Plant Hormones and Tropisms
Recap Plant Hormones and Tropisms for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) biology topic on how auxin controls phototropism and gravitropism; the required practical investigates the effect of light on seedling growth.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Plants produce hormones to coordinate and control growth and responses to light and gravity. This is a Triple-only topic and includes a required practical.
Auxin is the main plant hormone. It controls phototropism, the response to light, and gravitropism, the response to gravity. Auxin accumulates on the shaded side of a shoot, where it causes cells to elongate more, so the shoot bends towards the light.
In roots the effect is reversed: more auxin inhibits growth, so a root bends downwards towards gravity. Remembering that auxin promotes growth in shoots but inhibits it in roots is the key to answering almost any question here.
Revision notes
Phototropism in shoots
Light causes auxin to accumulate on the shaded side of the shoot tip.
Auxin promotes cell elongation, so cells on the shaded side grow longer than those on the lit side. This makes the shoot bend towards the light — positive phototropism.
Gravitropism in roots
Auxin accumulates on the lower side of a root.
In roots, auxin inhibits cell elongation, so the lower side grows less than the upper side and the root bends downwards — positive gravitropism. The opposite effect to shoots.
The required practical
Investigate the effect of light or gravity on newly germinated seedlings.
Control variables such as water, temperature and the type of seed. Use a control grown in all-round light. Measure the angle or direction of growth after a set period.
Key points
- Auxin is the main plant growth hormone.
- Phototropism is a response to light.
- Gravitropism is a response to gravity.
- Auxin accumulates on the shaded side of a shoot.
- In shoots auxin promotes cell elongation.
- In roots auxin inhibits cell elongation.
Worked examples
Example 1
Explain why a shoot bends towards the light. [3 marks]
Model answer
Auxin accumulates on the shaded side of the shoot (1 mark). Auxin promotes cell elongation, so the cells on the shaded side grow longer than those on the lit side (1 mark). This unequal growth makes the shoot bend towards the light (1 mark).
Example 2
Explain why a root grows downwards. [3 marks]
Model answer
Auxin accumulates on the lower side of the root (1 mark). In roots auxin inhibits cell elongation, so the cells on the lower side grow less than those on the upper side (1 mark), causing the root to bend downwards (1 mark).
Example 3
Name two variables that should be controlled when investigating phototropism in seedlings. [2 marks]
Model answer
The amount of water given to each seedling (1 mark) and the temperature at which they are kept (1 mark).
Common mistakes
Saying auxin always promotes growth.
It promotes elongation in shoots but inhibits it in roots — the effect is opposite.
Saying auxin gathers on the lit side.
It accumulates on the shaded side of the shoot.
Saying the shoot grows towards the light because it needs light.
Give the mechanism: unequal auxin distribution causing unequal cell elongation.
Confusing phototropism with gravitropism.
Photo is light; gravi is gravity.
Exam tips
- Always give the mechanism, not just the outcome.
- Remember auxin's opposite effects in shoots and roots.
- Name control variables in the practical.
- Remember this is a Triple-only topic with a required practical.
Key terms
- Auxin
- The main plant hormone controlling growth responses.
- Phototropism
- A growth response to the direction of light.
- Gravitropism
- A growth response to gravity.
- Cell elongation
- The lengthening of cells, causing growth.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.