Control of blood glucose

• • all cells need a constant energy supply to work
  • blood glucose concentration must be carefully controlled
  • the concentration of the blood is normally around 90 mg per 100 cm3 of blood
  • it is monitored by the cells in the pancreas
  • blood glucose concentration rises after eating food containing carbohydrate - falls after exercise, as more glucose is used in respiration to release energy
• The hormonal system controls blood glucose concentration using 2 hormones - inulin and glucagon
• like all hormones - insulin and glucagon are chemical messengers that travel in the blood to their target cells (effectors)
• where are insulin and glucagon secreted from?


  clusters of cells in the pancreas called the islets of Langerhans
• The islets of Langerhans:

  • contain beta cells and alpha cells
  • beta cells secrete insulin into the blood
  • alpha cells secrete glucagon into the blood
• insulin and glucagon act on effectors, which respond to restore the blood glucose concentration to the normal level
• Insulin:

  • lowers blood glucose concentration when it is too high
  • binds to specific receptors on the cell membranes of muscle cells and liver cells (hepatocytes)
  • increases the permeability of muscle-cell membranes to glucose so that cells take up more glucose
  • involves increasing the number of channel proteins in the cell membranes
• insulin also activates:

  • enzymes in the muscle and liver cells that convert glucose into glycogen
  • the cells are able to store glycogen in their cytoplasm as an energy source
  • the process of forming glycogen from glucose is called
  • glycogenesis
  • insulin also increases the rate of respiration of glucose, especially in muscle cells
• Glucagon:

  • glucagon raises the blood glucose concentration when it is too low
  • it binds to specific receptors on the cell membranes of liver cells and activates enzymes that break down glycogen into glucose
  • the process of breaking down glycogen is called glycogenolysis
• Glucagon also activates:

  • enzymes that are involved in the formation of glucose from glycerol (component of lipids) and amino acids
  • the process of forming glucose from non-carbohydrates is called gluconeogenesis
  • glucose decreases the rate of respiration of glucose in cells
• Because they travel in the blood to their target cells, the responses produced by hormones are slower than those produced by nervous impulses

  BUT - also means that responses to hormones can occur all over the body if their target cells are widespread, unlike nervous impulses that are localised to one area

  hormones are not broken down as quickly as neurotransmitters though, so their effects tend to last for longer
• Negative feedback mechanisms keep blood glucose concentration normal
• Rise in blood glucose concentration:

  • when the pancreas detects blood glucose concentration is too high
  • beta cells secrete insulin and alpha cells stop secreting glucagon
  • insulin then binds to receptors on liver and muscle cells (the effectors)
  • the liver and muscle cells respond to decrease the blood glucose concentration e.g. glycogenesis is activated
  • blood glucose level returns to normal
• • normal blood glucose concentration
  • rise in blood glucose concentration
  • pancreas detects change
  • pancreas secretes insulin and stops glucagon secretion
  • liver and muscle cells respond
  • take up more glucose
  • glycogenolysis activated
  • cells respire more glucose
  • blood glucose concentration falls
• Fall in blood glucose concentration:

  • when the pancreas detects blood glucose is too low
  • alpha cells secrete glucagon and beta cells stop secreting insulin
  • glucagon then binds to receptors on the liver cells (the effectors)
  • the liver cells respond to increase the blood glucose concentration e.g. glycogenolysis is activated
  • blood glucose concentration returns to normal
• • normal blood glucose concentration
  • fall in blood glucose concentration
  • pancreas detects change
  • pancreas secretes glucagon, stops insulin secretion
  • liver cells respond:
  • glycogenolysis is activated
  • gluconeogenesis is activated
  • cells respire less glucose
• 'genesis' = making - glycogenesis means making glycogen
• 'lysis' means splitting - glycogenolysis means splitting glycogen
• 'neo' means new - gluconeogenesis means making new glucose
• Glucose transporters:

  • glucose transporters are channel proteins which allow glucose to be transported across a cell membrane
• skeletal and cardiac muscle cells contain a glucose transporter called?

  GLUT4
• • when insulin levels are low, GLUT4 is stored in vesicles in the cytoplasm of cells
  • but when insulin binds to receptors on the cell-surface membrane, it triggers the movement of GLUT4 to the membrane
  • glucose can then be transported into the cell through the GLUT4 protein by facilitated diffusion
• facilitated diffusion transports large or charged particles across a membrane down a concentration gradient from a higher concentration to a lower concentration - passive process - does not require energy
• Adrenaline:

  • a hormone that is secreted from the adrenal glands (just above the kidneys)
  • secreted when there is a low concentration of glucose in the blood, when stressed or when exercising
• Adrenaline binds to:

  • receptors in the cell membrane of liver cells and does 2 things to increase blood glucose concentration:
  • activated glycogenesis (breakdown of glycogen to glucose)
  • inhibits glycogenesis (synthesis of glycogen from glucose)
• Adrenaline also activates glucagon secretion and inhibits insulin secretion - increases glucose concentration
• Adrenaline gets the body ready for action by making more glucose available for muscles to respire
• Second messengers:

  • both adrenaline and glucagon can activate glycogenolysis inside a cell even though they bind to receptors on the outside of the cell
  • do this by the second messenger model
• What is the second messenger model?

  the binding of the hormone to cell receptors activates an enzyme on the inside of the cell membrane, which then produces a chemical known as a second messenger - the second messenger activates other enzymes in the cell to bring about a response
• The receptors for adrenaline and glucagon have specific tertiary structures that make them complementary in shape to their respective hormones
• To activate glycogenolysis:

  • adrenaline and glucagon bind to their receptors and activate an enzyme called adenylate cyclase
  • activated adenylate cyclase converts ATP into a chemical called cyclic AMP (cAMP) - second messenger
  • cAMP activates an enzyme called protein kinase A
  • protein kinase A activates a chain of reactions that breaks down glycogen into glucose (glycogenolysis)
• adrenaline and glucagon bind to receptors on the cell membranes of liver cells
• adenylate cyclase = adenylyl cyclase
• Glycogenesis:

  • converts glucose to glycogen
  • activated by insulin
  • inhibited by adrenaline
• Glycogenolysis:

  • converts glycogen into glucose
  • activated by glucagon and adrenaline
• Gluconeogenesis:

  • converts glycerol/ amino acids to glucose
  • activated by glucagon