WATER POTENTIAL
References: Campbell/Reese Biology 9e , p. 768-771; AP Lab #1; AP Investigation #4,
LabBench
I. Introduction
A. Water and solutes can move by bulk flow
B. Diffusion
However, you don’t get the whole picture if water movement is discussed based entirely
on concentration.
Movement will be from high free energy to low, or from high water potential to low
water potential.
II. Water Potential
A. Water potential
1. Osmotic potential ( s ) This depends on ____________ It is always negative, in an
inverse relation to molarity. As molarity increases, the osmotic potential (  s )
decreases.
2. Pressure potential ( p ) results from _______________________.
Water Potential
=
Pressure Potential
+
Osmotic Potential
 =p+s
B. Water potential is expressed in bars or atmospheres or megapascals (MPa).
1MPa = 10 bars
C. Water potential measures_________________________________________
D. In plant cells, the movement of water into the cell causes it to swell, and the cell
membrane pushes against the cell wall, causing an increase in pressure.
E. Eventually, this positive turgor pressure builds up to oppose the more negative
osmotic potential of the cell.
F. The water potential of pure water in a container open to the atmosphere is ___
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EXAMPLES from AP Laboratory I
Sketch the following set-up:
Cell: Osmotic Potential ( s ) = -8 bars; Pressure Potential ( p ) = 2 bars
Calculate Water Potential of the cell:
_______________
Beaker: Osmotic Potential must be calculated; It contains .2M NaCl at 27oC
Osmotic Potential
=-iCRT
i = ionization constant, which is 2 for NaCl
C = molar concentration
R = .082 (pressure constant)
T = temperature in degrees K, which = degrees Celsius + 273
Osmotic Potential
=(
)(
)(
)(
)
=_________
So, the water potential of the cell is _____________
water potential in the beaker is_____________
Water will go______ of the cell because____________________________________.
When will it stop? When pressure is decreased by ____________________.
For the lab, find the molar concentration of sucrose where the potato cores do not
change mass; calculate osmotic potential.
Questions:
1. If one solution has an osmotic potential of -5 bars, and another has an osmotic
potential of -8 bars, which one has the greater molarity?
2. Which one is hypertonic to the other?
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3. Which one has greater water potential?
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