CHAPTER III 



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CARBOHYDRATES 



Composition, Occurrence, General Function. The carbohy- 

 drates are compounds of carbon, hydrogen and oxygen in which 

 the hydrogen and oxygen are present usually in the same pro- 

 portions as in water, hence the name of the group. A few 

 carbohydrates do not conform to this general statement, for 

 example Rhamnose C C H 12 5 . Some substances not carbohy- 

 drates do contain hydrogen and oxygen in this proportion, such 

 as acetic acid C 2 H 4 2 and lactic acid C 3 H 6 3 . Thus, this char- 

 acteristic is not distinctive of the group. Carbohydrates vary 

 considerably in their properties. They are found both in plants 

 and in animals, but chiefly in the former, in which they form 

 a considerable part of the structural frame work. In animals 

 they serve mainly as a fuel to be oxidized for the production 

 of heat or the performance of mechanical work, but they may 

 be laid away as a reserve store to be called on in case of need. 



Structure of the Carbohydrates. The carbon atoms in the 

 carbohydrate molecule are linked together in a long chain. It 

 has been shown that the molecule contains several hydroxyl 

 groups, and that in the simpler members of the group at least 



H 



I I 



there is either an aldehyde C = or a ketone C = group. 



i i 



The remaining valencies of the carbon atoms in the chain hold 

 the hydrogen atoms not accounted for. Thus the formula 

 for glucose, one of the most important carbohydrates, has been 

 shown to be as follows: 



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