ABSTRACT
The last several decades have seen an evolution of the concept of cell membranes from simple physical separations between “inside” and “outside” to very active regions where many diverse biochemical reactions are taking place. 1 In fact, biomembranes have now been identified as the location where the cell generates and utilizes a great deal of its energy. For example, the generation of ATP via both photosynthetic and oxidative phosphorylation takes place at the membrane of the thylakoid or mitochondria, respectively. 2 – 14 Research has further provided evidence that a common feature in biological energy transduction processes is the transport of protons across the biomembrane. Mitchell’s 15 – 17 chemiosmotic hypothesis describes the coupling of energy transduction to a “protonmotive force” across the membrane. This force is composed of two parts: an electric field generated by an excess of charge on one side of the membrane, and a pH gradient resulting from a difference in proton concentrations. The transport of protons across the biomembrane and “down” the pH gradient supplies the driving force for ATP synthesis in much the same way that passage of electrons through a filament provides the energy necessary to cause a lightbulb to glow. Indeed, the storage of electric energy in a charged capacitor provides a convenient analogy to the utilization by the cell of a pH gradient in a membrane as a storehouse of chemical energy.
