||"Membrane potential changes require charge transfer across the membrane, which means that electroneutrality cannot be exact – but the error is
small, as illustrated by considering the net charge transfer required to move the membrane potential from 0 mV to the equilibrium potential for K+, Ek. For a spherical cell of radius, r, equal to 10µm and containing 150 mM K+ (and an equal concentration of negative charge), there is 4XpXr^3/3X[K+] or 6.28X10^-13 moles of K+ in the cell. The net charge transfer (in moles of charge) is 4XpXr^3XCmXEk/F where F is Faraday's constant, or 1.23X10^-17 moles [and Cm, membrane capacitance, is ~1µF/cm^2]. The net K+ movement needed to establish the membrane potential is ~0.002% of the total amount of K+ in the cell! Membrane potential changes results from very small
net charge movements across the membrane, it is reasonable
to invoke electroneutrality."