76 PHYSIOLOGICAL CHEMISTRY 



animals, they are important constituents of the supporting 

 framework or protective covering. The members of this group 

 are made up of several molecules of monosaccharide united with 

 loss of water to form the larger polysaccharide molecule. The 

 commoner individual polysaccharides yield only one kind of 

 monosaccharide when hydrolized, thus differing from the other 

 material bases which, on hydrolysis yield varying kinds of sim- 

 pler units. Since the number of monosaccharide molecules which 

 make up a polysaccharide molecule is unknown, it is customary 

 to express the formula with the indefinite coefficient n. 



Starch (C 6 H 10 5 ) n . Starch is a plant product, and is found 

 stored in leaves, seeds, fruits, tubers, etc. Grains contain as 

 much as 50-70% of their dry weight; potatoes contain from 

 15-30% of their wet weight. Starch forms granules of more or 

 less characteristic shapes which are serviceable in determining 

 the source of the starch. Thus potato starch appears as egg- 

 shaped granules which often show concentric lines. Starch is 

 prepared from potatoes or grain by grinding the material, filter- 

 ing through sieves to remove the coarse debris and allowing the 

 suspended starch particles to settle. It forms a white amorphous 

 powder which does not dissolve in cold water. If boiled with 

 water, the granules are broken open and the starch forms an 

 opalescent solution. Starch is insoluble in alcohol and ether. 

 A starch solution rotates the plane of polarized light to the 

 right. It will not reduce Fehling's, or similar solutions, and 

 will not ferment. The characteristic starch test is the formation 

 of a blue color on the addition of a few drops of iodine solu- 

 tion. This color disappears on heating, but reappears if the 

 solution is cooled. Alkali and alcohol also destroy the blue 

 color. On boiling with dilute acids starch is broken down to 

 glucose. The starch passes through various intermediate stages, 

 the nature of which may be followed by the iodine test. If 

 portions of the hydrolizing mixture are tested with iodine from 

 time to time, the blue color soon gives place to red. This cor- 

 responds to the stage known as erythrodextrin (from the Greek 

 word meaning "red")- On further hydrolysis, the iodine test 



