# Water Potential Notes

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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
How is water potential calculated?
physical pressure.
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 less 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
Look at the conditions on the right and
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?
0 MPa since the cell wall applied no pressure if flaccid
b) What is the water potential of the cell before it was placed in the beaker?
Ψs + Ψp = Ψ
-0.9 + 0 = -0.9MPa
c) What is the water potential in the beaker containing the sucrose?
Ψs + Ψp = Ψ
-0.5 + 0 = -0.5MPa
d) How will the water move?
From high to low water potential (from beaker to cell)
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?
Equilibrium tells you that the water potential of both solutions must be the same. If the outer solution is -0.5MPa then the cytoplasm must
also be -0.5MPa.
Ψ = Ψs + Ψp
-0.5MPa = -0.9 MPa + Ψp
Ψp = 0.4, a positive number makes sense since the cytoplasm’s potential to apply pressure is
increased if the cell wall is pushing on it.
f) Is the cell now turgid/flaccid/plasmolysed?
turgid
g) Is the cell hypotonic or hypertonic with respect to the outside?
It is still hypertonic since not much water would have
entered before pressurizing the cell causing the
membrane to push back.
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?
0 MPa since the cell wall applied no pressure if flaccid
b) What is the water potential of the cell before it was placed in the beaker?
Ψs + Ψp = Ψ
-0.3 + 0 = -0.3MPa
c) What is the water potential in the beaker containing the sucrose?
Ψs + Ψp = Ψ
d) How will the water move?
-0.7 + 0 = -0.7MPa
From high to low water potential (from the cell to the beaker)
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?
Ψp = 0, the plant cell wall is not exerting pressure on the cytoplasm since
water is moving from the cell into the beaker
f) Also, what is the cell’s final water potential when it is in equilibrium?
Water potential of cell = water potential of beaker -0.7 MPa
g) Is the cell now turgid/flaccid/plasmolysed?
plasmolysed
h) What is the cell’s solute potential when it is in equilibrium?
Ψ = Ψs + Ψp
-0.7MPa = -0.7 MPa + 0MPa Water potential of cell = water potential of beaker
I) Is the cell hypotonic or hypertonic with respect to the outside?
hypotonic
J) If it is hypo/hyper (choose one) tonic – this means that its water potential is higher/lower (choose one) than the outside.
hypotonic &
higher
Review Questions:
Water will move out from hypo- to hypertonic
The cell will crenate.
Review Questions:
The solution is hypertonic
as the solute
concentration is higher
making the water
concentration lower than
inside the cell.
Osmosis will take place
since membrane is not
permeable to sugar. Water
will diffuse out of the cell
causing it to crenate.
-24.8 bars
Ψs + Ψp = Ψ
-24.8 bars + 0 = -24.8 bars
-4.9 bars
Into the cell: from high water potential to low
water potential
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