CARBOHYDRATES 45 



The preceding technical discussion of the chemical consti- 

 tution and reactions of the hexoses has been presented, not because 

 it has any direct connection with the occurrence or functions of 

 these compounds in plant tissues, but for the purpose of giving to 

 the student a graphic conception of the structure and properties 

 of these simple carbohydrates, as a basis for the understanding of 

 the nature, properties, possible chemical reactions, syntheses, 

 etc., of the more complex types of carbohydrates, which, along 

 with these simple monosaccharides, constitute the most important 

 single group of organic components of plants. 



THE OCCURRENCE AND PROPERTIES OF MONOSACCHARIDES 



Only two monosaccharides occur as such in plants. These are 

 glucose and fructose. All the other hexoses, whose structure is 

 shown on pages 37 and 38, occur in plants only as constituents of 

 the more complex saccharides, in glucoside-formations, or as 

 the corresponding polyatomic alcohols. 



The aldo-hexoses which occur most commonly in plants, either 

 free or in combination, are d-glucose, d-mannose, and d-galactose; 

 while d-fructose and d-sorbose are the common keto-hexoses. 



Glucose (often called also dextrose, fruit sugar, or grape sugar) 

 occurs widely distributed in plants, most commonly in the juices of 

 ripening fruits, where it is usually associated with fructose and 

 sucrose, the two hexoses being easily derived from sucrose by 

 hydrolysis. Glucose is also produced by the hydrolysis of many 

 of the more complex carbohydrates, by the action either of enzymes 

 or of dilute acids; lactose, maltose, raffinose, starch, and cellulose, 

 as well as many glucosides all yielding glucose as one of the products 

 of their hydrolysis. In all such cases, it is d-glucose which is 

 obtained. 



Glucose is a crystalline solid (although it does not form such 

 sharply denned crystals as does sucrose, or " granulated sugar"), 

 which is easily soluble in water. It usually appears on the market 

 in the form of thick syrups, which are produced commercially by 

 the hydrolysis of starch with dilute sulfuric acid, removal of the 

 acid after the hydrolysis is complete, and evaporation of the 

 resulting solution to the desired syrupy consistency. (Since 

 corn starch is commonly used as the raw material for this process, 

 these syrups are often spoken of as " corn syrup.") The sweet- 

 ness of glucose is about three-fifths that of ordinary cane sugar. 



