170 THE CARBOHYDRATES 



then acidified with acetic acid and dialysed, the last traces of 

 copper being removed electrolytically. The further purifica- 

 tion of the glycogen is effected by repeated alternate pre- 

 cipitation by alcohol and solution in water. 



According to these same authors, some of the glycogen of 

 the yeast cell occurs in the plasma and is readily extracted by 

 water, while another portion, which is less readily extracted by 

 water, is associated with the cell wall. The former modifica- 

 tion, which resembles the amylose constituent of the starch 

 granule, produces in water a faintly opalescent solution which 

 gives a pure red colour with iodine but no precipitate. The 

 other modification, associated with the cell wall, appears to be 

 a phosphoric ester of the form associated with the plasma and 

 is comparable with amylohemicellulose ; its solution in water 

 is opalescent and gives with iodine a reddish-brown precipitate. 



Properties. 



Pure glycogen is a snow-white amorphous solid. It is 

 readily soluble in hot water, forming an opalescent solution, 

 from which it may be precipitated again by alcohol, provided 

 small quantities of dissolved salts are present ; lOO c.c. of a 

 I per cent solution when mixed with 200 c.c. of absolute 

 alcohol remain clear, but on adding 003-005 gram of sodium 

 chloride, an immediate precipitate is formed. Glycogen is 

 strongly dextro-rotatory, a^ = + 198-9°, and is coloured red 

 to brown by iodine ; it does not reduce Fehling's solution, but 

 is broken down by diastase into dextrin and maltose, and by 

 acids into glucose. 



The fact that cold concentrated hydrochloric acid con- 

 verts glycogen into triamylose is considered by Pringsheim to 

 establish the identity of glycogen with amylopectin of starch. 



Identification. 



1. The opalescent appearance of its aqueous solution is 

 characteristic ; it is strongly dextro-rotatory. 



2. A brown coloration is given with iodine solution. 



3. A white precipitate is given with basic lead acetate in 

 strong solutions only. 



