THE CARBOHYDRATES 



61 



particular form which are assimilable, and therefore of physiological im- 

 portance. The small differences in the configuration of .the four d-sugars 

 can be readily seen if their graphic formulse be compared : 



CHO 

 I 

 H.C.OH 



. I 

 HO.C.H 



I 

 H.C.OH 



I 

 H.C.OH 



I 

 CH 2 OH 



d-glucose 



CHO 



I 

 HO.C.H 



I 

 HO.C.H 



I 

 H.C.OH 



H.C.OH 



I 

 CH 2 OH 



d-mannose 



CH,OH 



CO 



I 

 HO.C.H 



I 



CHO 



H.C.OH 



HO.C.H 





THE PENTOSES. C 6 H 10 5 x 



These bodies occur largely in plants in the form of complex polysaccharides, the 

 pentosanes, which give pentoses on hydrolysis with acids. Two forms of pentose 

 have been found in the animal body, namely, i-arabinose, which has been isolated 

 from the urine in cases of pentosuria, and 1- xylose (or d-ribose, Levene), which occurs 

 built up into the nucleic acid molecule of the pancreas and perhaps other organs. The 

 pentoses can apparently be utilised by herbivora as food-stuffs. We know nothing as 

 to the part they play in the animal body or as to the causation of the rare condition of 

 pentosuria. Since however they are reducing substances and the presence of pentose 

 in urine might therefore lead to a suspicion of diabetes, it is necessary to mention the 

 tests by which the presence of pentoses may be detected. The two following are the 

 chief tests for pentoses : 



(1) The solution supposed to contain a pentose is mixed with an equal volume of 

 concentrated hydrochloric acid. To the mixture is added a small quantity of solid 

 orcin and the whole is heated. If pentose is present the solution becomes at first 

 reddish-blue and later bluish-green. The colour can be extracted on shaking the 

 fluid with amyl alcohol, the solution, on spectroscopic examination, showing an absorp- 

 tion band between C and D. 



(2) Instead of adding orcin, we may add phloroglucin to the mixture of hydrochloric 

 acid and pentose. The solution on heating becomes first cherry red and then cloudy. 

 On shaking with amyl alcohol a red solution is obtained which shows a band between 

 D and E. 



THE HEXOSES AND THEIR DERIVATIVES 



The most important of the carbohydrates belong to this class and are 

 either hexoses or formed by a combination of two or more hexose molecules. 

 They are divided into three main groups : 



(1) Monosaccharides, with the formula C 6 Hi 2 6 , examples of which ?,re 

 glucose, fructose, &c. 



(2) Disaccharides, which are derived from two molecules of a monosac- 

 charide with the elimination of a molecule of water, as follows : 



2C 6 H 12 6 -H 2 0=C 12 H 32 O n . 

 (Examples, maltose and cane sugar.) 



