Explain why a plant shoot grows faster on the shaded side than on the illuminated side when light shines from one direction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A gardener notices that seedlings growing on a windowsill always bend towards the light over several days.
Model answer (4 marks)
Auxin produced in the shoot moves downwards and away from the light source, so it accumulates on the shaded side. The higher concentration of auxin on that side stimulates cell elongation. Cells on the shaded side therefore elongate more rapidly than those on the illuminated side, causing the shoot to bend towards the light.
Examiner tips
- Use the word ‘auxin’ and explain its movement away from light
- Show the link between higher auxin concentration and cell elongation
- State that the shaded side grows faster, leading to bending
- Keep answer concise – 4 marks only
Common mistakes
- Confusing phototropism with gravitropism; mentioning gravity instead of light direction
- Failing to mention auxin or its role in cell elongation
- Using vague terms like ‘hormone’ without specifying auxin
Mark scheme (4 marks)
- Auxin moves away from the light / accumulates on the shaded side
- Higher concentration of auxin on the shaded side
- Auxin promotes/stimulates cell elongation
- Shaded side elongates more / grows faster causing the shoot to bend towards the light
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →