A student placed cress seedlings on a windowsill so that light came from one direction only. After several days, the shoots had bent towards the light. Explain why the shoots bent towards the light.

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).

Cress seedlings were kept in a box with a hole cut in one side to allow light to enter from one direction only. After five days, the shoots had grown towards the hole.

Model answer (5 marks)

The shoot senses the light at its tip and synthesises auxin. The hormone is transported to the shaded side of the shoot, where it accumulates. Auxin stimulates cell elongation, so cells on the dark side grow longer than those on the light side. The unequal growth causes the shoot to bend towards the light (phototropism).

Examiner tips

  • Use the word ‘detects’ and ‘synthesises’ for auxin production
  • Show the direction of auxin movement (shaded side)
  • Explain that longer cells on the dark side give the bend
  • Link the unequal growth to the final bending

Common mistakes

  • Confusing light side with shaded side for auxin accumulation
  • Failing to mention auxin synthesis at the tip
  • Omitting the unequal cell elongation step

Mark scheme (5 marks)

  1. The stimulus (light) is detected and auxin is produced at the shoot tip
  2. Auxin moves to / accumulates on the shaded (darker) 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)

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