Chemical Constituents of the Animal Body. 199 



CH 3 (OH) 



H(OH) 



CH 2 (OH) 

 CO- 



CHO CH.(OH) 



Amongst the tetroses which are found native are 

 erythrose and threose, and amongst the pentoses, arabinose, 

 xylose, ribose. The hexoses include the natural products 

 glucose, mannose, and galactose, all of which are aldehydes 

 (or aldoses), and laevulose and sorbose, which are ketones 

 (ketoses). All the native monosaccharoses are optically 

 active. They are crystalline substances, readily soluble in 

 water, less soluble in alcohol, and practically insoluble in the 

 majority of organic solvents. Nearly all the above sugars 

 may be represented by the general empirical formula 

 C n nH 2 0. A few sugars are also known which cannot be 

 represented by this formula, such as rhamnose, which is a 

 methyl pentose. 



Isomerism in the Group of Monosaccharoses. If the 

 formula representing the structure of glucose is examined, 

 it will be found to contain four asymmetric carbon atoms. 

 No less than sixteen stereoisomerides are capable of exist- 

 ence, composed of eight pairs of enantiomorphically related 

 substances. 



These stereoisomerides are the following : 

 CHO CHO CHO CHO 



CH 2 OH DB.OH 



Glucoses. 



