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 selectively 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

Water molecules move from solution one to solution two.

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 selectively permeable.
  • Water will move into and out of cells by osmosis.
  • The net movement of water will be from the high water/low solute concentration into the low water/high solute concentration region.

Plant cells

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:

View of a plant cell

  • pure water
    • In pure water, the cell contents - the cytoplasm and vacuole - push against the cell wall and the cell becomes turgid
  • 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.

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 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.