Water and osmosis

Movement into and out of cells · Osmosis · note 1 of 4

Spec B3.2.1, B3.2.2Download all Osmosis notes (PDF)Download all notes (PDF)6 pages

Water and osmosisSpec B3.2.1, B3.2.2

In short

Osmosis is the diffusion of water through a partially permeable membrane; water is also the solvent that carries dissolved substances in organisms.

Osmosis

Water diffuses through partially permeable membranes by osmosis. A partially permeable membrane lets small molecules such as water through, but not larger molecules such as sucrose or proteins. Water moves into and out of cells by osmosis through the cell membrane.

Water molecules pass through the partially permeable membrane, but the larger sugar molecules cannot. There is a net movement of water from the dilute solution into the concentrated solution.

Common mistake:

Osmosis is only about water. Do not write that 'sugar moves by osmosis' or that osmosis is 'water moving through any membrane'.

Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026