What does endosmosis mean?
Endosmosis is a noun denoting the inward flow of a fluid—most often water—through a semipermeable membrane into a cell or enclosed space, driven by differences in solute concentration. It names one direction of the broader process of osmosis: when the solution inside a cell is more concentrated than the surrounding medium, water moves inward by endosmosis, causing the cell to swell and, in plants, to become firm and turgid. Its counterpart, exosmosis, describes the same movement directed outward, as when a cell placed in salty water loses fluid. Endosmosis appears chiefly in biology, botany, and physiology textbooks, and in laboratory descriptions of membrane experiments; everyday scientific writing tends to use 'osmosis' alone, specifying direction only when it matters. Though technical in register, the word neatly captures a phenomenon essential to life—the constant, silent traffic of water that plumps cells, supports stems, and keeps tissues functioning.
nounThe movement of a fluid, especially water, into and through a membrane from outside toward the inside of a cell or organism, driven by osmotic pressure. It is the inward-flowing counterpart of osmosis.
- The osmotic flow of water or other fluid through a semipermeable membrane in an inward direction, into a cell or cavity (chiefly in biology and physiology).
- By extension, any inward passage or absorption of a substance resembling such osmotic flow (rare, figurative).
"Water entered the cell by endosmosis until the membrane swelled taut."
"Placed in fresh water, the amoeba swells as endosmosis draws liquid steadily through its membrane."
"The physiologist measured the rate of endosmosis across the artificial membrane under differing salt concentrations."
The plural is grammatically regular but uncommon; the word is nearly always used as an uncountable noun referring to the general process.
"The experiment compared the endosmoses observed at three different concentrations."
Every time a plant cell stiffens with water it owes its firmness to endosmosis — the invisible inward tide that keeps lettuce crisp and flowers upright.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of endosmosis
Endosmosis was coined in the nineteenth century by combining the Greek prefix 'endo-' (within) with 'osmosis', itself derived from Greek 'ōsmos' (a push or impulse). It was formed to name the inward-directed half of osmotic flow, alongside its mirror term exosmosis ('exo-', outside). The word shares its root with other 'endo-' formations such as endoderm and endocrine, all built on the same Greek notion of interiority.
How endosmosis is actually used
A technical term confined largely to biology, botany, and older physiological literature; in modern writing scientists usually simply say 'osmosis' and specify direction. The word carries no emotional connotation and would sound pedantic outside scientific contexts. Its antonym exosmosis is similarly rare today.
Easily confused with endosmosis
Exosmosis is the outward flow of fluid through a membrane out of a cell or organism, whereas endosmosis is the corresponding inward flow.
Osmosis is the general two-way diffusion of solvent through a semipermeable membrane; endosmosis specifically denotes the direction of flow toward the inside.