Osmosis

Osmosis is the spontaneous net movement of a solvent through a selectively permeable membrane that restricts the passage of dissolved substances. In biological systems, the solvent is water. When temperature and pressure are equal on both sides, water generally moves toward the solution containing a higher concentration of impermeant solute particles. More precisely, its direction is determined by differences in solvent chemical potential, which depend on both solution composition and pressure. Osmosis influences cell volume, plant water relations, and membrane-based separation processes. (openstax.org)

Physical basis

Osmosis is related to diffusion, but specifically concerns solvent transport across a selective barrier. Solvent molecules move in both directions; “net movement” denotes the difference between these opposing flows. A membrane need not be biological: synthetic materials can also allow water to pass while retaining particular dissolved molecules or ions. Selectivity depends on the membrane’s structure and interactions with the substances crossing it, rather than simply on particle size. (openstax.org)

In thermodynamics, the driving quantity is the solvent’s chemical potential. Dissolving a substance ordinarily lowers the solvent’s thermodynamic activity relative to its pure state, whereas increasing pressure raises its chemical potential. Net solvent transfer continues until these influences balance. At thermodynamic equilibrium, solvent chemical potentials are equal across the membrane, even if solute concentrations remain unequal. Thus, osmosis does not necessarily equalize concentrations on the two sides. (goldbook.iupac.org)

A familiar demonstration uses a U-shaped tube containing pure water and a sugar solution separated by a membrane impermeable to sugar. Water enters the solution, raising its liquid level. The resulting hydrostatic pressure difference eventually balances the osmotic tendency, stopping net transfer without stopping molecular exchange. (openstax.org)

Osmotic pressure and quantitative description

Osmotic pressure, symbolized by $\Pi$, is the excess pressure required on a solution to maintain osmotic equilibrium with pure solvent across a membrane permeable only to that solvent. For an ideal dilute solution,

where $c_{\mathrm{particles}}$ is the molar concentration of independently moving solute particles, $R$ is the gas constant, and $T$ is absolute temperature. The relationship is commonly called the van ’t Hoff equation. (goldbook.iupac.org)

Osmotic pressure is a colligative property: in the dilute ideal limit, it depends on the number of dissolved particles rather than their chemical identity. An electrolyte can therefore produce a larger effect than an equal molar concentration of a nondissociating substance because it separates into ions. For a solute concentration $c$, the equation is often written $\Pi=icRT$, where the van ’t Hoff factor $i$ accounts for the effective number of particles produced per formula unit. Interactions between particles cause departures from ideal behavior. (openstax.org)

For nonideal solutions, solvent activity provides the more general description. With the incompressible-fluid approximation,

where $a_w$ is water activity and $\overline V_w$ is its partial molar volume. This expression connects the concentration-based approximation with the thermodynamic definition. (goldbook.iupac.org)

Cell membranes and tonicity

The cell membrane separates a cell from its surroundings and permits different substances to cross at different rates. Water crosses the lipid bilayer and, in many cells, passes rapidly through aquaporins, specialized membrane proteins forming water channels. These channels facilitate movement along an existing driving gradient rather than pumping water against it. (openstax.org)

Tonicity describes a solution’s effect on cell volume. A hypotonic environment causes net water entry; a hypertonic environment causes net water loss; an isotonic environment produces no sustained net volume change. Animal cells can swell and rupture in sufficiently hypotonic conditions or shrink in hypertonic conditions. Plant cell walls constrain expansion and allow internal pressure to develop. (openstax.org)

Tonicity is not identical to total osmotic concentration. Its sustained effect depends on which solutes remain effectively impermeant to the particular membrane. Osmoregulation maintains water and solute balance and contributes to homeostasis. Although water movement itself is passive, maintaining the solute gradients that influence it can require energy-consuming transport processes. (arxiv.org)

Plant water relations

Plant physiology describes water movement using water potential, a measure incorporating the effects of solutes, pressure, gravity, and interactions with surrounding materials. Water moves from higher to lower water potential. Adding solute lowers the solute component, while positive hydrostatic pressure raises the pressure component. Concentration alone therefore cannot determine water movement in every plant tissue. (openstax.org)

Osmotic entry into plant cells generates turgor pressure, which supports nonwoody tissues. Water loss reduces turgor and can cause wilting; sufficiently severe loss can separate the membrane from the cell wall. Osmosis also links water in xylem to sugar transport: sugar accumulation in conducting tissues lowers water potential, drawing in water and helping generate the pressure that drives transport. (openstax.org)

Technological applications

Reverse osmosis applies pressure sufficient to overcome the opposing osmotic pressure difference, moving water from a concentrated feed through a selective membrane toward a less concentrated product stream. It is widely used in desalination and water purification. Dissolved salts remain largely in the concentrated stream, while the water passing through the membrane is collected separately. (openstax.org)

Practical systems must manage membrane fouling and mineral scaling, which impair performance and require pretreatment, cleaning, or membrane replacement. Other osmotic technologies use concentration differences to draw water into concentrated solutions or to investigate energy recovery from salinity differences. These applications exploit the same coupling between membrane selectivity, solvent activity, and pressure that governs osmosis in cells. (usbr.gov)

References

1. 2 Passive Transport - Biology 2eopenstax.org
2. 4 Colligative Properties - Chemistry 2eopenstax.org
3. Osmosis, from molecular insights to large-scale applicationsarxiv.org
4. 1 Osmoregulation and Osmotic Balance - Biology 2eopenstax.org
5. The Nobel Prize in Chemistry 2003nobelprize.org
6. 5 Transport of Water and Solutes in Plants - Biology 2eopenstax.org
7. Yuma Area Office: Reverse Osmosisusbr.gov
8. Scaling resistant RO/NF membraneusbr.gov