THE CARBOHYDRATES 21 



Dextrin is the name given to the intermediate products in the 

 hydrolysis of starch, and two chief varieties are distinguished 

 erythro-dextrin, which gives a reddish-brown colour with iodine solu- 

 tion ; and achroo- dextrin, which does not. 



It is readily soluble in water, but insoluble in alcohol and ether. 

 It is gummy and amorphous. It does not give Trommer's test, nor 

 does it ferment with yeast. It is dextro-rotatory. By hydrating 

 agencies it is converted into glucose. 



Glycogen, or animal starch, is found in liver, muscle, colourless 

 blood corpuscles and other tissues. 



Glycogen is a white tasteless powder, soluble in water, but it 

 forms, like starch, an opalescent solution. It is insoluble in alcohol 

 and ether. It is dextro-rotatory. With Trommer's test it gives a 

 blue solution, but no reduction occurs on boiling. 



With iodine solution it gives a reddish or port-wine colour, very 

 similar to that given by erythro-dextrin. Dextrin may be distin- 

 guished from glycogen by (1) the fact that it gives a clear, not an 

 opalescent, solution with water ; and (2) it is not precipitated by basic 

 lead acetate as glycogen is. It is, however, precipitated by basic lead 

 acetate and ammonia. (3) Glycogen is precipitated by 55 per cent, 

 of alcohol ; the dextrins require 85 per cent, or more. 



Cellulose. This is the colourless material of which the cell-walls 

 and woody fibres of plants are composed. By treatment with 

 strong mineral acids, it is like starch, converted into glucose, but 

 with much greater difficulty. The various digestive ferments have 

 little or no action on cellulose ; hence the necessity of boiling starch 

 before it is taken as food. Boiling bursts the cellulose envelope of 

 the starch grains, and so allows the digestive juice to get at the 

 starch proper. Cellulose is found in a few animals, as in the test or 

 outer investment of the Tunicates. 



Salting out of the Colloid Carbohydrates. By saturating solutions of 

 the colloid carbohydrates (starch, soluble starch, glycogen, and some varieties 

 of dextrin) with such neutral salts as magnesium sulphate or ammonium 

 sulphate the carbohydrate is thrown out of solution in the form of a white 

 precipitate. The remaining carbohydrates (sugars and some of the smaller 

 moleculed dextrins like achroo -dextrin) are not precipitated by this means. 

 We shall find in connection with the proteins that this method, known as 

 ' salting out,' is one largely employed there for precipitating and distinguish- 

 ing between classes of proteins. The student is therefore warned that a 

 precipitate obtained under such circumstances will not necessarily indicate 

 the presence of protein. 



Further information regarding the carbohydrates is given in Lesson XIII, 



