The concentrations of sodium and potassium are fairly constant. however, there is a transmembrane potential charge difference that exists across the membrane. how is this possible? the concentrations of sodium and potassium are fairly constant. however, there is a transmembrane potential charge difference that exists across the membrane. how is this possible? the concentrations of sodium and potassium are not equal inside and outside the cell. the cell membrane is impermeable to sodium and potassium. leak channels allow sodium and potassium to move across the membrane, causing the difference in electric charge. the cell is more negative outside than inside.