Osmosis
The net passive diffusion of solvent molecules (water) across a selectively permeable membrane from lower solute concentration toward higher solute concentration.
Governing formula
Π = i C R T (van 't Hoff Equation)
SI unit
Pa or kPa (Osmotic Pressure)
In depth
Driven by chemical potential gradients. Osmotic pressure Π represents the hydrostatic pressure required to prevent net solvent influx across the membrane. Aquaporin protein channels facilitate rapid transmembrane water transport.
Examples in the real world
Red blood cells lysing in hypotonic solutions (hemolysis) or shrinking in hypertonic saline solutions (crenation).