Central Role of Glucose in Carbohydrate Metabolism
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 con- verted into glucose in the liver. Therefore, little fructose and galactose are present in the circulating bloodGlucose 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. There- fore, glucose-6-phosphate can be degraded to glucose and phosphate, and the glucose can then be transported through the liver cell membrane back into the blood.
Once again, it should be emphasized that usually more than 95 per cent of all the monosaccharides that circulate in the blood are the final conversion product, glucose.
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