AQA A Level Biology复习笔记6.4.3 Insulin

The Action of Insulin

 

  • 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
  • If the concentration of glucose in the blood increases above a certain level, this can also disrupt the normal function of cells, potentially causing major problems
  • 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 liver, muscle and fat cells act as the effectors in response to insulin

Secretion of insulin

  • When the blood glucose concentration increases to above the normal range it is detected by the β cells in the pancreas
  • In response to the stimulus, they secrete the hormone insulin
    • Glucose is absorbed in the β cells via carrier proteins (facilitated diffusion)
    • This causes insulin-containing vesicles to move towards the cell-surface membrane where they release insulin into the capillaries

     

  • Once in the bloodstream, insulin circulates around the body
  • It stimulates the uptake of glucose by muscles cells, fat cells and the liver

Action of insulin

  • Muscle cells, fat storage cells, adipose tissue and liver cells possess glucose transporter proteins in their surface membranes
    • They are the target cells of insulin

     

  • These membrane proteins allow for the uptake of glucose molecules via facilitated diffusion
    • The rate of glucose uptake for these cells is limited by the number of glucose transporter proteins present

     

  • The glucose transporter proteins on target cells are insulin-sensitive
  • Insulin binds to specific receptors on the membranes of target cells
    • This stimulates them to activate/add more glucose transporter proteins to their cell surface membrane which increases the permeability of the cells to glucose
    • As a result, the rate of facilitated diffusion increases

     

Insulin-and-glucose-transporters

As the number of glucose transporter proteins in the membrane increases the permeability of the cell to glucose increases

 

  • Insulin also helps to increase the uptake of glucose in the liver by stimulating glycogenesis
    • Once glucose has entered a liver cell an enzyme rapidly converts it to glucose phosphate
    • Different enzymes then convert glucose phosphate into glycogen
    • This helps to lower glucose concentration within the liver cell
    • A steep diffusion gradient is maintained between the blood in the capillaries and the liver cells

     

Negative-feedback-control-of-blood-glucose

When the blood glucose concentration gets too high more insulin is secreted. The action of insulin on target cells helps to remove glucose from the blood.

 

 

 

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