6.4.6 Control of Blood Glucose Concentration

Key Processes in the Liver to Regulate Blood Glucose:

1. Glycogenesis:

• The conversion of glucose to glycogen for storage when blood glucose levels are high.

2. Glycogenolysis:

• The breakdown of glycogen into glucose, which is released into the blood when glucose levels are low.

3. Gluconeogenesis:

• The synthesis of glucose from non-carbohydrate sources, such as amino acids and glycerol.

Response to High Blood Glucose Levels:

When glucose levels are high, for example after a carbohydrate-rich meal:

• Beta cells in the islets of Langerhans in the pancreas detect the rise in glucose.
• Insulin is secreted by beta cells, which:
  • Inhibits alpha cells from secreting glucagon.
  • Travels in the bloodstream to target cells (e.g., hepatocytes, fat, and muscle cells). Effects of Insulin:

4. Binding to receptors on the target cell membrane activates adenyl cyclase, converting ATP into cyclic AMP (cAMP).
5. cAMP stimulates enzyme-controlled reactions, causing:

• Opening of glucose channels in cell membranes, allowing more glucose to enter cells.
• Conversion of glucose to glycogen or fats for storage.
• Increased glucose usage in respiration.

Response to Low Blood Glucose Levels:

When glucose levels are low, such as during fasting:

• Alpha cells in the pancreas detect the fall in blood glucose and secrete glucagon, which:

  • Inhibits beta cell action.
  • Stimulates hepatocytes to:
  • Break down glycogen into glucose (glycogenolysis).
  • Produce glucose from non-carbohydrate sources (gluconeogenesis).
  • Release glucose into the bloodstream. Alternative Energy Sources:

• Cells switch to using fatty acids and amino acids for respiration instead of glucose.

Adrenaline's Role in Blood Glucose Control:

Adrenaline can also increase blood glucose levels using a secondary messenger system:

6. Adrenaline binds to receptors on the liver cell membrane, causing the receptor to change shape.
7. This activates adenyl cyclase, which converts ATP into cAMP (the secondary messenger).
8. cAMP activates protein kinase enzymes, which:

• Catalyse the breakdown of glycogen into glucose (glycogenolysis).
• Release glucose into the blood.