How It Works: Insulin and Glucagon as Antagonistic Hormones

How It Works: Insulin and Glucagon as Antagonistic Hormones

Insulin and glucagon are antagonistic hormones — they have opposite effects on blood glucose and work together to maintain it within the normal range (approximately 4–7 mmol/L). This is a classic negative feedback system in which the pancreas acts as both receptor and coordination centre.

The role of insulin (released by beta cells): When blood glucose rises above the set point (e.g. after a meal), beta cells in the islets of Langerhans in the pancreas secrete insulin into the bloodstream. Insulin binds to receptor proteins on liver and muscle cells, making them more permeable to glucose. Glucose enters these cells and is converted to glycogen (a polysaccharide) for storage — this process is called glycogenesis. Blood glucose falls back to normal. Insulin also increases the rate of cellular respiration in muscle cells, using up more glucose.

The role of glucagon (released by alpha cells): When blood glucose falls below the set point (e.g. after exercise or fasting), alpha cells in the pancreas secrete glucagon. Glucagon acts on liver cells, stimulating the breakdown of glycogen back into glucose — this process is called glycogenolysis. Glucose is released from the liver back into the bloodstream, raising blood glucose toward normal.

Why antagonistic control? Having two opposing hormones allows the system to respond rapidly in both directions. If only insulin existed, blood glucose could only be lowered. If only glucagon existed, it could only be raised. The antagonistic pair creates a precise two-way control system, keeping glucose within the narrow range that cells need for efficient respiration.