Lesson 4 of 4 / Cell signalling and glucose control
Glucagon signals a rise in blood glucose
How do antagonistic hormones stabilise one variable?
I can outline glucagon GPCR signalling and compare glucose feedback pathways.
I can explain insulin and glucagon receptor pathways and blood-glucose control.Syllabus
Syllabus 9477, Core 3(p). Explain insulin and glucagon receptor pathways and blood-glucose control.
About 7 min
Make a guess. You are not marked.
Glucagon raises blood glucose. Can this still be called negative feedback?
Show the answer
It opposes a fall in glucose.
Negative feedback means the response opposes the change. After a fall, the correction raises the level.
The key idea
Glucagon acts through a G-protein-linked receptor, promoting hepatic glucose release; its effect opposes insulin in glucose homeostasis.
Explore the idea
Glucagon: correct a fall in glucose
A fall in blood glucose stimulates pancreatic alpha cells to secrete glucagon. Follow the signal in a liver cell.
The liver can release glucose to the blood. Muscle uses its glycogen primarily for its own needs; this diagram does not assign the same glucagon response to muscle.
Original pathway schematic. Receptor shapes and intermediate numbers are simplified; the two receptor classes remain distinct. The feedback arrow means the corrected glucose deviation reduces the stimulus for secretion, not that glucose physically travels back along this drawn route. Other regulatory inputs are omitted.
Explanation
When blood glucose falls, pancreatic alpha cells release glucagon. It binds a G-protein-linked receptor on liver cells. The ligand-induced receptor conformational change activates a G-protein relay and downstream signalling that alters enzyme activity.
The liver responds by increasing glycogen breakdown and glucose production, increasing glucose release into the blood. This response differs from insulin-promoted storage. Muscle glycogen is primarily used locally; do not generalise glucagon-driven blood-glucose release equally to every tissue.
In a common explanatory pathway, the G-protein relay activates adenylyl cyclase and cAMP-mediated signalling. The syllabus requires the receptor-to-response outline but not every specific messenger or kinase in the insulin/glucagon pathways. The separate cAMP lesson teaches the general second-messenger principle.
As glucose rises towards the regulated range, the stimulus for glucagon secretion falls. Insulin and glucagon are antagonistic in their effects on blood glucose, but they are not enzymes that directly destroy each other. Regulation depends on changing secretion and tissue responses, not maintaining a perfectly unchanging glucose value every second.
Step by step
- 1
Identify the low-glucose signal
Distinguish alpha from beta cells.
- 2
Trace receptor to liver response
Explain increased breakdown/production and release.
- 3
Close the feedback loop
Restoring glucose reduces the original stimulus.
Worked example
Work through the evidence
Why is glucagon action an example of negative feedback even though it increases blood glucose?
One way to explain it
It opposes the initiating fall. Negative feedback means reducing the deviation from the regulated range, not always decreasing the variable.
Why this answer works
- Identify the starting disturbance.
- A corrective increase can be negative feedback.
Watch out for this
A student says: "Negative feedback always means the response lowers the variable." What is wrong with this?
Show the answer
It means the response opposes the deviation; after a fall, the corrective response raises the variable.
Check your understanding
Which pairing is correct?
Show the answer
Glucagon from alpha cells promotes liver glucose release after a fall
This helps restore blood glucose towards its regulated range.
Quick recall
How do antagonistic hormones stabilise one variable?