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The Cori cycle, which is also referred to as Lactic acid cycle, is a metabolic pathway where lactate that is produced through anaerobic glycolysis taking in the muscle moves to the liver to be converted into glucose. The glucose produced is then taken back to the muscles to be converted again to lactate. They activities in the muscle get their energy from the breakdown of molecules of glycogen through the glycogenolysis. This process results in the production of a glucose molecule in the glucose-1-phosphate (G-1-P) form. The G-1-P is then converted by the phosphoglocomutase enzyme to G-6-P, which is readily used in the glycolysis process. In case the concentration of G-6-P is high, it can provide muscles with ATP through pentose phosphate pathway (Nelson & Cox, 2005).
When the muscle is doing work, there is the need to keep replenishing the ATP store. When there is enough oxygen supply, the muscles get the ATP from pyruvate, a product of glycolysis that is fed into the Krebs cycle. In situations when the supply of oxygen is not sufficient, especially during intensive activities of the muscle, the energy is produced through anaerobic process. Pyruvate is converted into lactate through the lactic acid fermentation using an enzyme called lactate dehydrogenase. The process produces NAD+ through NADH oxidation, and this enables more glycolysis to take place. This is the first half of the Cori cycle (Ophardt, 2003).
The produced lactate is usually taken up by the liver instead of piling up inside the cells of the muscle. The movement of the lactate molecule to the liver starts the other half of the Cori cycle and the lactate molecule undergoes the gluconeogenesis process. The process converts the lactate molecule first into pyruvate, which is then converted to glucose. The produced glucose is then taken to the muscles through the blood to go through glycolysis reactions in the muscles. If the activities done by the muscles are over, the glucose is used in replenishing the glycogen supply through glycogenesis (Ophardt, 2003).
After the Cori cycle is complete, there is a production of 2 ATP molecules at the expense of 6 ATP molecules that are used in the gluconeogenesis process. This means that it is hard to maintain the Cori cycle indefinitely. The Cori cycle has a number of importances in the muscle that is doing intense activities. First, the cycle prevents lactic acidosis from occurring in the muscle when the muscle is under anaerobic conditions. The cycle also helps in the production of ATP when the muscle is still active. The functioning of Cori cycle is more effective when the activities in the muscle have stopped. This enables the oxygen debt to be settled, and this enables both the electron transport chain and the Krebs cycles are able to effectively produce energy. The steps taken in Cori cycle are illustrated in figure 1.
Figure 1: The Cori cycle
When the body is at under steady-state exercise or rest conditions, the lactate produced in the blood is balanced with the lactate that is removed from blood (Brooks, 2000). The lactate threshold is a term that refers to the intensity of exercise at which there is a rapid rise in the level of blood lactate (Robergs & Roberts, 1997). Lactate threshold is affected by factors such as the rate of lactate removal, rate of recruitment of the fast-twitch motor units, rate of blood flow, level of oxygen in the blood and the balancing of glycolysis and the mitochondrial respiration (Kravitz & Dalleck, 2005).
Reference List
Brooks, G. A., 2000. Intra- and extra-cellular lactate shuttles. Medicine and Science in Sports and Exercise, 32(4), p. 790–799.
Kravitz, L. & Dalleck, L., 2005. Lactate Theshold Training. [Online] Available at: http://www.unm.edu/~lkravitz/Article%20folder/lactatethreshold.html[Accessed 24 June 2013].
Nelson, D. L. & Cox, M. M., 2005. () Lehninger Principles of Biochemistry Fourth Edition.. 4th ed. New York: W.H. Freeman and Company.
Ophardt, C., 2003. Cori Cycle. [Online] Available at: http://www.elmhurst.edu/~chm/vchembook/615coricycle.html[Accessed 24 June 2013].
Robergs, R. & Roberts, S., 1997. Exercise Physiology: Exercise, performance, and clinical applications. St Louis: Mosby.
