In humans and other mammals, much of the lactic acid produced in anaerobic respiration is later eliminated by aerobic respiration of the lactic acid to carbon dioxide and water. However, some of the lactic acid produced by exercising skeletal muscles is delivered by the blood to the liver. Within the liver cells under these conditions, the enzyme lactic acid dehydrogenase (LDH) converts lactic acid to pyruvic acid. This is the reverse of the step of anaerobic respiration shown in figure 5.3, and in the process NAD is reduced to NADH + H+. Unlike most other organs, the liver contains the enzymes needed to take pyruvic acid molecules and convert them to glucose 6-phosphate, a process that is essentially the reverse of glycolysis.
Glucose 6-phosphate in liver cells can then be used as an intermediate for glycogen synthesis, or it can be converted to free glucose that is secreted into the blood. The conversion of noncar-bohydrate molecules (not just lactic acid, but also amino acids and glycerol) through pyruvic acid to glucose is an extremely important process called gluconeogenesis. The significance of this process in conditions of fasting will be discussed in a later section on amino acid metabolism.
During exercise, some of the lactic acid produced by skeletal muscles may be transformed through gluconeogenesis in the liver to blood glucose. This new glucose can serve as an energy source during exercise and can be used after exercise to help replenish the depleted muscle glycogen. This two-way traffic between skeletal muscles and the liver is called the Cori cycle (fig. 5.5). Through
Chapter Five the Cori cycle, gluconeogenesis in the liver allows depleted skeletal muscle glycogen to be restored within 48 hours.
1. Define the term glycolysis in terms of its initial substrates and products. Explain why there is a net gain of two molecules of ATP in this process.
2. Discuss the two meanings of the term anaerobic respiration. As the term is used in this text, what are its initial substrates and final products?
3. Describe the physiological functions of anaerobic respiration. In which tissue(s) is anaerobic respiration normal? In which tissue is it abnormal?
4. Describe the pathways by which glucose and glycogen can be interconverted. Explain why only the liver can secrete glucose derived from its stored glycogen.
5. Define the term gluconeogenesis and explain how this process replenishes the glycogen stores of skeletal muscles following exercise.
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