CYTOPLASM 



155 



ticular with SCN, I, Br, but also with CI, which is held to be re- 

 sponsible for the swelling of the cytoplasm of haloph^nies (Stocker, 

 1928). In the series I,Br,Cl,F, chlorine often takes a similar optimal 

 place to potassium in the alkaline metals and Ca in the alkaline earths 

 (PiRSCHLE, 1930). 



Fig. loi. Change in swelling of a gel frame consisting of polypeptide chains 



under the influence of chlorides of the alkali series at various pn values ; 



Q = degree of swelling, a) Hofmeister's series; dotted line = behaviour 



expected at pn = I.E. P.; instead, one finds b) transitionary series. 



III. Heteropolar valency bonds. If all acid and basic groups in the 

 cytoplasm exactly cancel each other out, the isoelectric state (im- 

 properly called "isoelectric point" I.E. P.) is attained and nearly all 

 properties of protoplasm reach extreme values : the degree of swelling 

 becomes a minimum, the danger of setting a maximum ; the stability 

 is low, the electric charge and the migration in an electric field become 

 zero by definition, etc. 



If then positive and negative end groups of the side chains occupy 

 suitable relative positions, they can enter into salt-like bonds (Fig. 

 96 III, p. 145). Their electric charges are neutralized and the hydration 

 of the region in question is reduced to a minimum. The salt bonds 

 cannot be broken as readily by neutral salts as the heteropolar cohesive 

 bonds. Something more drastic is required, viz., the concentration of 

 the H ions (pn), must be changed. Some of the intermolecular salt 

 bridges are then hydrolyzed and a certain number of the bound 

 carboxyl and amino groups become free. If hydrolysis is achieved by 

 H ions, i.e., if the p^ of the liquid in which the cytoplasm is examined 

 drops below the I.E. P., the dissociation of the free COOH groups 

 is diminished, that of the amino groups (-NH3OH) is increased. Thus 

 the cytoplasm acquires a positive electric charge and behaves like a 

 complex cation. Conversely, if the p^ of the medium is greater than 

 the I.E. P. of the cytoplasm, the dissociation of the COOH groups is 



