6.4.6 Control of Blood Glucose Concentration

• The liver plays a vital role in the conversion between glycogen and glucose
  • The conversion between these molecules helps to regulate blood glucose concentration

• Both insulin and glucagon have specific receptors on the membranes of liver cells
• When these hormones bind to their receptors they trigger several processes within the liver
  • Glycogenesis
  • Glycogenolysis
  • Gluconeogenesis

Glycogenesis

• Glycgogenesis is the synthesis of glycogen from glucose molecules
• Insulin triggers this process after it detects an increased blood glucose concentration
• The synthesis of glycogen removes glucose molecules from the bloodstream and decreases the blood glucose concentration to within a normal range
• Glycogen acts as a compact and efficient carbohydrate storage molecule

Glycogenolysis

• Glycogenolysis is the breakdown of glycogen to produce glucose molecules
• Glucagon triggers this process after it detects a decreased blood glucose concentration
• It activates enzymes within the liver that breakdown glycogen molecules into glucose
• The breakdown of glycogen releases more glucose molecules to the bloodstream and increases the blood glucose concentration to within the normal range

Gluconeogenesis

• Gluconeogenesis is the synthesis of glucose molecules from non-carbohydrate molecules
• Glucagon also triggers this by activating enzymes within the liver
• These enzymes convert other molecules, such as fatty acids and amino acids, into glucose molecules
• Glucose molecules are released into the bloodstream which increases the blood glucose concentration to within the normal range

• There are three ways in which glucose can enter the bloodstream:
  • Absorption in the gut following carbohydrate digestion
  • Hydrolysis of glycogen stores
  • Non-carbohydrates such as lipids, lactate and amino acids that have been converted to glucose

• The amount of glucose that gets absorbed into the blood from the products of digestion can vary substantially
  • Some meals may be much more carbohydrate-rich than others

• Control systems within the body help to manage the concentration of glucose in the blood via the hormones insulin and glucagon
• When there is excess glucose in the blood from a carbohydrate-dense meal it is removed
  • This occurs through increased glucose uptake into muscle, fat and liver cells and glycogenesis

• When there is insufficient glucose in the blood for metabolic needs it is rapidly released from storage molecules
  • This occurs through glycogenolysis and gluconeogenesis

• The levels of insulin and glucagon present in the blood are constantly regulated and adjusted in order to maintain the blood glucose concentration at a mostly constant level