A scientist discovered that when the growing tip of a young plant shoot is removed, the shoot stops bending towards light. When the tip is replaced, the shoot bends towards light again. Explain how the shoot normally bends towards light, including the role of auxin.

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).

Auxin is a plant hormone produced in the tip of a shoot. It controls the rate of cell elongation in the shoot. Auxin moves away from the light source, accumulating on the shaded side of the shoot.

Model answer (5 marks)

Auxin is produced in the shoot tip.
Auxin is transported down the shoot and accumulates on the shaded side.
Auxin stimulates cell elongation.
Cells on the shaded side elongate more than those on the light side.
The unequal growth bends the shoot towards the light (phototropism).

Examiner tips

  • Use the exact terms: auxin, shoot tip, shaded side, cell elongation, unequal growth, phototropism.
  • Show the causal chain: production → transport → accumulation → elongation → bending.
  • Keep the answer concise – one sentence per point to hit all 5 marks.

Common mistakes

  • Confusing light side with shaded side; writing that auxin accumulates on the light side.
  • Using vague words like ‘growth’ instead of ‘cell elongation’.
  • Omitting the step that cells on the shaded side elongate more than those on the light side.

Mark scheme (5 marks)

  1. Auxin is produced in the tip of the shoot
  2. Auxin moves towards / accumulates on the shaded side of the shoot
  3. Auxin causes cells to elongate / grow longer
  4. Cells on the shaded side elongate more than cells on the light side
  5. This unequal growth causes the shoot to bend towards the light (phototropism)

Key terms in this question

auxin

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