Osmosis is the diffusion of water through a semipermeable membrane from a region of higher water concentration to lower water concentration. If a concentrated solution is separated from a dilute solution by a membrane that allows water but not other molecules to pass through, water will diffuse through the membrane from the dilute to the concentrated solution until concentrations equalize. Cell membranes are selectively permeable, allowing water and some molecules to diffuse across while blocking other molecules, regulating the movement of water and substances into and out of cells.