Osmosis is a special type of diffusion of liquids. Osmosis can also be defined as the movement of water molecules from a region of higher free energy to a region of lower free energy through a semi permeable membrane until they reach a state of dynamic equilibrium.
The potential with which the water molecules diffuse through the membrane is called osmotic potential. The osmotic potential is otherwise called water potential (or) chemical potential. The water potential of pure water at 25°C is zero. When a solute (sugar or salt) is added to pure water, the water potential is decreased and it becomes negative as the free energy decreases. Always water moves from a region of higher water potential to a region of lower water potential. The symbol for water potential is Ψ and the unit is pascal.
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