What is Water Potential?

What is Water Potential?
Water potential
• the force responsible for movement
of water in a system
• Has the symbol psi
• Is measured in bars or megapascals
It is a measure of the free
energy of water which is less
when it is has to surround
solutes.
We are
really stuck
here, YUK!
Well I told
you not to
come here!
But, I
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join the
party!
Has two components:
• Solute potential (also called osmotic
potential) џs which is determined by
solute concentration
• Pressure potential џp which results
from exertion of pressure on
membranes/walls as water moves in
or out; can be positive or negative
The water potential of
pure water is given the
value ZERO
• Because pure water has the highest
concentration of water molecules, and
thus the highest water potential, the
water potential of all other solutions
must be lower than zero i.e. negative.
Pure water:
=0
Adding solute decreases
water potential!
• The more solute there is present in a
solution the more negative it
becomes.
• So, solute potential will be a
negative number if not pure water.
So hypertonic solutions have negative solute potentials.
water potential = solute potential +
pressure potential
(s is pi on your paper)
Water moves from areas
of higher water potential
to areas of lower water
potential (i.e. towards the
more negative,
concentrated region).
water always "falls"
from a high to a low
water potential
This will occur until the
water potential inside the
cell equals the water
potential outside of the
cell.
If this makes no sense
whatsoever the key
information to learn is:
• The equation given
• the water potential of pure water is
zero
• water moves from areas of higher
water potential to areas of lower
water potential (i.e. towards the
more negative region)
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 -0.3
bars.
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?
d) How will the water move?
e) Is the cell hypotonic or hypertonic with
respect to the outside initially?
f)If it is hypo/hyper (choose one) tonic – this
means that its water potential is
higher/lower (choose one) than the
outside.
A solution in a beaker has sucrose dissolved in water with
a solute potential of -0.5 bars. A flaccid cell is placed in
the above beaker with a solute potential of -0.9 bars.
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?
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?
So, what happens when a potato cell
in put in pure water?
• Water will move in or out until the wp of
the cell will equals the wp surrounding
the cell.
• The pressure potential will increase to
balance out the solute potential to equal
to zero which is the wp of pure water.
• No more net movement of water occurs
So how can we determine the
water potential of potato cells?
We place potato cells in different molarities
of sucrose. When enough solute is added
outside of the potato cells to result in NO
more NET movement of water, that is the
molarity of the potato cells.
How do you go from molarity of a
solution to the solute potential to
figure the wp?
• Another equation
solute potential = -iCRT
I = ionization constant (1 for sucrose)
C = molar concentration of sucrose (in this case where
no net gain/loss of water occurs)
R = pressure constant (0.0831 liter/bars/mole 0K for
sucrose)
T = temperature Kelvin (273 + C)
Units will cancel out to equal bars.
So what is the solute potential of a 0.1
M solution of sucrose at 22 C?
•
•
•
•
Solute potential = -iCRT
i (ionization constant) = 1
R = 0.0831 (from handbook)
T = temp K (273 + C of solution)
Ωs
=
Ωs =
- (1) (0.1) (0.0831) (295)
- 2.45 bars
So you will graph the results of the
change in weight of the potato cells
in different molarity solutions of
sucrose after overnite.
• Where the line crosses the graph at the X
axis, representing no gain or loss of water,
will be the molarity of the potato cells.
• Then substitute in the equation for solute
potential (-iCRT)
So, how do you get the water potential?
Once you determine the solute potential, plug into
the equation to determine the water potential. The
pressure potential will be zero since water is at
equilibrium (no net movement in or out.