C. ADRIAN M. HOGBEN 



199 



If a postulate is introduced that there exists within the complex membrane 

 a region or micelle whose pH is maintained distinctively different from that of 

 the external bathing solutions, and if the boundaries of that region differ ap- 

 preciably in their relative permeability to ionized and unionized particles 

 (fig. 3b), there will be a net flux of a weak electrolyte independent of the pH 

 of the external bathing solutions. Across the face of the micelle that is relatively 

 impermeable to ions, there will be net movement till a steady state concen- 

 tration difference is attained, as depicted in figure 3a. However, across the 



< > JU. < > 



.Hi 



Ac ds 



(b) 



Bases 



Fig. 3. Micellar model of weak electrolyte distribution, a: Osterhout's model of the steady 

 state distribution of a weak acid, pKa 4, across a cell membrane separating two solutions of 

 difTerent pH, the cell membrane being freely permeable to the unionized moiety, b: A trans- 

 port micelle whose interior pH differs from that of the external bulk solution and of whose 

 boundaries that on the left is freely permeable to unionized particles (light arrow) and that 

 on the right is equallj' permeable to both unionized and ionized (dark arrow) particles. The 

 movement and distribution across the left boundarj^ would correspond to the oil barrier of 

 a developing a concentration gradient while the free movement across the right boundary 

 would tend to dissipate the concentration dififerences developing on the left, such that weak 

 acids would be pumped from right to left and weak bases from left to right. 



opposite face of the micelle which does not discriminate between ionized and 

 unionized particles, there will be net movement tending to dissipate the con- 

 centration difference building up across the other face. The overall result will 

 be a continued pumping of weak electrolyte from one side of the epithelium to 

 the other and the energy for the uphill transfer will come from the maintenance 

 of the distinctive pH within the micelle. 



In two instances, carbonic and lactic acids, there is a net movement from the 

 secretory to the nutrient solution independent of the pH of the bulk bathing 

 solutions. The secretion of weak electrolytes by the dog stomach is compatible 



