Definition · Plain-language
Osmosis
Osmosis is the movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one.
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Water moving across a membrane
Osmosis is a special kind of diffusion that involves only water. It happens when two solutions of different concentration are separated by a partially permeable membrane — a barrier that lets water through but holds back dissolved substances. Water moves from the side with more water (the dilute solution) to the side with less water (the concentrated solution), evening out the difference. Because the dissolved substance cannot cross the membrane, water moves instead. Osmosis is passive, meaning it happens on its own without the cell spending energy.
Osmosis in cells
Cell membranes are partially permeable, so osmosis controls how water enters and leaves every cell. If a cell sits in a dilute solution, water moves in by osmosis and the cell swells; in a concentrated solution, water moves out and the cell shrinks. In plants, this is how root cells absorb water from the soil, and the water pressure inside the large vacuole keeps cells firm and the plant upright. When plant cells lose too much water by osmosis, the plant wilts. Animal cells, lacking a wall, can burst or shrivel if osmosis is not balanced.
Why water moves and the solute does not
The reason osmosis happens is the partially permeable membrane. Its tiny pores let small water molecules pass freely but block the larger dissolved particles. Since the dissolved substance cannot move to even out the two concentrations, water moves instead, flowing towards the more concentrated solution. The driving force is sometimes described in terms of water potential: water always moves from a higher water potential (more water) to a lower one (less water). The movement continues until both sides reach the same concentration.
Key facts
At a glance
- Definition: movement of water across a partially permeable membrane
- Direction: from dilute (more water) to concentrated (less water)
- Type: a special case of diffusion, involving only water
- Energy: none needed — it is passive
- In plants: how roots absorb water; keeps cells firm
- Stops when: the concentrations on both sides are balanced
Common misconceptions
What people often get wrong
Often heard: In osmosis, the dissolved substance moves to even out the concentrations.
Actually: It is the water that moves, not the dissolved substance. The partially permeable membrane blocks the solute, so water crosses instead, flowing towards the more concentrated side to balance the two.
Often heard: Osmosis needs energy from the cell to happen.
Actually: Osmosis is passive and needs no energy. Water moves down its concentration gradient on its own. This is different from active transport, which moves substances against the gradient and does require energy.
Often heard: Osmosis can move any dissolved particle, not just water.
Actually: Osmosis specifically describes the movement of water. The movement of dissolved particles from high to low concentration is ordinary diffusion. Osmosis is the water-only special case across a partially permeable membrane.
Going deeper
