1. Open you lab notebook to a new page, record today’s date, and the title “Diffusion Through A Membrane.”
2. Write down the definition of diffusion:
Diffusion is the movement of molecules from a high concentration to a low concentration. Many molecules
(movement of solutes) diffuse across cell membranes. The size of the molecules is what determines what can and cannot diffuse across a semi-permeable membrane.

3. In today’s lab you will be working with iodine, glucose, and
starch. Form a hypothesis by predicting which molecule(s) – iodine, glucose, or starch – will diffuse across a semipermeable membrane.
4. Sketch the following diagram and label as shown:

DIRECTIONS:
1. Seal one end of the tube by folding the end over and tying it closed with a piece of string. The goal is to make the end completely leak-proof.
2. Pour glucose solution into the tube until it is about ¼ full. Next add enough starch solution to fill the tube about halfway.
3. Tie off the top of the tube in the same way that you tied off the bottom. The tube should not leak from either end.
5. Gently mix the contents of the tube by turning it upside down and back again.

6. Rinse off the “cell” you’ve just made by holding it under running tap water.
7. Place the “cell” in a beaker and add tap water until the “cell” is just covered.
8. Add iodine to the water in the beaker (enough to make the water an amber color ~ about 30 drops).
9. Make an initial sketch of the beaker and “cell” in your lab notebook. Indicate the contents and color of the beaker and the “cell.” Set the beaker aside and leave it undisturbed for at least 10 minutes.
10. Notate all observations on your drawing sheet.
11. Discard “cell”, save all beaker solution for Glucose determination of lab.

11. Place 3 pipettes of beaker solution into test tube.
12.Place 3 pipettes of Benedicts Solution into test tube.
13.Place test tube into beaker of water on hot plate.
14.Heat test tube for 5 minutes.
15.Observe test tube and record all observations.

OVERVIEW OF OSMOSIS AND DIFFUSION

• What happened to the Glucose(sucrose)?
• How can we tell if it diffused out of the cell?
• Indicator?
• Write down in your lab notebook:
– What happened to the Glucose(sucrose)?
– How can we prove what happened to the
Glucose(sucrose)?
– Record your observations and findings in your notebook.

The following should be completed in the lab section of your notebook:
Diffusion Through A Membrane
1. Prepare a labeled sketch of the initial and final state.
2. Identify and describe the two chemical indicators used in this lab.
3. What is the best explanation for the color change that occurred inside the “cell.”
4. Did any starch diffuse out of the “cell?” Explain how you can tell.
5. Did any glucose diffuse out of the “cell?” Explain how you can tell.
6. Which substance(s) diffused through the membrane?
7. Which substance(s) did not diffuse through the membrane?
8. Explain why some substances were able to pass through the membrane while others were not able to?

9. During this lab activity, one group of students followed the directions incorrectly. They poured the iodine into the “cell” and filled the beaker with starch and glucose solution.
State how these results would differ from those obtained by students in the class who followed the directions correctly.
10. In many animals, glucose, rather than starch, is transported by the blood through the body to all the cells. Starches in many foods are digested to yield glucose.
Based on what you learned in this lab activity, explain why the digestion of starch to glucose is necessary.

Diffusion of Water Across a Membrane
Red Onion in Water
Magnified 400X
Red Onion in Salt Solution
Magnified 400X

Red Onion After Water Wash
Magnified 400X