Diffusion and Osmosis Lab
Overview
In this laboratory you will investigate the processes of diffusion and osmosis in a model membrane
system.
Introduction
Many aspects of the life of a cell depend on the fact that atoms and molecules have kinetic energy and
are constantly in motion. This kinetic energy causes molecules to bump into each other and move in
new directions. One result of this molecular motion is the process of diffusion.
In this activity you will observe the diffusion of molecules throughn dialysis tubing, an example of a
semi-permeable membrane. The size of the minute pores in the dialysis tubing determines which
substances can pass through the membrane.
Diffusion is the movement of molecules of a substance from an area of higher concentration of that
substance to areas of lower concentration.
Osmosis is a special case of diffusion. Osmosis is diffusion of water through a selectively permeable
membrane. When two solutions have the same concentration of solutes, they are said to be isotonic to
each other. If the two solutions are separated by a semi-permeable membrane, water will move
between the two solutions, but there will be no net change in the amount of water in either solution.
The solutions are described as isotonic to each other. If two solutions differ in the concentration of
solutes each has, the one with more solute is hypertonic to the one with the less solute. The solution
that has less solute is hypotonic to the one with more solute. These terms can only be used to compare
solutions.
A solution of starch and glucose will be placed inside the dialysis tubing. Distilled water will be placed in
a beaker, outside the dialysis bag. After 30 minutes have passed, the solution inside the dialysis tubing
and the solution in the beaker will be tested for glucose and starch.
Pre-Lab questions
1.
2.
3.
4.
Explain what is meant by the fluid mosaic nature of a cell membrane.
Why is the membrane of the cell described as semi-permeable?
How would you test for the presence of starch?
How would you test for the presence of glucose?
I.
DIFFUSION
Materials:
30 cm piece of dialysis tubing
timer
250 mL plastic beaker
balance
15 mL glucose/starch solution
distilled water
IKI solution
Benedict’s solution
Procedure:
1. Obtain a 30 cm piece of 2.5 cm dialysis tubing that has been soaking in water. Tie off one end of
the tubing to form a bag. To open the other end of the bag, rub the end between your fingers
until the edges separate.
2. Place 10 to 15 mL of a glucose/starch solution in the bag. Tie off the other end of the bag,
leaving suffiecient space for the expansion of the contents in the bag. Rinse the tubing with
distilled water and pat dry. Weigh your dialysis bag.
3. Observe and record the color of the solution in your lab notebook.
4. Test the glucose/starch solution for the presence of glucose using Benedict’s test. Record your
results in your lab notebook.
5. Fill a plastic 250 mL beaker two thirds full with distilled water. Add approximately 4 mL of IKI
solution to the distilled water and record the color of the solution. Test this solution for glucose
and record your results.
6. Immerse your dialysis tubing into the beaker solution.
7. Allow your set-up to stand for 30 minutes or until you observe a distinct color change in the bag
or in the beaker. Record the final color of the solution in the bag, and of the solution in the
beaker.
8. Rinse off your dialysis tubing and pat dry. Weigh your dialysis bag.
9. Test the liquid in the beaker and in the bag for the presence of glucose. Record your results.
10. Answer the following questions and write your answers in your notebook:
a. What substances entered the bag and which left the bag? How do you know?
b. Why did the substances that moved do so?
c. Compare the substances present to the size of the pores in the dialysis bag.
d. After weighing your dialysis tubing, was there a difference in mass? What would
account for this?
II.
OSMOSIS
Materials:
Three 30 cm lengths of dialysis tubing
sharpie marker
Distilled water
balance
0.4 M sucrose solution
three pices of lab tape
0.6 M sucrose solution
0.8 M sucrose solution
Three 250 mL plastic beaker
Procedure:
1. Obtain three 30 cm lengths of presoaked dialysis tubing.
2. Tie a knot in one end of each piece of dialysis tubing to form three bags. Pour approximately
20 mL of each of the following solutions into separate bags:
a)
b)
c)
d)
Distilled water
0.4 M sucrose solution
0.6 M sucrose solution
0.8 M sucrose solution
Remove most of the air from each bag by drawing the dialysis tubing bewteen two fingers.
Tie off the other end of the bag. Leave sufficient space for the expansion of the contents in
the bag. (The solution should fill only about one-third to one-half of the piece of tubing.)
3. Rinse each bag with distilled water to remove any sucrose spilled during filling
4. Carefully blot the outside of each bag and measure the mass. Record the initial mass of each
bag, expressed in grams.
5. Fill three 250 mL beakers two thirds full with distilled water. Using lab tape label each beaker as
containing distilled water, 0.4 M solution or 0.8 M solution.
6. Immerse each bag into one of the beakers of distilled water. Be sure to completely submerge
each bag.
7. Let them stand overnight.
8. The following day, remove the bags from the water. Carefully rinse and blot dry.
9. Determine the mass of each bag. Record your data in your lab notebook. Obtain data from the
other lab groups in class and average the results.
An example of a data table using individual data
Contents of
Dialysis Bag
Mass –initial
Mass – final
Mass – change
% Change in Mass
Distilled water
0.4 M sucrose
0.6 M sucrose
0.8 M sucrose
To calculate: % Change in Mass = [Final mass – Initial mass] X 100 / Initial Mass
10. Show your work for at least one of your calculations for percent change in mass.
11. Graph both your individual data and the class average using the percent change in mass.
12. Answer the following questions in your lab notebook:
a. Explain the relationship between change in mass and the molarity of sucrose within
the dialysis bags.
b. Describe how the results would have differed if you had used 0.4 M sucrose in the
beakers instead of distilled water? Explain why you think this.
c. Why is the percent change a more appropriate measure to use to compare the
dialysis bags than the raw change in mass?