Transcript Water Potential

Water Water Water
Potential
What are you water potential?
• Measures the tendency of water to
leave one place in favor of another
– Measured in bars
• High water potential = water molecules
can easily move about
• Low water potential = water molecules
cannot easily move about
Water Potential
• Pure water  water potential of zero
• Determines where water is going to flow
due to
– Osmosis
– Gravity
– Pressure
– Surface tension
Lets go to our packet ‘o fun
So What Does This Mean?
• The solvent is pulling the water toward
it
• High solute concentration = low water
potential
• Low solute concentration = high water
potential
Review
• Moves from hypotonic to hypertonic
Review
• Hypotonic
– Solution= less solute; more water
– Cell= more solute; less water
• Hypertonic
– Solution= more solute; less water
– Cell= less solute; more water
Lets go back to our packet ‘o fun
Temperature
• High temperature = more water
potential
– Why?
Water molecules can move easily
• Low temperature = less water potential
– Why?
Water molecules cannot move easily
What Will Happen?
• When you put a pipette in water?
Water will go naturally up, against
gravity, without pressure (-)
• What do you have to do to get the
water out?
Put pressure on it (+)
Water
• Concentration of solute is related to
osmotic pressure…
• Pressure related to pressure potential…
• Pressure raises water potential
The 3 Factors
• What affects water potential?
– Temperature
– Solutes
– Pressure
Pressure Potential
ψp
• A force that resists water flow due to a
container (cell) being full.
• Important in plant cells because they
are surrounded by a cell wall
• Water enters plant cell, its volume
increases and the living part of the cell
presses on the cell wall
Pressure Potential
ψp
• The cell wall gives very little so pressure
builds up…oh no!
• This has the tendency to stop more water
entering the cell and stops cell from
bursting
• When plant cell is inflated with water, it is
turgid
– Pressure potential is called turgor pressure
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 =
Molar concentration Yikes, what's that??????
R =
Pressure constant = 0.0831 liter bar/mole K
T =
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
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.
Molarity
Another way of expressing
concentration, 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
151.91g
x 98.4g = 0.647 moles ÷ 2.00L= 0.324 mol/L
EXAMPLE PROBLEMS
• Calculate the solute potential of a .1M NaCl
solution at 22 degrees Celsius.
• If the concentration of NaCl inside the
plant cell is .15M, which way will the water
flow if the cell is placed in the .1M
solution?