60 CARBOHYDRATES 



it forms an opalescent colloidal solution. With iodine, glycogen gives 

 a red-brown color, which is made somewhat more pronounced by the 

 addition of sodium chloride. It does not reduce Fehling's solution. It 

 is hydrolyzed by the action of dilute acids to maltose, and finally to 

 glucose. Like starch, glycogen is not fermented by yeast, but is readily 

 hydrolyzed by starch-splitting enzymes. Diastase, ptyalin, and amy- 

 lopsin convert it into maltose. 



Bacterial dextrans 



The dextrans ^ are polysaccharides produced by several species of 

 bacteria, notably Leucanostoc mesenteroides. Composed entirely of 

 glucose, they are high molecular weight (several million) , water-soluble 

 substances, which can be precipitated from aqueous solutions by adding 

 an equal volume of alcohol. The glucose units are attached to each other 

 by a-l,6-linkages in chains which have many branches. At the branch 

 points, a-l,4-linkages occur. The structure is thus the reverse of that 

 of glycogen and amylopectin, where the chains are held together by 

 a-l,4-linkages, and a-l,6-links are found only at branch points. 



Bacterial dextrans, like other glucosans, can be hydrolyzed with acids 

 to form glucose. Dextran degraded by. partial hydrolysis to an average 

 molecular weight of about 100,000 has been used in the form of a 6 per 

 cent solution in 0.9 per cent saline solution as a substitute for plasma in 

 blood transfusions. Although by no means a complete substitute for 

 plasma or whole blood, such solutions do have considerable value for body 

 fluid replacement in cases of severe burns, shock, blood loss, and the 

 like. One of the main problems in such cases is to prevent further loss 

 of fluid from the body, and this can only be done if the fluid used for the 

 transfusion contains a nondiffusible solute which gives it an osmotic 

 pressure similar to that normally caused by the blood serum proteins. 

 The dissolved substance must have about the right molecular weight, 

 must remain in the blood for a day or two, must not cause too great 

 viscosity, and must not be toxic or produce any undesirable side effects. 

 Partly hydrolyzed dextran is one of the most satisfactory materials of 

 a number which have been investigated for this purpose. 



Cellulose 



Cellulose consists of an unbranched chain of D-glucose units joined by 

 y8-l,4-linkages. Thus it closely resembles amylose except for the /3- 

 linkage and a much higher molecular weight. Many efforts have been 

 made to determine the number of glucose units in the chain, and values 

 ranging all the way from a few hundred to several thousand have been 



^ Do not confuse with dextrins. 



