CHAPTER III. 

 THE CARBOHYDRATES; THE MONOSACCHARIDES. 



GENERAL CHARACTERISTICS. 



The Carbohydrates are extremely abundant in nature, and play an 

 exceedingly important part in the life-cycle. In vegetable tissues they 

 are of importance, not only as foodstuffs and reserve materials, but also 

 as structural materials. For example, the walls of plant-cells are 

 usually composed of cellulose, a complex carbohydrate. In the animal 

 economy the carbohydrates are chiefly of importance as food and 

 reserve-materials and afford a very important source of kinetic energy 

 to our tissues. 



The carbohydrates owe their name to the fact that all of them contain 



carbon and in all of them, moreover, the proportion of hydrogen to 



oxygen is the same as it is in water, namely, 2 to 1. This is not a very 



satisfactory definition of the group, however, since many substances 



are known which correspond to such a definition and yet are most 



distinctly not carbohydrates. In more exact terms it may be said that 



carbohydrates are aldehyde and ketone derivatives of the polyatomic 



ilcohols. The majority of the naturally occurring simple sugars 



:ontain six atoms of carbon and are termed Hexoses, although some 



Contain five atoms of carbon and are termed Pentoses. From the simple 



ftonosaccharides, more complex sugars, the Disaccharides, are formed 



>y the combination of two molecules of monosaccharide with the 



Jimination of a molecule of water. More complex carbohydrates 



till, the starches and dextrines, collectively termed the Polysaccharides, 



,re derived from the monosaccharides by the combination of a variable 



lumber of sugar molecules, with the elimination of a corresponding 



lumber of molecules of water. 



It is only within comparatively recent times that the artificial 

 ynthesis of sugar has been accomplished, but within the brief period 

 >f thirty years nearly all of the natural sugars have been synthesized, 

 ,nd the light which the consequent accessions to our chemical knowl- 

 dge have thrown upon the function and transformations of the carbo- 

 tydrates in living organisms is so great, that today we are in a position 

 o interpret countless phenomena which were entirely obscure before 

 hese discoveries had been made. 



The first sugar to be synthesized was Glycerose, which was prepared 

 >y Emil Fischer in 1890. This sugar, which contained, however, only 

 hree atoms of carbon (formula (CH 2 OH) 2 CO was prepared by the 

 entle oxidation of the triatomic alcohol, glycerol (C 3 H 5 (OH) 3 . This 

 ynthesis is particularly interesting because it establishes a connection 

 etween the carbohydrates and the fats, since all of the naturally 

 ccurring fats contain a glycerol radical. From this sugar it was found 



