Electrogenic transporters cause charge build up across the membrane, which inhibits further transport. The combined gradients of coupling ion and substrate (illustrated here for a symporter with red/blue arrow), and the applied membrane potential (voltage, green arrow) therefore influence the direction of substrate flux across the membrane. Depending on the magnitude of these opposing forces, three outcomes can occur: (a) at applied voltages that are insufficient to oppose the diffusional force of the substrates, influx of substrate occurs. (b) At the equilibrium, or reversal, potential, Erev, the applied voltage exactly opposes the diffusional force of the substrates resulting in no net flux. (c) At higher applied voltages, the voltage overcomes the electrochemical gradient imposed by the coupling ion and substrate gradients, reversing the direction of flux and efflux occurs.