CARBOHYDRATES. 25 



We know absolutely nothing with regard to the formation of sugars of 

 the five carbon series in the animal organism and concerning their signifi- 

 cance in the organism. It would seem most likely that the source of their 

 presence is to be sought in the diet, although such a relationship has not 

 yet been definitely traced. 



We now come to that group of monosaccharides which is most important 

 for the animal organism, namely, the hexoses of the general formula 

 C^H 12 06- We have already seen that, according to the rule of Le Bel and 

 van 't Hoff, there are sixteen possible isomeric aldehydes having this gen- 

 eral formula. We need consider here only the mannoses, the glucoses, the 

 galactoses, and the fructoses. Before taking up these sugars in detail we 

 will mention some of the more important general reactions of sugars to 

 the extent necessary for us to understand the physiological behavior of 

 different kinds of sugars. 



The simple sugars, in accordance with their aldehyde or ketone nature, 

 are very readily oxidized. They reduce, therefore, metallic oxides on 

 warming their alkaline solutions. Some of the qualitative and quanti- 

 tative methods of analysis, such as those of Fehling, Trommer, and 

 Bottcher, are based upon this property. 



On heating a solution of sugar in caustic soda, or potash, a decomposition 

 takes place (Moore's test). The liquid turns brown, and among other 

 substances, lactic acid, catechol, and formic acid are formed. 



If half a cubic centimeter of a dilute, aqueous solution of d-glucose is 

 treated with a few drops of a ten per cent solution of a-naphthol in alco- 

 hol and then one cubic centimeter of concentrated sulphuric acid cau- 

 tiously added, the zone of contact becomes reddish violet in color. On 

 shaking, the mixture assumes this color (Molisch). This test is often 

 used for detecting the presence of sugar in proteins, etc., and depends upon 

 the formation of furfurol by the action of the concentrated sulphuric acid 

 upon the sugar. 



If a sugar solution is evaporated to dryness and the residue heated 

 somewhat, or if the sugar itself is at once exposed to direct heat, carboni- 

 zation takes place with a characteristic odor. The mass is called caramel. 



An important reaction, and one which has become of great consequence 

 in the investigation of the different kinds of sugars, is the combining of 

 many sugars with hydrazines in acetic acid solution, water being eliminated 

 and hydrazones formed. The most important of these compounds are 

 those with phenyl-hydrazine. 1 If an approximately ten per cent aqueous 

 solution of glucose, for example, is treated with a solution of phenyl- 

 hydrazine in acetic acid and then heated on the water-bath for ten or 

 fifteen minutes, fine yellow needles are deposited whose composition 



1 Emil Fischer: " Verbindungen des Phenylhydrazins mit den Zuckerarten," Ber. 

 17, 579 (1884), and 20, 821 (1887). 



