IJZ FINE-STRUCTURE OF PROTOPLASM II 



framework acts as an anion, the order in which ions furtlier swelling 

 is as follows 



Li > Na > K > Rb > Cs. 



In an acid medium, however, in which the gel framework behaves 

 like a cation: 



Li < Na < K < Rb < Cs. 



Now one would expect that in the isoelectric, i.e., uncharged state, 

 the gel frame would show the same degree of swelling in all neutral 

 chlorides, since in that case no electrostatic attractive forces are 

 operating. This is not so, however; one finds so-called fransitionarj 

 series which are of special importance for biology : 



Li > Na > K < Rb < Cs. 



This result is not easily comprehensible after what has been said 

 before. For, if one plots the degree of swelling against the atomic 

 weight of the cations, one obtains a descending curve in alkaline 

 solutions (gel framework negative) and an ascending curve in acid 

 solutions (gel frame positively charged; Fig. loia). For this reason 

 one would expect a horizontal hne if the pjj of the swelling medium 

 has been adjusted to the isoelectric point (L E. P.) of the protein. 

 However, the experiment yields a minimum curve in which K holds 

 a special place. 



By using ion models, however, it is possible to understand these 

 relations, too. It follows from the ion mobilities that the two ions in 

 KCl are of equal size. For this reason they are adsorbed in the same 

 way by an isoelectric framework. In LiCl and NaCl, however, Li and 

 Na are adsorbed to a smaller extent than CI because of their large 

 hydration layer. Consequently, the molecular framework assumes a 

 weakly negative charge and is more strongly hydrated than in KCL 

 Conversely, in RbCl and CsCl the cations are more easily adsorbed 

 than the CI ions, which again results in a weak electric charge of the 

 gel, accompanied by increased hydration (Fig. loi b). 



Since the isoelectric point of cytoplasm usually lies in the weakly 

 acid region, cations have a discharging effect on it. As a rule, therefore, 

 cations cause less swelling than water. Anions, on the contrary, in- 

 crease hydration as a result of their similar electric charge, so that 

 cytoplasm often swells considerably (cap-plasmolysis, p. 197) in par- 



