Explain how the unequal distribution of auxin in a shoot tip causes the shoot to grow towards a light source coming from one direction only.

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).

Plants produce a hormone called auxin in their shoot tips. When light hits a shoot from one side, auxin moves away from the light source towards the shaded side of the shoot.

Model answer (5 marks)

Auxin is produced in the shoot tip and, when light falls on one side, it is transported laterally away from the illuminated side towards the shaded side of the shoot tip.

This lateral movement creates a higher concentration of auxin on the shaded side than on the illuminated side.

Auxin promotes cell elongation, so cells on the shaded side elongate more than cells on the illuminated side.

The unequal elongation of cells causes the shoot to bend or curve towards the light source, producing positive phototropism.

Examiner tips

  • Use the word ‘laterally’ to show auxin movement; mention ‘higher concentration’ on shaded side; state that auxin stimulates cell elongation; explain unequal elongation leads to bending; link to phototropism.
  • Keep the answer concise – one sentence per point – and use exact terminology from the mark scheme.

Mark scheme (5 marks)

  1. Auxin moves (laterally) from the illuminated side to the shaded side of the shoot tip
  2. This results in a higher concentration of auxin on the shaded side than the illuminated side
  3. Auxin promotes / stimulates cell elongation
  4. Cells on the shaded side elongate more than cells on the illuminated side
  5. This unequal elongation causes the shoot to bend / curve towards the light source (phototropism)

Key terms in this question

auxin · shoot tip

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