Osmosis
Osmosis is the net movement of a solvent—usually water in living systems—through a selectively permeable membrane when conditions differ across it. When water can pass more readily than particular dissolved solutes, it moves toward the side with lower water potential, often the more concentrated solution. Osmosis influences cell volume and plant water balance and underlies processes such as reverse osmosis for water treatment.
How osmosis works
A selectively permeable membrane lets some substances cross more readily than others. If water can cross but a dissolved solute cannot, water molecules move in both directions through the membrane, but a difference in water potential produces a net flow. In many aqueous solutions, this net movement is from the more dilute side toward the side with the greater concentration of solutes that cannot cross freely. The flow tends toward equilibrium; it can also be balanced by pressure opposing the movement. Osmosis is related to diffusion, but the term specifically describes net solvent movement across a membrane.
Osmosis in cells
Water crosses many cell membranes through aquaporins, channel proteins that facilitate its passage. Tonicity describes how the effective solute concentration outside a cell affects its volume. In a hypotonic environment, water enters the cell and it may swell; in a hypertonic environment, water leaves and the cell may shrink. In an isotonic environment there is no net water movement, although water molecules continue to move across the membrane. Plant cell walls limit expansion, and water entering plant cells contributes to turgor pressure, which helps keep non-woody tissues firm.
Osmotic pressure and applications
Osmotic pressure is the pressure that must be applied to a solution to prevent net solvent movement across a semipermeable membrane. In reverse osmosis, pressure greater than the system's osmotic pressure is applied to drive water through a membrane opposite the direction of natural osmotic flow. The membrane retains many dissolved salts and other substances, so reverse osmosis is used in water purification, including seawater desalination.
History
The British chemist Thomas Graham introduced the general term “osmose,” now written “osmosis,” in 1854. German plant physiologist Wilhelm Pfeffer carried out an early thorough study of the process in 1877.
Key facts
- What moves
- A solvent—usually water in biology—moves across a selectively permeable membrane.
- Net direction
- Net flow follows a difference in water potential; for many solutions, water moves toward the side with more solute that cannot cross freely.
- Cell response
- Hypotonic surroundings tend to make cells take up water, while hypertonic surroundings tend to make them lose water; isotonic surroundings produce no net water movement.
- Osmotic pressure
- The pressure required to prevent net solvent movement across a semipermeable membrane.
- History
- Thomas Graham introduced the term “osmose” in 1854; Wilhelm Pfeffer thoroughly studied the process in 1877.
Timeline
1854
Thomas Graham introduced the term “osmose,” now called osmosis.
1877
Wilhelm Pfeffer conducted an early thorough study of osmosis.
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References
- 1.Osmosis Definition, Examples, & Facts
Encyclopaedia Britannica · other · reliability: high · retrieved 21 October 2026
Supports: Defines osmosis and describes semipermeable membranes, the history of the term, osmotic pressure and reverse osmosis.
- 2.5.2 Passive Transport - Biology for AP® Courses
OpenStax, Rice University · other · reliability: high · retrieved 21 October 2026
Supports: Explains osmosis across cell membranes, aquaporins, tonicity and cell responses in hypotonic, hypertonic and isotonic environments.
- 3.Osmotic Pressure and Its Biological Implications
International Journal of Molecular Sciences (MDPI) · other · reliability: high · retrieved 21 October 2026
Supports: Peer-reviewed review of osmotic pressure, its relationship to osmosis and its biological implications for cells.
- 4.Osmotic pressure Description, Types, Measurement, & Applications
Encyclopaedia Britannica · other · reliability: high · retrieved 21 October 2026
Supports: Defines osmotic pressure and explains its relation to osmosis, reverse osmosis and applications such as desalination.
References are suggested by the research engine and may be incomplete. Unverified URLs are omitted rather than guessed.
Article history
- Version 121 October 2026 · EditorCreated by Zapia