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, what looks like, a stiff plastic wrap (cellophane). However,
what you can’t see, is that there are microscopic pores (very small openings) that allow only small molecules to
pass through but block larger molecules from moving. 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.
Experiment 1. Diffusion:
For experiment one, the solution in the balloon (solution A) will contain glucose, starch and water. The beaker
will contain just water and iodine (solution B). Over time, depending on the concentration gradient (the
difference in concentration of a a solute or solvent in solutions A and B) and the size of the molecules,
molecules could enter the balloon, exit the balloon or not be able to cross the dialysis tubing.
Experiment 2. Osmosis:
For the second experiment you will fill one balloon with one of three sucrose solutions (0.0M, 0.5M or 1.0M).
This concentration will be assigned to your group. Each balloon will be soaked in a cup containing 0.5M
sucrose solution for 30 minutes. (each group will have one of the concentrations) 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.
Bio A – The Cell
Pre-Lab Questions: to be collected the day of the lab. No late assignments will be collected;
1. How is osmosis different from diffusion?
2. Unlike a real cell membrane there is only one characteristic that will determine whether a molecule will
pass through the dialysis tubing. What is that characteristic?
3. If a molecule moves OUT of the dialysis balloon, where is the concentration of that molecule higher?
4. Think back to past labs to answer the following:
a. What happens when iodine comes in contact with starch?
b. What can we use to determine if glucose is present in a solution?
5. For part two we will test 0.0M, 0.5M and 1.0M sucrose solutions in the balloon. All will be soaked in
cups filled with 0.5M sucrose solution.
a. Can sucrose move through of the pores in the tubing?
b. Can water move through the pores in the tubing?
c. Draw a picture of the 1.0M balloon sitting in the 0.5M beaker. Label which is hypotonic and
which is hypertonic. Then draw an arrow indicating where you think the water will go.
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?
Bio A – The Cell
Protocol:
Experiment 1:
1. Fill a cup with 250mL of water.
2. Add 30 drops of iodine to the cup of water.
3. Test the iodine water for glucose with a glucose test strip. Record the glucose reading in your data table.
4. Follow the steps under ***Making a dialysis balloon*** to make a dialysis tubing balloon, filling the
balloon with 7mL of Starch and Glucose solution.
5. After you’ve checked for leaks and record the starting colors of the cup and balloon.
6. Place your balloon in the beaker and set aside for about 20minutes
7. While experiment 1 is “working” you may begin setting up experiment 2.
8. After about 20 minutes test the cup again for glucose and note any changes in color
9. Record your data and any and all observations on the data page.
Experiment 2:
1. Fill the labeled cups with 250 mL of 0.5M sucrose solutions.
2. Follow the steps under ***Making a dialysis balloon*** to make another balloon and fill it with 7mL
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. Record your data on the data page (not in the table).
4. Place your balloon in to the cup of 0.5M sucrose solution and set aside for 30 minute. While you are
waiting, don’t forget about experiment 1!!!
5. After 30 minutes take your cup over to the scale. When you are ready, take out your balloon, zero the
scale and weigh it. Record your data on the data page (not in the table).
6. Place your data on the board for experiment two and copy the class data into your chart.
***Making a dialysis balloon: *** (you need to work with a partner to do this)
1. pre-soak 15cm strips of tubing in water (done for you)
2. fold about 2cm of the tubing up and tie off the bottom of the tube (tie it above the fold, but below the
edge of . 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.
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. Wipe up all spills with a DAMP paper towel (remember this is sugar water, it will get sticky!!)
Bio A – The Cell
Lab Note Book must include the information on pages 4-6. Please WRITE OUT as shown i
n the rough draft copies below. YOU MAY NOT CUT AND PASTE.
1. OVERALL TITLE OF THE LAB ACTIVITY
2. Title of Experiment 1.
3. Purpose:
4. Experimental set up (explain and create a diagram)
5. RESULTS (ALL measurements require proper units)
Initial Glucose concentration of water in the cup: ___________________
Final Glucose concentration of water in the cup : ____________________
Initial color of iodine/water: __________
Final color of iodine/water: _________
Initial color of balloon: _________________
Final color of balloon: _________________
6. Title of Experiment 2.
7. Purpose:
8. Experimental set up (explain and create a diagram)
9. RESULTS :
Remember the concentration of the solution IN Your BALLOON is unknown. The concentration of each
BEAKER is provided in the chart.
Record your data below: (correct units are required)
a. Concentration of solution in your balloon: ___________
-
Initial mass: _______________ Final mass: _______________
CALCULATE the CHANGE in MASS of your balloon : Final mass – Initial mass :
Change in mass: ______________ (pos or neg depending on if it increased or decreased)
PERCENT CHANGE IN MASS
Change in mass divided by Initial mass. Put this number on the blan
below in the table on the whiteboard _____________________
Bio A – The Cell
Class data: Fill this in from the table on the whiteboard
Percent Change in Mass of balloons in each of the following beakers
0.0M
0.5M
1.0M
Group 1:
Group 2:
Average:
1. Graph your averages from the CLASS data following all the directions on your graphing rules sheet
(including that whole “must be on graph paper” thing!!!). Excel graphs are acceptable but must still
follow all the rules! You are graphing your independent and dependent variables and NOT using a bar
graph!!!
Bio A – The Cell
2. Although we only tested 3 different concentrations, a best fit line allows us to estimate what the change
in mass would have been for any concentration between 0.0M and 1.0M. With a best fit line we can
find a value on either either the X or Y axis if we have the opposite value. The values shown below can
be “matched” to the Y values by simply finding the Y coordinate (% change) for the X value listed.
You can also find the X value, given the Y value listed.
3. Estimate what the percent change in mass would have been for each of the following using your graph or
what Molarity is expected when a given percent change in weight occurs.
0.4M __________
0.6M __________
0.2M __________
0.8M __________
3. Now identify 3 percentages that are in the range of your data, and estimate the molarity of a balloon that
has had that percent change (use different percentages than the examples above).