Explain why auxin causes differential growth on the shaded side of a shoot tip when the shoot is exposed to unilateral light.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When a plant shoot is exposed to light from one side only, auxin (IAA) redistributes laterally across the shoot tip. The shaded side elongates faster than the illuminated side, causing the shoot to bend towards the light source.
Model answer (4 marks)
Auxin is produced in the cells of the illuminated side of the shoot tip and then moves laterally across the tip to the shaded side. The shaded side therefore has a higher concentration of auxin than the illuminated side. Auxin promotes cell elongation by stimulating the loosening of cell walls, which allows cells to take up more water and expand. Because the shaded side elongates more than the illuminated side, the shoot bends towards the light (positive phototropism).
Examiner tips
- Use the word ‘auxin’ and ‘IAA’ correctly; mention lateral redistribution.
- Explain that higher auxin concentration leads to cell‑wall loosening and water uptake.
- Show the link between greater elongation on the shaded side and bending towards light.
- Keep the answer concise – 4 points, one sentence each.
Mark scheme (4 marks)
- Auxin moves laterally from the illuminated side to the shaded side of the shoot tip
- The shaded side therefore has a higher concentration of auxin than the illuminated side
- Auxin promotes cell elongation by stimulating loosening of cell walls, allowing cells to take up more water and expand
- Greater elongation on the shaded side compared to the illuminated side causes the shoot to bend towards the light (positive phototropism)
Key terms in this question
auxin · unilateral light · differential growth · shoot tip
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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