Explain why the roots of a seedling grow downwards and the shoot grows upwards when the seedling is placed horizontally on damp soil.

Cambridge International IGCSE Biology (0610) — 14.5 Tropic responses · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The response is a gravitropic reaction.
1. Auxin accumulates on the lower side of a horizontally placed seedling.
2. In the root, the higher auxin concentration inhibits cell elongation on the lower side, so the lower side grows more slowly.
3. The upper side elongates faster, causing the root to bend downwards (positive gravitropism).
4. In the shoot, the higher auxin concentration stimulates cell elongation on the lower side, so the lower side grows faster.
5. The upper side elongates less, causing the shoot to bend upwards (negative gravitropism).

Examiner tips

  • Use the word ‘gravitropism’ early. Include the auxin distribution and its effect on growth. Show the opposite responses in root and shoot. Keep it concise, 4–5 short sentences.

Common mistakes

  • Confusing auxin as a growth promoter in both root and shoot. Mixing up positive and negative gravitropism. Omitting the role of auxin distribution.

Mark scheme (4 marks)

  1. Identifies that this growth response is gravitropism
  2. States that auxins accumulate on the lower side of the horizontal root and shoot
  3. States that in the root, the higher concentration of auxins on the lower side inhibits growth, so the lower side grows more slowly, causing the root to bend / grow downwards (positive gravitropism)
  4. States that in the shoot, the higher concentration of auxins on the lower side stimulates growth, so the lower side grows more quickly, causing the shoot to bend / grow upwards (negative gravitropism)

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