Water potential, symbol ψ (the Greek letter psi), is a measure of the tendency of water molecules to move out of a solution. It depends on how many free water molecules there are and how much pressure is acting on the water. It is measured in units of pressure, usually kilopascals (kPa).By definition, pure water at atmospheric pressure has a water potential of 0 kPa. This is the highest possible water potential at atmospheric pressure.When a solute such as sucrose or salt dissolves in water, water molecules cluster around the solute particles. Fewer water molecules are free to move, so the water potential falls below zero: it becomes negative. The more concentrated the solution, the more negative its water potential.For example:
• pure water: ψ = 0 kPa
• a dilute solution: ψ = −300 kPa
• a concentrated solution: ψ = −700 kPaWater always moves from a region of higher water potential (less negative) to a region of lower water potential (more negative). So water moves from pure water into any solution, and from a dilute solution into a concentrated one.Worked example: a cell with ψ = −400 kPa is placed in a solution with ψ = −250 kPa. −250 kPa is higher (less negative) than −400 kPa, so water moves from the solution into the cell.Remember: a more negative number means a lower water potential, even though the magnitude is bigger.
• pure water: ψ = 0 kPa
• a dilute solution: ψ = −300 kPa
• a concentrated solution: ψ = −700 kPaWater always moves from a region of higher water potential (less negative) to a region of lower water potential (more negative). So water moves from pure water into any solution, and from a dilute solution into a concentrated one.Worked example: a cell with ψ = −400 kPa is placed in a solution with ψ = −250 kPa. −250 kPa is higher (less negative) than −400 kPa, so water moves from the solution into the cell.Remember: a more negative number means a lower water potential, even though the magnitude is bigger.