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

 

Controlling-blood-glucose-concentration-1Controlling-blood-glucose-concentration-2

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

Negative-feedback-control-of-blood-glucose

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.

 

转载自savemyexams

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