Revise: Transport across membranesOsmosis

Cells can gain or lose water by the process of osmosis. This depends on the water concentration of the solution inside the cell compared to water concentration of the solution outside the cell.

The water concentration can be thought of as the proportion of a solution that is water. Solutions with a high concentration of solute molecules, such as sugars or salts, have a low concentration of water molecules and vice versa.

Definition of osmosis

Osmosis can be defined in two ways:

• The net movement of water from a region of high water concentration to a region of low water concentration through a selectively permeable membrane.
• The net movement of water from a region of low solute concentration to a region of high solute concentration through a selectively permeable membrane.

Both definitions say the same thing in different ways. A high solute concentration means there is a lower proportion of water in the solution.

This means osmosis is a special case of diffusion: the diffusion of water. In the diagram above solution two gains water faster than it loses sugar.

This is because the selectively permeable membrane lets water molecules pass through much more rapidly than it lets sugar molecules pass through.