A gardener notices that the roots of seedlings always grow downwards into the soil, even when the seeds were planted at different angles. Explain how auxin controls this response in roots.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.7 Plant responses · Explain · 5 marks · View as Markdown

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

Auxin is a plant hormone that is produced in the tips of shoots and roots. It moves to the lower side of roots in response to gravity.

Model answer (5 marks)

1. Auxin is synthesised in the shoot and root tips and, when a root is exposed to gravity, it is redistributed to the lower side of the root.
2. The higher concentration of auxin on the lower side inhibits cell elongation there.
3. On the upper side the auxin concentration is lower, so cells elongate more rapidly.
4. The unequal elongation causes the root to bend, with the lower side shortening relative to the upper side.
5. This bending is a positive gravitropic (geotropic) response, ensuring roots grow downwards into the soil.

Examiner tips

  • Use the word ‘redistributed’ to show understanding of auxin movement. Mention ‘inhibits cell elongation’ rather than just ‘stops growth’. Explain the result as a bending of the root. Finish with ‘positive gravitropism’ to link to the stimulus.”]
  • common_mistakes
  • :
  • Saying auxin ‘stops’ growth on the lower side instead of ‘inhibits elongation’. Forgetting to mention the redistribution of auxin to the lower side. Using ‘negative’ gravitropism instead of ‘positive’ for roots.

Mark scheme (5 marks)

  1. Auxin moves to the lower side of the root (due to gravity / unequal distribution of auxin)
  2. The higher concentration of auxin on the lower side inhibits / slows cell elongation on that side
  3. The upper side of the root has a lower concentration of auxin, so cells elongate more on the upper side
  4. The difference in growth / elongation between the two sides causes the root to bend / curve downwards
  5. This is a gravitropic / geotropic response (positive gravitropism in roots)

Key terms in this question

auxin

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