Chapter 7 Lab Detecting Diffusion

Name Class

Problem How can you determine whether solutes are diffusing across a membrane?

Introduction A cell membrane is a selectively permeable barrier. Some particles can pass through the cell membrane while other particles are held back. Solutes that can move across the membrane generally do so by diffusion. When solutes diffuse, they move from an area of high concentration to an area of lower concentration. Diffusion of certain solutes can be facilitated by protein channels that act like tunnels through the cell membrane. In this lab, you will use dialysis tubing to represent the cell membrane. Dialysis tubing is a synthetic membrane used to filter wastes from the blood of a person who has experienced kidney failure. The tubing has small openings, or pores, that allow the passage of relatively small molecules. You will use indicators to determine which molecules diffuse through the pores.

Skills Focus Use Models, Infer, Compare and Contrast

Materials • dialysis tubing • 1% starch solution • scissors • iodine solution • metric ruler • forceps • 250-mL beakers • 15% glucose solution • twist ties • glucose test strip •10-mL graduated cylinders

Safety Iodine solution can irritate the eyes and skin and can stain clothing. Wear gloves, safety goggles, and a laboratory apron while handling any solution that contains iodine. Rinse off any solution that spills on your skin or clothing. Do not direct the points of the scissors toward yourself or others. Use the scissors only as instructed. Pre-Lab Questions 1. Draw Conclusions How will you know whether starch has diffused across the membrane in Part A? How will you know whether iodine has diffused across the membrane?

2. Draw Conclusions How will you be able to tell whether glucose has diffused across the membrane in Part B?

3. Use Analogies How is a window screen similar to a cell membrane?

Procedure In Part A, you will determine whether the dialysis membrane is permeable to starch or to iodine. In Part B, you will determine whether the membrane is permeable to glucose. NOTE: Iodine will turn blue-black when it comes into contact with starch. Part A: Testing Permeability to Starch and Iodine 1. Put on your goggles, apron, and gloves. Cut a 15-cm length of dialysis tubing. 2. Add 50 mL of water to a 250-mL beaker. 3. Soak the tubing in the water for one minute. 4. Remove the tubing from the water. Fold up 1 cm of the tubing at one end. Use a twist tie to tightly seal off the folded end. 5. Roll the unsealed end of the tubing between your fingers until it opens. 6. Pour 3 mL of starch solution into the tubing. 7. Fold down 1 cm of the tubing at the open end. Use a second twist tie to tightly seal this end. 8. Use tap water to thoroughly rinse the outside of the tubing. Be sure to rinse the twist ties as well. 9. Place the tubing in the beaker. The tubing must be completely covered with water. 10.Add 4 drops of iodine solution to the water in the beaker. Record your initial observations in Data Table 1. Wait 10 minutes, and then record your final observations. 11.Use a forceps to remove and dispose of the tubing as instructed by your teacher.

Data Table 1 Inside Tubing Outside Tubing Color Is starch Is iodine Color Is starch Is iodine present? present? present? present? Initial

Part B: Permeability to Glucose 12.Repeat Steps 1–5 with a new 15-cm length of dialysis tubing and a clean beaker. 13.Use a clean graduated cylinder to pour 3 mL of glucose solution into the tubing. 14.Repeat Steps 7–9 with the new tubing. 15.After 10 minutes, use a glucose test strip to test the liquid outside the tubing. Record your results in Data Table 2.

Data Table 2

Inside Tubing Outside Tubing

Is glucose present? Is glucose present?

Initial Final Analyze and Conclude 1. Infer In Part A, what happened to the iodine and the starch? In Part B, what happened to the glucose?

2. Apply Concepts Use what you know about the structure of starch and glucose molecules to explain your results.

3. Infer What substance other than iodine moved across the membrane in Part A? In which direction did this substance move, and why?

4. Predict Red blood cells are placed in water that has been distilled so that there are no solutes dissolved in the water. Are the red blood cells likely to swell up or shrink? Why?

5. Use Models Describe two functions of a cell membrane that cannot be modeled with dialysis tubing.

6. Perform Error Analysis When Alyssa did Part A of the experiment, she observed the solution outside the tubing turn black. What might have happened?