Osmosis

Osmosis, a special type of diffusion that involves the spontaneous passage of water or other solvents through a semipermeable membrane from a region of higher solvent concentration to one of lower solvent concentration. Unlike diffusion osmosis occurs only across a selectively permeable membrane, which permits water or other solvent molecules to pass while restricting solutes. An example of such a membrane is the cell membrane, which is permeable to water and to certain small molecules, including oxygen and carbon dioxide, but not to larger solutes such as sucrose, proteins, and starch.

History

The process of osmosis was first thoroughly studied in 1877 by a German plant physiologist, Wilhelm Pfeffer. Earlier researchers had made less accurate studies of leaky membranes (e.g., animal bladders) and the passage through them in opposite directions of water and escaping substances. The general term osmose (now osmosis) was introduced in 1854 by a British chemist, Thomas Graham.

Osmosis in living organisms

Osmosis is essential to life because it helps maintain the balance of fluids within cells, thereby supporting their structure and normal function. By regulating water movement across cell membranes, osmosis prevents cells from swelling or shrinking excessively. Plants absorb water from the soil mainly through osmosis, while mineral nutrients are absorbed through diffusion and active transport. In humans and other animals, osmosis regulates the exchange of water between cells and surrounding body fluids, facilitating nutrient absorption and waste removal. An imbalance in this process may result in various health issues, such as dehydration, edema, and electrolyte imbalances.

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chemical analysis: Osmosis

Osmotic pressure

If a solution is separated from the pure solvent by a membrane that is permeable to the solvent but not the solute, the solution will tend to become more dilute by absorbing solvent through the membrane. This process can be stopped by increasing the pressure on the solution by a specific amount, called the osmotic pressure. Dutch-born chemist Jacobus Henricus van ’t Hoff showed in 1886 that if the solute is so dilute that its partial vapor pressure above the solution obeys Henry’s law (i.e., is proportional to its concentration in the solution), then osmotic pressure varies with concentration and temperature approximately as it would if the solute were a gas occupying the same volume. This relation led to equations for determining molecular weights of solutes in dilute solutions through effects on the freezing point, boiling point, or vapor pressure of the solvent.

Reverse osmosis

While osmosis naturally moves solvents across a membrane from the side of higher solvent concentration to the side where the solvent concentration is lower, the application of pressure in reverse osmosis forces the solvent to flow in the opposite direction—against the concentration gradient—thus resulting in the filtration of the solute from the solvent, leaving behind the solutes. The membrane allows passage of small solution components, such as fresh water, while preventing passage of larger molecules, like salts and other impurities. Reverse osmosis is an effective means of concentrating impurities, recovering contaminated solvents, cleaning up polluted streams, and desalinizing seawater, and it is often used as an alternative to distillation for water purification.