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What is osmosis?

Osmosis is the net movement of water molecules from a dilute solution to a concentrated one through a partially permeable membrane.

Osmosis explained

Water moves down its own concentration gradient — from higher to lower water potential — until the potentials are equal or pressure stops further movement.
Plant cells become turgid in dilute solutions and plasmolysed in concentrated ones; animal cells, lacking a wall, can burst or crenate instead.

Key formula

high water potential → low water potential

Worked example

Potato cylinders placed in concentrated sucrose lose mass because water leaves the cells by osmosis.

Examiner tip

Osmosis moves water only. If a question involves ions or glucose, the answer is diffusion or active transport instead.

Osmosis: common questions

Is osmosis active or passive?
Passive — it needs no ATP because water moves down a water potential gradient.
What is water potential?
A measure of the tendency of water to move; pure water has the highest value, zero, and solutions have negative values.

Related terms

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

Active transport

Active transport moves substances against a concentration gradient using carrier proteins and energy from ATP.

Enzyme

An enzyme is a biological catalyst — a protein that speeds up a specific reaction by lowering its activation energy.

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