Chapter 14 Glycolysis, Gluconeogenesis, and the Pentose
Chapter 14 Glycolysis, Gluconeogenesis, and the Pentose
Chapter 14 Glycolysis, Gluconeogenesis, and the Pentose
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168<strong>Chapter</strong> <strong>14</strong> <strong>Glycolysis</strong>, <strong>Gluconeogenesis</strong>, <strong>and</strong> <strong>the</strong> <strong>Pentose</strong> Phosphate Pathwaypyruvate to lactate, <strong>and</strong> is <strong>the</strong>reby recycled to NAD + .47. Fates of pyruvate under anaerobic conditions: fermentationPage: 523 Difficulty: 2Explain with words, diagrams, or structures why lactate accumulates in <strong>the</strong> blood during bursts ofvery vigorous exercise (such as a 100-meter dash).Ans: During vigorous exercise, <strong>the</strong> cardiovascular system cannot deliver O 2 to <strong>the</strong> muscle tissue fastenough to maintain aerobic conditions. As glycolysis proceeds under anaerobic conditions, NAD + isconverted to NADH (during <strong>the</strong> glyceraldehyde 3-phosphate dehydrogenase reaction), but <strong>the</strong> muscletissue has no O 2 to which NADH can pass electrons. To recycle NADH to NAD + , which is essentialfor continuing glycolysis, electrons from NADH are used to reduce pyruvate to lactate.48. Fates of pyruvate under anaerobic conditions: fermentationPage: 523 Difficulty: 2Describe <strong>the</strong> fate of pyruvate, formed by glycolysis in animal skeletal muscle, under two conditions:(a) at rest, <strong>and</strong> (b) during an all-out sprint. Show enough detail in your answer to explain whypyruvate metabolism is different in <strong>the</strong>se two cases.Ans: (a) At rest, plenty of O 2 is being delivered to <strong>the</strong> muscle, <strong>and</strong> pyruvate formed during glycolysisis oxidized to acetyl-CoA by <strong>the</strong> pyruvate dehydrogenase complex. Acetyl groups <strong>the</strong>n enter <strong>the</strong>citric acid cycle <strong>and</strong> are oxidized to CO 2 (b) Under <strong>the</strong> conditions of all-out exertion, skeletal musclecannot be supplied with enough O 2 to keep metabolism completely aerobic; under <strong>the</strong>se conditions,muscle tissue must function anaerobically. Pyruvate is reduced to lactate to recycle NADH, formedby glycolysis, to NAD + , so that glycolysis can continue.49. Fates of pyruvate under anaerobic conditions: fermentationPage: 523 Difficulty: 2During strenuous activity, muscle tissue dem<strong>and</strong>s large quantities of ATP, compared with restingmuscle. In white skeletal muscle (in contrast with red muscle), ATP is produced almost exclusivelyby fermentation of glucose to lactate. If a person had white muscle tissue devoid of <strong>the</strong> enzymelactate dehydrogenase, how would this affect his or her metabolism at rest <strong>and</strong> during strenuousexercise?Ans: Lactate dehydrogenase allows cells to pass electrons from NADH to pyruvate, thus regeneratingNAD + for continued glycolysis under anaerobic conditions. The lack of this enzyme would cause nosignificant problems at rest because aerobic red muscle tissue would function well. During strenuousexercise, however, <strong>the</strong> absence of lactate dehydrogenase would severely reduce <strong>the</strong> ability of muscleto perform anaerobically.