AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Unequal distributions of auxin cause unequal growth rates in plant roots…
Full specification wording
Control and coordination: Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.
Auxin is a plant hormone made in the shoot and root tips.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Auxin is a plant hormone made in the shoot and root tips. It moves away from the tip and can spread unevenly when a plant is stimulated by light or gravity. Auxin causes cells to elongate, so a higher auxin concentration on one side makes those cells grow more than cells on the other side. This unequal growth bends the organ. In a shoot lit from one side, auxin moves to the shaded side, cells there elongate more, and the shoot bends towards the light. In a root placed horizontally, auxin gathers on the lower side, but high auxin inhibits root cell elongation, so the upper side grows more and the root bends downwards. The same hormone can therefore produce different responses in roots and shoots because their sensitivity to auxin differs.
What earns the marks
- Auxin is produced in the shoot and root tips and moves to other regions of the plant.
- Auxin causes cells to elongate, so a higher auxin concentration leads to more growth on that side.
- In a shoot, auxin moves to the shaded side, cells elongate more there, and the shoot bends towards the light.
- In a root, high auxin inhibits cell elongation, so the lower side grows less and the root bends downwards.
- Unequal auxin distribution therefore causes unequal growth rates, which produce the bending seen in tropisms.
- Roots and shoots respond differently to the same auxin concentration because their sensitivity to auxin differs.
Common mistakes and how to put them right
Saying auxin always increases growth.
Put it right: Auxin increases elongation in shoots but inhibits elongation in roots at higher concentrations.
Placing auxin on the lit side of a shoot.
Put it right: Auxin moves to the shaded side, where cells elongate more and the shoot bends towards the light.
Writing that auxin is made in the leaves.
Put it right: Auxin is produced mainly in the shoot and root tips.
Exam tips
- Describe the sequence: auxin moves, auxin becomes unevenly distributed, cells on one side elongate more, and the organ bends.
- State the direction of bending and link it to the advantage, such as a shoot bending towards light for photosynthesis.
- For roots, remember that high auxin inhibits elongation, so the side with less auxin grows more and the root bends downwards.
Try the lesson
The start of the Control and coordination: Unequal distributions of auxin cause unequal growth rates in plant roots and shoots. lesson, made from checked teaching notes for this specification point.
Have a go first
An unequal distribution of which substance causes unequal growth rates in plant roots and shoots?
Listen
Auxin is a plant hormone made in the shoot and root tips. It moves away from the tip and can spread unevenly when a plant is stimulated by light or gravity. Auxin causes cells to elongate, so a higher auxin concentration on one side makes those cells grow more than cells on the other side. This unequal growth bends the organ. In a shoot lit from one side, auxin moves to the shaded side, cells there elongate more, and the shoot bends towards the light. In a root placed horizontally, auxin gathers on the lower side, but high auxin inhibits root cell elongation, so the upper side grows more and the root bends downwards. The same hormone can therefore produce different responses in roots and shoots because their sensitivity to auxin differs. A strong answer earns marks for these points. Auxin is produced in the shoot and root tips and moves to other regions of the plant. Auxin causes cells to elongate, so a higher auxin concentration leads to more growth on that side. In a shoot, auxin moves to the shaded side, cells elongate more there, and the shoot bends towards the light. In a root, high auxin inhibits cell elongation, so the lower side grows less and the root bends downwards. A common slip: Saying auxin always increases growth. Instead, auxin increases elongation in shoots but inhibits elongation in roots at higher concentrations. Exam tip: Describe the sequence: auxin moves, auxin becomes unevenly distributed, cells on one side elongate more, and the organ bends.
Key words
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Auxin and cell elongation
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Unequal growth
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Tropisms
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Plant coordination
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See every definition
- Auxin and cell elongation
- Auxin is a plant hormone made in the shoot and root tips that causes cells to elongate, so a higher auxin concentration leads to more growth on that side.
- Unequal growth
- A higher auxin concentration on one side makes those cells grow more than cells on the other side, and this unequal growth bends the organ.
- Tropisms
- Unequal auxin distribution causes unequal growth rates, which produce the bending seen in tropisms.
- Plant coordination
- Auxin is a plant hormone made in the shoot and root tips that moves away from the tip and can spread unevenly when a plant is stimulated by light or gravity.
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