How do plants survive extreme environments and respond to light and gravity?
Explain how plants are adapted to survive in extreme environments, how plant hormones such as auxins control growth in phototropisms and gravitropisms, and the commercial uses of plant hormones.
A focused answer to Edexcel GCSE Biology 6.14B to 6.16B, covering adaptations to extreme environments, how auxins control phototropism and gravitropism, and the commercial uses of auxins, gibberellins and ethene.
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What this dot point is asking
Edexcel statements 6.14B to 6.16B are Biology only. They want you to explain how plants are adapted to extreme environments, how auxins control growth in phototropism (response to light) and gravitropism (response to gravity), and the commercial uses of auxins, gibberellins and ethene.
Adapting to extreme environments
These are all examples of structure suiting the environment, the same idea as specialised cells, applied to whole plants.
Auxin and phototropism
In positive phototropism (shoots growing towards light):
- Auxin is made at the shoot tip and moves down the shoot.
- When light comes from one side, auxin moves to the shaded side.
- The extra auxin makes the cells on the shaded side elongate more.
- Because one side grows faster, the shoot bends towards the light, so the leaves get more light for photosynthesis.
Auxin and gravitropism
Auxin also controls the response to gravity:
- In a shoot, auxin gathers on the lower side and makes it grow more, so the shoot bends upwards (negative gravitropism).
- In a root, auxin gathers on the lower side but here it inhibits growth, so the upper side grows more and the root bends downwards (positive gravitropism), towards water and anchorage.
The opposite effect of auxin in shoots and roots is a common exam point: the same hormone promotes elongation in shoots but inhibits it in roots.
Commercial uses of plant hormones
Try this
Q1. State where auxin is produced in a plant. [1 mark]
- Cue. At the tips of shoots and roots.
Q2. Give one commercial use of ethene and one of auxins. [2 marks]
- Cue. Ethene: ripening fruit. Auxins: selective weedkiller or rooting powder.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksExplain how auxin causes a plant shoot to grow towards a light source coming from one side.Show worked answer →
A 4-mark explain question rewards the uneven distribution of auxin and its effect.
- Auxin is made at the tip of the shoot and moves down it.
- When light shines from one side, the 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 light side.
- Because one side grows faster, the shoot bends towards the light (positive phototropism).
Markers reward auxin accumulating on the shaded side, increased cell elongation there, and the resulting bend towards the light. Saying the plant simply moves towards light, without the auxin mechanism, scores little.
Edexcel 20212 marksGive two commercial uses of plant hormones and name a hormone used for each.Show worked answer →
A 2-mark question rewards two correct hormone-and-use pairs.
Auxins are used as selective weedkillers (herbicides) and as rooting powders to make cuttings grow roots.
Ethene is used to ripen fruit (such as bananas) during storage and transport, and gibberellins are used to end seed dormancy, promote flowering or produce seedless fruit.
Markers reward any two valid uses, each matched to the correct hormone. A use with the wrong hormone, or a vague answer like plant hormones help farming, does not score.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Biology (1BI0) specification — Pearson (2016)