With the development of radioactive tracers, it was shown that cells are not impermeable to . This was difficult to explain with the membrane barrier theory, so the sodium pump was proposed to continually remove as it permeates cells. This drove the concept that cells are in a state of dynamic equilibrium, constantly using energy to maintain ion gradients. In 1935, discovered ATP and its role as a source of energy for cells, so the concept of a metabolically-driven sodium pump was proposed.