Download Diffusion through a Membrane

Laboratory Activity
Accelerated Biology
Name:_________________________
Date:______________
Diffusion through a Membrane
Introduction:
Molecules are constantly moving. Diffusion occurs when the molecules of a substance move from high
concentrations, where there are more molecules, to low concentrations, where there are fewer
molecules. Diffusion occurs because collisions between moving molecules cause them to move further
apart. Diffusion will occur until the molecules are evenly distributed over an area. At this point, the
concentration of molecules is equal over an area, and equilibrium has been reached.
Cells are surrounded by a cell membrane that is selectively permeable. It only allows some molecules to
come in and out of the cell. As part of this lab activity, we will build a model of a cell using dialysis
tubing as an artificial membrane. Like a cell membrane, an artificial membrane only lets certain
molecules pass through it. Dialysis tubing is made of cellulose and has pores, or small holes, in its
surface. Smaller molecules can move more easily through the pores of an artificial membrane than larger
ones. Today we will observe if small and large molecules are able to diffuse through our artificial cell
membrane.
Prelab Questions: Answer in your lab notebook. Show completed questions in order to get materials
for lab.
1. Define diffusion.
2. Starch is a polysaccharide and a large molecule made of hundreds of atoms, whereas iodine is a small
molecule made of only a few atoms. Based on this information, form a hypothesis about whether or not
these substances would be able to pass through an artificial membrane. Explain your reasoning.
Objectives:
At the end of this activity, you should be able to
 Demonstrate how to test for starch using chemical indicators
 Explain diffusion through a membrane
Materials
20 cm piece of dialysis tubing
Starch Solution
Distilled Water
Lugol’s Iodine Solution
Graduated Cylinder
250 mL Beaker
Funnel
Procedure:
1. Take 20 cm of soaked dialysis tubing. Pull the two ends of the tube gently apart, forming it into a
tube. If you are having difficulty opening the tubing, try rubbing the ends under water.
2. Tie a knot in one end of the tubing. Pull on the knot to secure it, but don’t yank too hard because
you could tear the tubing.
3. Pour about 5 mL of the starch solution into the tubing. You can use a funnel to make this easier.
4. Tie off the top of the tube in the same way you tied off the bottom. Leave enough room so that
the tubing is about half full.
5. Rinse off your filled tube by holding it under gently running water.
6. Place the tubing in a 250 mL beaker and add enough distilled water (water with no solutes) until
the tubing is just covered.
7. Add 2 mL of Lugol’s Iodine solution to the water. The water should now be an amber color.
8. Record the initial color of the solutions in the beaker and in the dialysis tubing in table 1.
9. Leave the beaker undisturbed for at least 15 minutes.
10. While you are waiting, observe the beakers of iodine in water and starch solution with iodine
added at the front desk. Record the colors as answers to the questions in the results section.
11. After 15 minutes, record the color of the solution in the beaker and the color of solution in the
bag in table 1.
12. Pour all fluids down the sink. Discard the tubing in the trashcan. Wash all glassware.
Results: Record in your lab notebook.
1. What color is iodine when no starch is present?
2. What color is iodine when it is mixed with starch?
Table 1: Record and complete in your lab notebook.
Initial
Contents
Bag (Cell)
Starch
Beaker
Solution Color: Iodine Test
Initial
Final
Water and Iodine
Analysis: Answer in your lab notebook.
1. What is the best explanation for the color change that occurred inside of the “cell”?
2. Did any starch diffuse out of “the cell”? Explain how you can tell.
3. Why didn’t starch leave the bag and enter the beaker?
4. Copy the following diagrams into your lab notebook. In the “Initial State” diagram, molecules of
the Lugol’s Iodine Starch Indicator Solution are indicated with the letter “I”. Using the letters “S”
for starch molecules, indicate the areas where iodine and starch molecules are located in both
diagrams.
Initial
Final
5. Did the iodine molecules that moved through the membrane move from a higher concentration
to a lower concentration? Explain your reasoning.
6. How is the dialysis tubing like a cell membrane?
Reasoning/Conclusion
In your lab notebook, draw your conclusions about this lab. Be sure to write in complete sentences in
paragraph form.