LECTURE III. 



CARBOHYDRATES. 

 II. 



POLYSACCHARIDES. 



THE polysaccharides, or compound sugars, which we now have to 

 consider, can be regarded, as we have seen, as glucosides of sugar itself; in 

 other words, they are formed from simpler sugars with elimination of 

 water; and, conversely, by the action of hydrolyzing agents (chemicals or 

 ferments), they may be broken down into their separate components, i.e., 

 simple sugars. In discussing the monosaccharides, we often found oppor- 

 tunity to point out how widely distributed these complicated sugars are, 

 for the simple sugars themselves in some cases only occur in nature in this 

 state. Biologically this group assumes a distinctive position. The animal 

 and vegetable organisms store their reserves of carbohydrates in this 

 form. On the other hand, many representatives of the class may be 

 looked upon as the intermediary products between the more complicated 

 and simpler sugars, and owe their origin to a progressive, spontaneous 

 hydrolysis. In the center of all these processes of transformation taking 

 place in the animal organism, we find the hexoses, especially glucose; 

 whether a complicated sugar molecule such as starch breaks down, or 

 whether such a one as glycogen is formed, for example. In the plant 

 organism the relations are to some extent similar, except that here, as has 

 been previously mentioned, the sugars of the five-carbon series are more 

 common. It is, however, still an open question as to whether the 

 simple pentoses here take such a central position in the metabolism 

 of carbohydrates as the hexoses in the animal system, for up to the 

 present time the pentoses are known almost exclusively in the form of 

 polysaccharides (pentosans, etc.), concerning the formation of which our 

 knowledge is still very limited. 



The group of polysaccharides is subdivided, according to the number of 

 sugar molecules which enter into their composition, into di-, tri-, tetrasac- 

 charides, etc., and the true polysaccharides. 



The disaccharides * consist of two molecules of the simple sugar minus 



1 Here only the hexobioses are considered, i.e., those composed of two molecules of 

 hexose. There are also bioses built up of sugars containing fewer carbon atoms, for 

 example, gluco-apiose, which is built up from /?-oxymethyl-tetrose (apiose) and glucose, 

 and is obtained from the glucoside in Petroselinum apiin. Again, we have the mano- 

 rhamnoses prepared from strophanthin. 



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