Osmosis is the passage of a solvent (usually water in physiology) through a semi-permeable (better described as a differentially permeable) membrane from a less concentrated to a more concentrated solution until both solutions are of the same concentration.
The free movement of solvent presupposes that both the more and less concentrated solutions exist in open systems. That is, they are not enclosed. In physiology this is rarely, if ever, the case.
Consider a cell such as an erythrocyte in pure water. The cell contents are very much more concentrated than the solution in which it is suspended, so water enters. The rapid build up of fluid inside the cell increases the pressure causing the weak cell membrane to rupture. Conversely, if the erythrocyte is placed in a moderately concentrated salt or sugar solution, waters leaves the cell, reducing the internal volume and pressure causing the membrane to collapse. The erythrocyte takes on a crinkled appearance.
Now consider a bacterial cell which, like plants cells, has a tough cell wall exterior to the cytoplasmic membrane. If a bacterial cell is placed in pure water, water enters, the volume increases and the cytoplasmic membrane presses tightly against the interior of the cell wall just like a bicycle inner tube inside a tyre. In this case, the cytoplasmic membrane does not rupture because there comes a point when the pressure inside the cell reaches a point which prevents any further entry of water. This pressure is known as the osmotic pressure.
Definition of osmotic pressure
The osmotic pressure of a solution is that pressure which must be exerted
on a solution to prevent any net movement of solvent between the solution
and its pure solvent when these are separated by a semi-permeable membrane.
