THE DISACCHARIDES 373 



H 



H— C— OH 



I 



HO— C — H 



I 

 H— C —OH 



I 

 H— C —OH 



1 

 H— C-OH 



i 

 H 



Fructose is levorotatory (hence the name "levulose"), its specific rotatory 

 power at 20° C. being — 92.5°. It is especially abundant in many fruits in 

 which it often exceeds the amount of either glucose or sucrose present. Fruc- 

 tose may be one of the simple sugars produced directly in the process of 

 photosynthesis. Fructose can probably be oxidized in respiration, and un- 

 doubtedly is readily translocated from cell to cell in plants. Condensation 

 of fructose molecules produces inulin, an important storage carbohydrate in 

 some species of plants. Fructose is also one of the hydrolytic products of 

 sucrose and of several of the tri- and tetrasaccharides. 



The Disaccharides. — The three most important disaccharides found in 

 plants are sucrose ("cane sugar"), maltose and cellobiose. 



Sucrose is produced by the condensation of equimolar quantities of glu- 

 cose and fructose (Chap. XX). Under certain conditions digestion of 

 sucrose to glucose and fructose occurs under the influence of the enzyme 

 sucrase (Chap. XXVII). IMost plant tissues contain larger quantities of 

 sucrose than any of the other sugars. In sugar cane stalks from 12 to 20 

 per cent of the fresh weight is sucrose ; in the roots of the sugar beet, it repre- 

 sents from 15 to 20 per cent of the fresh weight. Considerable quantities 

 of sucrose are also found in the sap of the sugar maple and in certain of the 

 sorghums. 



The probable structural formula of sucrose is as follows: 



CHOH CHOH /CHOH CHOH 



CH-CHaOH 

 CH2OH 

 The right hand portion of the sucrose molecule may be regarded as de- 



