4.3 Osmosis and water potential

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane.

Water potential, given the symbol Ψ and measured in kilopascals, is the tendency of water to move out of a solution. Pure water at atmospheric pressure has a water potential of zero. Adding any solute lowers the water potential, so every solution has a negative water potential.

A cell placed in a solution of higher water potential than its own gains water. An animal cell, having no cell wall, may continue to take in water until it bursts — lysis. A plant cell in the same solution becomes turgid instead: the cellulose cell wall resists further expansion and the pressure potential rises until net water movement stops.

A cell placed in a solution of lower water potential loses water. An animal cell crenates, shrinking and becoming wrinkled. A plant cell becomes plasmolysed as the protoplast pulls away from the cell wall.