Glycogen Is Stored in Liver and Muscle
After absorption into a cell, glucose can be used immediately for release of energy to the cell, or it can be stored in the form of glycogen, which is a large polymer of glucose.
All cells of the body are capable of storing at least some glycogen, but certain cells can store large amounts, especially liver cells, which can store up to 5 to 8 per cent of their weight as glycogen, and muscle cells, which can store up to 1 to 3 per cent glycogen. The glycogen molecules can be polymerized to almost any molecular weight, with the average molecular weight being 5 million or greater; most of the glycogen precipitates in the form of solid granules.
This conversion of the monosaccharides into a high-molecular-weight precipitated compound (glycogen) makes it possible to store large quantities of carbohy-drates without significantly altering the osmotic pres-sure of the intracellular fluids. High concentrations of low-molecular-weight soluble monosaccharides would play havoc with the osmotic relations between intracel-lular and extracellular fluids.
The chemical reactions for glycogenesis are shown in Figure 67–4. From this figure, it can be seen that glucose-6-phosphate can become glucose-1-phosphate; this isconverted touridine diphosphate glucose, which is finally converted into glycogen. Several specific enzymes are required to cause these conversions, and any monosaccharide that can be converted into glucose can enter into the reactions. Certain smaller com-pounds, including lactic acid, glycerol, pyruvic acid, and some deaminated amino acids, can also be converted into glucose or closely allied compounds and then con-verted into glycogen.
Glycogenolysis means the breakdown of the cell’sstored glycogen to re-form glucose in the cells. The glucose can then be used to provide energy. Glycogenol-ysis does not occur by reversal of the same chemical reactions that form glycogen; instead, each succeeding glucose molecule on each branch of the glycogen polymer is split away by phosphorylation,catalyzed by the enzyme phosphorylase.
Under resting conditions, the phosphorylase is in an inactive form, so that glycogen will remain stored. When it is necessary to re-form glucose from glycogen, the phosphorylase must first be activated. This can be accomplished in several ways, including the following two.
Activation of Phosphorylase by Epinephrine or by Glucagon. Twohormones, epinephrine and glucagon, can activate phos-phorylase and thereby cause rapid glycogenolysis. The initial effect of each of these hormones is to promote the formation of cyclic AMP in the cells, which then initiates a cascade of chemical reactions that activates the phosphorylase.
Epinephrine is released by the adrenal medullaewhen the sympathetic nervous system is stimulated. Therefore, one of the functions of the sympathetic nervous system is to increase the availability of glucose for rapid energy metabolism. This function of epineph-rine occurs markedly in both liver cells and muscle, thereby contributing, along with other effects of sym-pathetic stimulation, to preparing the body for action.
Glucagon is a hormone secreted by the alpha cells of the pancreas when the blood glucose concentration falls too low. It stimulates formation of cyclic AMP mainly in the liver cells, and this in turn promotes conversion of liver glycogen into glucose and its release into the blood, thereby elevating the blood glucose concentration..
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