ATP is present everywhere in the cytoplasm and nucleoplasm of all cells, and essentially all the physiologic mechanisms that require energy for operation obtain it directly from ATP (or another similar high-energy compound—guanosine triphosphate [GTP]). In turn, the food in the cells is gradually oxidized, and the released energy is used to form new ATP, thus always maintaining a supply of this substance. All these energy transfers take place by means of coupled reactions.

The principal purpose of this chapter is to explain how the energy from carbohydrates can be used to form ATP in the cells. Normally, 90 per cent or more of all the carbohydrates utilized by the body are used for this purpose.

Central Role of Glucose in Carbohydrate Metabolism

As explained in Chapter 65, the final products of carbohydrate digestion in the alimentary tract are almost entirely glucose, fructose, and galactose—with glucose representing, on average, about 80 per cent of these. After absorption from the intestinal tract, much of the fructose and almost all the galactose are rapidly converted into glucose in the liver. Therefore, little fructose and galactose are present in the circulating blood. Glucose thus becomes the final common pathway for the transport of almost all carbohydrates to the tissue cells.

In liver cells, appropriate enzymes are available to promote interconversions among the monosaccha-rides—glucose, fructose, and galactose—as shown in Figure 67-3. Furthermore, the dynamics of the reactions are such that when the liver releases the monosaccha-rides back into the blood, the final product is almost entirely glucose. The reason for this is that the liver cells contain large amounts of glucose phosphatase. Therefore, glucose-6-phosphate can be degraded to glucose



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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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