☀ Lit side
Less auxin. The cells lengthen less, so this side grows more slowly.
Plant Hormones » Plant Hormones and Tropisms
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.5.4.1 (Biology only)
- How plants use hormones to coordinate and control growth
- What phototropism and gravitropism (geotropism) are
- How unequal distribution of auxin causes unequal growth in shoots and roots
- The jobs of gibberellins and ethene (Higher tier only)
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Unlock This CoursePlants respond too
Animals have a nervous system and hormones. Plants have no nerves, but they still sense their surroundings and respond. A plant on a windowsill bends towards the glass. A seed planted upside down still sends its root downwards. These responses are not random. Plants produce hormones to coordinate and control their growth and their responses.
Plant responses are usually slow growth changes, not fast movements. The plant grows more on one side than the other, so the whole shoot or root turns.
Key terms:
- Tropism: a growth response of a plant towards or away from a stimulus.
- Phototropism: a growth response to light.
- Gravitropism (geotropism): a growth response to gravity.
- Auxin: a plant hormone that controls growth in shoots and roots.
Phototropism: growing to the light
Plants need light for photosynthesis, so a shoot that grows towards light has a better chance of making food. Shoots grow towards the light.
Auxin makes the cells in a shoot lengthen. If light shines from one side, the auxin is not spread out evenly. More auxin collects on the shaded side of the shoot.
☾ Shaded side
More auxin. The cells lengthen more, so this side grows faster.
The shaded side grows faster than the lit side, so the shoot bends towards the light. This is an unequal distribution of auxin causing unequal growth rates.
Worked example
A seedling grows on a windowsill. Light comes from the right. Auxin collects on the left, the shaded side. The left cells lengthen faster than the right cells, so the shoot bends to the right, towards the window. If you turn the pot round, new growth bends back towards the window again.
Notice what is being compared each time. The lit side and the shaded side both have the same type of cell and the same hormone. The only difference is how much auxin each side gets. That is why unequal distribution of auxin is such a useful idea in exams.
Gravitropism: roots down, shoots up
Gravity is a stimulus too. Roots grow downwards, in the direction of gravity. This carries the roots into the soil, where they take in water and mineral ions and hold the plant firmly. Shoots grow upwards, against gravity. This lifts the leaves up towards the light.
Auxin is again the key. If a shoot or root lies on its side, gravity makes auxin collect on the lower side.
🌱 In a shoot
More auxin on the lower side makes those cells grow faster. The lower side lengthens more, so the shoot bends upwards.
🌿 In a root
More auxin on the lower side slows the growth of those cells. The upper side grows faster, so the root bends downwards.
This is the clever part: the same hormone on the same side has the opposite effect in a root and a shoot. In a shoot, more auxin means more growth. In a root, more auxin means less growth on that side.
Worked example
A seedling is laid on its side in the dark. Gravity pulls auxin to the lower side of the shoot. The lower cells lengthen faster than the upper cells, so the shoot curves upwards. In the same seedling, the root's lower side grows more slowly than its upper side, so the root curves downwards. Both responses are caused by unequal distribution of auxin.
Why these responses matter
Plants cannot move away, so growth is how they respond. Growing towards light helps leaves capture more light for photosynthesis. Growing roots downwards helps a young seedling reach water and mineral ions and anchor itself. Growing shoots upwards lifts them out of the soil.
- Shoots: towards light, away from gravity.
- Roots: downwards, in the direction of gravity.
Other plant hormones (Higher tier only)
Higher tier only
Gibberellins are important in initiating (starting) seed germination. Ethene controls cell division and the ripening of fruits. You do not need to know how gibberellins or ethene work, only what they do.
Putting it together
- Plants produce hormones to coordinate and control growth and responses.
- Phototropism is a response to light. Gravitropism (geotropism) is a response to gravity.
- Auxin is not shared out evenly, so one side of a root or shoot grows faster than the other.
- That unequal growth turns the shoot or root.
Common mistakes
Saying that plants "move towards the light". They do not move. One side grows faster, so the shoot bends. Another mistake is thinking auxin is made only on the shaded side. Auxin moves, and it ends up in a greater amount on the shaded side. Finally, do not say that auxin speeds up growth in roots on the lower side: it slows it.
Exam-style question
A student places a seedling on its side in a dark cupboard. After two days the shoot has bent upwards.
(a) Name the hormone that caused the shoot to bend. [1 mark]
(b) Explain why the shoot bent upwards. [3 marks]
(c) Name this type of response. [1 mark]
Model answer
(a) Auxin. (1)
(b) Gravity caused auxin to collect on the lower side of the shoot (1). The cells on the lower side grew faster than the cells on the upper side (1). This unequal growth made the shoot bend upwards (1).
(c) Gravitropism (geotropism). (1)
Exam tip
For "explain" questions about bending, always say where the auxin collects and which side grows faster. Marks are lost for saying only that the plant "grows towards" something.