CARBOHYDRATES 



53 



This constitutes what was termed a single "sugar" group 

 of atoms, hence, monosaccharide. It is found free in nature 



in all parts of the plant, but for the most part it occurs 

 associated with an equal quantity of levulose in such sweet 

 fruits as grapes, cherries, and pears. It is also found in 

 onions, as has been mentioned. Dextrose occurs as gluco- 

 sides 1 in combination with different kinds of compounds 

 such as alcohols, acids, and aldehydes, which hydrolyze 

 naturally under the action of enzymes to form glucose and 

 the other compounds. It is formed in nature (Section 23) 

 by the condensation of formaldehyde in the leaf, and by 

 hydrolytic enzyme action on such storage forms of carbo- 

 hydrates as starch and sucrose (Section 24). 



Physiologically dextrose is the usual transport form of 

 carbohydrates, but occasionally it is the storage form, as in 

 the onion. Dextrose is a nearly white solid, easily soluble 

 in water and in hot alcohol, insoluble in ether, crystallizes 

 as a hydrate, CeHioOe.H-iO, from water, and as an anhydride 

 from alcohol (Fig. 11). It is not as sweet as ordinary sugar. 

 It is dextrorotatory, from which fact it gets the name dex- 

 trose, "right sugar." The specific rotation of ordinary 

 dextrose is +52.5°. Since it contains the aldehyde group 

 it is easily oxidized to various compounds. This oxidation is 

 measured by the equivalent reduction of what is called 



1 The formation of a glucoside can best be seen from a graphic formula : 



H H 



I I 



H— C— O— H H— C— O— H 



I I 



H— C— O— H H— C— O— H 



-O— R = H— C— O 



H— C— O- I H + H 

 H— C— O— H 

 H— C— O— H 



H— C I =0 



.J 



+ H.0 



O— K 



R represents the alkyl group of an alcohol, acid, aldehyde, or ketone. 



