Bio H– The Cell
Diffusion and Osmosis Lab:
The movement of molecules from an area of high concentration to low concentration is called Diffusion.
Diffusion can happen anywhere: in a room, in a beaker or even across a membrane in a cell. As long as
molecules are moving from high concentration to low concentration, it is a form of diffusion.
In this lab, you will experiment with both diffusion, the movement of the solutes, and osmosis, the
diffusion of water molecules themselves. The basic set up requires the use of something called dialysis tubing.
Dialysis tubing is a thin plastic tube of, what looks like, plastic wrap. In the tubing there are microscopic pores
that allow certain molecules to pass through, and others to get blocked. Different tubing can have different
sized pores and therefore can be used for different purposes. Take a look at the diagram below to examine your
set up.
Beaker with iodine and
water
Movement of molecules
Starch,
glucose
and water
Dialysis tubing “balloon”
Above is a diagram of how your lab set up should look. FOLLOW THE DIRECTIONS CAREFULLY.
Bio H– The Cell
Part I: Diffusion of Starch and Glucose
1.
Make sure you have everything from the list below at your station. If you are missing anything, let your
teacher know.
a. 1 empty beaker
b. 1 dropper bottle of iodine
c. 1 package of Glucose test strips
d. 2 pieces of string
e. 1 long neck funnel
f. 1 10mL graduated cylinder
2. Bring your empty beaker up to the sink in the back and fill it with 250mL of tap water.
3. Add 30 drops of iodine to your beaker of water. This should be enough to make it a nice
yellowy/orange.
4. Measure your iodine/water beaker with a glucose test strip. Record the initial color of the beaker
solution and glucose test results on the data sheet.
Making the balloon:
1. Go to the front of the room to pick up 1 dialysis tube. They have been presoaked for you to save time.
2. Fold up one end of the tube about ½ in and tie it with one piece of string as demonstrated at the
beginning of class.
3. Rub your fingers together on the other end of the tube to open it up and place the long neck of the funnel
into the tube.
4. Send another group member up front to get 7mL of starch/glucose solution with the graduated cylinder.
5. Slowly pour the starch/glucose solution into the funnel to fill the balloon.
6. Gently squeeze out the air.
7. Leaving as much room as possible (but no air) fold the other end of the balloon over ½ inch and tie it off
with the remaining string.
8. Dip your balloon into the beaker of water to wet it and take it immediately over to the scale up front to
weigh it. Make sure the scale is set to “zero” before weighing.
Experiment:
1. Place your balloon into the beaker of water and iodine and record your initial weight on your data sheet.
2. You may work on part II while you wait for 15 minutes.
3. After 15 minutes, reweigh your balloon.
4. Test the solution in the beaker again for glucose.
5. Record all your observations and results on the data sheet.
Clean up:
1. Throw out your balloon.
2. Empty your beaker into the sink IN THE BACK and rinse your beaker.
3. With a WET PAPER TOWEL, wipe up any drips or spills at your station.
4. Dry your station.
5. Make sure your station is properly set up as it was in the beginning
Bio H– The Cell
Data part I:
Initial color of the iodine/water mixture: ___________________
Final color of the iodine/water mixture: ___________________
Was there glucose in the iodine/water mixture at the beginning? ______________
Was there glucose in the iodine/water mixture at the end? ___________________
Initial Mass of balloon: _____________
Final Mass of balloon: ______________
Initial color of balloon: _________________
Final color of balloon: _________________
Part II: Using Osmosis to determine the concentration of a solution
Students in a class employed a similar technique to identify the concentration of a solution of sucrose. Using
the same set up that you did in part I, they filled six dialysis balloons with a sucrose solution of unknown
concentration. They then placed each of the six balloons into a beaker containing sucrose solution of six
different known concentrations. The six known concentrations are listed in the table below.
They weighed each balloon prior to placing them into their respective beakers and then allowed the “mystery”
balloons to soak for 15 minutes. At the end of the 15 minutes they weighed the balloons again. All of their data
is presented below.
Weight of balloons in each of the following beakers
Solution B (in
beaker)
0.0M
0.2M
0.4M
0.6M
0.8M
1.0M
Initial
Final
9.4 g
10.6g
9.6g
10.2g
9.5g
9.8g
9.6g
9.3g
9.5g
8.9g
9.4g
8.5g
% change in
weight
1. Calculate the percent change in weight for each balloon. Use +/- to indicate whether the weight
increased or decreased.
2. Graph the percent change in weight. Include all appropriate parts of a graph.
Bio H– The Cell
IN YOUR LAB JOURNALS
Prelab –
Read ENTIRE LAB including procedure and questions (you should know what we expect you to get out
of this lab before diving in)
Purpose: what were the goals of this lab (each part)
Variables: indentify dependent and independent variables for each part
Introduction: any background information on diffusion or these molecules needed to understand this lab PLUS a
brief description, in your own words, of how the experiment will be done.
Procedure: summarize in your words so you know what you did (a couple of sentences, not step by step)
Data: All data tables, calculations and graphs
Conclusion: What does your data tell you? Why is this significant? How does this relate to real cells?
Scientific support for the statements you have made. Also consider the discussion questions below
Discussion questions part I:
1. Which molecules in part I were able to permeate the dialysis membrane? Explain how you were able to
tell.
2. What criteria determine whether molecules travel through the dialysis tubing? How does this differ
from a REAL cell?
3. How does what you know about these molecules provide an explanation for why some moved and other
didn’t.
Discussion questions part II:
1. Why is it important to look at the percent change as opposed to just the simple change in weight?
2. Define isotonic point:
a. How would this appear on a graph that looks at percent change in mass?
3. What is the concentration of the mystery solution? Explain.
4. Which balloon(s) experienced the greatest rate of diffusion?
a. What relationship do you see between the rate of diffusion and the steepness of the concentration
gradient? (in other words, does how does rate of diffusion change as the difference in
concentration between the balloon and the beaker increases?)
5. To do this experiment the dialysis tubing must be semi-permeable. It allows water to pass, but not the
sucrose used to make the sugar solution. Explain how our results might have been different if both
sucrose and water were able to move. (address the direction of both molecules as well as the overall
effect on the weight change)