Explain how a shoot tip responds to light coming from one direction only.

Pearson Edexcel International GCSE Biology (4BI1) — 2.14 Coordination and response: flowering plants · Explain · 4 marks · View as Markdown

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

When light falls on a plant shoot from one side only, the shoot bends towards the light source. This response is controlled by a hormone produced in the shoot tip.

Model answer (4 marks)

Auxin is produced in the shoot tip. When light falls from one side, auxin is redistributed and accumulates on the shaded side of the shoot. The higher concentration of auxin on the shaded side stimulates greater cell elongation there. The unequal elongation causes the shoot to bend towards the light (positive phototropism).

Examiner tips

  • Use the word ‘auxin’ and mention it is produced in the shoot tip.
  • Explain the redistribution to the shaded side and the resulting cell elongation.
  • Show the link between unequal elongation and bending towards light.
  • Use the phrase ‘positive phototropism’ to summarise the response.

Common mistakes

  • Confusing auxin with another hormone such as gibberellin.
  • Failing to state that auxin accumulates on the shaded side.
  • Not linking higher auxin concentration to increased cell elongation.

Mark scheme (4 marks)

  1. Auxin (is the hormone that) is produced in the shoot tip
  2. Auxin moves to / accumulates on the shaded (darker) side of the shoot
  3. The higher concentration of auxin on the shaded side causes cells on that side to elongate more
  4. Unequal elongation causes the shoot to bend towards the light (positive phototropism)

Key terms in this question

shoot tip

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