
  3. The higher concentration of auxin on the shaded side causes greater / faster cell elongation on that side
  4. Unequal elongation causes the shoot to bend / curve towards the light (phototropism)

Key terms in this question

auxin

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