Download Water Potential Notes

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Water potential is a concept
that helps to describe the
tendency of water to move
from one area to another,
particularly into or out of cells.
oWater molecules move randomly.
oWhen water is enclosed by a membrane some of
the moving water molecules will hit the membrane,
exerting pressure on it.
oThis pressure is known as water potential.
It is measured in units of pressure, can
be measured in kPa, MPa, bar.
Pure water has a water potential of
zero.
A solution will have a lower
concentration of water molecules so it
will have a negative water potential.
water movement
water potential

Solute concentration
pressure
Water potential determines the rate
and direction of osmosis.
PROBLEM – What about physical pressure?
physical pressure.
It is easy to say that water will move from
high concentration to low concentration…but
look at the cell to the right…can the water
keep going in?
NO, as water enters and the cytoplasm begins to put pressure on the cell
wall, the cell wall will begin push back (3rd law of motion) – physical
pressure.
The combined effects of solute concentrations and physical pressure are given in a
single measurement called the WATER POTENTIAL (Ψ; psi) for a given solution (EACH
SOLUTION IS ASSIGNED A WATER POTENTIAL).
WATER POTENTIAL
physical pressure.
How is water potential calculated?
Ψ = Ψs + Ψp
Ψ = water potential of the given solution
1. The potential pressure, in Mpa, that the water
will exert on another solution.
2. Water always move from solutions of higher water potential (higher
pressure) to solutions of lower water (lower pressure) of course.
3. Water potential determines the direction of water movement
4. Largest water potential = 0 Mpa = DISTILLED WATER
It has the greatest available free water molecules  greatest
potential to move and do work  greatest pressure potential to
exert on another solution
WATER POTENTIAL
physical pressure.
How is water potential calculated?
Ψ = Ψs + Ψp
Ψ = water potential of the given solution
Ψs = solute potential (osmotic potential)
1. Ψs = defined as 0 MPa for pure water; it becomes more and more
negative as more and more solute is added since the potential pressure the
water would exert on a neighboring solution should decrease if the water
concentration is falling.
2. Always either zero or negative
3. Makes sense since the more solute, the more likely you are to bring
water to you and therefore the lower the potential pressure exerted by the
water.
WATER POTENTIAL
physical pressure.
How is water potential calculated?
Ψ = Ψs + Ψp
Ψ = water potential of the given solution
Ψs = solute potential (osmotic potential)
Ψp = [physical] pressure potential
1. Ψp = potential pressure the water will apply on a neighboring
solution due to physical forces
2. Ψp can be positive or negative since the physical pressure
can be greater than atmospheric pressure (positive; i.e. in a
turgid plant cell) or less than atmospheric pressure (negative;
i.e. xylem cells when water is flowing through by transpiration).
Quantitative analysis of
WATER POTENTIAL
Look at the four conditions on the right and
explain what you are observing in terms of
water potential.
Water is moving from higher water potential to lower
water potential whose values are dependent on solute
potential and pressure potential.
More solute - more negative Ψs
More pressure - increases Ψp
Less pressure - decreases Ψp
Quantitative analysis of
WATER POTENTIAL
Hypertonic
Hyp0tonic
Look at the conditions on the right and left: explain what you
are observing in terms of water potential.
The cell is at equilibrium when the cell wall pushes back with
the equivalent pressure potential of 0.7 MPa
Quantitative analysis of WATER POTENTIAL
1. A solution in a beaker has sucrose dissolved in water with a solute
potential of -0.5MPa. A flaccid cell is placed in the above beaker
with a solute potential of -0.9MPa.
a) What is the pressure potential of the flaccid cell before it was
placed in the beaker?
b) What is the water potential of the cell before it was placed in the
beaker?
c) What is the water potential in the beaker containing the sucrose?
Quantitative analysis of WATER POTENTIAL
d) How will the water move?
e) What is the pressure potential of the plant cell when it is in
equilibrium with the sucrose solution outside? Also, what is its
final water potential when it is in equilibrium?
f) Is the cell now turgid/flaccid/plasmolysed?
g) Is the cell hypotonic or hypertonic with respect to the outside
prior to equilibrium?
Quantitative analysis of WATER POTENTIAL
2. A solution in a beaker has sucrose dissolved in water with a solute potential of
-0.7MPa. A flaccid cell is placed in the above beaker with a solute potential of 0.3MPa.
a) What is the pressure potential of the flaccid cell before it was placed in the
beaker?
b) What is the water potential of the cell before it was placed in the beaker?
c) What is the water potential in the beaker containing the sucrose?
Quantitative analysis of WATER POTENTIAL
d) How will the water move?
e) What is the pressure potential of the plant cell when it is in equilibrium with the
sucrose solution outside? Think carefully – does the plant cell wall change shape?
f) Also, what is the cell’s final water potential when it is in equilibrium?
g) Is the cell now turgid/flaccid/plasmolysed?
Quantitative analysis of WATER POTENTIAL
h) What is the cell’s solute potential when it is in equilibrium?
I) Is the cell hypotonic or hypertonic with respect to the outside prior
to equilibrium?
J) If it is hypo/hyper (choose one) tonic – this means that its water
potential is higher/lower (choose one) than the outside.
Review Questions:
Review Questions:
Ans: .8 bars
Ans: -4.9 bars