A seedling is grown in a windowless box. A small hole is cut in one side of the box so that light enters from one direction only. After several days, the shoot tip is removed from the seedling and the seedling is left to continue growing. Explain why the shoot no longer bends towards the light after the tip has been removed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A seedling is grown in a windowless box with a small hole cut in one side, so that light enters from one direction only. After several days, the shoot tip is removed.
Model answer (4 marks)
Auxins are produced at the shoot tip.
Without the tip, no auxins are produced, so auxins are absent.
Auxins normally move to, or accumulate on, the shaded side of the shoot, causing unequal growth.
Without auxins there is no differential growth, so the shoot grows straight and does not bend towards the light.
Without the tip, no auxins are produced, so auxins are absent.
Auxins normally move to, or accumulate on, the shaded side of the shoot, causing unequal growth.
Without auxins there is no differential growth, so the shoot grows straight and does not bend towards the light.
Examiner tips
- Use the exact phrase ‘auxins are produced at the shoot tip’ to gain the first mark. Show the causal chain: tip → auxin production → auxin distribution → unequal growth → bending. Mention ‘shaded side’ to hit the second point. Conclude that without auxins there is no unequal growth, so no bending for the final mark.
Common mistakes
- Confusing auxin with other hormones such as gibberellins. Saying the tip is removed but still saying auxins are produced. Forgetting to mention that auxins accumulate on the shaded side.
Mark scheme (4 marks)
- Auxins are produced at the shoot tip
- Without the tip, no auxins are produced / auxins are absent
- Auxins normally move to / accumulate on the shaded side of the shoot, causing unequal growth
- Without auxins there is no differential / unequal growth, so the shoot grows straight / does not bend
Key terms in this question
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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