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Principles of Osmosis
Osmosis will occur if a vessel is separated into two compartments by a semipermeable membrane, both compartments are filled to the same level with a solvent, and solute is added to one side. The level of the liquid on the side containing the solute will rise as the solvent flows from the side of its higher concentration to the side of lower concentration. If an external pressure is exerted on the side containing the solute, the transfer of solvent can be stopped and even reversed (reverse osmosis). Two solutions separated by a semipermeable membrane are said to be isotonic if no osmosis occurs. If osmosis occurs, transfer of solvent is from the hypotonic solution to the hypertonic solution, which has the higher osmotic pressure.
The minimum pressure necessary to stop solvent transfer is called the osmotic pressure. Since the osmotic pressure is related to the concentration of solute particles, there is a mathematical relationship between osmotic pressure, freezing-point depression, and boiling-point elevation. Properties such as osmotic pressure, freezing point, and boiling point, which depend on the number of particles present rather than on their size or chemical nature, are called colligative properties. For dilute solutions the mathematical relationship between the osmotic pressure, temperature, and concentration of solute is much like the relation between pressure, temperature, and volume in an ideal gas (see gas laws). A number of theories explaining osmotic pressure by analogy to gases have been devised, but most have been discarded in favor of thermodynamic interpretations using such concepts as the entropy of dilution.
Biological Importance of Osmosis
Osmosis and dialysis are of prime importance in living organisms, where they influence the distribution of nutrients and the release of metabolic waste products. Living cells of both plants and animals are enclosed by a semipermeable membrane called the cell membrane, which regulates the flow of liquids and of dissolved solids and gases into and out of the cell. The membrane forms a selective barrier between the cell and its environment; not all substances can pass through the membrane with equal facility. Without this selectivity, the substances necessary to the life of the cell would diffuse uniformly into the cell's surroundings, and toxic materials from the surroundings would enter the cell.
If blood cells (or other cells) are placed in contact with an isotonic solution, they will neither shrink nor swell. If the solution is hypertonic, the cells will lose water and shrink (plasmolyze). If the solution is hypotonic (or if pure solvent is used) the cells will swell; the osmotic pressure that is developed may even be great enough to rupture the cell membrane. Saltwater from the ocean is hypertonic to the cells of the human body; the drinking of ocean water dehydrates body tissues instead of quenching thirst.
In plants osmosis is at least partially responsible for the absorption of soil water by root hairs and for the elevation of the liquid to the leaves of the plant. However, plants wilt when watered with saltwater or treated with too much fertilizer, since the soil around their roots then becomes hypertonic.
the passage of a substance, usually a solvent, through a semipermeable membrane that separates a solution from a pure solvent or two solutions of different concentrations from each other. A semipermeable membrane allows small molecules of a solvent to pass through but is impermeable to larger molecules. The concentrations on both sides of such a membrane can be equalized only by unilateral diffusion of the solvent. Hence, osmosis always proceeds from a pure solvent to a solution or from a dilute solution to a more concentrated one. Osmosis within an enclosed volume of liquid is called endosmosis, and osmosis to the outside is called exosmosis.
A solvent is transported through a membrane by osmotic pressure, which is equal to the excess external pressure that has to be exerted by the solution in order to stop osmosis, that is, to create the condition of osmotic equilibrium. Excess pressure above the osmotic pressure may lead to reversal of osmosis, or reverse diffusion, of the solvent. In cases where a membrane is permeable not only to a solvent but also to some dissolved substances, diffusion of the solutes from the solution into the solvent is called dialysis, which is used to separate polymers and colloidal systems from low-molecular admixtures.
Osmosis was first observed by J. A. Nollet in 1748 but was not thoroughly studied until a century later. A major factor in biological processes, it is widely used in laboratory work to study various biological structures and to determine the molecular characteristics of polymers and the concentration of solutions. Osmotic phenomena are sometimes utilized in industry, for example, to produce certain polymeric materials or to purify highly mineralized water by reverse osmosis.
L. A. SHITS