Neurons and muscle cells are unussual in having large membrane potentials that can change rapidly under certain circumstances. Electrical potential and its changes are basis for signalling ability of neurons and contractile function of muscle. The greater the concentration difference, the greater the net transfer Possible to treat net movement as result of a force propelling ions down concentration gradient. Charge separation generates electrical field that spans membrane.
Pumps move only a few hundred ions per second.
To small a number to have effect even over a few minutes, by working continuously, hour after hour, ion pumps have significant effect. Eventually equilibrium between concentration gradient and charge gradient is reached. Membrane potential established by purely passive means. In real neurons more than one type of ion can cross membrane. Cl- ions can cross membrane, but don't contribute to membrane potential in most neurons distributed according to magnitude of resting potential.