DISTURBANCES OF CARBOHYDRATE METABOLISM 291 



ketone, or CO group. Sugars containing the former are known as aldoses 

 and those with the latter as ketoses. Aldoses and ketoses which differ only 

 in this respect, having the remainder of the groups similarly arranged, 

 are closely related and yield the same phenylosazone. The property of 

 reducing solutions of metallic salts depends upon the presence of the alde- 

 hyde or of the ketone group. The carbohydrates that we shall have to 

 consider in this chapter are the pentoses, hexoses and heptoses, containing 

 five, six and seven carbon atoms, respectively, arid the derivatives and com- 

 binations of these simple sugars. Although all the sugars of any one class, 

 such as the hexoses, have the same empirical chemical formula, C C H 12 O 6 

 in the case of the hexoses, many of their properties are quite different. 

 These differences are connected with differences in the arrangement of 

 the atoms within the molecule. 



All the sugars are optically active, that is, are capable of rotating the 

 plane of polarized light. Each simple sugar has its optical antipode, or 

 isomer, which rotates polarized light to the same degree, but in the opposite 

 direction. The following diagrams represent the accepted molecular con- 

 stitution of some of the sugars and may be useful in the discussion that 

 is to follow. 



COOH 

 HOCH 

 HOCH 



HCOH 

 HOCH 



CHO 

 d-glycuronic acid 



The prefixes d and I do not refer to the actual rotatory activity of 

 the sugars. They were introduced by Emil Fischer, and are based upon 

 the derivation of the sugar from the common dextrorotatory sugars, glucose 

 and galactose, and their optical antipodes. Therefore, though levulose 

 is levorotatory, it is known as ^-fructose, because it may easily be derived 

 from, and has the same configuration as, d-glucose. Similarly, the actual 

 rotation of polarized light by the arabinoses and xyloses is the opposite of 

 that indicated by the prefixes. 



Because optical activity is a property of all sugars, the terms 



