English: Fig. 1. Fluxes and concentration profiles in a membrane and the surrounding solutions. In Fig. a, a driving force is applied to a system initially at equilibrium: the flux of a selectively permeating species in the membrane, ${\displaystyle J_{1}^{m}}$, is higher than its flux in solution, ${\displaystyle J_{1}^{s}}$. Higher flux in the membrane causes decreasing concentration at the upstream membrane/solution interface, and increasing concentration at the downstream interface (b). Concentration gradients gives rise to diffusion transport, which increases the total flux in solution and decreases the flux in the membrane. In steady state, ${\displaystyle J_{1}^{s}=J_{1}^{m}}$.