Osmotic Pressure

∞ generated and posted on 2016.12.16 ∞

Force exerted across membranes by movement of water down its concentration gradient.

Osmosis can be viewed either as water movement towards regions of less water or more dissolved substances and Osmotic Pressure is in effect the intensity of this water movement with greater intensity associated with greater differences between where water is coming from and where it is going to in terms of amount of water/amount of dissolved substances present.

Osmotic pressure can be determined by countering this force literally with a pressure, such as can be generated using a piston> or, more commonly, as a column of water. That force that exactly counters the net movement of water across the membrane is deemed the osmotic pressure. The higher the solute concentration difference across the membrane then the greater the osmotic pressure.

Figure legend: Osmotic pressure is typically measured using a U-tube apparatus. Here the higher solute concentration is found to the right of a dashed line, which represents a semipermeable membrane. Water flows osmotically therefore from the left to the right and as a consequence the water in the column to the right rises. This continues until the water has sufficiently risen that the pressure exerted by the force of gravity exactly counters the force exerted osmotically by water across the membrane. That pressure that exactly balances the force of gravity is known as osmotic pressure which in turn is a measure of the force exerted by water molecules as they attempt to move down their concentration gradient.

See also osmosis, osmolarity, and osmotic lysis.