THE CARBOHYDRATES AND THEIR METABOLISM 243 



COH 



HCOH 



I 

 CH 2 OH 



d-Glycericaldehyde 



or 

 d-Glycerose 



COH 

 HOCH 



CH 2 OH 



1-Glycericaldehyde 



or 

 1-Glvcerose 



CH 2 OH 



CO 



I 

 CH 2 OH 



Dihydroxy acetone 



All of these substances are intermediary products in the metabolism of 

 carbohydrates, and are of interest on that account. 



There are four possible aldotetroses of which three are known, but they 

 have not been found to occur in nature in the free states. 



COH 

 HOCH 

 HOCH 



CH 2 OH 



1-Erythrose 



COH 

 HCOH 

 HCOH 



CH 2 OH 



d-Erythrose 



COH 

 HCOH 

 HOCH 



CH 2 OH 



1-Threose 



COH 

 HOCH 



HCOH 



I 

 CH 2 OH 



d-Threose 



The alcohol of erythrose, erythritol, has been obtained from various alga? 

 and mosses. 



Disaccharides 



These sugars contain twelve carbon atoms and are made up of two 

 hexoses united by an oxygen atom. When acted upon by hydrolytic agents, 

 they take up one molecule of water and are converted into the hexoses of 

 which they are composed. 



The hexoses in these carbohydrates are bound together in much the 

 same way as they are in the glucosides; hence the aldehyde or ketone 

 radical of one of the hexoses is the point of union, while the ketone or 

 aldehyde radical of the other hexose may or may not remain free. 



Those disaccharides that have a potentially free aldehyde or ketonic 

 group give the typical reactions of the hexoses, such as reduction of alkaline 

 copper and other metallic hydroxides and combination with hydrocyanic 

 acid. They exhibit mutarotation and exist in two forms which are in 

 equilibrium in aqueous solution. The union of the two hexoses is similar 

 to that found in the case of the glucosides. In fact, many of them are 

 hydrolyzed by certain glucosidases. 



When an aldehyde or ketone group is free, as in maltose, phenyl osa- 

 zones, that are only slightly soluble but difficult to purify, are obtained. 

 The hydrazones are almost all easily soluble in water. The disaccharides 



