Osmosis is the diffusion of water molecules, from a region of higher concentration to a region of lower concentration, through a selectively permeable membrane.

A dilute solution contains a high concentration of water molecules, while a concentrated solution contains a low concentration of water molecules.

Eventually, the concentration either side of the membrane will be the same. At this point, there will be equal movement of water molecules in both directions. The solution is said to be in equilibrium – there is therefore no net movement in one direction.

Osmosis in cells

The results of osmosis are different in plant and animal cells.

Plant cells

Plant cells have a strong cellulose cell wall on the outside of the cell membrane. This supports the cell and stops it bursting when it gains water by osmosis.

A plant cell in a dilute solution (higher water concentration than the cell contents)

Water enters the cell by osmosis. The cytoplasm pushes against the cell wall and the cell becomes turgid.

A plant cell in a concentrated solution (lower water concentration than the cell contents)

Water leaves the cell by osmosis. The cytoplasm shrinks and pulls away from the cell wall. This process is called plasmolysis. The cell becomes flaccid and the plant wilts.

Turgid plant cells play an important part in supporting the plant.

Animal cells

Animal cells do not have a cell wall. They change size and shape when put into solutions that are at a different concentration to the cell contents.

For example, red blood cells:

• gain water, swell and burst in a more dilute solution (this is called haemolysis)
• lose water and shrink in a more concentrated solution (they become crenated or wrinkled)

These things do not happen inside the body. Osmoregulation involving the kidneys ensures that the concentration of the blood stays about the same as the concentration of the cell contents.