What does exosmosis mean?
Exosmosis refers to the movement of a liquid—most commonly water—outward through a semipermeable membrane, traveling from an area of lower solute concentration toward one of higher concentration. It is simply osmosis described directionally: where ordinary osmosis might draw water into a cell, exosmosis draws it out. This distinction matters most in biology and physiology, where it explains familiar phenomena such as why salted vegetables release their juices, why plants wilt in saline soil, and how organisms that live in salty environments must guard against constant water loss. Exosmosis is a technical term, confined largely to textbooks and scientific writing, and is nearly always discussed alongside its counterpart, endosmosis, which names the reverse inward flow. Though rarely heard in casual conversation, the word offers a precise label for a process that shapes everything from food preservation to kidney function, making it a useful term for anyone studying life at the cellular level.
nounThe outward flow of a fluid through a semipermeable membrane from a region of lower solute concentration to one of higher concentration; essentially osmosis in the outward direction.
- The passage of a fluid, especially water, outward through a semipermeable membrane from a region of lower to higher solute concentration.
"In freshwater fish placed in seawater, exosmosis draws water out of their cells, threatening dehydration."
"When vegetables are sprinkled with salt, exosmosis pulls water out of their cells, causing them to soften."
"The physiologist measured the rate of exosmosis across the artificial membrane under varying salt concentrations."
Rarely pluralized in practice, since the process is usually discussed in general rather than countable terms; the plural appears mainly in comparative scientific contexts.
"The experiment recorded differing rates of exosmoses across several membrane types."
Every prune-like fingertip after a long bath and every wilted lettuce leaf owes its fate to exosmosis — water quietly leaking outward across cell membranes.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of exosmosis
Exosmosis derives from Greek roots: 'exo-' meaning outside or outward, and 'ōsmos' meaning impulse or push, combined with the noun-forming suffix '-osis.' The word was coined in the nineteenth century alongside endosmosis as scientists formalized the study of diffusion through membranes, with 'osmosis' itself emerging from the same Greek root. Related terms include osmotic and endosmosis, all descendants of that same Greek origin.
Related word forms
How exosmosis is actually used
A technical term used almost exclusively in biology, physiology, plant science, and chemistry. It is rare in everyday English and typically appears only when contrasting with endosmosis; in most contexts writers simply say 'water moves out by osmosis.' The adjective form 'exosmotic' is correspondingly uncommon.
Easily confused with exosmosis
Endosmosis is the inward osmotic flow of water into a cell or solution, whereas exosmosis is its opposite — the outward flow across a membrane.
Exocytosis is active transport of material out of a cell via vesicles fusing with the membrane, while exosmosis is passive diffusion of water through a semipermeable membrane.