Explain how auxin causes a plant shoot to grow towards a light source. In your answer, refer to the distribution of auxin and the effect it has on cells.

AQA GCSE Biology (8461) — 4.5.4 Plant hormones (Bio only) · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Light causes auxin to move to the shaded side of the shoot. The higher concentration of auxin on the shaded side makes cells there elongate more than cells on the light side. As a result, cells on the shaded side are longer, so the shoot bends or curves towards the light source.

Examiner tips

  • Use the word ‘cause’ to link light to auxin movement. Mention ‘higher concentration’ and ‘elongate more’. Show the final effect: bending towards light. Keep answer concise to fit 4 marks.

Common mistakes

  • Confusing the direction of auxin movement (light side instead of shaded side). Failing to state that cells on the shaded side elongate more. Omitting the final bending of the shoot.

Mark scheme (4 marks)

  1. Light causes auxin to move to / accumulate on the shaded side of the shoot
  2. Higher concentration of auxin on the shaded side causes cells on that side to elongate more / grow faster
  3. Cells on the shaded side are longer than cells on the light side
  4. The shoot bends / curves towards the light source

Key terms in this question

auxin

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