Whole-body metabolism is coordinated across organs. Two classic inter-organ cycles - the Cori cycle and the glucose-alanine cycle - redistribute carbon and nitrogen between working muscle and the liver.
The Cori and glucose-alanine cycles
Between organs
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Pyruvate: the shared junction ATP gained by muscle ATP spent by liver Nitrogen carried by alanine
Why muscle exports the problemHard-working muscle outruns its oxygen supply, so it must regenerate NAD⁺ by reducing pyruvate to lactate. Lactate is a dead end in muscle: nothing can be done with it locally, so it leaves in the blood.
What the liver gets out of itNothing, energetically. The liver spends 6 ATP to rebuild one glucose from two lactate, and the muscle got only 2 ATP from burning it. The cycle is a deliberate loss that shifts the metabolic burden to the organ that has oxygen to spare.
What alanine addsThe glucose-alanine cycle carries the same carbon but also ferries nitrogen. Muscle transaminates pyruvate to alanine to move amino groups safely, and the liver strips them off and feeds them straight into the urea cycle.
Both cycles are the same trick: send the carbon somewhere that can afford to fix it. The Cori cycle moves lactate, the glucose-alanine cycle moves lactate's nitrogen-carrying twin. In each case the muscle keeps working anaerobically and the liver pays the bill.
Both cycles are clever ways of linking anaerobic glycolysis (fast ATP, produces lactate) in one tissue with gluconeogenesis (makes glucose from non-carb sources) in another. They show how glycolysis and gluconeogenesis - seemingly opposite pathways from sections 9.2-9.6 - work as a team across the whole body during exercise or fasting.
The Cori Cycle
Muscle at high workload (sprinting) produces lactate via anaerobic glycolysis. Lactate travels through blood to the liver. The liver converts lactate → pyruvate (by lactate dehydrogenase) and runs gluconeogenesis to make glucose, which is released into blood and can return to muscle.
Net energy transfer: muscle gains 2 ATP per glucose (glycolysis). Liver spends 6 ATP per glucose (gluconeogenesis). The muscle “borrows” energy from the liver during intense work - the liver pays the bill.
The Glucose-Alanine Cycle
A parallel cycle that transports nitrogen from muscle to liver. During exercise and fasting, muscle protein is broken down for energy. Amino acid carbons are burned locally, but nitrogen (as NH3) is toxic. Muscle transaminates pyruvate with NH3 to form alanine, which travels through blood to the liver.
In the liver, alanine is transaminated back, releasing NH3 (entering the urea cycle) and producing pyruvate (entering gluconeogenesis to make glucose). The glucose returns to muscle. So the cycle transports BOTH nitrogen (safely to the urea cycle) AND carbon (to make glucose).
What does the Cori cycle transport, and between which organs?
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The Cori cycle transports lactate from muscle (where anaerobic glycolysis produces it) to the liver (where gluconeogenesis converts it back to glucose). The glucose is then released into blood and can return to muscle. The cycle lets muscle keep running anaerobically while the liver pays the ATP cost of regenerating glucose.
What does the glucose-alanine cycle accomplish?
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It simultaneously transports nitrogen (as -NH2 on alanine) and carbon (pyruvate backbone) from muscle to liver. In the liver, alanine is transaminated to pyruvate (feeding gluconeogenesis) and the nitrogen enters the urea cycle for safe disposal. The glucose produced in the liver returns to muscle. It couples amino acid catabolism with glucose production.
Why is the Cori cycle energetically unfavorable for the body as a whole?
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Muscle gains 2 ATP per glucose via glycolysis, but the liver must spend 6 ATP to reconvert lactate back to glucose via gluconeogenesis. Net: -4 ATP per glucose across the whole body. The cycle is run anyway because it is essential - it lets muscle continue working under anaerobic conditions and keeps blood lactate from dangerously accumulating.
