AQA A Level Biology复习笔记6.4.4 Glucagon

The Action of Glucagon


  • If the concentration of glucose in the blood decreases below a certain level, cells may not have enough glucose for respiration and may not be able to function normally
  • The control of blood glucose concentration is a key part of homeostasis
  • Blood glucose concentration is controlled by two hormones secreted by endocrine tissue in the pancreas
  • This tissue is made up of groups of cells known as the islets of Langerhans
  • The islets of Langerhans contain two cell types:
    • α cells that secrete the hormone glucagon
    • β cells that secrete the hormone insulin


  • These α and β cells act as the receptors and initiate the response for controlling blood glucose concentration
  • The control of blood glucose concentration by glucagon can be used to demonstrate the principles of cell signalling
    • α and β cells in the pancreas act as the receptors
    • They release the hormone glucagon
    • Liver cells act as the effectors in response to glucagon


The second messenger model

  • If a decrease in blood glucose concentration occurs, it is detected by the α and β cells in the pancreas:
    • The α cells respond by secreting glucagon
    • The β cells respond by stopping the secretion of insulin


  • The decrease in blood insulin concentration reduces the use of glucose by liver and muscle cells
  • Glucagon (the first messenger) binds to receptors in the cell surface membranes of liver cells
  • This binding causes a conformational change in the receptor protein that activates a G protein
  • This activated G protein activates the enzyme adenylyl cyclase
  • Active adenylyl cyclase catalyses the conversion of ATP to the second messenger, cyclic AMP (cAMP)
  • cAMP binds to protein kinase A enzymes, activating them
  • Active protein kinase A enzymes activate phosphorylase kinase enzymes by adding phosphate groups to them
  • Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes
  • Active glycogen phosphorylase enzymes catalyse the breakdown of glycogen to glucose
    • This process is known as glycogenolysis and it occurs via the second messenger model


  • The enzyme cascade described above amplifies the original signal from glucagon and results in the releasing of extra glucose by the liver to increase the blood glucose concentration back to a normal level



The second messenger model. Glucagon acts as the first messenger and cAMP acts as the second messenger


  • Glucagon and insulin are both involved in the negative feedback loop that controls blood glucose concentration
  • When blood glucose concentration is low, more glucagon is secreted and less insulin is secreted
  • When blood glucose concentration is high, more insulin is secreted and less glucagon is secreted


The corrective mechanisms work together to regulate blood glucose concentration

Exam Tip

Make sure you know where this response to a decrease in blood glucose concentration occurs! The enzyme cascade only occurs in liver cells, there are no glucagon receptors on muscle cells.