Describe how auxin causes a plant root to respond to the force of gravity.

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).

When a plant root is placed horizontally, it changes its direction of growth within a few hours so that it grows downwards into the soil.

Model answer (4 marks)

Auxin is redistributed by gravity to the lower side of the root.
The higher auxin concentration on the lower side inhibits cell elongation there.
On the upper side, the lower auxin level allows cells to elongate more.
The unequal cell elongation bends the root downwards, giving positive gravitropism.

Examiner tips

  • Use the word ‘redistributed’ to show auxin movement. Mention ‘inhibits’ for the lower side and ‘promotes’ for the upper side. Explain that unequal elongation causes bending. Keep the answer concise and use the exact terms from the mark scheme.

Common mistakes

  • Confusing auxin’s effect on shoots (promotion) with roots (inhibition). Forgetting to state that the lower side has higher auxin concentration. Using vague terms like ‘stops growth’ instead of ‘inhibits cell elongation’.

Mark scheme (4 marks)

  1. Auxin moves to / accumulates on the lower side of the root (due to gravity)
  2. Higher concentration of auxin on the lower side inhibits / slows cell elongation on that side
  3. Lower concentration of auxin on the upper side allows / promotes greater cell elongation on that side
  4. Unequal elongation causes the root to bend / grow downwards (positive gravitropism)

Key terms in this question

auxin

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