OSMOSIS

The diffusion of water across a selectively permeable membrane from an area of higher concentration to an area of lower concentration is osmosis (Gr. osmos, pushing). Osmosis is just a special type of diffusion, not a different method. The term tonicity (Gr. tonus, tension) refers to the relative concentration of solutes in the water inside and outside the cell. For example, in an isotonic (Gr. isos, equal tonus, tension) solution, the solute concentration is the same inside and outside a red blood cell. The concentration of water molecules is also the same inside and outside the cell. Thus, water molecules move across the plasma membrane at the same rate in both directions, and there is no net movement of water in either direction. In a hypertonic (Gr. hyper, above) solution, the solute concentration is higher outside the red blood cell than inside. Because the concentration of water molecules inside the cell is higher than outside, water moves out of the cell, which shrinks. This condition is called crenation in red blood cells. In a hypotonic (Gr. hypo, under) solution, the solute concentration is lower outside the red blood cell than inside. Conversely, the concentration of water molecules is higher outside the cell than inside. As a result, water moves into the cell, which swells and may burst