What does exosmotic mean?
Exosmotic is an adjective describing processes involving exosmosis — the outward flow of water or another fluid across a semipermeable membrane, typically out of a cell, vessel, or compartment, in the direction dictated by a difference in solute concentration. When the surrounding solution is more concentrated than a cell's interior, water moves outward by exosmotic flow, causing the cell to shrink or lose turgor; this is why plants wilt in salty soil and why tissues firm up again when placed in fresh water. The word pairs with its mirror image, endosmotic, which describes inward flow, and both derive from the same root as the everyday scientific staple 'osmosis'. Though once standard vocabulary in nineteenth-century physiology, when researchers like Dutrochet first charted these membrane currents, exosmotic today appears mainly in historical accounts, plant science, and specialist discussions of membrane transport — a precise, if somewhat old-fashioned, way of naming which way the water flows.
adjectiveRelating to or involving exosmosis: the outward flow of a fluid through a semipermeable membrane from a region of lower to higher solute concentration. Used chiefly in older physiology and biology texts as the counterpart of endosmotic.
- Of or relating to exosmosis: the outward passage of fluid through a semipermeable membrane, especially out of a cell or vessel.
- (Historical, in early physiology) Describing the direction of diffusion observed in the classic membrane experiments of the nineteenth century, contrasted with endosmotic.
"The exosmotic movement of water from the cells caused the tissue to become flaccid."
"In a hypertonic solution, the exosmotic loss of water causes red blood cells to shrink and crenate."
"Early experimenters such as Dutrochet distinguished endosmotic from exosmotic currents across animal membranes."
Every time a plant wilts, exosmotic force is at work — water quietly streaming out through cell membranes, one of the first phenomena nineteenth-century biologists ever measured through a membrane.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of exosmotic
Exosmotic combines the Greek prefix exo-, meaning 'outside', with -osmotic, from Greek osmos, meaning 'push' or 'impulse' — the same root that gave English the word osmosis. The term arose in early nineteenth-century physiology alongside endosmotic, as naturalists such as Henri Dutrochet investigated how fluids pass through membranes in opposite directions. Its cognates include osmotic, osmosis, and other exo- compounds like exothermic and exoskeleton.
Related word forms
How exosmotic is actually used
A technical term of physiology and biology, now largely confined to historical and specialist literature; modern writing usually speaks simply of osmotic efflux or outward osmosis rather than using 'exosmotic'. Its opposite is 'endosmotic' (or endosmosis).
Easily confused with exosmotic
Endosmotic describes the inward flow of fluid through a semipermeable membrane, whereas exosmotic describes the outward flow.