Osmotic Pressure

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Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semi-permeable membrane. Osmotic pressure arises from the tendency of a pure solvent to move

through a semi-permeable membrane and into a solution containing a

solute to which the membrane is impermeable. This process is important in biology as the cell's membrane is selective toward many of the solutes found in living organisms.

Osmosis is a selective diffusion process driven by the internal energy of the solvent molecules. It is convenient to express the available energy per unit volume in terms of "osmotic pressure". It is customary to express this tendency toward solvent transport in pressure units relative to the pure solvent.

If pure water were on both sides of the membrane, the osmotic pressure

difference would be zero. But if normal human blood were on the right

side of the membrane, the osmotic pressure would be about seven

atmospheres.

Thermal Energy

The energy which drives the fluid transfer is the thermal energy of the water molecules, and that energy density is higher in the pure solvent since there are more water molecules.

Osmotic Gradient

The osmotic gradient is the difference in concentration between twosolutions on either side of a semi-permeable membrane, and is used to tell the difference in percentages of the concentration of a specific particle dissolved in a solution. Usually the osmotic gradient is used while comparing solutions that

have a semipermeable membrane between them allowing water to diffuse

between the two solutions, toward the hypertonic solution (the solution

with the higher concentration). Eventually, the force of the column of

water on the hypertonic side of the semipermeable membrane will equal

the force of diffusion on the hypotonic (the side with a lesser

concentration) side, creating equilibrium. When equilibrium is reached,

water continues to flow, but it flows both ways in equal amounts as well

as force, therefore stabilizing the solution.

Hypertonicity is the presence of a solution that causes cells to shrink.

Hypotonicitiy is the presence of a solution that causes cells to swell.

Isotonicity is the presence of a solution that produces no change in cell volume.

Osmotic Pressure