Cori and Alanine Cycles

Expansion

Cori cycle

1. Muscle glycolysis produces lactate under anaerobic conditions
2. Lactate travels to the liver
3. The liver converts it back to glucose by gluconeogenesis
4. Glucose returns to muscle

Glucose-alanine cycle

1. Muscle transaminates pyruvate to alanine, using the amino group from protein breakdown
2. Alanine travels to the liver
3. The liver converts alanine back to pyruvate and then glucose, and disposes of the nitrogen as urea

The alanine cycle therefore does two jobs at once: it returns carbon and safely transports nitrogen without releasing free ammonia into the blood.

Both cycles are energetically costly overall: gluconeogenesis consumes six ATP equivalents while glycolysis yielded only two. The point is not efficiency but redistribution, allowing muscle to work anaerobically while the liver, which has ample oxygen, pays the bill.

This is why the liver is central to exercise and starvation metabolism, and why hepatic failure causes hypoglycaemia and lactic acidosis.