Osmotic pressure

Osmotic pressure

Osmotic pressure is the hydrostatic pressure produced by a difference in concentration between solutions on the two sides of a surface such as a semipermeable membrane.

Osmotic potential is the opposite of water potential, with the former meaning the degree to which a solvent (usually water) would want to stay in a liquid.

The osmotic pressure Π of a dilute solution can be calculated using the formula

Pi = i M R T,


i is the dimensionless van 't Hoff factor
M is the molarity
R=0.08206 L · atm · mol-1 · K-1 is the gas constant
T is the thermodynamic (absolute) temperature

This equation gives the pressure on one side of the membrane; the total pressure on the membrane is given by the difference between the pressures on the two sides. Note the similarity of the above formula to the ideal gas law and also that osmotic pressure is not dependent on particle charge. This equation was derived by van 't Hoff.

Osmotic pressure is an important factor affecting cells. Osmoregulation is the homeostasis mechanism of an organism to reach balance in osmotic pressure.

  • Hypertonicity is a solution that causes cells to shrink. It may or may not have a higher osmotic pressure than the cell interior since the rate of water entry will depend upon the permeability of the cell membrane.
  • Hypotonicity is a solution that causes cells to swell. It may or may not have a lower osmotic pressure than the cell interior.since the rate of water entry will depend upon the permeability of the cell membrane.
  • Isotonic is a solution that produces no change in cell volume.

When a biological cell is in a hypotonic environment, the cell interior accumulates water, water flows across the cell membrane into the cell, causing it to expand. In plant cells, the cell wall restricts the expansion, resulting in pressure on the cell wall from within called turgor pressure.


Osmotic pressure is the basis of filtering ("reverse osmosis"), a process commonly used to purify water. The water to be purified is placed in a chamber and put under an amount of pressure greater than the osmotic pressure exerted by the water and the solutes dissolved in it. Part of the chamber opens to a differentially permeable membrane that lets water molecules through, but not the solute particles. The osmotic pressure of ocean water is about 27 atm. Reverse osmosis desalinators use pressures around 70 atm to produce fresh water from ocean salt water.

Osmotic pressure is necessary for many plant functions. It is the resulting turgor pressure on the cell wall that allows herbaceous plants to stand upright, and how plants regulate the aperture of their stomata. In animal cells which lack a cell wall however, excessive osmotic pressure can result in cytolysis.

For the calculation of molecular weight by using colligative properties, osmotic pressure is the most preferred property.

Potential osmotic pressure

Potential osmotic pressure is the maximum osmotic pressure that could develop in a solution if it were separated from distilled water by a selectively permeable membrane. It is the number of solute particles in a unit volume of the solution that directly determines its potential osmotic pressure. If one waits for equilibrium, osmotic pressure reaches potential osmotic pressure.

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