Osmosis is the diffusion of water molecules, from a region where the water molecules 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

Step one

The beaker contains water and sugar molecules

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. In theory, the level of solution two will rise, but this will be opposed by gravity and will be dependent on the width of the container.

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. Root hair cells, if the soil is wet or moist, 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 and supports the cell and stops it bursting when it gains water by osmosis.

If plant cells are placed in solutions of increasing solute concentration:

Pure water

View of a plant cell

In pure water, the cell contents (the cytoplasm and vacuole) 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. They shrink and pull away from the cell wall. The cell becomes flaccid. It is becoming plasmolysed.

Highly-concentrated solution

In a very concentrated solution, the cell undergoes full plasmolysis as the cells lose more water.

Plants would be exposed to higher concentrations of solutes if there was less water in the soil - for instance, if plants were not watered, or plants in drought conditions. Plant cells would then lose water by osmosis.

Aquatic, freshwater plants placed in the sea, or a seaweed in a rock pool where the water evaporated in the Sun, would also lose water by osmosis.

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 could:

  • lose water and shrink
  • gain water, swell and burst in a more dilute solution
Red blood cells in different solutions

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.