CARBOHYDRATES 



59 



Carbohydrates may be classified according to ihc nature oC the rings of 

 the acetal and semiacetal forms. They may he grouped according to 

 the configuration about tlie semiacetal (or acetal) atom, the number 

 one carbon. They may be divided into reducing and non-reducing 

 carbohydrates, depending on the presence or absence of free carbonyl 

 or semiacetal groups. The small, water-soluble representatives are also 

 known as sugars, most of them yielding solutions with a sweet taste. 



Beyond these descrijJtive terms are others. One of these is wide- 

 spread and specifies the number of individual carbohydrate units into 

 which the parent molecule may be hydrolyzed. The root word 

 saccharide is used as the suffix and signifies carbohydrate. If the origi- 

 nal compound is a simple carbonyl or semiacetal sugar incapable of 

 hydrolysis, it is termed a monosaccharide. When a sugar hydrolyzes 

 into two such monosaccharide molecules, it is a disaccharide, into three 

 a trisaccharide, and so forth. The giant carbohydrate molecules hy- 

 drolyze into many monosaccharide units and are called polysaccha- 

 rides. The classes undergoing hydrolysis may yield only a single kind 

 of monosaccharide, or they may yield more than one kind, depending 

 on the nature of the parent carbohydrate. 



With several asymmetric centers present, a variety of diastereo- 

 isomers can exist. These forms differ from one another chemically, 

 physically, and biologically and are distinguished by means of different 

 names like glucose and galactose. However, mirror-image sugars are 

 known and are described by optical rotations or more commonly by 

 conventional representations of their configurations. This system is 

 applied to the individual monosaccharides even when they are com- 

 ponent parts of larger molecules, never to the larger units themselves. 

 The atom conventionally chosen for the representation key is the 

 asymmetric carbon atom of highest number, always counting from the 

 carbonyl, semiacetal, or acetal group, or the end nearest one of these, 

 along the carbon chain. Using glucose as an example, 



H 



OH 



CHO 

 HOCH 



HCOH 

 HOCH 

 HOCH 



CHO 



HCOH 



HOCH 



HCOH 



HCOH 



HCOH 

 HOCH 



HCOH 



I 

 HC 



O H 

 C 



OH 



CHoOH 



C O 



H 



OH H 



H 



P 

 OH 



C- 



-C 



CH2OH 



L-glucose 



CH2OH 



D-glucose 



CH2OH 



a-D-glucose 



H 



OH 



a-D-glucose 



