How do plants respond to light and gravity, and how does the hormone auxin control their growth?
Plant responses to light and gravity as tropisms, phototropism and gravitropism, and the role of the plant hormone auxin in controlling the direction of growth.
A focused answer to the WJEC GCSE Biology section 2.5 topic on plant responses, covering tropisms, phototropism and gravitropism, and the role of the plant hormone auxin in controlling the direction of plant growth in shoots and roots.
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What this dot point is asking
WJEC wants you to know that plants respond to light and gravity by tropisms, to name phototropism and gravitropism, and to explain how the plant hormone auxin controls the direction of growth.
Tropisms
Plants cannot move, but they respond to their surroundings by changing the direction in which they grow. A response of this kind is a tropism.
The two tropisms you need are:
- Phototropism: a growth response to light. Shoots grow towards light (positive phototropism), which helps them photosynthesise.
- Gravitropism (geotropism): a growth response to gravity. Roots grow downwards (positive gravitropism), which anchors the plant and reaches water; shoots grow upwards (negative gravitropism).
The role of auxin
Plant tropisms are controlled by a plant hormone called auxin. Auxin is produced at the tips of shoots and roots and moves back along them. Auxin makes plant cells elongate (grow longer).
The key idea is that auxin becomes unevenly distributed when there is a one-sided stimulus, causing uneven growth that bends the plant.
How a shoot bends towards light
- Auxin is made at the shoot tip.
- Light shines from one side, and auxin moves to the shaded side.
- The higher auxin concentration on the shaded side makes those cells elongate more.
- The shaded side grows faster than the lit side, so the shoot bends towards the light.
This response means leaves receive more light for photosynthesis, improving the plant's chance of survival.
Why each tropism helps the plant survive
A tropism is useful because it points growth in the direction that helps the plant most.
- Shoots growing towards light (positive phototropism) lets the leaves absorb more light, so the plant can photosynthesise faster and make more glucose.
- Roots growing downwards (positive gravitropism) anchors the plant firmly and helps the roots reach water and minerals deeper in the soil.
- Shoots growing upwards (negative gravitropism) lifts the leaves and flowers above the ground, towards the light and where insects can reach the flowers.
Because these responses increase the chance of survival, plants that respond well to light and gravity grow and reproduce more successfully.
Investigating phototropism
A common classroom investigation grows seedlings in different light conditions to show phototropism.
- Grow several identical seedlings in pots.
- Place one pot in all-round light, one in light from a single side, and one in the dark as a control.
- Leave them for a few days and observe the direction of growth.
The seedlings lit from one side bend towards the light; those in all-round light grow straight up; and those in the dark grow tall and pale (etiolated). To keep the test fair, the seedlings should be the same age and species and given the same water and temperature, so only the light direction changes.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksA shoot is lit from one side and grows towards the light. Explain how auxin causes this response.Show worked answer →
A 4-mark explain question on phototropism.
Auxin is made at the tip of the shoot and moves down the shoot. When light comes from one side, auxin moves to the shaded side of the shoot. The higher concentration of auxin on the shaded side makes those cells grow (elongate) more than the cells on the lit side. The uneven growth makes the shoot bend towards the light.
Markers reward: auxin moves to the shaded side; auxin makes cells elongate/grow more; the shaded side grows faster; the shoot bends towards the light. This is a positive phototropism.
WJEC style2 marksState what is meant by a tropism and give one example.Show worked answer →
A 2-mark definition and example.
A tropism is a growth response of a plant towards or away from a stimulus, where the direction of growth depends on the direction of the stimulus. One example is phototropism, the growth of a shoot towards light; another is gravitropism, the growth of a root downwards in response to gravity.
Markers reward: a directional growth response to a stimulus, plus a correct named example (phototropism or gravitropism). Calling it a "movement" rather than growth is a common slip.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)