196 



ELECTROLYTES IN BIOLOGICAL SYSTEMS 



ternative of paired ion transport demands that one of the ions diffuse backward 

 along the concentration gradient created by its transport. For active transport, 



NUTRIENT 



NUTRIENT 



HCO 



SECRETORY 



H ♦CI" 



H -►CO, 



Hco; 



>C02+ HjO^^HjCO, 



I 



CI" H+ + HCOj 

 HCI 



Fig. 2. Three models of paired ion exchange across the gastric mucosa which would gen- 

 erate a diffusion potential. Forced ionic movement mediated by a hypothetical carrier X is 

 indicated by heavy lines and net passive diffusion along a concentration gradient is indicated 

 by a broken line, a: Redox anion pump. Two iron carrier molecules are indicated. The ferric 

 form of the carrier transfers Cl~ from the nutrient to the secretory surface. At the secretory 

 surface the carrier is reduced, generating H"*" and liberating Cl~. At the nutrient surface the 

 carrier is oxidized with formation of 0H~, and the carrier takes up Cl~. To generate a mucosal 

 current, some of the H"*", the H^ formed by the lower carrier in the figure, diffuses back to 

 the secretory solution and is neutralized by 0H~, with a net negative charge transfer as- 

 sociated with Cl^. b: Bicarbonate bi-ionic potential. This model is similar to the redox anion 

 pump except for the diffusing ion. Bicarbonate ions, removing some of the 0H~ ions in the 

 nutrient solution, diffuse into the secretory solution. There HCOs^ is neutralized by H"*", 

 and CO2 redistributes by diffusion. The net result is a transfer of a CI" ion and net negative 

 charge from the nutrient to secretory solution, c: Forced anion exchange. In this model there 

 is forced exchange of CI" against HCOs". As in 26, the mucosal current arises from HCO3 

 diffusion from the nutrient to secretory solution. This HCOs" is continuously recycled. This 

 model would generate H"*" if, in addition to HCOs^ diffusing from the nutrient solution, fur- 

 ther HCO3" was formed in the secretory solution from CO2. This additional HCOs" would 

 be accepted by the carrier, releasing an additional CI which would be paired against H"*" 

 derived from COo. 



we can confine our consideration to the members of the carbonic acid system. 

 It is least likely that 0H~ is the passively diffusing ion because of the pre- 

 vailing pH's. 



