250-mL beakers. iodine solution metric ruler. 10-mL graduated cylinders pipettes. (Read the Procedure first to answer the Questions)

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1 Detecting Diffusion 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. Materials dialysis tubing 250-mL beakers starch solution scissors dialysis clips iodine solution metric ruler 10-mL graduated cylinders pipettes Pre-Lab Questions Name Block (Read the Procedure first to answer the Questions) 1. How will you know whether iodine has diffused across the membrane of a bag filled with starch? 2. Does diffusion require energy? What do you call processes like this? 3. What does it mean if a membrane is selectively permeable? 4. Which particles do you think will be small enough to pass through the membrane? Starch or Iodine? 5. In this lab there are no protein doorways for large particles to pass through in the dialysis tubing, but if there were, which particles would pass through these doorways? AND what would that type of cell transport be called? Procedure NOTE: Iodine will turn blue-black when it comes into contact with starch. stamp Testing Permeability to Starch and Iodine 1. Pick up your piece of dialysis tubing. 2. Add 200 ml of water to a 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 dialysis clip to tightly seal off the folded end. 5. Roll the unsealed end of the tubing between your fingers until it opens.

2 6. Use a pipette to squirt 5 ml of starch solution into the tubing. 7. Fold down 1 cm of the tubing at the open end. Use a second dialysis clip to tightly seal this end. 8. Use tap water to thoroughly rinse the outside of the tubing. 9. Place the tubing in the beaker. The tubing must be completely covered with water. 10. Add drops of iodine solution to the water in the beaker until the water turns a dark brown color. Record your initial observations in Data Table Wait 10 minutes, and while you do this, take your self quiz with your lab partner. 12. Then remove the dialysis tubing from the beaker. 13. Record your final observations in Data Table Remove the dialysis tubing, take off the clips and throw away the tubing. 15. Rinse the dialysis clips, pour out the water that was in your beaker, and put the clips in the beaker with a new dialysis tube for the next class to use. Data Table 1 Inside Tubing Outside Tubing Color Is starch Is iodine Color Is starch Is iodine Initial observations (0 minutes) Final observations (10 minutes) Analyze and Conclude (answer in COMPLETE SENTENCES) 1. What happened to the iodine? In which direction did it flow? 2. What happened to the starch? In which direction did it flow? _ 3. Use what you know about the size of starch and iodine molecules to explain the color change in your bag. 4. What substance other than iodine moved across the membrane? 5. In which direction did this substance move? Into or out of the bag and why? 6. Describe one type of diffusion that cannot be modeled with dialysis tubing. What is missing from the dialysis tubing that prevents this from being modeled?

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5 Name Class Date Extend Your Inquiry Different amounts of starch are dissolved in two beakers of water. How could you use dialysis tubing to determine which starch solution is more concentrated? 48