Download Water Potential

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
(ψ)
• Water potentials
are a way of measuring the freeenergy 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
Ho w So l u t e Co n c en t r a t i o n Af f ec t s
Wa t er Po t en t i a l
12
Wa t er Po t en t i a l
10
8
6
4
2
0
1
2
3
4
5
6
7
8
So l u t e Co n c en t r a t i o n
9
1
11
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