Knowing that the concentration of different ions are not the same on the inside and the outside of the cell, yet we know that the important nature of the membrane is its permeability. The sodium-potassium pump is used to keep an osmotic balance across the membrane, by keeping the concentration of sodium ions low on the inside, because the sodium and potassium ions have the same charge, but sodium is smaller molecule and has a bigger charge density. That makes sodium more attracted to water molecules, and so the sodium will have a larger diameter, and that makes it harder to cross the inside of the membrane, that is why it is restricted from the inside. Because there is little sodium ions on the inside there has to be plentiful cations on the