CARBOHYDRATES 61 



These last two systems of classification are ofteti combined to specify 

 both the number of oxygen atoms and the type of carbonyl group in- 

 volved. Thus the above aldoses are aldotrioses and have in this case 

 the common name glyceraldehyde. Dihydroxyacetone by this nomen- 

 clature is a ketotriose, the only such sugar possible in this instance. 

 By analogy, glucose is an aldohexose, and so on for the other monosac- 

 charides. 



A few sugars are methyl derivatives; these are simply called 

 methylaldopentose and the like. In one important natural five-carbon 

 sugar, deoxyribose, the second carbon is missing the oxygen atom. 

 This exceptional compound is usually included among the pentoses, 

 nevertheless. Sugars with amino groups are considered as a separate 

 class at present regardless of their relationships with the other carbo- 

 hydrates. 



To summarize, the most common classification in use groups the 

 carbohydrates into mono-, di-, tri-, and polysaccharides. These classes 

 are then divided further, the monosaccharides according to both the 

 number of oxygen atoms and the type of carbonyl group, the disac- 

 charides into reducing and non-reducing sugars, and the polysac- 

 charides according to the sizes of the monosaccharide subunits. For 

 this last classification step the polysaccharides made up of pentoses 

 are pentosans, those made up of hexoses are hexosans. 



IMPORTANT CARBOHYDRATES 



The number of theoretically possible carbohydrates is astronomical. 

 Fortunately for the chemist and biochemist the number occurring 

 naturally seems to be limited, and these are of the greatest interest, 

 although some others have been synthesized. Even of the known nat- 

 ural carbohydrates, space dictates that only a few can be discussed in- 

 dividually. The choice is quite arbitrary, so the literature must be 

 consulted for anything like a comprehensive treatment of the subject. 

 Moreover, a number of derivatives of carbohydrates play important 

 biochemical roles. Some of these are discussed in later chapters. 



L-Arabinose 



This important aldopentose is obtained on hydrolysis of a variety of 

 vegetable gums and occurs in legumes in small amounts and in urine 

 in certain pathological states. As is true of most other sugars, it is 

 used by a number of species as both an energy and a carbon source. 

 The optical isomer D-arabinose also occurs naturally in the form of a 



