IGCSE Biology / Chemical coordination in plants
Shoots growing towards light revision notes
Edexcel International GCSE Biology (4BI1), chapter 12: Chemical coordination in plants, specification point 2.83, 2.84, 2.85. Free lesson.
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Phototropism is a shoot’s growth response to light. Auxin makes the shaded side grow longer, bending the shoot towards light and helping its leaves photosynthesise.
What phototropism means
A tropism is a growth response in which the direction of growth depends on the direction of a stimulus. Phototropism is a response to light. Shoots grow towards light.
How auxin bends a shoot towards light
When light shines from one side, the shoot tip detects it. Auxin is made at the growing tip and moves away from the light, collecting on the shaded side. It causes cells there to elongate, so that side becomes longer and the shoot bends towards the light. The sequence is: light from one side, the tip detects the light and produces auxin, auxin collects on the shaded side, shaded-side cells elongate, then the shoot bends towards the light. A shoot curves towards its shorter side. The longer shaded side is therefore what makes the shoot bend towards the light.
Why growing towards light helps
A shoot that grows towards light gets more light on its leaves. This can increase the rate of photosynthesis, so more glucose is made and the shoot can grow more. Growing out from under a neighbouring plant also helps the shoot avoid staying in shade.
What the coleoptile experiments showed
A coleoptile is the protective sheath around the shoot of a seedling. Experiments with oat coleoptiles tested which parts detect light and how the growth signal travels. An intact coleoptile lit from one side bends towards the light. This is the control for comparison. Removing the tip stops bending and greatly reduces growth, showing that the tip is needed. An opaque cap over the tip stops bending, showing that the tip detects the light. An opaque sleeve around the base does not stop bending when the tip is exposed, showing that the base does not need to see the light. Putting agar…