In membrane biophysics sometimes used interchangeably with cell potential, but applicable to any lipid bilayer or membrane. Hence every organelle and every membranous compartment (such as a synthetic vesicle) has a transmembrane potential (although the size of this potential may be zero). The transmembrane potential is the voltage "drop" or the difference in voltage between one face of a bilayer and its immediate opposite face. The property need not be uniform throughout the cell or compartment, but under some conditions may vary between one patch of membrane and another. A localized change in potential occurs at the synapse of nerve cell, for example, with the opening of ion channels by neurotransmitter. Likewise during an action potential, the magnitude of the transmembrane potential will vary in time and space along a nerve fiber. Strictly, it is the transmembrane potential or transmembrane electric field, and not the cell potential that controls the transmembrane flow of charged solutes and the activity of voltage-gated ion channels.

Compare electrochemical potential