semipermeability

Semipermeability is the property of a membrane that allows certain substances to pass through while restricting others. A semipermeable (or selectively permeable) membrane enables diffusion of small or nonpolar molecules such as oxygen and carbon dioxide; it hinders many ions, sugars, and larger polar molecules. Water often moves across semipermeable membranes by osmosis, from regions of low solute concentration to high solute concentration, contributing to hydration and fluid balance in biological systems.

Transport across semipermeable membranes occurs mainly by passive diffusion, with or without protein channels, and by facilitated diffusion; impermeable substances require carriers or channels. The lipid bilayer’s properties, along with embedded proteins, determine permeability. Factors affecting permeability include molecular size, charge, polarity, lipophilicity, temperature, and membrane composition. Permeability is often described quantitatively by permeability coefficients and flux according to Fick’s laws.

In biology the plasma membrane is semipermeable, regulating ion gradients, nutrient uptake, and waste removal. Semipermeable membranes are central to processes such as osmoregulation and neural signaling. In medicine and industry, synthetic semipermeable membranes are used in dialysis, microfiltration, and desalination to separate substances by size or chemistry.