In biology, water is essential. It acts as a solvent, meaning other substances can dissolve in it. This is vital for processes like digestion, excretion, and the transport of substances around an organism.

Water is constantly moving into and out of cells. This movement happens across the cell's partially permeable membrane. The special name for this movement of water is osmosis.

For your exam, you need a precise definition of osmosis. Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. 'Water potential' is a measure of the concentration of free water molecules; a dilute solution has a high water potential, while a concentrated solution has a low water potential.