A farmer grows a crop of sunflowers in a large open field. During the growing season, the farmer observes that the young sunflower shoots consistently face east in the morning and west in the evening, tracking the movement of the Sun across the sky. Explain how auxin brings about this directional growth response in the young sunflower shoots.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Auxin is a plant hormone produced at the tip of a shoot. It moves away from regions of high light intensity towards regions of lower light intensity, causing unequal distribution across the shoot.
Model answer (5 marks)
Auxin is produced at the shoot tip.
When a shoot is exposed to light, auxin is transported away from the illuminated side and towards the shaded side.
The shaded side therefore has a higher auxin concentration.
Higher auxin levels stimulate greater cell elongation on that side.
Unequal elongation bends the shoot so it points towards the light, and as the Sun moves, auxin redistribution continues, making the shoot track the Sun.
When a shoot is exposed to light, auxin is transported away from the illuminated side and towards the shaded side.
The shaded side therefore has a higher auxin concentration.
Higher auxin levels stimulate greater cell elongation on that side.
Unequal elongation bends the shoot so it points towards the light, and as the Sun moves, auxin redistribution continues, making the shoot track the Sun.
Examiner tips
- Use the exact sequence of points from the mark scheme
- Show the cause–effect chain: production → redistribution → differential growth → bending
- Mention the continuous change as the Sun moves
Common mistakes
- Confusing light with darkness as the direction of auxin movement
- Saying auxin causes cell division instead of elongation
- Omitting that the redistribution is continuous during the day
Mark scheme (5 marks)
- Auxin is produced at the tip of the shoot
- Auxin moves away from the light / towards the shaded side of the shoot
- The higher concentration of auxin on the shaded side causes cells on that side to elongate more
- Unequal cell elongation causes the shoot to bend / curve towards the light
- As the Sun moves across the sky throughout the day, the position of the light source changes, so auxin redistribution changes continuously, causing the shoot to track the Sun
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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