Glucose is stored in the liver and muscles of animals and humans as the branched polymer glycogen; the equivalent polymer in plants is starch. Glycogen is more branched than amylopectin and has 10 to 18 long chains of a-D-glucopyranose residues (in a(1 ® 4) glucoside links) with branching by a(1 ® 6) glucoside bonds ( Fig. 3.5). Although it occurs in concentrations of up to 6% of liver mass but only 1% of muscle, muscle mass is so much greater that it represents 3 to 4 times as much glycogen as in the liver. Muscle glycogen is mainly used by the muscle, but liver glycogen is for storage, export, and the maintenance of blood glucose. The liver has glycogen stores for only 12 to 18 hours of fasting; it then becomes glycogen depleted.
Figure 3.5. A highly schematic diagram of a portion of a glycogen molecule. Each solid black circle represents a glucose moiety joined by 1®4 and 1®6 links. The branching of the structure, caused by the 1®6 links, is more random than is shown.
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