Restoring and maintaining readiness.
Sodium is in high concentration outside the cell and low inside.
It does not cross the cell membrane readily unless a specialized gate is opened. Potassium is in high concentration inside the cell and low outside. It can pass the cell membrane much more easily than sodium. Chloride ion is in high concentration outside because of its negative charge, it is repelled by the negatively charged protein. There is a pump in the cell membrane that pumps 3 molecules of sodium out and two molecules of potassium in with every use of an ATP molecule.