Diffusion and Osmosis
Introduction:
A cell is a dynamic, living thing. To continue to live, it must continuously exchange materials with its
environment, through its cell membrane. In this lab, you will learn about diffusion, one of the major
ways that things get in and out of cells. Osmosis is a special example of diffusion, the diffusion of water.
Materials:
Methyl Orange powder
Potassium permanganate crystals
Solution of starch and glucose in distilled water
Iodine (starch test) solution
petri dish containing agar
dialysis tubing
string
100ml and 10 ml graduated cylinders
Procedures:
Part 1: Physical diffusion
Diffusion is defined as the movement of molecules from an area of high concentration to an area of low
concentration without the expenditure of energy. If particles are tightly packed together, their motion
will cause them to collide frequently and spread apart. Two factors determine how much and easily
particles will spread apart or diffuse.
The first factor you will discover in the following experiment.
1. Get a Petri dish containing agar. Draw a line on the bottom of the dish, using a grease pencil,
dividing the dish in half.
2. Next, draw a small circle near the outer edge of each section – see figure 1.
3. Using the flat edge of a toothpick, place a small sample of methyl orange
onto the agar in one of the little circles. Place an equal sized sample of p
potassium permanganate, using another toothpick, into the other circle..
4. The molecular weight of methyl orange is 327g,
and potassium permanganate is 158g.
5. Set the petri dish aside for about an hour, then measure the distance that each chemical
diffused from its original edge with your ruler. Record this in your lab.
6. Measurements:
a. Methyl orange___________________ potassium permanganate_________________
b. What relationship exists between rate of diffusion and molecular weight?
Part 2: Dialysis and Osmosis
Dialysis is the diffusion of dissolved substances through a differentially permeable membrane while
osmosis is the diffusion of water. In this part of the lab you will learn how these processes operate in a
cell by making a cell model and subjecting it to a concentration gradient. Dialysis tubing is a cellophane
membrane which has been specifically manufactured to contain microscopic pores, and thus to mimic
cell membrane.
1. Obtain a 15cm length of dialysis tubing. Soak it in a beaker of tap water for about 30 seconds to
soften it. Tie off one end of it very securely with string.
2. Using a funnel, fill the tubing with the glucose-starch solution to within 2cm of the top. Leave
an air space in the cell.
3. Very securely tie off the top end of the tube. Check for any leaks, but avoid touching the
surface of the tubing as much as possible.
4. Rinse your “cell” under the tap to remove any spilled solution on the outside.
5. Place your “cell in a 100ml graduated cylinder and just cover it with distilled water.
6. Measure 3ml of iodine solution in a 10ml graduated cylinder (Avoid contacting this solution
with your hands or clothes), and pour the iodine into the water surrounding your “cell”. Note
the time on the clock as you add the iodine. Wait 20 minutes. At the end of 20 minutes observe
the results and record them in the data table 1. While waiting for the 20 minutes, continue
with the other parts of this lab.
Beginning time is:
7. While waiting, get a test tube and add about starch solution to the depth of 1cm.
8. Add one drop of IKI solution to the starch and gently mix the two together. What do you
observe?
9. Empty, clean, and return the test tube.
10. When the 20 minutes is up, record your observations in the data table below.
Start
After 20 minutes
Color inside cell
Color outside cell
11. Based on what you have seen in the “cell” and the test tube, what substance(s) diffused into the
cell? Why do you think so?
12. What substance(s) moved out of the “cell”? Why do you think so?
13. You have not tested for the presence of glucose outside the cell. Why would this information be
useful?
14. Has the “cell” swollen, shrunk, or stayed the same size? Explain any change observed in terms of
osmosis.
15. What can you conclude about the size of the pores in the tubing compared to the size of water,
starch, glucose, and iodine particles?
16. This membrane, like cell membrane, is “selectively permeable”. What does this term mean?