CARBOHYDRATES 



IOI 



Dextrins are a series of intermediate poly-saccharides between the di- 

 saccharides and the starches. They are non-crystalline, are soluble in 

 water, and are precipitated on the addition of alcohol in excess. They are 

 dextrorotatory and are not fermented by yeast. Their power to reduce 

 alkaline copper solution has been questioned, but they yield osazones which 

 are relatively soluble. They have a slightly sweet taste. The more com- 

 plex dextrins give a red reaction with iodine. 



Starch is found in various parts of plants, especially in the tubers and 

 seeds. It is a form of storage carbohydrate, and serves as a source of ma- 

 terial for the development of the young plant. Starch is obtained commer- 

 cially from potatoes, rice, corn, wheat, sago, and the like. It constitutes the 

 greater proportion of our food. 



Starch as obtained is a soft white powder which on microscopic examina- 

 tion is found to consist of small granules. These are often characteristic in 

 shape and size according to the origin of the material. The granules appear 

 to be built up of concentric layers of two varieties of starch. The more sol- 

 uble form, known as amylose, is ensheathed by the less soluble variety of 

 starch, the amylopectin. This sheath is ruptured by boiling the aqueous sus- 

 pension of starch granules and an opalescent solution or starch paste is 

 obtained. On standing in dilute solution, the opalescent material settles to 

 the bottom, but the clear fluid above still gives the blue reaction with iodine. 

 This color is characteristic for starch; it disappears on heating, but returns 

 when the liquid cools. Starch will not diffuse through a semi-permeable 

 membrane. On boiling with dilute mineral acids starch is hydrolyzed to dex- 

 trose. The dextrins and maltose are formed as intermediate products. 

 With amylolytic enzymes, the change practically only goes as far as the 

 maltose stage. 



Glycogen (animal starch) is the reserve form of carbohydrates in 

 animals. It is synthesized from dextrose and can again be hydrolyzed to 

 dextrose for transportation or for oxidation to yield energy to the tissues. 

 The following table shows the per cent, and distribution of glycogen in the 

 various tissues of the dog (Schondorff) : 



