Osmosis is a special term used for the diffusion (the passing through) of water through cell membranes. Water passes by diffusion from a region of higher to a region of lower concentration. Note that the key is the concentration of water, NOT the concentration of any solutes (a dissolved substance in a solution) present in the water. The diffusion of water does not need to be facilitated (aided) because cell membranes are freely permeable to water. Water is never transported actively; that is, it never moves against its concentration gradient. However, the concentration of water can be altered by the active transport of solutes and in this way the movement of water in and out of the cell can be controlled.

Osmosis is important!

A report in the 23 April 1998 issue of The New England Journal of Medicine tells of the life-threatening complications that can be caused by an ignorance of osmosis.

• Large volumes of a solution of 5% human albumin are injected into people undergoing a procedure called plasmapheresis.

• The albumin is dissolved in physiological saline (0.9% NaCl) and is therefore isotonic to human plasma (the large protein molecules of albumin have only a small osmotic effect).

• If 5% solutions are unavailable, pharmacists may substitute a proper dilution of a 25% albumin solution. Mixing 1 part of the 25% solution with 4 parts of diluent results in the correct 5% solution of albumin.

• BUT, in several cases, the diluent used was sterile water, not physiological saline.

• SO, the resulting solution was strongly hypotonic to human plasma.

• The Result: massive, life-threatening hemolysis (the bursting of red blood cells) in the patients.