Osmosis

Osmosis is the spontaneous movement of a solvent through a selectively permeable membrane from a region of lower solute concentration to higher solute concentration, driven by differences in chemical potential. The membrane is permeable to the solvent but restricts most solutes, so water moves to dilute the more concentrated side. Osmosis continues until the chemical potentials equilibrate or until opposing forces counter the flow.

The pressure required to stop net solvent movement is called osmotic pressure. For dilute solutions, osmotic pressure is proportional to solute concentration and temperature (the van't Hoff relation).

In physiology, solutions can be described as hypertonic, hypotonic, or isotonic relative to a cell. In a hypertonic solution, cells lose water and shrink; in a hypotonic solution, cells absorb water and may swell or lyse; isotonic solutions exert similar osmotic pressure to the cell interior, causing little net water movement. Plant cells rely on osmosis to generate turgor pressure that supports structure; animal cells rely on regulated osmolarity to maintain volume.

Osmosis is exploited in nature and technology. Aquaporin channels facilitate rapid water movement in many membranes; osmosis underlies plant water uptake and kidney function. In industry, osmosis is used in desalination and dialysis-related processes, notably reverse osmosis, where high pressure drives water through a membrane, separating water from solutes.

The phenomenon was investigated in the 19th century by Wilhelm Pfeffer, who introduced the term osmosis, and later explained by Jacobus van 't Hoff's thermodynamic description.