A gardener notices that shoots of a climbing plant always grow towards the light coming through a window. Explain how the unequal distribution of auxin in the shoot brings about this response.

Pearson Edexcel A-Level Biology A (9BI0) — 4.7 Plant responses · Explain · 5 marks · View as Markdown

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

The gardener observes that, over several days, the shoots consistently bend towards the window regardless of how the pot is turned.

Model answer (5 marks)

Light causes auxin to move to the shaded side of the shoot
The shaded side therefore has a higher concentration of auxin than the illuminated side
Auxin promotes cell elongation in the shoot
Cells on the shaded side elongate more (or faster) than cells on the illuminated side
This unequal elongation causes the shoot to bend (curve) towards the light (phototropism)

Examiner tips

  • Use the exact phrase "light causes auxin to move to the shaded side" to hit the first point. Show the chain: higher auxin → more cell elongation → bending towards light. Keep the answer concise – one sentence per point – to fit the 5‑mark structure.
  • common_mistakes
  • :
  • Mixing up the direction of auxin movement (writing that it moves to the illuminated side). Failing to mention that the higher auxin concentration is on the shaded side. Using vague wording such as "growth towards light" without linking it to auxin and cell elongation.

Mark scheme (5 marks)

  1. Light causes auxin to move to / accumulate on the shaded side of the shoot
  2. The shaded side therefore has a higher concentration of auxin than the illuminated side
  3. Auxin promotes / stimulates cell elongation in the shoot
  4. Cells on the shaded side elongate more / faster than cells on the illuminated side
  5. This unequal elongation causes the shoot to bend / curve towards the light (phototropism)

Key terms in this question

auxin · unequal distribution

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