Osmotic potential (π) is the part of water potential that is due to dissolved solutes. It is now usually called solute potential, written ψs. Both names describe the same idea: how much the solutes lower the water potential.Solute molecules attract water molecules, so fewer water molecules are free to move. This lowers the tendency of water to leave the solution. So:
• pure water has a solute potential of 0 kPa;
• any solution has a solute potential below zero, so solute potential is always zero or negative;
• the more solute dissolved, the more negative the solute potential.Solute potential depends on the number of dissolved particles, not their type. A solution of salt that splits into two ions has a more negative solute potential than a sugar solution of the same molar concentration.Comparing solutions: solution X has ψs = −400 kPa and solution Y has ψs = −900 kPa. Y is more negative, so Y has the higher solute concentration. If they were separated by a partially permeable membrane, water would move from X to Y.In a plant cell, the solute potential comes mainly from sugars, ions and other solutes in the cell sap of the vacuole. Cells can change it: by actively pumping ions into the vacuole or making sugars, they make the solute potential more negative. Water then enters the cell by osmosis. This is how guard cells take in water to open stomata.Note: some older books define osmotic pressure as a positive number. In the water potential system used here, solute potential is given as a negative value in kPa.
• pure water has a solute potential of 0 kPa;
• any solution has a solute potential below zero, so solute potential is always zero or negative;
• the more solute dissolved, the more negative the solute potential.Solute potential depends on the number of dissolved particles, not their type. A solution of salt that splits into two ions has a more negative solute potential than a sugar solution of the same molar concentration.Comparing solutions: solution X has ψs = −400 kPa and solution Y has ψs = −900 kPa. Y is more negative, so Y has the higher solute concentration. If they were separated by a partially permeable membrane, water would move from X to Y.In a plant cell, the solute potential comes mainly from sugars, ions and other solutes in the cell sap of the vacuole. Cells can change it: by actively pumping ions into the vacuole or making sugars, they make the solute potential more negative. Water then enters the cell by osmosis. This is how guard cells take in water to open stomata.Note: some older books define osmotic pressure as a positive number. In the water potential system used here, solute potential is given as a negative value in kPa.