Cori Cycle
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The Cori cycle (also known as the lactic acid cycle) is a biochemical pathway that is used to deal with the excess lactate produced by muscle cells. During muscular activity, muscle feeds on glucose molecules to produce pyruvate via glycolysis. If oxygen is present, pyruvate is then shuttled into the Krebs cycle via pyruvate dehydrogenase. Pyruvate is then converted into acetyl-CoA, after which it can undergo aerobic metabolism to produce large amounts of ATP. Unfortunately, vigorously exercising muscle may not have sufficient oxygen, which is required for aerobic metabolism downstream of the Krebs cycle. In such an oxygen-deficient state, pyruvate instead undergoes anaerobic metabolism to generate ATP. This leads to the generation of lactate, an accumulation of which can lead to lactic acidosis. Since muscle cells have no mechanism to deal with this accumulation of lactate, the body must find a different way to get rid of it. Using the Cori cycle, lactate from muscle is shuttled via the bloodstream to the liver. The liver contains the enzymes necessary for gluconeogenesis, so it has a mechanism to convert the accumulated lactate back into glucose. The Cori cycle then sends this glucose back out to the muscle, which can then use it for additional energy. The Cori cycle unfortunately requires a net amount of 4 ATP to function, so it is not an infinite energy machine. More specifically, 6 ATP are required for gluconeogenesis in the liver, and 2 ATP are later yielded by glycolysis in muscle. You can think of this as moving energy from the liver to the muscle, which occurs at the expense of 4 ATP. In addition to just moving energy, the Cori cycle also has the added benefit of removing accumulated lactate. In summary, the Cori cycle disposes of accumulated lactate in muscle while also shuttling additional ATP from the liver back to the muscle.