Control of blood glucose

• Blood glucose concentration involves a negative feedback system
• Blood glucose concentration is strictly controlled and within the range of 80-100mg per 100cm3
• Low levels of blood glucose concentration leads to hypoglycaemia
• High levels of blood glucose concentration leads to hyperglycaemia
• Pancreas:

  • Blood glucose concentration controlled by pancreas
  • Glucose receptor cells monitor blood glucose concentration
  • Pancreas has endocrine cells which secrete hormones
• Insulin and glucagon have antagonistic effects
• Alpha and beta cells are found in the Islets of Langerhans in the pancreas
• Alpha cells secrete the hormone glucagon
• Beta cells secrete the hormone insulin which lowers blood glucose levels
• Insulin (peptide hormone):

  • Insulin binds to complementary receptors on the cell-surface membrane of target cells
  • Controls the uptake of glucose by regulating the inclusion of glucose carrier proteins in the cell-surface membrane of target cells
  • Insulin also activates enzymes that stimulate conversion of glucose to glycogen (glycogenesis)
  • Decreases blood glucose
• Glucagon (peptide hormone):

  • Glucagon binds to receptors on the cell-surface membrane on target cells
  • Activates enzymes involved in the hydrolysis of glycogen to glucose (glycogenolysis)
  • Activates enzymes involved in the conversion of glycerol and amino acids into glucose (gluconeogenesis)
  • Increases blood glucose concentration
• Explain the action of glucagon:

  • Works by activating enzymes
  • Hydrolysis of glycogen to glucose (glycogenolysis)
  • Gluconeogenesis
• Describe the role of glycogen formation and its role in lowering blood glucose concentration:

  • Glucose concentration in cells
  • Below that in blood plasma
  • Creates glucose concentration gradient
  • Glucose enters the cell and leaves the blood by facilitated diffusion using a carrier protein
• Describe how blood glucose can be increased using hormones:

  • Release of glucagon
  • Lead to the formation of glucose in liver cells
  • From amino acids or fatty acids
• Insulin and glucagon levels fluctuate and can both be present in the blood at the same time
• Adrenaline - Hormone released by adrenal glands above the kidneys
• Adrenaline can increase blood glucose concentration
• Second messenger model:

  • Adrenaline and glucagon (first messenger) both bind to specific membrane protein receptors on the target cell
  • Hormone-receptor complex is formed and changes the tertiary structure of the receptor protein
  • Hormone-receptor complex activates adenylate cyclase to convert ATP into cAMP (second messenger)
  • cAMP causes a series of chemical changes
  • cAMP activates protein kinase enzymes and this converts glycogen to glucose
• Second messenger model (simplified):

  • Adenylate cyclase activated
  • cAMP produced
  • Activates protein kinase in cell
  • More glycogenolysis and gluconeogenesis
• Factors influencing blood glucose concentration:

  • Diet
  • Glycogenolysis
  • Gluconeogenesis
• Diabetes - Disease caused by failure of glucose homeostasis
• Symptoms of diabetes:

  • Thirst due to osmosis of water from cells to blood
  • More urine produced due to more water in blood
  • Poor vision due to osmotic loss of water from eye lens
  • Tiredness due to loss of glucose in urine
  • Muscle wastage due to gluconeogenesis
• Type I diabetes:

  • Insulin-dependent diabetes
  • Severe insulin deficiency due to autoimmune killing of beta cells
  • Could be caused by a virus or faulty gene
• Type II diabetes:

  • Non-insulin-dependent diabetes
  • Insulin is produced but the insulin receptors in the target cells are unresponsive
  • Lack of sensitivity to insulin
• In diabetes, the blood glucose concentration may be so high that the kidney cannot reabsorb the glucose through the PCT so glucose is lost in urine
• Binding of insulin leads to an increase in the rate of respiration in cells:

  • Insulin leads to more carrier proteins for glucose
  • More glucose enters the cell for aerobic respiration so more ATP
• Explain how glucagon would affect a person's blood glucose concentration:

  • Stimulates release of glucose from cells using carrier proteins
  • Gluconeogenesis
  • Increases blood glucose concentration
• Why pancreas transplants are not used for treatment of type II diabetes:

  • Cells do not respond to insulin (cells less sensitive to insulin)
  • Insulin is still produced