Insulin (AQA A Level Biology)

• 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

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

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.