Water and osmosis

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

Spec 3.2.1, 3.2.2, 3.2.3, 3.2.6Download all Osmosis notes (PDF)Download all notes (PDF)6 pages

Water and osmosisSpec 3.2.1, 3.2.2, 3.2.3, 3.2.6

In short

Osmosis is the diffusion of water through a partially permeable membrane, which lets small molecules such as water through but not larger ones such as sucrose. Water is also a solvent, which matters for digestion, excretion and transport. Plant cells take in water by osmosis, and water pressing outwards on the cell wall makes the cells firm, supporting the plant.

Water as a solvent

Water is a solvent: many substances dissolve in it. This is important in organisms in three ways.

Digestion: food molecules dissolve in water so enzymes can break them down and the products can be absorbed.
Excretion: waste substances such as urea dissolve in water and are removed in urine.
Transport: substances dissolve in the water of blood plasma (animals) and of sap in xylem and phloem (plants) and are carried around the organism.

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'.

Support in plants

Plant cells take in water by osmosis. The water inside the cells presses outwards on the cell wall. This pressure of water pushes the cell wall outwards and makes the cells firm. The firm cells support the plant, especially the stems and leaves of non-woody plants.

If a plant loses water, the pressure falls, the cells become soft and the plant wilts.

Exam tip:

For plant support, say 'water pressing outwards on the cell wall'. 'The water makes the cell stiff' alone does not gain the mark.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

How does osmosis differ from diffusion?

Osmosis is a type of diffusion that involves only water moving through a partially permeable membrane. Diffusion is the net movement of any particles, such as oxygen or carbon dioxide, down a concentration gradient. Both are passive, so neither needs energy from respiration. Never write that sugar moves by osmosis.

How does osmosis affect plant cells?

Plant cells in a dilute solution take in water by osmosis, and the water presses outwards on the cell wall, making the cells firm. Firm cells support the stems and leaves of non-woody plants. In a concentrated solution, plant cells lose water, the pressure falls, the cells become soft and the plant wilts.

Why don't plant cells burst in pure water?

Supplement Plant cells do not burst in pure water because the strong cell wall resists the pressure. Water enters by osmosis, the vacuole swells and presses the cell contents against the wall, and the cell becomes turgid. Animal cells have no cell wall, so if they take in too much water they can burst.

All 5 questions on Osmosis