Diffusion in a one-dimensional system with a thin membrane (which is treated as a partially permeable wall) for the discrete and continuous time and space variables is discussed. The internal structure of the membrane is not explicitly involved into consideration. Starting from microscopic models of diffusion we obtain the boundary condition at the membrane for macroscopic diffusion equation.

PACS numbers: 05.40.+j, 05.60.+w, 02.30.--f