202 



FINE-STRUCTURE OF PROTOPLASM 



II 



account only the ultrafilter action (Ullrich, 1936b); yet lipid solu- 

 bility is also included, if one realizes that the molecular framework, 

 especially in its outer regions, contains lipids and phosphatide mole- 

 cules which are located within the meshes. Wilbrandt (1935) there- 

 fore rightly remarks that no sharp 

 distinction can be made between the 

 effects of filter action and solubility. 



A colloid framework in the form of 

 a polyvalent immobile ion, which is in 

 contact with a true solution, represents 

 a DoNNAN system, even though no 

 semi-permeable wall is present. For, as 

 required for a Donnan equilibrium, the 

 migration of the colloid framework into 

 the surrounding solution is impossible, 

 whereas its mobile ions can move freely 

 (Fig. 117). This consideration makes a 

 theory of selective permeability possible. 

 Suppose an anionic, molecular frame- 

 work R in the form of a potassium salt 

 KR is in contact with a KCl-solution. 

 Let A be the number of dissociation 

 points of the framework anion, i.e., the concentration of the potassium 

 capable of dissociation, y the concentration of the KCl penetrated 

 into the meshes of the framework, and c the KCl-concentration of 

 the outer solution. Then the ion product [K] • [CI] equals (y + A)y 

 inside, and c^ outside the framework. Accordingly, one obtains 

 Donnan's law^: (y + A)y = c^. 



Donnan's exchange mechanism therefore applies to our framework 

 structures, since the immobile anion R expels the mobile anion CI 

 from the meshes of the framework. As follows from Table XXIII, 

 the CI concentration, y, in the framework decreases rapidly with in- 

 creasing A. Thus, in order to establish Donnan equilibria in the cyto- 

 plasm, no semi-permeable membranes are required: the plasma gel as a 

 whole acts as a gigantic, immobile and polyvalent colloid ion. 



1 Usually the equilibrium is formulated in a more complicated way (Hober, 1922, p. 219) : 

 (KCl — y)/y = (KR + KC1)/KC1. In this less convenient form KCl = c -f- y and KR = 

 A, which gives the above formula. 



Fig. 117. Donnan equilibrium be- 

 tween a molecular framework R 

 with anionic dissociative groups 

 (A) and a solution of KCl ; (c) and 

 (y) are the outer and inner equili- 

 brium concentrations. 



