PENTOSES. 207 



is known as to how this cleavage occurs. According to BUCHNER and 

 MEISSENHEIMER l the fermentation with the enzymes contained in the 

 bacteria produces chiefly the racemic, inactive form of the acid. This 

 also occurs as a rule by the action of living bacteria. In reference to the 

 formation of lactic acid within the organism see Chapter X. 



The monosaccharides are colorless and odorless bodies, neutral in 

 reaction, with a sweet taste, readily soluble in water, generally soluble 

 with difficulty in absolute alcohol, and insoluble in ether. Some of them 

 crystallize well in the pure state. They are strong reducing substances. 

 They reduce metallic silver from ammoniacal silver solutions and they 

 also reduce other metallic oxides such as copper, bismuth and mercury 

 oxides, on heating in alkaline solution. This behavior is of great 

 importance in the detection and quantitative estimation of the sugars. 



The simple varieties of sugar occur in part in nature as such, already 

 formed, which is the case with both of the very important sugars, glucose 

 and fructose. They also occur in great abundance in nature as more 

 complex carbohydrates (di- and polysaccharides) ; also as ester-like 

 combinations with different substances, as so-called glucosides. 



Among the groups of monosaccharides known at the present time, 

 those containing less than five and more than six carbon atoms in the 

 molecule have no great importance in biochemistry, although they are 

 of high scientific interest. Of the two groups the hexoses are the more 

 abundant and are of special interest. The pentoses are becoming of 

 greater importance, not only for the chemistry of plants, but also for 

 the chemical processes in the animal body. 



Pentoses (C 5 Hi 5 ). 



As a rule the pentoses do not occur as such in nature. They are 

 obtained from animal tissues, organs and fluids as cleavage products 

 of the nucleic acids, or nucleoproteins. The pentoses are chiefly obtained 

 from the plant kingdom, where nucleic acids also occur, by the hydro- 

 lytic cleavage with dilute mineral acids, of more complex carbohydrates, 

 the so-called pentosans. The pentosans exist very widely distributed 

 in the plant kingdom, and are of especially great importance in the build- 

 ing up of certain plant constituents. Methyl pentosans and methyl 

 pentoses also occur in the plants, and of these, the methyl pentose, rham- 

 nose, which occurs in several glucosides, must be specially mentioned. 



The pentoses were first found in the animal kingdom by SALKOWSKI 

 and JASTROWITZ in the urine of a person addicted to the morphine habit, 



1 Ann. d. Chem. u. Pharm., 349, 125 (1906). 



