The simple osmosis definition is this: water moves through a thin barrier, called a membrane, from the side where less is dissolved in it to the side where more is dissolved. Drop a shrivelled raisin into a glass of water overnight and it plumps up, because water slips in through its skin toward the sugar inside.
The barrier has to be selectively permeable, meaning small water molecules can pass but larger dissolved particles, such as sugar, cannot get through easily. No pump and no energy are needed; the water simply drifts until both sides are closer to balanced. Outside biology class, people also say they learned something by osmosis when they picked it up without studying.
Entry at a glance
Meaning
Passive flow of water across a membrane toward higher solute concentrationPart of speech
NounPronunciation
oz-MOH-sis (US also ahs-MOH-sis)Word forms
osmotic (adjective), osmotically (adverb); plural osmosesOrigin
From osmose, short for earlier endosmose, built on Greek ōsmos, a pushField
Biology, chemistry, physicsFigurative sense
Absorbing ideas or skills gradually, without effort
The osmosis definition, step by step
Picture a bag made of a thin membrane, filled with sugary water, sitting in a bowl of plain water.
- Water molecules are always jiggling and crossing the membrane in both directions.
- Inside the bag, many water molecules are busy surrounding sugar, so fewer are free to cross outward.
- More water therefore crosses inward than outward.
- The bag swells until pressure inside pushes back hard enough to balance the flow.
That push needed to stop the flow is called osmotic pressure. The key words to remember: solvent (the liquid doing the moving, usually water), solute (what is dissolved) and membrane (the barrier).
Hypotonic, hypertonic and isotonic
| Outside solution | What happens to an animal cell |
|---|---|
| Hypotonic (less dissolved outside) | Water rushes in; the cell swells and may burst |
| Hypertonic (more dissolved outside) | Water leaves; the cell shrinks and wrinkles |
| Isotonic (equal) | No overall change in size |
Red blood cells placed in pure water swell and can burst, while cells placed in very salty water shrink. Hospitals use drips matched to body fluids for exactly this reason. Plant cells have a stiff wall, so in plain water they become firm rather than bursting, which keeps leaves and stems upright. Keeping these water levels steady is part of homeostasis.
Osmosis examples you can see at home
- Sprinkle salt on sliced cucumber and, within minutes, a puddle of water collects on the plate.
- Limp spinach left in cold water for twenty minutes turns crisp again.
- A cut potato slice left in strong salt water turns floppy, while one in plain tap water stays stiff.
- Dried beans soaked overnight double in size before cooking.
- Sugar sprinkled over chopped strawberries draws out juice and makes a syrup.
Osmosis versus diffusion
Students often mix these up. Diffusion is any substance spreading from where there is more of it to where there is less, like perfume filling a room. It needs no membrane. Osmosis is a special case: only water (or another solvent) moves, and it moves across a selectively permeable membrane. Both are passive, meaning the cell spends no energy. When a cell must move something against the natural flow, it uses active transport and burns energy to do it.
Plants rely on osmosis to draw water from soil into root hairs, which then supports photosynthesis in the leaves.
Reverse osmosis in water purifiers
In reverse osmosis, pressure is used to force water the opposite way, from the salty or dirty side through a very fine membrane to the clean side. The membrane holds back many dissolved salts and impurities. This is how many home purifiers in India and desalination plants on coastlines produce drinking water. The trade off is that some water is rejected along with the impurities, and the process needs a pump.
History of the word
A short definition of osmosis hides a long history. The term grew out of endosmose, a word coined by a French scientist in the early nineteenth century to describe fluid passing inward through a membrane. It was later shortened to osmose, and English Latinised that into osmosis in the middle of the same century. The root is Greek ōsmos, a thrust or push, from a verb meaning to push. By around the start of the twentieth century, writers were also using osmosis loosely for ideas that soak into a person.
Learning by osmosis: the figurative use
Outside the lab, the osmosis meaning shifts: it describes taking something in gradually just by being around it. She picked up Spanish by osmosis while living with her cousins. Don't expect the interns to learn the billing process by osmosis; give them a guide. The phrase is usually light and often a little doubtful, hinting that real learning needs more than sitting near an expert. For other science words that crossed into daily talk, see catalyst, entropy and the science words hub.
Frequently asked questions
What is the easiest way to define osmosis?
To define osmosis in one line, call it water seeping across a thin barrier into the more concentrated solution. The water moves on its own, without the cell using energy.
Does osmosis need energy?
No. Osmosis is passive, like a ball rolling downhill. Active transport, by contrast, is when a cell spends energy to move substances against the natural direction.
What is the plural of osmosis?
The plural is osmoses, pronounced oz-MOH-seez, though it is rarely needed. Most writers talk about osmosis as an uncountable process.
Is osmosis only about water?
In biology it almost always means water, because water is the solvent in living things. In chemistry the term can cover other solvents crossing a membrane in the same way.
What does it mean to learn something by osmosis?
It means you absorbed it slowly through exposure rather than study. Someone who grew up in a kitchen might know cooking times by osmosis.
Why does salt kill garden slugs?
Salt on a slug's moist skin creates a very concentrated solution outside its body. Water rushes out of its cells by osmosis, and the slug dries out quickly.