the phospholipid bilayer of the cell membrane is, in itself, highly impermeable to ions. the complete structure of the cell membrane includes many proteins that are embedded in or completely cross the lipid bilayer. some of those proteins allow for the highly specific passage of ions, ion channels. leak potassium channels allow potassium to flow through the membrane in response to the disparity in concentrations of potassium inside (high concentration) and outside the cell (low). the loss of positive(+) charges of the potassium(k+) ions from the inside of the cell results in a negative potential there compared to the extracellular surface of the membrane. a much smaller "leak" of sodium(na+) into the cell results in the actual resting potential, about –70 mv, being less negative than the calculated potential for k+ alone, the equilibrium potential, about –90 mv. the sodium-potassium atpase is an active transporter within the membrane that pumps potassium (2 ions) back into the cell and sodium (3 ions) out of the cell, maintaining the concentrations of both ions as well as preserving the voltage polarization.