In either case, the direction of transfer is from the area of higher Concentration of the material transferred to the area of lower concentration. If a vessel is separated into two compartments by a semipermeable membrane, if both compartments are filled to the same level with a solvent, and if solute is added to one side, then osmosis will occur, and the level of the liquid on the side containing the solute will rise. If an external Pressure is exerted on the side containing the solute, the transfer of solvent can be stopped. The minimum pressure to stop solvent transfer is called the osmotic pressure.
Osmosis plays an important role in the control of the flow of liquids in and out of a living cell.