Explain gibberellin in barley germination
Describe how gibberellin from the embryo stimulates alpha-amylase production in the aleurone layer.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- A control pathway links a stimulus, receptor, coordination system and response.
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Quick readiness check
What is the role of alpha-amylase in barley germination?
What you will learn
- Relate water uptake and embryo activity to gibberellin release
- Explain how aleurone alpha-amylase mobilises stored starch for embryo growth
Gibberellin during barley germination
| Feature | Meaning |
|---|---|
| Water uptake | The barley seed imbibes water and the embryo becomes active |
| Signal | Gibberellin from the embryo acts on the aleurone layer |
| Enzyme | Aleurone cells produce and release alpha-amylase |
| Food reserve | Starch is hydrolysed to sugars used by the growing embryo |
Build the mechanism
When a barley seed takes up water, the embryo becomes active and produces gibberellin. Gibberellin signals to the aleurone layer, stimulating production and secretion of alpha-amylase. The enzyme hydrolyses starch in the endosperm to smaller sugars, which can be transported to and used by the embryo for respiration and growth. Gibberellin therefore coordinates mobilisation of a food reserve during germination.
Keep the sequence precise
Gibberellin is the signal; alpha-amylase is the digestive enzyme. The aleurone layer responds to the hormone, while starch is stored in the endosperm.
Pause and explain
A student says alpha-amylase stimulates the aleurone cells to release gibberellin. Correct the sequence.
Worked example
Which enzyme is stimulated in the aleurone layer, and what reserve does it digest?
Show the worked solution
- Gibberellin induces enzyme production.
- Alpha-amylase hydrolyses starch.
Answer The aleurone layer produces alpha-amylase, which hydrolyses starch in the endosperm to sugars available to the embryo.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Trace gibberellin action from an active embryo to a usable sugar supply.
Give me a hint
Name the first event, then follow each link in order.
Compare my reasoning
- Gibberellin acts on the aleurone layer.
- Alpha-amylase hydrolyses starch in the endosperm.
After water uptake activates the embryo, gibberellin signals the aleurone layer to produce alpha-amylase. The enzyme hydrolyses endosperm starch into sugars that can be used by the embryo.
Look for these in your work
- Mechanism stated
- Steps linked in order
A barley seed receives gibberellin, but alpha-amylase activity does not increase. Give a likely consequence for germination.
Give me a hint
Use the terms in the question and state what changes.
Compare my reasoning
- Starch mobilisation is reduced.
- Fewer sugars are available to fuel embryo growth.
Less starch is hydrolysed to sugars, so the embryo has a reduced supply of respiratory substrate and germination growth may be impaired.
Look for these in your work
- Evidence interpreted
- Biological direction explained
A student says alpha-amylase stimulates the aleurone cells to release gibberellin. Correct the sequence.
Give me a hint
Apply the mechanism to the new context without adding unsupported claims.
Compare my reasoning
- Gibberellin is produced by the active embryo.
- It stimulates the aleurone layer to produce alpha-amylase.
The active embryo produces gibberellin, which acts on the aleurone layer. The aleurone produces alpha-amylase, and the enzyme digests endosperm starch into sugars.
Look for these in your work
- Mechanism applied
- Limits preserved
Common mistakes
- Calling gibberellin the enzyme that digests starch
- Locating stored endosperm starch in the aleurone layer
What can you explain now?
What is the role of alpha-amylase in barley germination?
Compare with the explanation
It hydrolyses starch to sugars
Those sugars support the growing embryo.
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Make it stick
What is the signal, response tissue, enzyme and substrate in barley germination?
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