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Water Potential
Botanists use the term water potential when predicting the movement of water into and out of plant cells. Remember,
the movement of water across a semi-permeable membrane as it follows its own concentration gradient is osmosis. But
plants are a unique case because they have a cell wall which prevents a plant cell from bursting if it takes on too much
water. If a plant cell is placed in a hypotonic environment, it will take on water UNTIL water pressure inside the cell
builds up to a point that it cannot take on any more. This increase in water pressure will actually push water molecules
back out of the cell against water’s concentration gradient until the amount of water moving in due to solute difference
is equal to the water moving out due to water pressure—this is a dynamic equilibrium!
Water potential is a measure of how likely water is to move from one location (say outside the cell) to another (inside
the cell). For example, in the case above….a plant cell placed in a hypotonic solution…..water has a higher potential
(tendency) to move INTO the cell than it does to move OUT of it. Therefore, water potential is GREATER outside the cell
than inside the cell because the water outside the cell has the GREATER POTENTIAL TO MOVE. Just like any solute will
diffuse DOWN it’s concentration gradient—from high to low concentration, water will always move from an area of
greater water potential to an area of lesser water potential.
Water potential is made up of 2 parts---solute potential and pressure potential. Add both together to get the total
water potential. See below.
Note: (we use the greek letter “psi” or “Ψ” to represent water potential.)
Solute potential + Pressure potential = Water potential
Ψs + Ψ p = Ψ
Solute potential—( Ψs )—the greater the concentration of a solute, the lower the water potential.—an inverse
relationship. Think of it this way: if you place a plant cell in...