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
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.