Second Messenger Model of Adrenaline and Glucagon
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of blood glucose regulation. You can test your knowledge on this below.
What is glycogenolysis and when does it occur?
The breakdown of glycogen into glucose; occurs when blood glucose levels are low.
Which two hormones increase blood glucose concentration?
Glucagon and adrenaline.
How does insulin lower blood glucose concentration?
By increasing glucose uptake into cells and activating enzymes that cause glycogenesis (glucose → glycogen).
Topic Explainer Video
Check out this @LauraDoesBiology video that explains the second messenger model or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What Is the Second Messenger Model?
- Some hormones like adrenaline and glucagon are first messengers.
- They cannot enter target cells directly, so they trigger a second messenger inside the cell.
- (Remember, they are made of protein so are too large to cross the cell membrane).
- This activates enzyme cascades that lead to a cellular response (e.g. glycogenolysis).
Key Hormones Involved
- Adrenaline (from adrenal glands) – released during stress or low glucose.
- Glucagon (from α cells in pancreas) – released when blood glucose is low.
Step-by-Step: The Second Messenger Mechanism
1. Hormone binds to receptor
- Adrenaline or glucagon binds to a specific receptor on liver cell membrane.
2. Activation of adenylate cyclase
- This activates an enzyme called adenylate cyclase inside the membrane.
3. ATP → cAMP (cyclic AMP)
- Adenylate cyclase converts ATP into cyclic AMP (cAMP) – the second messenger.
4. cAMP activates enzyme cascade
- cAMP activates protein kinase A (PKA).
- This enzyme triggers glycogenolysis – the breakdown of glycogen into glucose.
5. Glucose diffuses into blood
- Blood glucose concentration rises.
Why Use a Second Messenger?
- Amplifies the signal: 1 hormone → many cAMP → massive enzyme response.
- Enables precise control over cell function and response intensity.
Does Insulin Use a Second Messenger?
Yes – but not in the same way as adrenaline or glucagon.
- Insulin DOES trigger intracellular signalling pathways, but it does not use the cAMP second messenger system like adrenaline or glucagon.
- Instead, insulin binds to a membrane receptor, which is a tyrosine kinase receptor – this activates a cascade of phosphorylation reactions inside the cell.
- This knowledge is not needed on the AQA specification!
Key Terms
- First messenger: A hormone that carries the signal to the target cell (e.g. adrenaline).
- Second messenger: A molecule inside the cell that transmits the signal (e.g. cAMP).
- Adenylate cyclase: The enzyme that converts ATP to cAMP.
- Protein kinase: An enzyme activated by cAMP that causes glycogen breakdown.
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Exam Tip
Glucagon and adrenaline do not enter the cell – they use second messengers. Always describe the cascade: hormone → receptor → adenylate cyclase → cAMP → protein kinase → glycogenolysis.
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Describe how adrenaline increases blood glucose concentration using the second messenger model. (5 marks)
- Adrenaline binds to a specific receptor on the liver cell membrane.
- This activates adenylate cyclase inside the cell.
- Adenylate cyclase converts ATP into cyclic AMP (cAMP).
- cAMP acts as a second messenger, activating protein kinase enzymes.
- These enzymes catalyse glycogenolysis (glycogen → glucose), raising blood glucose levels.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!