Interactive Lesson 14.5.1
Phototropism & Auxin: The Plant's Light-Seeking Mechanism
Discover how plants bend towards light through the action of the plant hormone Auxin
What is Phototropism?
Phototropism is a plant's growth response to the direction of light. Shoots show positive phototropism – they grow towards the light source.
The 5-Step Auxin Mechanism
1
Auxin is produced in the Shoot Tip
2
Auxin diffuses down the shoot
3
Light causes Auxin to move to the Shaded Side
4
Auxin stimulates Cell Elongation
5
Shaded side grows faster → Shoot bends towards light
Key Insight
Auxin doesn't make cells grow faster on one side – it makes them grow longer (elongate). The auxin-rich side has taller cells, causing the shoot to curve.
Exam Checklist: What You Must Know
✓ Definition: Phototropism is growth response to light direction
✓ Direction: Shoots show positive phototropism (towards light)
✓ Hormone: Auxin is the plant hormone responsible
✓ Production: Auxin made in shoot tip
✓ Movement: Auxin moves to shaded side
✓ Action: Auxin causes cell elongation
✓ Result: Shaded side grows more → shoot bends toward light
✓ Evidence: Removing tip stops bending (no auxin)
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