THE CARBOHYDRATES 67 



THE HEXOSES AND THEIR DERIVATIVES 



The most important of the carbohydrates belong to this class 

 and are either hexoses or formed by a combination of two or more 

 hexose molecules. They are divided into three main groups : 



(1) Monosaccharides, with the formula C 6 H 12 6 , examples of 

 which are glucose, fructose, &c. 



(2) Disaccharides, which are derived from two molecules of a mono- 

 'saccharide with the elimination of a molecule of water, as follows : 



2C 6 H 12 6 -- H 2 = C 12 H 22 O n . (Examples, maltose and cane sugar.) 



(3) Polysaccharides, composed of three or more molecules of a mono- 

 saccharide. The number of molecules which are associated in the 

 compounds of this group may be very large. We can regard their 

 general formation as represented by the following equation : 



nC 6 H 12 6 - nH 2 = (C 6 H 10 5 ) n . 

 (Examples, starch, dextrin, &c.) 



THE MONOSACCHARIDES 



Only four hexoses out of the large number which have been 

 synthetised are assimilable by the animal body. These are mannose, 

 glucose, galactose, and fructose, the three former being aldoses, while 

 the last is a ketose. All of them are derivatives of d-glucose. They 

 may be synthetised in several ways. The most interesting, because it 

 probably represents the mechanism of synthesis of hexoses in plants, 

 is the formation from formaldehyde. In alkaline solutions formalde- 

 hyde polymerises with the formation of a mixture of hexoses known 

 as acrose. From this mixture a-acrose can be isolated by the forma- 

 tion of its osazone and the reconversion of this osazone into sugar. 

 It is found to be identical with i-fructose. If a solution of this 

 i-fructose be treated with yeast, d-fructose is fermented, leaving 

 1-fructose behind. For the preparation of d-fructose it. is necessary 

 to convert the inactive sugar into the corresponding acid, mannonic 

 acid. This with strychnine or morphia forms salts which can be 

 separated into the d- and 1- groups by fractional crystallisation. From 

 the d- modification d-mannose can be obtained, and this by conversion 

 into the osazone and reconversion into a sugar gives d-fructose. 



All the monosaccharides, however many carbon atoms they 

 contain, present certain general reactions determined by their 

 chemical composition. 



(a) Like ordinary aldehydes and ketones, the sugars act as strongly 

 reducing substances, and, like aldehydes, reduce ammoniacal solution 

 of silver to metallic silver, and many of the higher oxides of metals 



