Explain how auxin causes a plant shoot to grow towards a light source.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Auxin is produced at the tip of the shoot and is transported downwards.
1. The hormone is carried more to the shaded side of the stem.
2. Auxin stimulates cell elongation on that side.
3. The shaded side therefore grows faster and becomes longer than the light side.
4. The unequal growth bends the shoot so it curves towards the light source.
1. The hormone is carried more to the shaded side of the stem.
2. Auxin stimulates cell elongation on that side.
3. The shaded side therefore grows faster and becomes longer than the light side.
4. The unequal growth bends the shoot so it curves towards the light source.
Examiner tips
- Use the exact sequence: production → distribution → cell elongation → bending.
- Show the cause–effect chain in order, as the mark scheme expects.
Common mistakes
- Confusing light side with shaded side; the hormone moves to the shaded side.
- Omitting the step that auxin causes cell elongation or the final bending of the shoot.
Mark scheme (4 marks)
- Auxin is produced at the tip of the shoot and moves/distributes to the shaded side
- Auxin causes cells to elongate / grow longer on the shaded 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
- Revision notes: Plant hormones (Bio only) →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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