A farmer grows a crop of tomato plants in a large greenhouse. The farmer notices that the tomato plants near the edges of the greenhouse grow with their stems bent towards the glass walls, while plants in the centre grow upright. Explain why the edge plants grow bent towards the glass walls.

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).

Tomato plants are flowering plants that show tropisms in response to environmental stimuli. The hormone auxin controls the direction of growth in plant shoots.

Model answer (5 marks)

Light enters the greenhouse mainly from the glass walls, so the plants at the edges receive more light on the side facing the walls than on the opposite side.

Auxin is redistributed away from the illuminated side of the shoot and accumulates on the shaded side.

The higher auxin concentration on the shaded side stimulates greater cell elongation there.

Because cells on the shaded side grow faster than those on the light side, the stem bends towards the light source.

This bending towards the light is a positive phototropic response mediated by auxin.

Examiner tips

  • Use the word ‘phototropism’ and ‘auxin’ – key terminology. Explain the direction of light, auxin movement, cell elongation and the resulting bend. Show the causal chain: light → auxin redistribution → differential growth → bend.
  • common_mistakes
  • :
  • Confusing the direction of auxin movement (must be away from light). Forgetting to mention that the bend is towards the light, not away. Using vague terms like ‘growth’ without specifying cell elongation or auxin.

Mark scheme (5 marks)

  1. Light is coming from one direction (through the glass walls / unequally / more light from the side)
  2. Auxin moves away from / is redistributed away from the light source, to the shaded side of the shoot
  3. The higher concentration of auxin on the shaded side causes cells on that side to elongate more / grow faster
  4. Because cells on the shaded side are longer / grow faster than cells on the light side, the shoot bends towards the light
  5. This response is called phototropism / the shoot is positively phototropic

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