What is the role of carbohydrate metabolism during muscle work?
Glycogen, the major storage form of carbohydrate, is the major source of ATP generation when physical activity is of short duration and high intensity (lifting heavy weight) or when anaerobic conditions exist (running sprints).
Glycogen is mobilized to form glucose-6-phosphate by glycogenolysis in a process started by the enzyme myophosphorylase. Glucose and glucose-6-phosphate are metabolized through a series of reactions in the glycolytic pathway to pyruvate. Under aerobic conditions, pyruvate enters the Krebs or tricarboxylic acid (TCA) cycle and is metabolized to carbon dioxide and water ( Fig. 72.2 ). Under aerobic conditions, a net of 36 molecules of ATP are generated for each molecule of glucose. However, under anaerobic conditions, pyruvate is converted to lactate and does not enter the Krebs cycle. Under these conditions, only two molecules of ATP are generated for each glucose molecule. Anaerobic glycogenolysis can supply energy to the muscle for only several minutes until the muscle fatigues, whereas there are sufficient muscle glycogen stores to supply energy for up to 90 minutes under aerobic conditions.