SECTION IV 

 THE METABOLISM OF CARBOHYDRATES 



ALL the carbohydrates which are taken in with the food are 

 ultimately transformed in the alimentary tract, or in its walls, into 

 the three monosaccharides, glucose, fructose, and galactose. These 

 three, together with mannose, are the only sugars which are directly 

 fermentable and directly assimilable by higher animals. A con- 

 sideration of their structural formulae shows that they are fairly 

 easily interconvertible, galactose presenting the greatest differences 

 from the general type. This conversion actually takes place in watery 

 solution. If a solution of any one be left for some months, it will be 

 found to contain the representatives of all four. 



Since these monosaccharides, for the greater part glucose, must 

 enter the blood in large quantities during the absorption of a heavy 

 carbohydrate meal, one would expect to find a greater proportion 

 in the blood during such a meal than during a period of starvation. 

 The amount of reducing sugar in the blood, however, is practically 

 constant, and varies between 0-1 and 0-15 per cent. 



Searching for the origin of this constant proportion of reducing 

 sugar, Claude Bernard found that the blood of the hepatic vein in a 

 fasting animal contained more sugar than the blood taken at the 

 same time from the portal vein. Although the reliability of this 

 experimental result has been put in doubt by more recent investigators, 

 it was important in that it attracted Bernard's attention to the liver. 

 If the liver be taken from an animal which has been dead for some 

 time, and extracted with water, the extract is found to contain a large 

 quantity of reducing sugar (glucose). If, however, it be removed 

 immediately the animal is dead, its vessels washed out with ice-cold 

 saline fluid, and be then cut up and thrown into boiling water, ground 

 and extracted, the extract, after separation of the coagulable proteins, 

 contains hardly a trace of sugar, and no more than is present in the 

 blood. The fluid is, however, opalescent ; and Bernard found that 

 this opalescence was due to the presence of a substance at that time 

 new to science, belonging to the class of polysaccharides. The sub- 

 stance he called gtycogen, i.e. the sugar-former. 



After a carbohydrate meal, glycogen may be present in very large 

 amounts in the liver, up to 12 per cent, of the weight of the fresh liver. 



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