Name: _____________________________ Class: ________ Date: _________________
Lab Bench: Osmosis & Diffusion
Exercise 1: Diffusion
1. What was this experiment used to illustrate? What principle were you supposed to
learn?
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2. What test can be used to show the presence or absence of starch? ________________
3. What would a positive starch test look like? A negative?
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4. Rank the following in order from smallest to largest. Use “1” for the smallest and “4”
for the largest molecule.
_____ water _____ starch _____ iodine _____ sugar
Exercise 2: Osmosis
1. What was the independent variable used? ___________________________________
2. What was the dependent variable was being studied? __________________________
3. List at least 3 controlled variables the experimenter would have used.
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4. List 2 possible sources of experimental error.
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Calculating Water Potential
1. Describe the procedure used to explore water potential and the way materials flow
across a cell membrane.
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Name: _____________________________ Class: ________ Date: _________________
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Name: _____________________________ Class: ________ Date: _________________
Water potential is calculated using the following formula:
Water potential ( ) =
pressure potential ( ) + solute potential ( )
Pressure potential ( ):
In a plant cell, pressure exerted by the rigid
cell wall that limits further water uptake.
Solute potential ( ):
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.
In sum, as solute is added, the water potential
of a solution drops, and water will tend to
move into the solution.
In this laboratory we use bars as the unit of measure for water potential; 1 bar =
approximately 1 atmosphere.
2. There are two components to water potential: solute concentration and pressure. How
do you think this fact affects the movement of water into and out of cells? For
example, can two solutions that differ in their solute concentration be at equilibrium
in terms of water movement?
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3. Can a solution with a molarity of 0.2 be in equilibrium with a solution with a molarity
of 0.4?
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Name: _____________________________ Class: ________ Date: _________________
Solute potential ( ) = –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 (from your
experimental data)
R=
Pressure constant = 0.0831 liter bar/mole K
T=
Temperature in degrees Kelvin = 273 + °C
of solution
Sample Problem
1. Will there be a net movement of water between two isotonic solutions? Why or why
not?
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2. Why don't red blood cells pop in the bloodstream?
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3. What will happen if a plant cell is placed in distilled water?
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4. The molar concentration of a sugar solution in an open beaker has been determined to
be 0.3M. Calculate the solute potential at 27 degrees. Round your answer to the
nearest hundredth. Show your work!
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Name: _____________________________ Class: ________ Date: _________________
5. The pressure potential of a solution open to the air is zero. Since you know the solute
potential of the solution, you can now calculate the water potential. (If you need to,
review the equation for calculating water potential.) What is the water potential for
this example? Round your answer to the nearest hundredth. You’re your work!!!
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Lab Quiz
1. Which beaker(s) contain(s) a solution that is hypertonic to the bag?
2. Which bag would you predict to show the least change in mass at the end of the
experiment?
3. Arrange the beakers in order of the mass of the bags inside them after the experiment
has run for 30 minutes. List the bag that loses the most mass first.
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Name: _____________________________ Class: ________ Date: _________________
4. In beaker B, what is the water potential of the distilled water in the beaker, and of the
beet core?
5. Which of the following statements is true for the diagrams?
a.
b.
The beet core in beaker A is at equilibrium with the surrounding water.
The beet core in beaker B will lose water to the surrounding environment.
c.
The beet core in beaker B would be more turgid than the beet core in beaker
A.
d.
The beet core in beaker A is likely to gain so much water that its cells will
rupture.
e.
The cells in beet core B are likely to undergo plasmolysis.
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