Water Potential

Water Potential Tendency of water molecules to move from high concentration of water to low concentration of water Hypotonichypertonic The physical property predicting the direction in which water will flow controlled by the solute concentration

Water potentials (ψ) are a way of measuring the free-energy of water Water will flow spontaneously from a high potential to a low potential like a ball rolling down a hill (ψ) = Greek letter psi

Water potential: The measure of the relative tendency of water to move from one area to another, and is commonly represented by the Greek letter Ψ (Psi) Measured in pressure units, kilopascals (kPa) Pure water has a water potential of 0 kPa Solutions have NEGATIVE water potentials (less than 0) The more negative the number, the lower the water potential of that solution (more hypertonic) Water moves by osmosis DOWN a “water potential gradient” from HIGH water potential (less negative) to low water potential (more negative) Modified definition of OSMOSIS (You need to know this one!): THE NET MOVEMENT OF WATER MOLECULES FROM A REGION OF HIGHER WATER POTENTIAL TO A REGION OF LOWER WATER POTENTIAL THROUGH A PARTIALLY PERMEABLE MEBRANE

Solute Potential The amount that the solute molecules LOWER the water potential of a solution ALWAYS negative Lower solute potential means MORE negative, more solute Higher solute potential, closer to zero, LESS negative (but still negative), less solute

In animal cells… Water potential = solute potential Only two factors are water potential and solute potential Plant cells have a cell wall …cell wall exerts pressure…new factor PRESSURE POTENTIAL

Pressure Potential PRESSURE POTENTIAL Contributions made by pressure to water potential Makes water potential less negative Always positive number Increase pressure, increase movement of water molecules from solution (increase water potential)

Water potential (ψ) = pressure potential (ψp ) + solute potential (ψs )

Physical pressure due to air & the container

Factors that determine plant water potential:  1) amount of solutes- increasing concentrations will lower  the free energy (water potential); termed osmotic potential (ψs) 2)  turgor pressure (ψp) in plant cell- positive pressure inside plant cells; increases free energy; loss of turgor = wilting

Plasmolysis A phenomenon in plant cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall when the cell loses water to a hypertonic environment. TURGID Central vacuole FULL of water, cell membrane pushed up against cell wall -cell wall is applying pressure to “protoplast” (living part of the cell within the cell wall) Pressure potential increases water potential inside the cell increase (until it equals water potential outside the cell, equilibrium) FLACCID Central vacuole has lost lots of water; cell membrane pulls away from cell wall  cell has plasmolyzed Incipient Plasmolysis  point at which pressure potential has just reached zero and plasmolysis is about to occur

Water Potential sometimes called…Osmotic Potential The tendency of water to move across a selectively permeable membrane into a solution Determined by measuring the pressure required to stop the osmotic movement of water into the solution.

Osmosis Review