What is osmosis?

Osmosis is the movement of water from a high concentration to a low concentration through a partially permeable membrane. It is a special type of diffusion.

You may get questions asking you to decide which way direction water will flow through a partially permeable membrane. Always remember that water will move from the side with highest water concentration (lowest solute concentration) to the side with the lowest water concentration (highest solute concentration).


Cell membranes exist in all cells. They are partially permeable membranes as they allow small molecules such as water molecules to diffuse through them, but not larger molecules such as sugars and salt ions. Osmosis can therefore happen in and out of cells, effecting the amount of water in the cell.

Plants are supported by the water pressure in their cells (this is useful because they do not have a skeleton). This movement of water in and out of plant cells is due to osmosis. If a cell is placed in pure water, water will move into the cell by osmosis. This is because it will move from a higher concentration in the pure water to a lower concentration in the cell, until the amount of water on each side of the permeable membrane is the same – until equilibrium is reached.

It can have consequences:

If too much water enters the plant cell it becomes turgid – completely full with water.

If too much water leaves the plant cell it becomes plasmolysed – the cell becomes limp and the cell membrane comes away from the cell wall.

A similar effect can be seen in animal cells, such as human red blood cells:

If too much water enters the animal cell it can burst – lysis

If too much water leaves the animal cell it can shrink – crenation

Animal cells burst and shrink, unlike plant cells, because they do not have the strong cell walls that plants have.

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