Download Water Potential - Phillips Scientific Methods

Water Potential
Osmosis & Plant cells
Plants & water potential

Plants can use the
potential energy in
water to perform
work.
 Tomato plant
regains turgor
pressure – cell
pushes against wall
due to uptake of
water
Plants & water potential
 The
combined effects of
1.) solute concentration
2.) physical pressure (cell wall)
can be measured as Water Potential 
 =
psi
 is measured in either kilopascals
(KPa) or megapascals (MPa)
 *1 Mpa = 10 atmospheres of pressure
Calculating Water Potential


=
P
+
S
Or
Water =
Potential
pressure +
potential
solute
potential
Solute Potential  S

Solute potential is also called the osmotic
potential because solutes affect the direction of
osmosis.

 S of any solution at atmospheric pressure is
always negative – why?

Answer = less free water molecules to do work
Solute Potential  S
 Solutes
bind water
molecules reducing
the number of free
water molecules 
lowers waters
ability to do work.
Pressure Potential P
P is the physical pressure on a solution.

P can be negative  transpiration in the
xylem tissue of a plant (water tension).
*Think of suction
P can be positive  water in living plant
cells is under positive pressure (turgid).
*Think of squeezing or applying force
Standard for measuring
 Pure
 Pure

water is the standard.
water in an open container has a
water potential of zero at one
atmosphere (atm) or one bar of
pressure.
Water Potential: an artificial model

(a) addition of
solutes on right side
reduces water
potential. S = -0.23

Water flows from
“hypo” to “hyper”

Or from hi  on left
to lo  on right
Water Potential: an artificial model



(b) adding +0.23 pressure with plunger  no net
flow of water
(c) applying +0.30 pressure increases water
potential solution now has  of +0.07
Water moves right to left
Water Potential: an artificial model

(d) negative
pressure or tension
using plunger
decreases water
potential on the left.

Water moves from
right to left
Water relations in plant cells

(b) Flaccid cell in pure water  Water
potential is into cell cell becomes turgid
Water relations in plant cells

(a) Flaccid cell placed in hypertonic solution
Water potential is out of cell  plasmolysis
Calculating Solute potential
 Need
solute concentration
 Use the equation
 S = - iCRT
i = # particles molecule makes in water
C = Molar concentration
R = pressure constant 0.0831 liter bar
mole oK
T = temperature in degrees Kelvin
= 273 + oC
Solve for water potential
(literal equation)
 Knowing
solute potential, water
potential can be calculated by inserting
values into the water potential
equation.

=
P
+
S
In an open container, P = 0
Water Potential problems
 When
calculating ‘Solute Potential’ (-iCRT),
the R value will depend on the pressure
units given in the problem. If ‘bars’ or ‘atm’
is used, R= 0.0821 L-atm (*bars) / mol 0K. If
kPa is used, R= 8.315 (see below)
UNIVERSAL GAS
CONSTANT
R=0.0821 Latm/molK
R=8.315 dm3kPa/molK
A few notes:
R
value will depend on the units used for
pressure. (see last slide)
 The ionization constant will vary,
depending on if the solute can ionize in
solvent or not. Ex- For NaCl (ionic bond),
when dissolved in water it forms 2 ions
(Na+ and Cl-), so the value is 2. Ex- For
glucose (covalent bond), does not ionize in
water, so the value is 1.
Let’s Take a class
(self graded) quiz!!
(A
Practice problem
 The
value for Ψ in root tissue was found to
be -3.3 bars. If you take the root tissue
and place it in a 0.1 M solution of sucrose
at 20°C in an open beaker, what is the Ψ
of the solution, and in which direction
would the net flow of water be?
Solution:
Ψ
s= -iCRT
 -(1)(0.1M)(0.0831 L bars/mol K)(293 K)=
-2.43 bars Yp=0, so Y=-2.43. The
movement will be into the root cells. Goes
from higher (less negative) to lower (more
neg).
 *M = mol/L
 HW- Do the Water Pot. Prac. Prob. sheet
Big Idea of Water Potential
So, we can now define osmosis as the
movement of water molecules from a
region of higher water potential
(hypotonic) to a region of lower water
potential (hypertonic) through a semipermeable membrane.
*Hypotonic has less neg value for solute
potential; Hypertonic has more neg value
Hints & reminders
1. Remember water always moves from [hi]
to [lo]. *Hi is less neg value in calculation
2. Water moves from hypo  hypertonic.
3. [Solute] is related to osmotic pressure.
Pressure is related to pressure potential.
4. Pressure raises water potential.
5. When working problems, use zero for
pressure potential in animal cells & open
beakers.
6. 1 bar of pressure = 1 atmosphere