Explain why a plant shoot placed in a box with a small hole on one side will grow towards the hole after several days.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A gardener places a young plant shoot inside a closed box. The box has a small hole on one side, allowing light to enter from that direction only. After several days, the shoot has bent and is growing towards the hole.
Model answer (4 marks)
Light enters the box only through the hole, so the shoot receives light from one direction.
Auxin is redistributed to the shaded side of the shoot tip.
The higher auxin concentration on the shaded side stimulates greater cell elongation there.
Cells on the lit side elongate less because auxin concentration is lower.
The shaded side becomes longer than the lit side, causing the shoot to bend and grow towards the light source (the hole).
Auxin is redistributed to the shaded side of the shoot tip.
The higher auxin concentration on the shaded side stimulates greater cell elongation there.
Cells on the lit side elongate less because auxin concentration is lower.
The shaded side becomes longer than the lit side, causing the shoot to bend and grow towards the light source (the hole).
Examiner tips
- Use the word ‘auxin’ and explain its redistribution to the shaded side. Show the link between higher auxin, greater cell elongation and bending. Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Confusing the direction of auxin movement (writing that auxin moves towards the light instead of the shaded side). Failing to mention that cells on the lit side elongate less. Using vague terms like ‘growth’ without specifying cell elongation.
Mark scheme (4 marks)
- Light comes from one direction (through the hole), so auxin moves/is redistributed to the shaded side of the shoot tip
- The higher concentration of auxin on the shaded side causes cells there to elongate more
- Cells on the lit side elongate less / grow more slowly because auxin concentration is lower there
- Because the shaded side is longer than the lit side, the shoot bends / curves towards the light (source / hole)
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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