Osmózis

Osmózis, or osmosis, is the spontaneous movement of a solvent, typically water, through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. The membrane allows the solvent to pass while restricting most solutes, enabling a net flow that tends to equalize solute concentrations on the two sides.

The process is driven by differences in chemical potential and can be quantified by osmotic pressure. For dilute solutions, osmotic pressure can be approximated by Pi = iCRT, where i is the van't Hoff factor, C is solute molarity, R is the gas constant, and T is temperature. Osmolarity, or osmolar concentration, reflects the total particle concentration contributing to osmotic pressure.

Whether water moves into or out of a cell depends on tonicity: a hypertonic solution has higher external solute concentration, causing water to leave the cell; a hypotonic solution causes water to enter; isotonic solutions yield no net water flow. In plant cells, osmosis generates turgor pressure against a rigid cell wall; excessive loss of water can lead to plasmolysis.

Practically, osmosis is fundamental to biology and medicine, influencing kidney function and plant hydration. It underpins techniques such as reverse osmosis, which uses applied pressure to overcome osmotic gradients for water purification. Membrane selectivity and temperature influence the rate and outcome of osmosis, but the basic mechanism is the movement of solvent, not solutes, across a selectively permeable barrier.