Osmosis is a special case of diffusion, which involves the movement of water molecules through a differentially permeable membrane from a region with a lot of water molecules to a region with fewer water molecules. A differentially permeable membrane is a membrane that allows some substances to pass through but not others. You may also find the membrane being called a semi-permeable or selectively permeable membrane. The cell membrane that surrounds biological cells is differentially permeable. A differentially permeable membrane contains minute pores. Water molecules are able to pass through these pores. However, the particles of many other substances, which may be dissolved in the water, are unable to pass through. When two solutions, e.g. sucrose solutions, which have different concentrations, are separated by a differentially permeable membrane, the water molecules will diffuse through the pores in the membrane from the more dilute solution to the more concentrated solution. The sucrose molecules, however, do not move because they are unable to pass through the pores in the membrane. The volume of the more dilute solution decreases and the volume of the more concentrated solution increases.