Carbohydrate Metabolism

Carbohydrate Catabolism In the previous sections, we described the major pathways of carbohydrate catabolism: the breakdown of glucose to pyruvate or lactate by way of the glycolytic pathway, and the metabolism of pyruvate to carbon dioxide and water by way of the Krebs cycle and oxidative phosphorylation.

Vander et al.: Human I I. Basic Cell Functions I 4. Protein Activity and I I © The McGraw-Hill

Physiology: The Cellular Metabolism Companies, 2001 Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

Body Function

Oxidative phosphorylation

(mitochondria)

FIGURE 4-24

Pathways of aerobic glucose catabolism and their linkage to ATP formation.

Oxidative phosphorylation

(mitochondria)

34 ATP

C6H1206 + 6 O2 + 38 ADP + 38 Pi -6 CO2 + 6 H2O + | 38 ATP

FIGURE 4-24

Pathways of aerobic glucose catabolism and their linkage to ATP formation.

The amount of energy released during the catabo-lism of glucose to carbon dioxide and water is 686 kcal/mol of glucose:

As noted earlier, about 40 percent of this energy is transferred to ATP. Figure 4-24 illustrates the points at which ATP is formed during glucose catabolism. As we have seen, a net gain of two ATP molecules occurs by substrate-level phosphorylation during glycolysis, and two more are formed during the Krebs cycle from GTP, one from each of the two molecules of pyruvate entering the cycle. The major portion of ATP molecules produced in glucose catabolism—34 ATP per molecule—is formed during oxidative phosphorylation from the hydrogens generated at various steps during glucose breakdown.

To reiterate, in the absence of oxygen, only 2 molecules of ATP can be formed by the breakdown of glucose to lactate. This yield represents only 2 percent of the energy stored in glucose. Thus, the evolution of aerobic metabolic pathways greatly increased the amount of energy available to a cell from glucose ca-tabolism. For example, if a muscle consumed 38 molecules of ATP during a contraction, this amount of ATP could be supplied by the breakdown of 1 molecule of glucose in the presence of oxygen or 19 molecules of glucose under anaerobic conditions.

It is important to note, however, that although only 2 molecules of ATP are formed per molecule of glucose under anaerobic conditions, large amounts of ATP can still be supplied by the glycolytic pathway if large amounts of glucose are broken down to lactate. This is not an efficient utilization of fuel energy, but it does permit continued ATP production under anaerobic conditions, such as occur during intense exercise (Chapter 11).

Glycogen Storage A small amount of glucose can be stored in the body to provide a reserve supply for use when glucose is not being absorbed into the blood from the intestinal tract. It is stored as the polysac-charide glycogen, mostly in skeletal muscles and the liver.

Glycogen is synthesized from glucose by the pathway illustrated in Figure 4-25. The enzymes for both glycogen synthesis and glycogen breakdown are located in the cytosol. The first step in glycogen synthesis, the transfer of phosphate from a molecule of ATP to glucose, forming glucose 6-phosphate, is the same as the first step in glycolysis. Thus, glucose 6-phosphate can either be broken down to pyruvate or used to form glycogen.

Note that, as indicated by the bowed arrows in Figure 4-25, different enzymes are used to synthesize and break down glycogen. The existence of two pathways

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Protein Activity and Cellular Metabolism CHAPTER FOUR

Protein Activity and Cellular Metabolism CHAPTER FOUR

Glycogen

(all tissues)

Glucose

(all tissues)

Glucose

Glucose 6-phosphate

(liver and kidneys)

(liver and kidneys)

Glycogen

Pyruvate

Glucose 6-phosphate

Pyruvate

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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