THE COMPONENTS OF PLANTS AND ANIMALS 9 



It occurs almost universally in the juices of plants along with 

 levulose and cane sugar, and is found also in small amounts in 

 the blood of mammals. Sixteen isomers of this compound 

 are possible, twelve of which are known. 



Galactose and mannose are isomers of dextrose, occurring in 

 nature only in combination as di- or polysaccharids. 



Levulose, or fruit sugar, is a ketose of sorbite, having the 

 formula CH 2 OH-(CH OH) 3 -CO-CH 2 OH, eight isomers 

 being theoretically possible. It occurs mixed with dextrose in 

 plant juices and in honey. 



The hexose monosaccharids are all soluble in water and 

 readily diffusible and have a more or less sweet taste. All 

 those found in nature are optically active, rotating the plane 

 of polarized light. Thus dextrose, as its name implies, has a 

 right-handed rotation and levulose a left-handed rotation. 

 They reduce an alkaline solution of metallic salts, especially of 

 copper, and this fact is utilized both as a qualitative test for 

 them and as a means of quantitative determination. They are 

 fermented by yeast, yielding as the chief products ethyl alcohol 

 and carbon dioxid. 



10. Pentoses. The pentoses are simple sugars, correspond- 

 ing to the hexoses but having the general formula C 5 Hi O5. 

 Those occurring in nature are aldoses. Like the hexoses, they 

 reduce metallic oxids, but unlike them they are not ferment- 

 able by yeast. 



Arabinose. By the hydrolysis of gum-arabic or cherry gum, 

 there is produced dextro-rotatory arabinose (/-arabinose). Levo- 

 rotatory arabinose (^-arabinose) has been prepared artificially. The 

 inactive or racemic form (-arabinose) has been found in human 

 urine in small amounts. 



Xylose. By the hydrolysis of wood gum there is produced a 

 dextro-rotatory pentose known as /-xylose. The levo-rotatory form 

 of the same sugar (d-xylose) is obtained in the hydrolysis of certain 

 nucleo-proteins, the pentose group seeming to be a constituent of 

 the molecule of those compounds. 



Rhamnose is a derivative of the pentose sugars in which an atom 

 of hydrogen has been replaced by methyl. It occurs somewhat 

 widely in the vegetable kingdom. 



