ELEMENTARY PHYSIOLOGICAL CHEMISTRY 167 



1. The Starches. These include ordinary starch and glycogen 

 (C 6 H 10 6 ) W . Starch is the most widely distributed carbohydrate in the 

 vegetable kingdom, for it is in this form that plants store up their excess 

 of nutriment. We store ours chiefly as fat. In plants, dextrose is 

 formed in the leaf by the chemical energy of the sun's rays acting on 

 the crude chemical substances absorbed by the roots. If the amount 

 of dextrose thus produced be in excess of the present needs of the 

 plant, it is stored up as starch. These starch grains may be 

 seen in various parts of the plant. They show under the 

 microscope concentric markings, produced by alternating layers of 

 starch granulose and starch cellulose. The former of these is more 

 easily hydrolysed than the latter, so that intact starch granules are less 

 easily hydrolysed than those which have been ruptured by boiling or 

 some such agency. 



The exact shape of starch grains varies according to the plant from 

 which they are obtained. In this connection they may be divided into 

 two groups : (1) a group in which their contour is even, such as wheat, 

 barley, arrowroot, potato ; (2) a group in which the contour is marked 

 by facets, either completely as in oats and rice, or only partially so, 

 as in tapioca and sago. 



EXPERIMENT IX. Examine some grains of wheat, a scraping of 

 potato, and some ground rice under the microscope. To do this, mix 

 the flour, etc., with a drop of water on a slide. 



Starch, like most other polysaccharides, is insoluble in cold water, 

 but it swells up in hot water, an opalescent solution being formed. 

 This is riot a true solution as only a trace is actually dissolved, the rest 

 being swollen up into transparent clumps which run together. 



EXPERIMENT X. Place some powdered starch in a test tube, and 

 half fill up with cold water no solution occurs now boil, when an 

 opalescent solution will be produced, and, if of sufficient concentration, 

 this will gelatinise on cooling. Try Trommer's test with this solution 

 no reduction occurs. By boiling, the cellulose layers of the granule 

 are ruptured, the granulose being liberated. 



EXPERIMENT XI. Place some of the solution in the mouth, and after 

 a minute or so transfer it again to the test tube : now apply Trommer's 

 test reduction occurs. 



Try the same experiment with some unboiled starch, and note that 

 with Trommer's test no reduction is effected (i.e. the resistent cellulose 

 layers have not been hydrolysed). 



The standard test for starch is with iodine solution. 



EXPERIMENT XII. To an opalescent solution of starch add a drop 



