The Origin of Bioelectrical Potentials in Plant and Animal Cells

The electrical potential difference across a plant or animalcell membrane can be caused by at least three different mechanisms,acting alone or in concert. First, a Donnan equilibrium canaccount for a sizable membrane potential without the participationof any active transport process. In a Donnan equilibrium themembrane potential is generated by the diffusion of permeatingions down their concentration gradients. The asymmetric distributionof permeating ions is caused by the presence of charged, nondiffusibleions, e.g., proteins inside the cell. The second mechanism isan electrically neutral ion pump, e.g., the coupled sodium-potassiumpump found in many types of cells. An electrically neutral pumpcan generate a large membrane potential if the membrane hasa high passive permeability to one of the actively transportedions, usually potassium. The third mechanism is an electrogenicion pump, which makes a substantial contribution to the membranepotential in several types of plant and animal cells. An electrogenicpump directly causes a net movement of charge across the cellmembrane. The membrane voltage generated by the pump then causesa passive flow of diffusible ions which partially short circuitsthe potential difference generated by the pump.