A student claims that the tropistic response of a shoot to light is entirely explained by the presence of auxin on the shaded side of the shoot tip. Evaluate this claim.

OCR A-Level Biology A (H420) — 5.5 Plant and animal responses · Evaluate · 4 marks · View as Markdown

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

Auxin (indole-3-acetic acid, IAA) is a plant growth hormone produced at the shoot tip. When a shoot is illuminated from one side, unequal distribution of auxin is thought to drive differential cell elongation, resulting in bending towards the light source.

Model answer (4 marks)

1. Auxin is produced at the shoot tip and, when the shoot is illuminated from one side, it redistributes to the shaded side.
2. The higher auxin concentration on the shaded side stimulates greater cell elongation there, so the shoot bends towards the light.
3. However, the claim is incomplete because it ignores other factors that influence phototropism.
4. For example, phototropins act as the light receptors that trigger auxin redistribution, and cytokinin or other growth inhibitors on the light side also affect bending.

Examiner tips

  • Use the 4-point structure: explain mechanism, show effect, critique claim, give additional factor.
  • Mention phototropins and cytokinin to demonstrate depth.
  • Keep sentences concise and use exact terminology (auxin, phototropism).

Common mistakes

  • Claiming auxin alone causes bending without mentioning phototropins.
  • Forgetting to note that cytokinin or other inhibitors also play a role.
  • Using vague terms like ‘hormone’ instead of ‘auxin (IAA)’.

Mark scheme (4 marks)

  1. Auxin redistributes to the shaded side of the shoot tip, causing greater cell elongation on that side
  2. This differential elongation causes the shoot to bend towards the light, which supports the claim
  3. The claim is incomplete because other factors or mechanisms also contribute to phototropism
  4. A named limitation or additional mechanism, such as the role of phototropins as the light receptors that initiate auxin redistribution, or evidence that cytokinin/other growth inhibitors on the light side also contribute to bending

Key terms in this question

auxin · tropistic response · shoot tip

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