CHAPTER LXXV 

 THE METABOLISM OF THE CARBOHYDRATES (Cont'd) 



FATE OF ABSORBED GLUCOSE. GLUCONEOGENESIS 



We may now consider what becomes of the sugar that is retained by 

 the liver and muscles. Two things may happen to it: It may become 

 stored, or it may become oxidized or split up. Of these processes, storage 

 occurs in both the liver and muscles, whereas oxidation occurs mainly if 

 not entirely in the muscles, although a certain amount of splitting of the 

 glucose molecule may also occur in the liver. 



Storage of Sugar. — For the present Ave shall consider the process of 

 storage of sugar and defer a consideration of its utilization until after we 

 have studied, not only the nature of the process by which the storage 

 occurs, but also the immediate destiny of the stored sugar. The storage 

 of sugar by the liver is brought about by its conversion into a polysac- 

 charide called glycogen. After an animal has been absorbing large quan- 

 tities of glucose, an acidified watery extract of a portion of liver made 

 immediately after death will be found to contain no more sugar than that 

 of a normal liver. On the other hand, it will be observed that the extract 

 is highly opalescent and yields on the addition of alcohol a copious precip- 

 itate, which on further purification can readily be shown to consist of a 

 polysaccharide — that is to say, of a starch-like substance which on hydrol- 

 ysis with mineral acid becomes entirely converted into sugar. If instead 

 of examining the liver immediately after death, it is allowed to stand for 

 some time, the yield of glycogen Avill greatly diminish, and in its place 

 will appear large quantities of glucose, indicating that some enzyme must 

 exist which attacks the glycogen after death and converts it into sugar, 

 This enzyme is called glycogenase. The existence of postmortem gh/co- 

 genolysis, as it is called, would seem to indicate that during life a con- 

 stant tendency for the glycogen in the liver to be attacked by glycogenase 

 is held in check by conditions which depend on the vital integrity of 

 the liver cell. It is evident that if anything should happen during life 

 to interfere with this inhibiting influence, the glycogen will become con- 

 verted into glucose, which on escaping into the blood will produce hyper- 

 glycemia and glycosuria. 



Sources of Glycogen. — In studying the sources of sugar in the animal 

 body it is of great importance that we should first of all know exactly the 



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