54 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., laevulose. 



(2) C 12 HO n '+ H 2 = C 6 H 12 6 + C 6 H 12 6 - 

 Cane-sugar Glucose. Lsevulose. 



(Disaccharide.) (Monosaccharides.) 



THE MONOSACCHARIDES. 



According to the number of carbon atoms which are present in 

 the molecule, the monosaccharides can be divided into trioses, tet- 

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

 oses only will be considered, as the remaining groups are of practi- 

 cally no significance as animal food-stuffs, and are in man, at least, 

 mostly eliminated through the kidneys as foreign matter. 



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 sub- 

 stances, 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, mannite, and dulcite. As has 

 been pointed out above, the monosaccharides are either aldehydes or 

 ketones, and we accordingly find that glucose, mannose, and galac- 

 tose represent the aldehydes (aldoses) of sorbite, mannite, and 

 dulcite, respectively, while Ia3vulose is the ketone (ketose) of 

 mannite. They can therefore be represented by the structural 

 formula : 



(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) (tevulose). 



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 



