Cells cannot accumulate very many separate glucose molecules, because an abundance of these would exert an osmotic pressure (see chapter 6) that would draw a dangerous amount of water into the cells. Instead, many organs, particularly the liver, skeletal muscles, and heart, store carbohydrates in the form of glycogen.
The formation of glycogen from glucose is called glycogenesis. In this process, glucose is converted to glucose 6-phosphate by utilizing the terminal phosphate group of ATP. Glucose 6-phosphate is then converted into its isomer, glucose 1-phosphate. Finally, the enzyme glycogen synthase removes these phosphate groups as it polymerizes glucose to form glycogen.
The reverse reactions are similar. The enzyme glycogen phosphorylase catalyzes the breakdown of glycogen to glucose 1-phosphate. (The phosphates are derived from inorganic phosphate, not from ATP, so glycogen breakdown does not require metabolic energy.) Glucose 1-phosphate is then converted to glucose 6-phosphate. The conversion of glycogen to glucose 6-phosphate is called glycogenolysis. In most tissues, glucose 6-phosphate can then be respired for energy (through glycolysis) or used to resynthesize glycogen. Only in the liver, for reasons that will now be explained, can the glucose 6-phosphate also be used to produce free glucose for secretion into the blood.
Figure 5.4 Glycogenesis and glycogenolysis. Blood glucose entering tissue cells is phosphorylated to glucose 6-phosphate. This intermediate can be metabolized for energy in glycolysis, or it can be converted to glycogen ( 1 ) in a process called glycogenesis. Glycogen represents a storage form of carbohydrates that can be used as a source for new glucose 6-phosphate (2) in a process called glycogenolysis. The liver contains an enzyme that can remove the phosphate from glucose 6-phosphate; liver glycogen thus serves as a source for new blood glucose.
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