CARBOHYDRATES 



67 



Starch is the major reserve substance of higher plants and is pro- 

 duced during photosynthesis. It is stored throughout the plant and 

 occasionally concentrated in roots, tubers, and seeds. Starch often 

 occurs in grains whose size and appearance are characteristic of the 

 plant source. These grains may burst on heating to yield a colloidal 

 solution or gel, depending on the concentration. The starch mole- 

 cules themselves are large and of two types differing enough so that 

 starch may be fractionated. One fraction called amylose is composed 

 of several hundred a-u-glucose subiuiits connected at the one and four 

 positions to form a long thread-like molecule without branches. Amy- 

 lose is readily dispersed in water and gives a typical blue color with 

 iodine but does not form the characteristic gel or starch paste. 



The other fraction of starch is amylopectin, which gives a purplish 

 color with iodine and is difficult to disperse in water. However, when 

 dispersed in sufficient concentrations, the amylopectin forms the typi- 

 cal starch gel. As in amylose, the compound is made up only of 

 glucose with most of the linkages connecting one and four positions. 

 However, amylopectin has a branched structure with the branches at- 

 tached through 1-6 links. The naturally occurring molecules seem to 

 differ in size but range up to perhaps 50,000 glucose subunits. 



Both amylose and amylopectin are hydrolyzed in acid solution to 

 D-glucose. If hydrolysis is interrupted early, large fragments are 

 formed called dextrins. These materials possess adhesive properties 

 and are used as adhesives on paper products. Starch breaks down into 

 dextrins on heating, especially in the presence of moisture. The re- 

 sulting dextrins then confer stiffness to clothes that have been starched 

 and ironed. Enzymes called amylases catalyze the hydrolysis of starch 

 to high yields of maltose, revealing the structural similarity between 

 these two carbohydrates. The structure of amylopectin is represented 

 below in schematic fashion with the ends of the structure shown con- 

 nected to systems of the same type. Amylose resembles amylopectin 

 in all respects except for the branches which are missing. Hence, amy- 



CH2OH 



amylopectin 



CH.,OH 



