Definition

Osmosis is the movement of water through a membrane that lets water pass but blocks bigger things like salt or sugar, with water moving toward the side that has more dissolved material. It is the diffusion of water across a selectively permeable membrane from a solution with lower solute concentration (more water) to one with higher solute concentration (less water), requiring no energy; cells in a hypotonic solution gain water and swell, cells in a hypertonic solution lose water and shrink, and cells in an isotonic solution keep their size. Physically it is the net flow of solvent down its own chemical potential gradient, which is equivalent to flow from lower to higher osmotic pressure. The osmotic pressure of a dilute solution follows the van't Hoff relation $\Pi = iMRT$, where $i$ is the number of particles per formula unit, $M$ is molarity, $R$ is the gas constant, and $T$ is absolute temperature. Water crosses the bilayer slowly on its own and rapidly through aquaporin channels, and organisms regulate cell volume with ion pumps, contractile vacuoles, cell walls, or kidneys.