3.6.7Inquiry Process
Phototropism — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 147 of 160
Section 1 of 5Auxin and Light
1
Auxin, mainly indole-3-acetic acid, is made in the shoot tip and moves downwards through the tissues.
2
Photoreceptors called phototropins detect blue light from one side of the shoot.
3
Light causes auxin efflux carriers, called PIN3, to move to the membrane on the shaded side of cells.
4
More auxin is therefore pumped towards the shaded side, giving a higher auxin concentration there than on the lit side.
5
Cells on the shaded side elongate faster than those on the lit side, so the shoot bends towards the light.
Section 2 of 5How Auxin Makes Cells Elongate
Proton pumps
- Auxin activates proton pumps in the plasma membrane, which move H⁺ into the cell wall and lower its pH.
Wall loosening
- The acid conditions activate enzymes called expansins that loosen the bonds between cellulose fibres in the wall.
Water uptake
- The cell takes in water by osmosis, and turgor pressure stretches the loosened wall.
Elongation
- Cells become longer rather than more numerous, so the side with more auxin grows more.
Perception
- The phototropin receptors detect blue wavelengths in particular, which is why blue light is the most effective stimulus.
Section 3 of 5Roots and Comparison
Roots and Comparison
Shoots
- Higher auxin concentration on the shaded side stimulates elongation, producing positive phototropism.
Roots
- The same concentrations inhibit elongation in root cells, so roots usually bend away from light or downwards.
Adaptive value
- Bending towards light helps leaves capture more light for photosynthesis.
Evidence
- Covering the shoot tip with foil prevents bending, showing the tip is where the light is perceived.
| Side of shoot | Auxin level | Growth |
|---|---|---|
| Lit side | Lower | Slower elongation |
| Shaded side | Higher | Faster elongation |
Section 4 of 5Worked Example
Question
- A shoot is 40 mm long.
- In light from one side, the cells on the shaded side elongate by 2.5 mm while those on the lit side elongate by 1.5 mm.
- Calculate the difference in growth and say which side becomes the outside of the curve.
- Shaded side growth = 2.5 mm, lit side = 1.5 mm.
- Difference = 2.5 − 1.5 = 1.0 mm.
- The longer side must be on the outside of the bend, so the shaded side forms the outside and the tip turns towards the light.
Answer The shaded side grows 1.0 mm more, so the shoot bends towards the light.
Section 5 of 5Exam Checklist & Tips
3.6.7 – Key Points for the Exam
- Auxin A plant hormone (IAA) that promotes cell elongation in shoots.
- PIN3 Efflux carrier proteins that move auxin out of cells in a particular direction.
- Phototropin Photoreceptor that detects blue light.
- Expansin Wall protein activated at low pH that loosens cellulose fibres.
Common Mistakes
- Saying auxin is destroyed by light; it is redistributed instead.
- Describing the bend as caused by cell division rather than elongation.
- Forgetting the role of H⁺ pumping and cell wall acidification.
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