Editing Cori cycle (section)

==Process==
[[File:Gerty Theresa Radnitz Cori (1896-1957) and Carl Ferdinand Cori - restoration1.jpg|thumb|[[Carl Cori]] and [[Gerty Cori]] jointly won the 1947 [[Nobel Prize in Physiology or Medicine]], for their discovery of the course of the catalytic conversion of glycogen, of which the Cori cycle is a part.]]
Muscular activity requires [[Adenosine triphosphate|ATP]], which is provided by the breakdown of [[glycogen]] in the [[skeletal muscles]]. The breakdown of glycogen, known as [[glycogenolysis]], releases [[glucose]] in the form of [[glucose-1-phosphate|glucose 1-phosphate]] (G1P). The G1P is converted to [[G6P]] by [[phosphoglucomutase]]. G6P is readily fed into [[glycolysis]], (or can go into the [[pentose phosphate pathway]] if G6P concentration is high) a process that provides ATP to the [[muscle cells]] as an energy source. During muscular activity, the store of ATP needs to be constantly replenished. When the supply of [[dioxygen|oxygen]] is sufficient, this energy comes from feeding [[pyruvate]], one product of glycolysis, into the [[citric acid cycle]], which ultimately generates ATP through oxygen-dependent [[oxidative phosphorylation]].

When oxygen supply is insufficient, typically during intense muscular activity, energy must be released through [[anaerobic metabolism]]. [[Lactic acid fermentation]] converts pyruvate to [[lactic acid|lactate]] by [[lactate dehydrogenase]]. Most importantly, fermentation regenerates [[Nicotinamide adenine dinucleotide|NAD<sup>+</sup>]], maintaining its concentration so additional glycolysis reactions can occur. The fermentation step oxidizes the [[NADH]] produced by glycolysis back to NAD<sup>+</sup>, transferring two electrons from [[NADH]] to reduce pyruvate into lactate. (Refer to the main articles on [[glycolysis]] and [[fermentation (biochemistry)|fermentation]] for the details.)

Instead of accumulating inside the muscle cells, lactate produced by anaerobic fermentation is taken up by the [[liver]]. This initiates the other half of the Cori cycle. In the liver, [[gluconeogenesis]] occurs. From an intuitive perspective, gluconeogenesis reverses both glycolysis and fermentation by converting lactate first into pyruvate, and finally back to glucose. The glucose is then supplied to the muscles through the [[bloodstream]]; it is ready to be fed into further glycolysis reactions. If muscle activity has stopped, the glucose is used to replenish the supplies of glycogen through [[glycogenesis]].<ref name="Elmhurst">"{{cite web | url = http://www.elmhurst.edu/~chm/vchembook/615coricycle.html | vauthors = Ophardt CE | title = Cori Cycle | archive-url = https://web.archive.org/web/20080423042037/http://www.elmhurst.edu/~chm/vchembook/615coricycle.html | archive-date= 23 April 2008 | access-date = 3 May 2008 | date = 2003 | work = Virtual Chem Book | publisher = Elmhurst College| pages = 1–3 }}</ref>

Overall, the glycolysis steps of the cycle produce 2 ATP molecules at a cost of 6 ATP molecules consumed in the gluconeogenesis steps. Each iteration of the cycle must be maintained by a net consumption of 4 ATP molecules. As a result, the cycle cannot be sustained indefinitely. The intensive consumption of ATP molecules in the Cori cycle shifts the [[metabolic]] burden from the muscles to the liver.