14 LECTURE II. 



In the course of the following discussion we shall see how closely the 

 development of the chemistry of carbohydrates follows the general develop- 

 ment of chemistry, and especially that of stereochemistry and the theory 

 of structure, and what a comprehensive outlook dawned all at once 

 for the whole field of biology. 



The carbohydrates are all composed of the elements carbon, hydrogen, 

 and oxygen, and these are the same elements that are found in fats. The 

 two classes of compounds, however, contain these elements in different 

 relative amounts. Oxygen and hydrogen in the former are present in the 

 ratio 1 : 2, which is the same as in water. This is the reason that the 

 name carbohydrates has been given to the group. Many other compounds 

 which do not belong to the sugar group, for example acetic and lactic 

 acids, are, however, also composed of the same elements and -in the same 

 ratio. Formerly, the carbohydrates were defined as containing six, or a 

 multiple of six, carbon atoms. This limitation was shown to be incorrect 

 by the discovery of sugars containing less than six atoms of carbon, and by 

 the synthesis of sugars with seven, eight, and nine carbon atoms. It is in 

 fact impossible to give a sharply-defined, satisfactory definition of a carbo- 

 hydrate, for to some extent the individual members of the group have very 

 different properties from one another. In general, the carbohydrates are 

 aldehyde or ketone derivatives of polyatomic alcohols. 



As is the case with almost all branches of physiological chemistry, so 

 here, as has already been indicated, it was only possible to obtain a clear 

 idea of the formation and transformations of carbohydrates in the animal 

 and vegetable organisms after the compounds in question had been pre- 

 pared synthetically. It was Emil Fischer who first succeeded in this 

 effort, by preparing from glycerol the same glycerol which we shall 

 meet with again in the discussion of fats by gentle oxidation, a sub- 

 stance with the typical properties of a sugar. This compound, called 

 glycerose, contains, to be sure, only half as much carbon as grape-sugar. As 

 it was found possible to prepare by the action of dilute alkali upon two 

 molecules of glycerose a true sugar with six atoms of carbon, there was 

 no longer any doubt that glycerose was to be regarded as a member of 

 the carbohydrate group. This synthesis is of especial value to us, as it 

 establishes a relation between the fats and the sugars. Finally, it was 

 even possible to effect the synthesis from the elements; for starting with 

 formaldehyde, CH^O, Emil Fischer succeeded by polymerization 

 (6 x CH 2 O = C6Hi 2 O 6 ) in obtaining the same sugar as that made from 

 the glycerose prepared from glycerol. 1 



This complete synthesis is particularly interesting to us, because some time 



1 Ber. 23, 2114 (1890); Die Chemie der Kohlehydrate und ihre Bedeutung fur die 

 Physiologic, Berlin, 1894; Synthesen in der Purin-imd Zuckergruppe, Braunschweig, 

 1903. 



