64 THE CARBOHYDRATES. 



there are no specific properties peculiar to these substances as a 

 class, it is impossible to give an adequate definition of what is meant 

 by the term carbohydrate. Chemically speaking, they are deriva- 

 tives of polyatomic alcohols, and of the nature of aldehydes or 

 ketones. They are conveniently divided into monosaccharides, 

 disaccharides, and polysaccharides. The disaccharides and poly- 

 saccharides differ from the monosaccharides in being more complex 

 substances and apparently built up from the monosaccharides 

 through a condensation of monosaccharine anhydrides to form a 

 double or a multiple group. Accordingly, on hydrolytic decom- 

 position they yield two or more monosaccharine molecules for 

 every original molecule, as is shown below : 



(1) C 6 H, 2 O 6 glucose, viz., Isevulose. 



(2) C 12 H 22 O n -f H 2 = C 6 H 12 6 + C 6 H ]2 6 . 

 Cane-sugar Glucose. Lsevulose. 



(Disaccharide.) (Monosaccharides.) 



THE MONOSACCHARIDES. 



According to the number of oxygen atoms which are present in 

 the molecule, the mouosaccharides can be divided into dioses, trioses, 

 tetroses, pentoses, hexoses, heptoses, octoses, etc. Of these, the hex- 

 oses and pentoses only will be considered, as the remaining groups 

 are not found in the animal body. 



The Hexoses. The most important representatives of the 

 hexoses are glucose, which is also termed dextrose ; Isevulose or 

 fructose ; mannose and galactose. Some of these, such as glucose 

 and Isevulose, are found free in nature, or they result as hydrolytic 

 decomposition-products from the more complex carbohydrates and 

 related nitrogenous substances, the so-called glucosides. They are 

 all derivatives of the stereo-isomeric hexatomic alcohols of the 

 composition CH 2 .OH (CH.OH) 4 -CH 2 .OH. Of these, three are 

 known to occur in the natural state, viz., sorbite, or glucite, man- 

 nite, and dulcite. As has been pointed out above, the monosac- 

 charides are either aldehydes or ketones, and we accordingly find 

 that glucose, mannose, and galactose represent the aldehydes 

 (aldoses) of sorbite, mannite, and dulcite, respectively, while Isevu- 

 lose is the ketone (ketose) of mannite. They can therefore be 

 represented by the structural formulae : 



(1) CH 2 (OH).CH(OH).CH(OH).CH(OH).CH(OH).CHO (glucose, mannose, and 



galactose). 



(2) CH 2 (OH).CH(OH).CH(OH).CH(OH).CO.CH 2 (OH) (Isevulose). 



As a matter of fact it is possible to transform these hexoses into 

 their corresponding alcohols by careful reduction, and vice versa. 



In accordance with their character as aldehydes or ketones, the 

 aldoses on oxidation yield oxyacids, which have the same number 



