How do plants sense and respond to their environment?
Plant growth responses (tropisms), the role of auxin (IAA) in phototropism and gravitropism, and the commercial uses of plant growth substances.
A CCEA A-Level Biology answer on plant growth responses (tropisms), the role of auxin in phototropism and gravitropism, and the commercial uses of plant growth substances.
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What this dot point is asking
CCEA wants you to describe plant growth responses (tropisms), explain the role of auxin (IAA) in phototropism and gravitropism, and outline the commercial uses of plant growth substances.
Tropisms
Shoots are positively phototropic (grow towards light, to maximise photosynthesis) and negatively gravitropic (grow upward, against gravity); roots are negatively phototropic and positively gravitropic (grow down into the soil to anchor the plant and reach water). Because plants have no nervous system, these responses are slow and brought about by growth, controlled by hormones rather than nerves.
The role of auxin
Auxin promotes elongation by causing the cell walls to loosen, so the cells take up water by osmosis and lengthen. The same hormone has opposite effects in shoots and roots because root cells are far more sensitive to auxin: the concentration that promotes shoot elongation is high enough to inhibit root elongation.
Commercial uses
Synthetic plant growth substances have many uses: rooting powders containing auxins promote root growth on cuttings so new plants can be propagated; selective weedkillers (synthetic auxins such as 2,4-D) make broad-leaved weeds grow too fast and die while leaving narrow-leaved cereals unharmed; and growth substances are used to control fruit ripening, to produce seedless fruit, and to time flowering for market.
Examples in context
Example 1. The Darwins' coleoptile experiments. Charles Darwin and his son showed that a grass coleoptile bends towards light only if the tip is exposed: covering the tip with an opaque cap stopped the bending, even though the rest of the shoot was lit. This classic experiment located the light-detecting region at the tip and pointed towards a chemical messenger (later identified as auxin) moving down from it. It is the experimental foundation of the modern auxin theory and a common CCEA practical context.
Example 2. Selective weedkillers on a lawn. A synthetic auxin sprayed on a cereal field or lawn is absorbed more by broad-leaved weeds (such as dandelions) than by narrow-leaved grasses. The weeds are driven into uncontrolled, excessive growth that exhausts and kills them, while the crop or grass survives. This shows a plant growth substance exploited commercially, and links the biology of auxin to agriculture.
Try this
Q1. Explain how auxin causes a shoot to bend towards light. [3 marks]
- Cue. Auxin moves to the shaded side and promotes cell elongation there, so that side grows faster and the shoot bends towards the light.
Q2. State one commercial use of plant growth substances. [1 mark]
- Cue. Rooting powders, selective weedkillers, or controlling fruit ripening.
Q3. Explain why the same auxin makes a shoot bend up but a root bend down. [2 marks]
- Cue. Roots are far more sensitive to auxin, so the concentration that promotes elongation in the shoot inhibits elongation in the root, giving the opposite bending.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20196 marksExplain how auxin (IAA) causes a shoot to grow towards a light source coming from one side.Show worked answer →
A 6-mark answer should follow auxin from synthesis to the bending response.
Auxin (IAA) is produced at the shoot tip.
When light shines from one side, auxin is redistributed away from the light, accumulating on the shaded side of the shoot.
Auxin promotes cell elongation: it makes the cell walls more flexible (loosening) so the cells take up water and elongate.
Because there is more auxin on the shaded side, the cells there elongate more than those on the lit side.
The shaded side therefore grows faster (lengthens more), so the shoot bends towards the light (a positive phototropism).
Markers reward auxin made at the tip, movement to the shaded side, promotion of cell elongation, faster growth on the shaded side, and bending towards the light.
CCEA 20214 marksDescribe an experiment using shoot tips and an opaque cap that would show that the shoot tip is needed for a phototropic response, and state the control needed.Show worked answer →
A 4-mark answer needs the manipulation, the comparison and a valid control.
Set up several identical shoots and illuminate them from one side.
In one group, cover the shoot tip with an opaque cap so no light reaches it; the shoot should not bend, showing the tip detects the light.
In another, remove the tip entirely; again no bending, showing the tip is the source of the response.
Control: an intact, uncapped shoot illuminated from the same side, which should bend towards the light, confirming the response is due to the tip and not the treatment.
Keep other variables the same (temperature, light intensity, type of shoot). Markers reward capping or removing the tip, the prediction of no bending, and a valid intact control.
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Sources & how we know this
- CCEA GCE Biology specification — CCEA (2016)