Osmosis (source code)
= Osmosis
{wiki}
<Osmosis> is net movement of <water> across a boundary whose permeability differs for <water> and relevant <solutes>. The direction depends on the <chemical potential> of <water>, including both dissolved material and <pressure>. A <cell wall> can let <water> reach the <cell membrane>, yet oppose expansion through <turgor pressure>. Zero net movement does not mean individual <water> <molecules> have stopped crossing.
Imagine pure <water> on side A and a <chemical solution> on side B of a <semipermeable membrane>. At equal <temperature> and <pressure>, dissolved particles lower the <chemical potential> of <water> in B, and net <water> transfer initially favors A to B. Retaining the <solutes> lets <water> redistribute without simply mixing everything. Increasing B's <pressure> opposes entry; enough <pressure> gives <thermodynamic equilibrium> with unequal <solute> <concentrations>. This is why “<water> moves toward more <solute>” requires a pressure condition.<Biology 2e by Mary Ann Clark and others/5.2 Passive Transport>[§5.2, Osmosis and Mechanism: selective passage and pressure opposing entry]{ref}
\Image[/osmosis-membrane-pressure-balance.png]
{title=<Selective permeability> and the <pressure> that balances <osmosis>}
{description=Original schematic of a boundary passing <water> and retaining the illustrated <solute>. Left: equal <pressures> allow net transfer toward the <chemical solution>. Right: excess <pressure> on the solution side balances the osmotic drive while <water> exchanges in both directions. Equal opposing arrows denote zero net transfer, not a measured molecular rate. Particle counts and dimensions are illustrative.}
At <thermodynamic equilibrium>, the <chemical potentials> of exchangeable <water> are equal. Individual <water> <molecules> still move and cross in both directions, with equal mean opposing transfer rates and zero net <flux>. A stationary liquid level alone does not prove this: a blocked route can also give no visible transfer. <Selective permeability> provides a route; <water potential> supplies the direction, and neither alone determines the speed. The <cell wall>, <turgor pressure> and <plasmolysis> articles show why a living boundary's mechanical support and its <selective permeability> must be distinguished.
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