Membrane Semi‐Permeability and Ion Selectivity in Chemiosmosis: Open Systems Volume Dependence of Electrolyte Chemistry

ABSTRACT The effect of pH changes in external ionic solutions on liposomes entrapping non‐permeable weak acid/base pairs was simulated using a simple “toy” model. Membrane semi‐permeability and selective passive diffusion lead to internal stationary states that inevitably exhibit electroneutrality breaking. The resulting electrostatic effects on species’ potentials and ion flow rates depend strongly on compartment volume, in agreement with the biological surface‐to‐volume rule. Although electrostatic effects on membrane stability, species’ chemical potentials, and transmembrane ion migration are negligible at very small volumes (pL range), the electrical potential differences between external and internal phases fall within the range observed in cellular compartments. Furthermore, proton gradients generate electrical potential differences across the membrane that drive electrons in the same direction as protons, offering a novel perspective on the proton's universal role as a driving force in living compartments via its coupling to redox reactions. Since all these effects are inherently tied to very small volumes, where electroneutrality deviations are possible, in contrast to macroscopic electrolyte solutions, this supports the emergence of life and cellular chemistry in small semi‐permeable compartments.