Osmotic pressure is important in living cells, because they are surrounded by a semipermeable cell membrane through which they communicate with the outside world. Cells are designed to function with a certain internal salt concentration.

If they are put in a concentrated brine solution they lose water through the membrane and shrivel; conversely, if they are placed in distilled water they take up more water and swell. If the osmotic pressure inside becomes too great for the membrane strength the cell ruptures. Plant cells have rigid cell walls of cellulose around them to protect them from such osmotic shock.

The fundamental idea behind all four colligative properties is that the molar free energy, chemical potential, or escaping tendency of solvent molecules must be the same in two phases if equilibrium is to exist between them.

When foreign molecules or ions are added to a liquid phase, the chemical potential of the liquid decreases in that phase. There is less tendency to migrate into a nearby vapor phase, solid phase, or to the other side of a membrane. To redress the balance and reestablish equilibrium, the temperature or the pressure of the solution must be adjusted.