98 THE CHEMICAL COMPOSITION OF THE BODY 



CARBOHYDRATES. 



The typical carbohydrates contain carbon combined with hydrogen and 

 oxygen in the proportion to form water. Other substances, such as acetic 

 acid, CH 3 COOH, lactic acid, CH 3 CHOHCOOH, and inosit, (CHOH) 6 , 

 which contain hydrogen and oxygen in the proportion to form water, are not 

 carbohydrates. Certain true carbohydrates also do not fulfill this condition. 

 Chemically, the carbohydrates are aldehyde or ketone derivatives of complex 

 alcohols; i.e., they have the structure 



R - CHOH - CHO or R - CO - CH 2 OH 



Aldose Ketose 



Accordingly, the carbohydrates are termed aldoses or ketoses. They 

 are commonly classified by the number of carbon atoms in the molecule; 

 e.g., pentoses are those containing five carbon atoms, and hexoses have six 

 carbons. Each member of the carbohydrate class, with the exception of 

 the pentoses, may be regarded as containing the saccharide group, C 6 H 10 O 5 . 

 The monosaccharides are then C 6 H 10 O 5 + H 2 O; the di-saccharides contain 

 two saccharide groups with water, (C 6 H 10 O 5 ) 2 +H 2 O, while the poly- 

 saccharides contain this group taken a large number of times, (C 6 H 10 O 5 ) n . 

 In general, the solubility of the saccharides varies inversely with the 

 number of saccharide groups present: the mono-saccharides, as a class, 

 being the most soluble and the poly-saccharides being the least so. On 

 boiling in the autoclave or with mineral acids, and by the action of amylo- 

 lytic and inverting enzymes, the poly-saccharides are, as a rule, hy- 

 drolyzed into the simple carbohydrates. The reaction may be indicated 



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



Poly-saccharide Water Mono-saccharide 



Simple poly-saccharides, and di-saccharides are formed as intermediate 

 products in the cleavage and in turn these are further hydrolyzed. 



C 12 H 22 U + H 2 , , 2C 6 H 12 O e 



Di-saccharide Water Mono-saccharide 



The color reactions with iodine, fermentation with yeast and bacteria, 

 the formation of characteristic crystalline osazones with phenylhydrazine 

 and the reducing of alkaline solutions of the oxides of metals like copper, 

 bismuth, mercury, and ammoniacal silver solutions, are the most widely 

 used reactions for testing and differentiating the carbohydrates. The 

 reduction of alkaline solutions of the metallic oxides is due to the easily 

 oxidized aldehyde and ketone structure of the sugar. 



