CARBOHYDRATES. 19 



carbon as well. 1 There appears really to be no limit to the applicability of 

 the synthesis. 



Perhaps this synthesis throws some light upon the formation of the 

 different sugars in plant organisms, for it must be often necessary for 

 them to build up from sugars containing a small number of carbon atoms 

 those with more carbon. The possibility of such transformations shows 

 that there is no sharp distinction between the separate sugar groups contain- 

 ing different numbers of carbon atoms, and unites, both chemically as well as 

 biologically, the different classes to a large unit which becomes even more 

 closely related by reason of the fact that the artificial representatives of 

 the same group such as grape-sugar, mannose, and fruit-sugar, can be 

 changed into one another by successive oxidation and reduction. 



The large number of sugars prepared synthetically, some of which have 

 not yet been found in nature, together with the natural sugars are sub- 

 divided into groups. We distinguish, in the first place, between the more 

 simple sugars called monosaccharides and compound sugars called poly- 

 saccharides. The latter may be regarded as formed from two or more 

 molecules of the former with elimination of water, and, as a matter of 

 fact, the simpler sugars may be formed from them by hydrolysis. 



The monosaccharides again are divided into subclasses governed by the 

 number of carbon atoms in the molecule. Thus we have a diose (glycol 



HCO 

 aldehyde, or glycolose, | ) which is the simplest possible sugar, 



CH 2 OH 



and trioses, tetroses, pentoses, hexoses, heptoses, etc. We have seen, further- 

 more, that, in general, a sugar is a polyatomic alcohol containing either a 

 ketone or aldehyde group. Corresponding to this, from the trioses on, we dis- 

 tinguish between al doses (aldehyde alcohols) and ketoses (ketone alcohols). 

 Again, an important type which we frequently meet with in nature is that of 

 the methyl derivatives of these sugars; thus we have fucose (from rockweed, 

 known botanically as fucus), rhodeose (from jalap) and rhamnose (pre- 

 pared from numerous plants), which are all designated as methyl pentoses. 



Of all the numerous sugars, but few are found in the animal organism, 

 and only a few play any considerable part as forms of nourishment, al- 

 though we must admit that our present knowledge concerning the phys- 

 iological significance of numerous sugars found distributed throughout 

 the vegetable kingdom, partly free and partly combined with other 

 substances, is still far from being complete. Members of the last-men- 

 tioned class of substances, the number of which is extremely large are 

 known as glucosides, and as such we designate all substances which are 

 more or less readily decomposed into a sugar on the one hand and a com- 

 pound either aromatic or aliphatic in nature on the other. Such decom- 



1 Emil Fischer: Ann. 270, 64 (1892). 



