ELEMENTARY CHEMICAL PHYSIOLOGY 219 



Glycogen (C fl H 10 O 5 ) M . Just as plants store up excess of carbo- 

 hydrate in the form of starch, so do animals store it partly in the 

 form of glycogen. The chief seats of this storage are the liver and 

 muscles. Glycogen forms an amorphous white powder. It is 

 soluble in water and the solution is opalescent. Solutions of 

 glycogen are dextro-rotatory and are precipitated by basic lead 

 acetate solution. 



EXPERIMENT VI. A simple method for the preparation of glycogen is 

 that introduced by Frankel. It consists in grinding up fresh liver or common 

 shell-fish, mussel or oyster in a mortar with about three times its volume of a 

 3 per cent, solution of tri-chloracetic acid. This reagent coagulates the 

 proteins. The glycogen is contained in the extract, and can be precipitated 

 by alcohol. 1 After collecting on a filter dissolve some of the glycogen in 

 water and notice that the solution is opalescent. Add to this a drop or two 

 of iodine solution : a port-wine colour results, which disappears on heating, 

 and returns on cooling. 



EXPERIMENT VII. Place 5 c.c. of glycogen solution in a test tube and add 

 ordinary alcohol carefully until a precipitate forms. Note approximately how 

 much alcohol requires to be added to obtain this (about 55 per cent.). 



EXPERIMENT VIII. Try Trommer's test with the glycogen solution ; no 

 reduction occurs, but the Cu(OH) 2 is held in solution. 



EXPERIMENT IX. To some of the glycogen solution add a few drops of 25 

 per cent. H 2 SO 4 and boil for about ten minutes ; dextrose is produced, as 

 can be shown by applying one of the reduction tests. 



EXPERIMENT X. Mix some glycogen solution with saliva and place the 

 test tube in water at body temperature. After about ten minutes apply one 

 of the reduction tests. It will be found that a reducing sugar has been 

 produced. 



The Dextrines (C 6 H 10 5 ) M . During the hydrolysis of starch and 

 glycogen dextrines are formed as intermediate products. 



There are several varieties of dextrine, amylodextrine, erythro- 

 dextrine and achroodextrine, which differ from one another in 

 molecular weight, colour reaction with iodine, etc. 



Dextrine is an amorphous powder, soluble in cold water forming 

 a clear solution and is not precipitated by basic lead acetate. 



EXPERIMENT XI. Add some iodine solution : a brownish red 

 colour, like that obtained with glycogen, results if erythrodextrine 

 be present. The colour disappears on heating and reappears on 

 cooling. The bluish violet tint frequently obtained is due to the 

 presence of starch. 



EXPERIMENT XII. Test reducing power of a solution of dextrine 

 before and after hydrolysis with acid. 



Pentoses. Besides the hexoses, animal tissues also contain small amounts 

 of pentoses, that is, sugars containing five carbon atoms, C 5 H 10 O 5 . Being 

 aldehydic in nature, they possess reducing powers and form osazone crystals. 

 They do not ferment with pure yeast, but they all rotate the plane of polarised 

 light. In the animal tissues pentoses do not exist in a free state, being, as 

 far as is known, bound to guanylic acid. They are very plentiful in plants, 

 where they exist as polysaccharides called pentosans. Thus, in gum arabic 

 there is a pentosan which yields Z-arabinose when hydrolysed by heating 

 with mineral acid, and in wood or bran another pentosane yields Z-xylose on 



1 Where not otherwise specified in these experiments, alcohol refers to the 

 commercial product containing from 92 to 96 per cent, pure alcohol. 



