Explain how the unequal distribution of auxin in a plant shoot causes the shoot to bend towards a light source.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A plant shoot is placed on a windowsill where light comes from one side only.
Model answer (4 marks)
Auxin is transported down the shoot and, when light is only on one side, it accumulates on the shaded side.
Higher auxin concentration on the shaded side stimulates greater cell elongation there.
The shaded side therefore grows faster and longer than the illuminated side.
This unequal growth causes the shoot to bend or curve towards the light source.
Higher auxin concentration on the shaded side stimulates greater cell elongation there.
The shaded side therefore grows faster and longer than the illuminated side.
This unequal growth causes the shoot to bend or curve towards the light source.
Examiner tips
- Use the word ‘auxin’ and ‘cell elongation’ – these are key terms.
- Explain the direction of auxin movement and the result of higher concentration.
- Show the link between faster growth on one side and bending.
- Keep the answer concise – 4 points, one sentence each.
Mark scheme (4 marks)
- Auxin moves to the shaded side of the shoot
- Higher concentration of auxin on the shaded side causes greater cell elongation on that side
- The shaded side grows faster/longer than the light side
- This unequal growth causes the shoot to bend/curve towards the light
Key terms in this question
Related
- All Eduqas GCSE Biology revision notes →
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- Decode the mark scheme abbreviations →