Explain how auxin controls the growth of a plant root growing downwards in response to gravity.

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).

Plants can detect and respond to gravity. Roots show positive geotropism, growing downward into the soil.

Model answer (4 marks)

Auxin is redistributed by gravity to the lower side of the root.
The lower side therefore has a higher concentration of auxin.
In roots, a high auxin concentration inhibits cell elongation.
The upper side, with less auxin, elongates more, causing the root to bend downwards.

Examiner tips

  • Use the word ‘redistributed’ to show gravity’s effect. Mention ‘higher concentration’ and ‘inhibits elongation’. Explain the resulting curvature. Keep each point brief but complete.

Common mistakes

  • Confusing auxin’s effect in shoots (promotes elongation) with its inhibitory effect in roots. Forgetting to mention the redistribution caused by gravity. Using vague terms like ‘more growth’ without specifying which side of the root.

Mark scheme (4 marks)

  1. Auxin moves to the lower side of the root (due to gravity / is redistributed unevenly)
  2. There is a higher concentration of auxin on the lower side of the root
  3. In roots, auxin inhibits / slows cell elongation (at higher concentrations)
  4. The upper side of the root elongates more / grows faster, so the root curves downwards

Key terms in this question

auxin

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