Osmoosiin

Osmoosiin is the process by which solvent molecules move across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. In biological and technical contexts, the solvent is usually water, and the membrane permits water but restricts most solutes. The driving force is the osmotic pressure generated by the difference in solute concentration on the two sides of the membrane. As water moves, the system tends toward equalization of chemical potential, though hydrostatic forces can oppose the flow.

In terms of terminology, solutions are described as isotonic, hypotonic, or hypertonic relative to a reference such as a cell interior. A hypotonic external solution causes water to enter cells, potentially causing swelling; a hypertonic external solution draws water out, causing shrinkage. Plant cells exhibit turgor pressure when the external solution is hypotonic, which helps maintain rigidity.

Quantitative descriptions include osmolarity and osmolality, and the van't Hoff relation approximates osmotic pressure for dilute solutions (π ≈ iCRT). However, real solutions deviate with non-ideal behavior.

Osmosis is essential in physiology: kidney water reabsorption, intestinal absorption, and cell volume regulation. In technology, osmosis is harnessed in processes such as dialysis and desalination, including reverse osmosis, where external pressure drives water through a membrane against the osmotic gradient.

Differences from diffusion: diffusion concerns the movement of solute molecules down concentration gradients, whereas osmosis concerns solvent movement across a membrane along an osmotic gradient.