Diffusion and Osmosis
Diffusion and osmosis are two vital processes in living organisms. Diffusion is the
movement of particles from an area of high concentration to an area of low concentration due to
the random action of individual particles. Osmosis is the diffusion of water molecules across a
membrane. Read sections 6.2 and 6.3 in the textbook and this article on membrane channels
(http://nobelprize.org/nobel_prizes/chemistry/laureates/2003/public.html) for a more in depth
discussion of diffusion and osmosis.
This laboratory exercise uses dialysis tubing to simulate processes happening across cell
membranes. Dialysis is used to filter toxins out of the blood in patients with dysfunctional and
nonfunctional kidneys (see sections 46.1, 46.2 and 46.3 for a discussion of blood filtration by the
kidneys).
Osmosis is also important in the transport of water and solutes in plants (See sections
32.1 and 32.2 for a discussion of this subject). In this lab, we will quantitatively investigate
osmosis in potatoes and visually examine the qualitative effects of hyperosmotic conditions in
onions.
Exercise A: Diffusion
1. You will need the following:
a. plastic cup filled to the top ridge with distilled water
b. 30 cm long piece of dialysis tubing
c. solution containing glucose and starch
d. iodine potassium iodide (IKI) (dyes starch deep purple to black)
e. glucose test strip
2. Moisten the dialysis tubing and make a knot in one end.
3. Place approximately 10-15 ml of glucose/starch solution in the bag.
4. Make a knot at the other end of the bag so that the bag is limp (it should not be like a
blown up balloon).
5. Rinse the bag off with tap water to remove any glucose/starch solution on the outside of
the bag.
6. Place the bag in distilled water.
7. Add a couple of drops of IKI to the distilled water so that it turns a pale yellow.
8. Wait at least 5 minutes.
9. Use the glucose test strip to test for glucose in the water
10. In your lab notebook, note any changes to the bag and to the water
Discuss the following questions with your lab partners. Summarize your answers in a brief
discussion of this demonstration in your lab notebook.
1. For each of the chemicals (IKI, glucose, starch, water) describe whether or not it crossed
the membrane.
2. Explain what evidence you have to support your conclusions in #1 above.
3. Comment on the size of each molecule in relationship to the membrane pores.
Exercise B: Osmosis
In this exercise, you will investigate the relationship between solute concentration and
osmosis. Because sucrose is too large to fit through the pores of the dialysis membrane it will
not be able to move down its concentration gradient. However, water is small enough to move
through the pores.
Some terms you should know:
Isotonic
Hypertonic
Hypotonic
1. You will need the following:
a. Six 30-cm long strips of dialysis tubing
b. Sucrose solutions of 0.2, 0.4, 0.6, 0.8 and 1M
c. Distilled water (0 M sucrose)
d. Six cups
2. Tie off one end of each dialysis tube.
3. Fill each tube with approximately 20 mL of solution so that you have six tubes each filled
with a different one of the six solutions.
4. Tie off the other end of the dialysis tube. Be sure to allow enough space for the bag to
swell.
5. Rinse each bag and blot with a paper towel.
6. Measure and record the mass of each bag.
7. Place each bag in a separate plastic cup and fill with distilled water.
8. Let the bags stand over night.
9. The following day, remove the bags from the water. Blot each bag with a paper towel.
10. Measure and record the final mass of each bag.
11. Express the change in mass as a percentage of the initial mass using the equation below.
Show your work.
%change in mass = (final mass – initial mass)/initial mass x 100
12. Graph the % change in mass as a function of sucrose molarity. Be sure to use the proper
x and y axes, include labels, units and a title. You may NOT connect the dots, but you
should draw a line of best fit.
13. Graph the class data similarly.
14. Tape or paste each graph into your lab notebook and describe each graph in text.
1.
2.
3.
4.
5.
6.
7.
8.
Discuss the following questions with your lab partner and summarize your conclusions in
your lab notebook:
Explain the relation relationship shown in your graph.
Imagine that you used 0.4 M sucrose in the cups instead of distilled water. Predict what
would have happened. Explain why.
What is the benefit of using % change in mass rather than simply change in mass?
Compare your individual data to the class data.
Explain why you data differs from the class data.
Explore sources of error in this experiment
Comment on which is likely to be a better representation of reality and explain why.
Propose an experiment that you could perform to answer a new question based on the
work you did here.
Exercise C: Water potential
Water potential is a measure of the tendency of water to leave one place in favor of
another. Water always moves from an area of higher water potential to an area of lower water
potential. For example, a dialysis tube filled with 1M sucrose has a higher sucrose concentration
than distilled water. It is therefore hypertonic to the water and has a lower water potential. The
distilled water is hypotonic to the bag and has a higher water potential. As a result, water will
move into the bag. Read sections 31.1 and 31.2 in your book for a more in-depth discussion of
water potential.
In this exercise, you will experimentally determine the water potential of potato cells.
1. You will need the following:
a. 6 plastic cups containing 0, 0.2, 0.4, 0.6, 0.8 and 1 M sucrose solution
b. 24 potato cores (appx. 5 mm diameter x 3 cm thick)
2. Divide your potato cores into six groups of four.
3. Measure and record the mass of each of your six groups of potato cores.
4. Place each group of potato cores in a different plastic cup.
5. Cover the plastic cups with plastic wrap.
6. Let stand over night
7. The following day, record the mass of each group of potato cores.
8. Determine the % change in mass as you did with the dialysis bags. Be careful with your
signs. Some of the values should be positive and some should be negative.
9. Graph your data and the class data and include a line of best fit.
10. Perform regression analysis on the data.
11. Determine the concentration of sucrose where the change in mass would be zero.
Exercise C (part 2): Calculating Water Potential from Experimental Data
The water potential of any solution is the sum of the pressure potential (Ψp)and the solute
potential (Ψs).
Ψ = Ψp + Ψs
For any system that is open to the atmosphere, Ψp = 0
Under these conditions, Ψ = 0 + Ψs or Ψ = Ψs
The solute potential of the sucrose solution can be calculated using the following formula:
Ψs = -iCRT
Where:
i = Ionization constant (for sucrose this is 1.0 because sucrose does not ionize in water.
C = Molar concentration (from step #10 in exercise C)
R = Pressure constant (R = 0.0831 liter bars/mole K)
T = Temperature in K (273 + oC)
Example:
A 1.0 M sucrose solution at 22 oC under standard atmospheric conditions
Ψs = -(1)(1.0 mole/liter)(0.0831 liter bars/mole K)(295 K)
Ψs = -24.51 bars
Therefore, Ψ = -24.51 bars
1. Determine the water potential of the potato cells.
2. If the potato were allowed to dehydrate by sitting in the open air, what would happen to
the water potential of the potato cells?
3. In your lab notebook, discuss the significance of the water potential of potato cells.
(Why do we care?)
Exercise D: Onion cell plasmolysis
Plasmolysis is the shrinking of the cytoplasm of a plant cell in response to water diffusing out
of the cell into a hypertonic solution. In plasmolysis, the cell membrane pulls away from the
cell wall.
1. Prepare a wet mount of the epidermis of a red onion. Observe the cells. Use a digital
camera to take a picture of fully hydrated red onion cells. Tape the picture in your lab
notebook.
2. Add 2 or 3 drops of 15% NaCl solution to one edge of the cover slip. Use a paper
towel on the opposite side of the cover slip to draw the fluid across the slide. Observe
the cells and take a picture for your lab notebook. Explain what happened.
3. Remove the cover slip and flood the onion epidermis with fresh water. Replace the
cover slip and observe. Describe and explain what happened.
4. In your lab notebook relate what you saw to water potential.
Questions for class discussion (you do not have to answer these in your lab notebook, but should
discuss them with your partners and be able to explain your answers to the class):
1. In the winter, grass often dies near roads that have been salted to remove ice. Explain
why this happens.
2. If you are on a lifeboat in the ocean, explain why you should not drink the water.
Describe a method for getting fresh water from ocean water.
3. If a plant cell has a lower water potential than its surrounding environment and if
pressure potential is zero, is the cell hypertonic or hypotonic to its environment? Will
it gain or lose water? Explain.
4. In the figure at right, the beaker is open to the atmosphere.
Initial values of
What is the pressure potential Ψp of the
Beaker contents
system?
0.4% sucrose Ψs = -4
5. Which has the highest water potential,
the beaker or the dialysis bag?
Dialysis Bag with
6. Which way will water diffuse? Why?
0.1% sucrose
solution
Ψs = -1, Ψp = 0