Glucagon (AQA A Level Biology): Revision Note

The action of glucagon

• A decrease in blood glucose concentration to a level that is below the normal range is detected by cells in the pancreas

• In response to this stimulus α cells in the pancreas secrete the hormone glucagon

• Glucagon causes an increase in blood glucose by activating enzymes that:

  • convert glycogen to glucose in glycogenolysis

  • convert glycerol and amino acids into glucose

The second messenger model of glucagon action

• The activation of glycogenolysis enzymes by glucagon occurs via a sequence of events known as the second messenger model

• Glucagon initiates this process as follows:

  1. glucagon binds to receptors on the cell surface membranes of target cells, e.g. liver cells

  2. this activates a G protein which in turn activates the enzyme adenylate cyclase

     • adenylate cyclase is also known as adenylyl cyclase

  3. active adenylate cyclase catalyses the conversion of ATP to cyclic AMP (cAMP)

     • cAMP is known as a second messenger

  4. cAMP activates the enzyme protein kinase A, which initiates a cascade of reactions that results in the activation of enzymes that break down glycogen to glucose