osmosis

Movement of water across a semipermeable membrane from a region of low solute concentration to high solute concentration. Nature seeking equilibrium.

mechanism

The membrane permits water molecules to pass but blocks larger solute molecules. Water flows to dilute the more concentrated side, equalizing osmotic pressure. No energy input required — driven by the entropy of mixing.

osmotic pressure

pi = iMRT (van’t Hoff equation). Proportional to solute concentration, temperature, and the dissociation factor. Measured in atmospheres or pascals.

biological role

• cell turgor: plant cells maintain rigidity through osmotic pressure against the cell wall
• kidney filtration: osmotic gradients in nephrons concentrate urine
• nutrient absorption: intestinal cells absorb water following solute transport
• red blood cells: isotonic solutions preserve shape; hypotonic solutions cause lysis, hypertonic cause crenation

applications

• reverse osmosis: applying pressure to force water through a membrane against the concentration gradient, desalination and water purification
• IV fluids: saline solutions matched to blood osmolarity
• food preservation: salt and sugar draw water out of microbes via osmosis

connections

Works alongside oxidation and fermentation as a fundamental chemical process in living systems. Membranes central to osmosis are built by polymerization of lipids.