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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 Potential
• We look at water movement in terms
of water potential. (ψ psi)
• Two factors:
– Solute concentration and pressure
• Pure water ψ =0
• The addition of solute lowers the water
potential. (negative number)
Water potential determines the rate
and direction of osmosis.
ψp
Pressure potential is important in plant cells
because they are surrounded by a cell wall
which, is strong and rigid.
ψp
When water enters a plant cell, its volume
increases and the living part of the cell
presses on the cell wall.
The cell wall gives very little and so
pressure starts to build up inside the cell.
ψp
This has the tendency to stop more water
entering the cell and also stops the cell from
bursting.
When a plant cell is fully inflated with
water, it is called turgid.
Pressure potential is called turgor pressure in plants)
ψp
Water potential (ψ ) =
pressure potential (ψp ) + solute (osmotic) potential (ψs)
Pressure potential (
ψp):
In a plant cell, pressure exerted by the
rigid cell wall that limits further water
uptake
Solute potential (ψs): The effect of solute concentration. Pure
water at atmospheric pressure has a
solute potential of zero. As solute is
added, the value for solute potential
becomes more negative. This causes
water potential to decrease also.
*As solute is added, the water
potential of a solution drops, and
water will tend to move into the
solution.
Water moves from a place of high water potential to a place of low water potential.
Water potential (ψ) =
pressure potential (ψp )
+ solute potential (ψs)
(osmotic)
This is an open
container, so the ψp = 0
This makes the ψ = ψs
The ψs =-0.23, so ψ is
-0.23MPa, and water
moves into the
solution.
Can a solution with a molarity of 0.2 be in equilibrium
with a solution with a molarity of 0.4?
YES!
Pressure
Two solutions will be at equilibrium when the water
potential is the same in both solutions. This does not
mean that their solute concentrations must be the
same, because in plant cells the pressure exerted by
the rigid cell wall is a significant factor in determining
the net movement of water.
Solute (osmotic) potential (ψs )= –iCRT
i =
The number of particles the molecule will make in water; for
NaCl this would be 2; for sucrose or glucose, this number is 1
C =
R =
T =
Molar concentration Yikes, what's that??????
Pressure constant = 0.0831 liter bar/mole K
Temperature in degrees Kelvin (273 + °C) of solution
Example Problem:
The molar concentration of a sugar solution in an open beaker
has been determined to be 0.3M. Calculate the solute potential
at 27°C degrees. Round your answer to the nearest hundredth.
What is the water potential?
Answer:
-7.48
Solute potential = -iCRT
= -(1) (0.3 mole/1) (0.0831 liter bar/mole K) (300
K)
= -7.48 bar
Water potential = -7.48 + 0, so water potential = -7.48
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So, we can now define osmosis as the
movement of water molecules from a
region of higher water potential to a
region of lower water potential through
a semi-permeable membrane.
Molarity
Another way of expressing
concentration, the way that we will
use most in this course, is called
molarity. Molarity is the number of
moles of solute dissolved in one liter
of solution. The units, therefore are
moles per liter, specifically it's
moles of solute per liter of solution.
Be very careful to
distinguish between moles
and molarity.
molarity =
moles of solute
liter of solution
"Moles" measures the amount or quantity
of material you have; "molarity" measures
the concentration of that material.
So when you're given a problem or some
information that says the concentration of the
solution is 0.1 M that means that it has 0.1
mole for every liter of solution; it does not
mean that it is 0.1 moles
What is the molarity of a solution made by
dissolving 2.5 g of NaCl in enough water to make
125 ml of solution?
molarity =
moles of solute
liter of solution
2.5 g NaCl x
1 mole of any substance has a mass
= to the atom’s atomic mass.
1 mole NaCl
58.5 g NaCl
molarity =
= 0.0427 mole
0.0427 mole NaCl
0.125 L
= 0.34 M NaCl
Molarity is
number of
moles per
Liters of
solution
EXAMPLE PROBLEMS
1- An AP Biology student dissolves 98.4 g of
FeSO4 in enough water to make 2.000 L of
solution. What is the molarity of the
0.324 M
solution?
1 mole .
151.91g
x 98.4g = 0.647 moles ÷ 2.00L= 0.324 mol/L
151.91g
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Potential Discussion