1. Water Potential
AP Biology

2. Water Potential – aka “Osmotic Pressure”
Day 13
The tendency of water to leave one place in favor of another place.
Water ALWAYS moves from an area of higher water potential to an area of lower water potential.
In animals….water moves into or out of a cell depends on solute concentration on either side of the cell membrane ( cells lyse or shrink)
In plants…..Affected by two factors because of cell wall:
Pressure exerted by cell wall on cell membrane (think balloon trapped in a box)
Amount of solute (ions)
Formula for water potential is:

3. In plants, pressure exerted by the rigid cell wall that limits further water uptake (also known as TURGOR PRESSURE)
Pure water has solute potential of ZERO. As solute is added, the value for solute potential becomes more negative. This causes the water potential to decrease also.
As solute is added, the water potential of a solution drops, and water will tend to move into the solution.
In the laboratory, we use “bars” as the
unit of measure for water potential.
1 bar = approximately 1 atm

4. Examples: Cell in distilled water.
Water potential outside = 0
Water potential inside = (-) since cells contain solutes
which have a negative solute potential
Water will move into the cell since inside cell is more negative
Over time, the inflow of water into the cell causes pressure to build (positive turgor or pressure) which will eventually override the negative from the solute potential.
Eventually the water potential inside and outside the cell will equal each other and net water movement ceases
Note, I said “net” water movement. Equilibrium still means movement is occurring. It just means the water going in equals the water moving out of the cell