Living cells "pump" singly ionized sodium ions, Na+, from the inside of the cell to the outside to maintain a membrane potential Δ V membrane = V in − V out = − 70 mV . It is called pumping because work must be done to move a positive ion from the negative inside of the cell to the positive outside, and it must go on continuously because sodium ions "leak" back through the cell wall by diffusion. How much work must be done to move one sodium ion from inside to the outside of the cell?