288 PRACTICAL PHYSIOLOGY 



CHAPTER III. 

 CARBOHYDRATES CONTINUED. 



III. POLYSACCHARIDES. 



A POLYS ACCHARIDE is the condensation product of more than two 

 monosaccharide molecules, and has accordingly the general formula, 

 (C 6 H 10 O 5 ) n , where n stands for a variable number. 1 Polysaccharides can 

 be hydrolysed, in which process they yield, first of all, polysaccharides 

 (dextrines) of lower molecular weight (i.e. with n of less value), then 

 disaccharides and, finally, monosaccharides. 



Thus, when acted on by diastatic ferments, dextrines (polysacchar- 

 ides of lower molecular weight) arid maltose (disaccharide) are formed. 

 When boiled with acid, on the other hand, the hydrolytic cleavage 

 goes further and, although dextrine and maltose occur as intermediary 

 products, yet the final product is monosaccharide. 



The most important members of this group are the starches, the 

 dextrines, glycogen, the celluloses, and the gums. They are very 

 widely distributed in vegetables, and constitute a most important class 

 of food-stuffs. 



General Characters. They do not form crystals, nor, with few 

 exceptions, are they soluble in cold water. Few possess any sweet 

 taste. As a rule they do not diffuse through parchment and are 

 therefore colloids. Their solutions are optically active. They do not 

 reduce metallic oxides in alkaline solution, they do not form osazones 

 and they cannot be fermented with yeast. Like other colloids, they 

 are precipitated when their solutions are saturated with certain neutral 

 salts, such as ammonium sulphate. They may be sub-divided into 

 three sub-groups, the starches, the dextrines and the celluloses. 



1. The Starches. These include ordinary starch and glycogen 

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

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

 their excess of carbohydrate. Animals store their excess of carbo- 

 hydrate partly as glycogen, but mainly as fat. If the amount of 

 dextrose produced in the leaves 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. In its minute structure the starch granule is 



1 It is impossible to give a definite value to n because the molecular weight is 

 unknown. The symbol n signifies that the formula within the brackets is to be 

 multiplied an indefinite number of times. 



