Osmosis

Osmosis is the spontaneous, net movement of a solvent across a selectively permeable membrane. This membrane allows the solvent to pass through but blocks the solute. The solvent moves from an area with a higher water potential (where the solute is less concentrated) to an area with a lower water potential (where the solute is more concentrated), working to balance the solute concentrations on both sides. This process can also be harnessed to perform work. The external pressure needed to stop this net solvent movement is called osmotic pressure. Because osmotic pressure depends only on the molar concentration of the solute, not its specific identity, it is a colligative property. The actual transport of the solvent occurs through viscous flow driven by a pressure gradient.

Osmosis is essential for life because biological membranes are semipermeable. These membranes generally block large, polar molecules like ions, proteins, and polysaccharides, while allowing non-polar or hydrophobic molecules (such as lipids) and small molecules (like oxygen, carbon dioxide, nitrogen, and nitric oxide) to pass. Permeability is influenced by a molecule's solubility, charge, and chemistry, as well as its size. Water molecules cross the phospholipid bilayer of the plasma membrane, tonoplast, or organelle membranes through aquaporins via a selective pore flow mechanism. This process is the main way water enters and leaves cells, and it largely maintains a cell’s turgor pressure through the interaction between the cell interior and its typically hypotonic surroundings.

The observation of osmotic flow dates back to ancient times, for instance, during the construction of the Egyptian pyramids. The first documented observation of osmosis was made in 1748. The word "osmosis" comes from the terms "endosmose" and "exosmose," coined by French physician René Joachim Henri Dutrochet from Greek words meaning "within," "outer," and "push." In 1867, Moritz Traube improved the measurement of osmotic flow by inventing highly selective precipitation membranes.

Osmosis involves a solvent moving toward a higher solute concentration across a semipermeable membrane. While water is the usual solvent in biology, osmosis can also occur in other liquids, supercritical liquids, and even gases. If a cell is placed in freshwater, water moves into it; if placed in saltwater, water moves out. When pure water is on both sides of the membrane, water molecules cross at equal rates in both directions, resulting in no net flow. This effect can be seen when potato slices are put in a high-salt solution: water leaves the potato, causing it to shrink and lose turgor pressure. The higher the salt concentration, the greater the loss in the slice's size and weight. Chemical gardens also demonstrate osmosis in inorganic chemistry.

The driving mechanism behind osmosis is not simply the dilution of water by solute or the solute's attraction to water—both ideas have been disproven. The diffusion model fails because osmosis can move water toward a region with a higher water concentration. The "bound water" model is refuted because osmosis is a colligative property, independent of solute size or how hydrophilic the solute is. A mechanical or thermodynamic explanation is needed: there is an interaction between solute and water that counteracts the pressure free solute molecules would otherwise exert. Heat from the surroundings can be converted into mechanical energy (like water rising), meaning osmotic solvent transport is a pressure-driven process, not random molecular diffusion. Thermodynamic explanations involve chemical potential, where the function of water on the solution side differs from pure water due to higher pressure and the presence of solute, keeping the chemical potential unchanged. The virial theorem shows that attraction between water and solute molecules reduces pressure, so water molecules in a solution exert less pressure on each other than in pure water. This allows pure water to push into the solution until pressure equilibrium is reached.

In living things, osmotic pressure is a major source of support for many plants. The osmotic entry of water increases turgor pressure against the cell wall until it matches the osmotic pressure, creating a steady state. If a plant cell is placed in a hypertonic solution (relative to its cytoplasm), water leaves the cell, causing it to shrink and become flaccid. In extreme cases, the cell undergoes plasmolysis, where the cell membrane pulls away from the cell wall.