Active transport, ion movements, and pH changes

The transport of substances across cell membranes may be the most fundamental activity of living things. When the substance transported is any ion there can be a change in the concentration of hydrogen ions on the two sides of the membrane. These hydrogen ion concentration changes are not caused by fluxes of hydrogen ions although fluxes of hydrogen ions may sometimes be involved. The reason for the apparent contradiction is quite simple. All aqueous systems are subject to two constraints: (1) to maintain the charge balance, the sum of the cationic charges must equal the sum of the anionic charges and (2) the product of the molar concentration of H⁺ and the molar concentration of OH^–, established and maintained by the association and the dissociation of water, remains always at 10^–14. As a consequence the concentrations of H⁺ and OH^– are determined uniquely by differences between the concentrations of the other cations and anions, with H⁺] and OH^–] being dependent variables. Hydrogen ions and hydroxyl ions can be produced or consumed in local reactions whereas any strong ions such as Cl^–, Mg²⁺, or K⁺ can be neither produced nor consumed in biological reactions. Further consequences of these truisms are outlined here in terms of the chemistry of the kinds of reactions which can lead to pH changes.