64 PHYSICOCHEMICAL BASIS OP PHYSIOLOGICAL PROCESSES 



particle assumes may be neutralized by a charge of opposite sign car- 

 ried by an ion present in the dispersion medium. The neutralization 

 of the electric charge causes coagulation of the suspensoids but not of 

 the emulsoids. Of the positive and negative ions into which the elec- 

 trolytes dissociate, the one producing the coagulation is that which is 

 opposite in sign to the electric charge of the colloidal particle.- 



A quantity of electrolyte which is capable of producing complete pre- 

 cipitation when added all at once to suspensoids will be ineffective when 

 added in small quantities at a time. This phenomenon, which is also 

 known to be exhibited when toxins and antitoxins are mixed together, is 

 probably owing to the fact that precipitation depends on inequality and 

 irregular distribution of electric charges, a condition which becomes 

 established when the electrolyte is suddenly added, -but not so when it 

 is gradually added. The particles in the latter case become, as it were, 

 acclimated to the electric charges introduced by the addition of the 

 electrolyte. 



Proteins as Colloids. The most prominent colloids in the field of bio- 

 chemistry are the proteins. On account of complexity of structure, 

 however, certain factors intervene which render the investigation of 

 their behavior very difficult. As we shall see later, proteins are made 

 up of combinations of amino acids, each of which contains basic (NH 2 ) 

 and acid groups (COOH). The various amino acids are linked together 

 in protein by the COOH of one uniting with the NH 2 of another, with 

 the elimination of water thus, CO ! OH + H ! HN but some NH 2 and 

 COOH groups are left uncombined. According to the relative number 

 of these uncombined radicles, the protein (or polypeptid, see page 636) 

 will exhibit faintly acid or basic or neutral properties. With acids, for 

 example, a salt will be formed by union with the NH 2 groups, which will 

 dissociate into the anion of the acid and a large organic cation; whereas 

 with alkalies union will occur with the COOH group, and the salt on 

 dissociating will form a small cation of the metal of the salt and a large 

 complex anion. We may therefore obtain the protein with either a 

 positive or a negative electric charge by altering the chemical nature of 

 the fluid in which it is dissolved, so that the reaction towards other 

 colloids and towards electrolytes will vary. 



One feature of proteins of importance in this connection is that known 

 as the isoelectric point, at which the protein exists with a maximum of 

 electrically neutral molecules. This point is reached by adding acid to 

 a protein solution. The acid represses the dissociation of the protein 

 acting as an acid, and therefore diminishes the number of free hydrogen 

 ions; and at the same time it combines with the NH 2 groups and neutral- 



