Osmosis is the diffusion of water molecules, from a region where they are in higher concentration, to a region where they are in lower concentration, through a partially permeable membrane.

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

Osmosis refers to the movement of water molecules only.

The slideshow shows an example of osmosis showing the direction of movement of water between two different concentrations of sugar solutions:

The process of osmosis

Water molecules move from solution 1 to solution 2 by osmosis

When the concentration of water is the same on both sides of the membrane, the movement of water molecules will be the same in both directions. There will be no net movement of water molecules.

Similar observations will be made with solutions containing different solutes, for instance, salt instead of sugar.

Osmosis across living cells

Cells contain dilute solutions of ions, sugars and amino acids. The cell membrane is partially permeable. Water will move into and out of cells by osmosis.

Plant cells

Isolated plant cells placed in a dilute solution or water will take in water by osmosis. If the soil is wet or moist then root hair cells will also take up water by osmosis. Leaf cells of land plants, unless it is raining or the humidity is high, will have a tendency to lose water.

Plant cells have a strong cellulose cell wall outside the cell membrane. The cell wall is fully permeable to all molecules. It supports the cell and stops it bursting when it gains water by osmosis.

Under normal circumstances, the cells in all organisms live in a safe equilibrium. They have enough water to live but not too much. They also have the correct amount of salts and sugars. If individual cells are placed in different solutions, we can see how important it is that equilibrium is maintained in living organisms.

Pure water

In pure water, the cell contents push against the cell wall and the cell becomes turgid. Fully turgid cells support the stems of non-woody plants.

Concentrated solution

In a more concentrated solution, the cell contents lose water by osmosis and the cells shrink. The cell membrance starts to pull away from the cell wall. The cell becomes flaccid.

Highly concentrated solution

In a very concentrated solution, the cell membrane pulls away from the cell wall completely; this is plasmolysis.

Diagram showing the effects of osmosis in animals

Animal cells

Animal cells also take in and lose water by osmosis. They do not have a cell wall, so will change size and shape when put into solutions that are at a different concentration to the cell contents.

For example, red blood cells:

Red blood cells in different solutionsRed blood cells lose water and shrink in a concentrated solution. They swell and burst in a solution that is too dilute.

In animals, the concentration of body fluids, blood plasma and tissue fluid must be kept within strict limits. If cells lose or gain too much water by osmosis, they do not function efficiently.