Trace glucagon cell signalling
Follow glucagon from receptor binding to cAMP, protein kinase A and glycogen breakdown.
Section 14.1 · Homeostasis in mammals
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not a practical skills certification.
Before you begin
- A control system detects a change, communicates information and changes an effector response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which enzyme is activated by cAMP in this pathway?
What you will learn
- Describe the glucagon receptor and G-protein signalling steps
- Explain cAMP amplification and the cellular response
Glucagon signal amplification
| Feature | Meaning |
|---|---|
| First messenger | Glucagon binds a cell-surface receptor |
| Second messenger | cAMP activates protein kinase A |
| Response | Cascade promotes glycogen breakdown |
Build the explanation
Glucagon binds to a complementary cell-surface receptor on a target liver cell, causing a conformational change. The receptor activates a G-protein, which stimulates adenylyl cyclase. Adenylyl cyclase produces cyclic AMP (cAMP), a second messenger that activates protein kinase A. Protein kinase A initiates an enzyme cascade; phosphorylation activates more enzymes, amplifying the signal. The final enzyme catalyses glycogen breakdown, helping raise blood glucose.
Apply the limits
The hormone is an extracellular first messenger; cAMP is an intracellular second messenger. Signal amplification comes from a cascade activating many downstream molecules. Do not put glucagon inside the cell or describe glycogen breakdown as glucose synthesis.
Pause and explain
One active kinase activates many enzyme molecules in a cascade. Explain how this can amplify the original signal.
Worked example
What is cAMP's role in the glucagon pathway?
Show the worked solution
- It is produced after G-protein activation of adenylyl cyclase.
- It activates protein kinase A inside the cell.
Answer cAMP is the second messenger that activates protein kinase A.
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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.
Order the signalling events after glucagon binds its receptor.
Give me a hint
Name the relevant structure or signal, then state its effect.
Compare my reasoning
- The receptor activates a G-protein, stimulating adenylyl cyclase.
- Adenylyl cyclase produces cAMP, which activates protein kinase A and an enzyme cascade.
Glucagon binding changes the receptor; the G-protein activates adenylyl cyclase; cAMP activates protein kinase A; the cascade activates glycogen breakdown.
Look for these in your work
- Correct mechanism
- Direction of response explained
Why is cAMP called a second messenger?
Give me a hint
Separate the observed change from the body's response.
Compare my reasoning
- Glucagon is the signal outside the cell.
- cAMP carries the signal inside the target cell.
Glucagon is the extracellular first messenger. cAMP is produced within the cell after receptor activation and relays the signal to intracellular targets, so it is a second messenger.
Look for these in your work
- Evidence interpreted
- Control step identified
One active kinase activates many enzyme molecules in a cascade. Explain how this can amplify the original signal.
Give me a hint
Apply the mechanism to the new context without claiming more than the evidence shows.
Compare my reasoning
- Each activated enzyme can act on multiple downstream molecules.
- Several cascade stages multiply the number of activated molecules.
A small receptor-triggered signal can activate many molecules at successive stages because each active enzyme acts on multiple targets, amplifying the response that leads to glycogen breakdown.
Look for these in your work
- New context applied
- Limits preserved
Common mistakes
- Calling cAMP a hormone
- Swapping adenylyl cyclase and protein kinase A
What can you explain now?
Which enzyme is activated by cAMP in this pathway?
Compare with the explanation
Protein kinase A
cAMP activates protein kinase A, initiating the cascade.
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Make it stick
How does glucagon receptor binding lead to glycogen breakdown?
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