Bio A – 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 of 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 the water molecules themselves). The basic set up requires the use of something called dialysis
tubing. Dialysis tubing is a thin plastic tube of cellulose. 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 solution B in it
Movement of molecules
Solution A
Dialysis tubing “balloon”
with solution A in it
Diffusion:
For experiment one, the solution in the balloon will contain glucose, starch and water. The beaker will contain
just water and iodine. Over time, depending on the concentration gradient and the size of the molecules,
molecules will either enter the balloon, exit or not move at all.
Osmosis:
Part II we will fill the balloon with different concentrations of sucrose solution (0.0M, 0.5M or 1.0M) and soak
each in a cup of 0.5M sucrose solution. Sucrose is too big to move through the pores of the tubing, but the
water in the solution is not. We can estimate the amount of water that moves by determining the mass of the
balloon before and after the diffusion period. We can then compare the change in mass of the balloon in each
of the different concentrations to see how the steepness of the gradient affects the rate of diffusion.
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Bio A – The Cell
Procedures
Experiment 1:
1. Fill a cup with water to the top of the ridges.
2. Test the water for the presence of glucose and record your data.
3. Add 20 drops of iodine to the cup of water.
4. Follow the steps under ***Making a dialysis balloon*** to make a dialysis tubing balloon, filling the
balloon with 5 mL of Starch and Glucose solution.
5. Rinse the balloon with fresh water.
6. After you’ve checked for leaks and record the starting colors of the cup and balloon.
7. Place your balloon in the beaker and set aside for about 20 minutes
8. While experiment 1 is “working” you may work on the preliminary questions.
9. After about 20 minutes test the cup again for glucose and note any changes in color
10. Record your data and any and all observations in your lab notebook.
Experiment 2:
1. Fill the labeled cups with 100 mL of 0.5M sucrose solutions.
2. Follow the steps under ***Making a dialysis balloon*** to make another balloon and fill it with 5mL
of your ASSIGNED concentration of sucrose solution.
3. After you’ve checked for leaks, dip your balloon in the cup of water to wet it and weigh your balloon.
Be sure to zero the balance before weighing. Record your data in your lab notebook.
4. Place your balloon in to the cup of 0.5M sucrose solution and set aside for 30 minutes.
5. After 30 minutes take your cup over to the balance. When you are ready, take out your balloon, zero the
balance and weigh the balloon. Record your data in the data chart.
6. Place your data on the board for experiment two and copy the class data into your chart.
***Making a dialysis balloon: ***
1. pre-soak 15cm strips of tubing in water (done for you)
2. fold about 1cm of the tubing up and tie off the bottom of the tube. Now it is sealed on one end
3. Rub the open end between your fingers to open it (it’s a pain, be patient)
4. Once open, insert the neck of the funnel and pour the proper volume of solution into the balloon.
5. Now remove as much air as possible, fold over the top of the tube and tie off. Be sure to leave as much
room for expansion as possible to get accurate results. Do not tie the tube immediately above the
solution!
6. You should now have a “balloon” tied on both ends with your solution in the middle. Check for leaks
before continuing. If your balloon is dripping, it needs to be retied.
Clean up:
1. Carry cups over to the garbage can and throw out the balloons.
2. Empty the contents of all cups into the sink and return them to your station.
3. Rinse the cup, funnel and graduated cylinder with clean water and leave them to drain on the paper
towel.
4. Wipe up all spills with a DAMP paper towel (remember this is sugar water, it will get sticky!!)
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Bio A – The Cell
Experiment one: ALL measurements require proper units. Set up your lab notebook to record…
Initial Glucose concentration of water
Final Glucose concentration of water
Initial color of balloon
Final color of balloon
Initial color of iodine/water
Final color of iodine/water:
Experiment two: Each group will do one balloon and the class will share data.
Initial mass:
Final mass:
Change in mass (subtract your initial mass from your final mass) – include whether the number is positive
or negative.
Determine your percent change in mass. To do this, divide your change in mass (above) by your initial mass.
This is the number you should put on the board.
Make a data table like the one below in your lab notebook.
Class data:
Percent Change in Mass of balloons in each of the following beakers
0.0M
0.5M
Group 1:
Group 2:
Average:
3
1.0M
Bio A – The Cell
Preliminary Questions:
1. How is osmosis different from diffusion?
2. What determines whether a molecule can pass through the dialysis tubing?
3. What determines the DIRECTION (into the balloon or out of the balloon) the molecules will move?
4. Think back to past labs to answer the following:
a. How will we tell whether the starch is moving?
b. What can we use to determine if the glucose leaves the balloon?
5. For part two we will test 0.0M, 0.5M and 1.0M solutions in the balloon. All will be soaked in 0.5M
sucrose solution. Predict what will happen to the mass of each balloon and provide a brief explanation.
0.0 M
0.5 M
1.0 M
6. If you were to graph your data from Experiment 2 (oh… hypothetically of course….)
a. What is the independent variable and therefore will be on the X axis?
b. What is the dependent variable and therefore will be on the Y axis of the graph?
7. Why are you using percent change in Experiment 2 rather than just the change in mass?
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Bio A – The Cell
Discussion Questions and Data Analysis:
The purpose of these experiments was:_______________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
Experiment 1:
Use your data to discuss what happened to each molecule below and why.
1. Did the glucose move in experiment 1? ____________ If so, in what direction? _____________
a. What data do you have to support your answer?
b. What does this tell us about the concentration of glucose inside the balloon compared to the
concentration of glucose in the cup?
2. Did the iodine move in experiment 1? ____________ If so, in what direction? _____________
a. What data do you have to support your answer?
b. What does this tell us about the concentration of iodine inside the balloon compared to the
concentration of iodine in the cup?
3. The starch did not move in experiment 1.
a. What data do you have to support the statement above?
4. Starch is a chain of glucose monomers, so they have a lot of similar properties. Why was glucose able
to move through the dialysis tubing while starch could not?
5. Was this experiment looking at osmosis or diffusion?
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Bio A – The Cell
Experiment 2
1. What is 0.0M sucrose solution better known as? (Is there ANY sucrose in it???)
2. Consider the 0.0M balloon in the 0.5M cup.
a. What happened to the mass of the balloon? _______________
b. Where did the water move? ________________
c. What term would be used to describe the solution in the cup relative to the balloon?
_____________________
3. Consider the 0.5M balloon in the 0.5M cup.
a. What happened to the mass of the balloon? _______________
b. Where did the water move? ________________
c. What term would be used to describe the BALLOON relative to the CUP? ________________
4. Consider the 1.0M balloon in the 0.5M cup
a. What happened to the mass of the balloon? _______________
b. Where did the water move? ________________
c. What term would be used to describe the BALLOON relative to the CUP? ________________
5. Although we only tested 3 different concentrations, the best fit line allows us to estimate what the
change in mass would have been for any concentration between 0.0M and 1.0M. Estimate what the
percent change in mass would have been for each of the following using your graph.
0.4M __________
0.6M __________
0.3M __________
0.7M __________
0.2M __________
0.8M __________
Use these numbers to help with the next three questions.
7. Since we all waited for the same length of time, the fact that the mass of some balloons changed more
than others suggests that diffusion was happening at a greater rate in these balloons than the others.
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Bio A – The Cell
Look at your numbers above. What happens to the rate of diffusion as we move away from the isotonic
point? Does it systematically increase or decrease?
8. The difference in concentrations between two solutions is the concentration gradient. How large this
difference is, is referred to as the steepness of the gradient. Write a rule relating the steepness of the
concentration gradient to the rate of diffusion.
9. Was experiment two looking at osmosis or diffusion?
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