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 we 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 mad? 

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

 some time, the yield of glycogen will 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 glyco- 

 genolysis, as it is called, would seem to indicate that during life also there 

 is a constant tendency for the glycogen in the liver to be attacked by 

 glycogenase, but that this is prevented 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 converted into glucose, which on escaping into the blood 

 will produce hyperglycemia and glycosuria. 



Sources of Glycogen. In studying the source of sugar in the animal 

 body it is important therefore that we should first of all know exactly the 



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