AP Biology Diffusion & Osmosis Labs INTRODUCTION The life of a cell is dependent on efficiently moving material into and out of the cell across the cell membrane. All cells need sugars and oxygen to make energy to fuel daily life. Cells also need raw materials to be able to repair themselves and to build new cells. And of course cells always need water to remain healthy. All these materials have to move into a cell to feed it. On the other hand, cells produce waste products during their daily activities. Cells make the waste product, carbon dioxide, when they make energy. They also make the waste product ammonia when they digest proteins. Both of these waste products have to be removed or the cell will be poisoned and die. Most of these materials move passively costing the cell no energy through the process of diffusion. Diffusion is the movement of molecules from an area of higher concentration of those molecules to an area of lower concentration. Another way to phrase this is that molecules move down the concentration gradient a metaphorical downhill from high concentration to low concentration. A good metaphor for this movement of molecules is what happens if you were to open a bottle of perfume in one corner of a room. It would not be long before someone in the opposite corner of the room would smell the perfume. The molecules moved from an area of higher concentration of perfume where the open bottle was to an area of lower concentration of perfume like the opposite corner of the room. Eventually a balance, or dynamic equilibrium, is reached. In other words, the concentration of perfume will be approximately equal throughout the room and no net movement of perfume will occur from one area to the other. Since all life takes place in water either external waters or internal waters we must also address the special case of the movement of water across cell membranes in our study of diffusion. The diffusion of water through a selectively permeable membrane is referred to as osmosis. As with the diffusion of solutes, water moves from a region of higher concentration of water to a region of lower concentration of water. This is often also stated as movement from a region of higher water potential to a region of lower water potential. Distilled water (pure water) has the highest concentration of water or the highest water potential. The concentration of water decreases as solutes like sugars and salts are dissolved in the water. In this experiment you will measure the diffusion of small molecules through dialysis tubing, an example of a selectively permeable membrane. Small dissolved molecules (solutes) and water molecules can move freely through a selectively permeable membrane, but larger molecules will pass through more slowly, or perhaps not at all. The size of the minute pores in the dialysis tubing determines which substances can pass through the membrane. We will explore the process of diffusion through a semi-permeable membrane in this activity. Pre-Lab Questions (Read the above information and use page 143 and Sections 7.2-7.5 in the textbook to answer the following questions) 1. What is kinetic energy, and how does it differ from potential energy? 2. Compare and contrast passive and active transport. 3. Compare and contrast diffusion and osmosis. 4. What is the difference between a monosaccharide and a polysaccharide? 5. To test for the presence of polysaccharides, Lugol s Solution (IKI) can be used. If Lugol s comes in contact with polysaccharides, the solution will change from an orange brown to a blue-black color. What are some examples of molecules could show a positive reaction for polysaccharides? Part 1 Day 1 Procedure: 1. Answer pre-lab questions and draw Table 1. 2. Place 200mL of tap water in a cup. 3. Add Lugol s iodine to the water in the cup. (CAREFUL: IKI WILL STAIN SKIN AND CLOTHES) Add only enough iodine to turn the water a medium amber color. Record this initial color of the solution in Section 1 of Table 1. 4. Obtain a piece of dialysis tubing that has been soaked in water until it is soft enough to work with. Tie off one end of the tubing (like a balloon) to form a bag. This will be our model cell. To open the other end, rub the end between your fingers until the edges separate.
5. Pour 25mL of the 15% glucose/ 1% starch solution in the cell using a funnel. Tie off the other end of the tubing with string, leaving sufficient space for expansion of the contents in the cell. In case any solution spilled on the outside, rinse off the cell under running water and set aside for now. Record the initial color of the solution in the cell in Section 1 of Table 1 (Experimental Observations). 6. Now immerse the dialysis bag in the cup. Allow the experiment to stand undisturbed overnight. 7. The teacher has completed the glucose and starch tests on the dialysis bag solutions and the beaker solutions. The resulting tests are on the teacher desk. Send one group member to view these results so you can record them in Table 1. 8. Table 2 is to be used to make predictions of what you expect to occur overnight in the beaker. Fill in the Day 1 Initial Observations columns (Column A) from your observations today. Then complete your predictions for each section in Column B. 9. Draw and complete the Day 1 Initial State diagrams in Figure 1.
Part 1 Day 2 Procedure: 1. It is now the second day. Record the final color of the solution in the cell and the final color of the solution in the beaker in Section 1 of Table 1. 2. Observe the colors of the solution in the cell and in the beaker as a test for starch and record the results for starch in Section 3 of Table 1. 3. Obtain a glucose test strip and test a sample of the liquid in the beaker for the presence of glucose. Record the results in Section 2 of Table 1. 4. Take a 2ml sample of the liquid in the cell and test for the presence of glucose in a test tube. Record the results in Section 2 of Table 1. 5. Complete the Day 2 Final State diagram in Figure 1. 6. Complete the Summary Questions. Part 1 Summary Questions: 1. Which substance(s) are entering the dialysis bag and which are leaving the bag? What experimental evidence supports your answer? 2. Explain the results you obtained by discussing concentration differences and membrane pore size. 3. Although we didn t measure it, what other molecule can we assume also moved across the membrane? Part 2: Pre-Lab Questions 1. You are in the hospital and need intravenous fluids. You read the label on the IV bag, which lists all of the solutes in the water. a. Why is it important for an IV solution to have salts in it? b. What would happen if you were given pure water in an IV? 2. How can you use weights of the filled cell models to determine the rate and direction of diffusion? What would be an appropriate control for the procedure you just described? 3. Which of the following molecules do you think will diffuse through the membrane and why: NaCL, sucrose, ovalbumin (egg white protein)? 4. If you put ovalbumin and glucose in the same dialysis tubing bag, do you think the protein would diffuse? Do you think the protein will affect the rate of diffusion for glucose? Part 2 Day 1 Procedure: (completed as an entire lab table) 1. Answer pre-lab questions above and draw Table 3. Section A 1. Obtain a piece of dialysis tubing that has been soaked in water until it is soft enough to work with. Tie off one end of the tubing (like a balloon) to form a bag. This will be our model cell. To open the other end, rub the end between your fingers until the edges separate. 2. Measure out 20 ml of one of the solutions on the middle lab station (your choice) and pour it into the dialysis bag using a funnel. Tie off the other end of the bag, leaving space for expansion of the contents in the bag. 3. In case any solution spilled on the outside, rinse off the model cell you just made by holding it under running water. 4. Carefully blot the outside of the cell with a paper towel. Mass the solution cell (in grams) on a scale and record the Day 1 Initial Mass in Table 3 (Osmosis Data). 5. Place the cell in an empty 250mL beaker. Now fill the beaker with water so that the cell is submerged. Label the beaker with tape (contents of beaker and contents of cell). Section B 6. Obtain a second piece of dialysis tubing. Tie off this tubing to make another cell. However, this time fill the bag with 10 ml of the solution you used in step 2 above and then 10 ml of a different solution. 7. Tie off the other end of the bag, leaving space for expansion of the contents in the bag. 8. In case any solution spilled on the outside, rinse off the model combined cell you just made by holding it under running water. 9. Carefully blot the outside of the cell with a paper towel. Mass the solution combined cell (in grams) on a scale and record the Day 1 Initial Mass in Table 3 (Osmosis Data).
10. Place the cell in an empty 250mL beaker. Now fill the beaker with water so that the combined cell is submerged. Label the beaker with tape (contents of beaker and contents of cell). Section C 11. Obtain a third piece of dialysis tubing. Tie off this tubing to make another cell. However, this time fill the bag with 20 ml of distilled water 12. Tie off the other end of the bag, leaving space for expansion of the contents in the bag. 13. Carefully blot the outside of the cell with a paper towel. Mass the solution combined cell (in grams) on a scale and record the Day 1 Initial Mass in Table 3 (Osmosis Data). 14. Place the cell in an empty 250mL beaker. Now fill the beaker with 1.0 M Sucrose solution so that the combined cell is submerged. Label the beaker with tape (contents of beaker and contents of cell). Section D 15. Obtain a fourth piece of dialysis tubing. Tie off this tubing to make another cell. However, this time fill the bag with 20 ml of something that will make this cell the control. 16. Carefully blot the outside of the bag with a paper towel. Mass the control cell (in grams) on a scale and record the Day 1 Initial Mass in Table 3. 17. Place the control cell in an empty 250mL beaker. Now fill the beaker with a water solution so that the control cell is submerged. Label the beaker with tape (contents of beaker and contents of cell). 18. Cover the beakers and allow the experiment to stand undisturbed overnight. 19. Complete the Prediction column in Table 3. 20. Follow Day 1 Clean-Up Procedures Table 3 Mass of Cell 1 Set Up Solution in cell; water in beaker Day 1 Initial Mass Predicted Change in Mass (+ or -) Day 2 Final Mass Change in mass % Change in mass 2 3 4 (Control) & in cell; water in beaker Water in cell; sucrose solution in beaker in cell; water in beaker Part 2 Day 2 Procedure: Retrieve your Osmosis beakers and carefully remove each cell from its beaker. Gently blot the outside of the cell with a paper towel. Mass each cell (in grams) on a scale and record the Day 2 Final Mass in Table 3. Calculate the Change in Mass for each cell. Record in Table 3. Change in Mass = Final Mass Initial Mass Calculate the percent change in mass using the following formula: Change in Mass =( Final Mass Initial Mass) x100 Initial Mass Record your results in Table 3. Answer the Summary Questions.
Part 2 Summary Questions: 1. Which pair(s) that you tested did not have a change in weight? How can you explain this? 2. Describe what happened to each cell and explain why you think these results occurred using evidence. 3. What kind of solution was cell 1 placed into? (hypertonic, hypotonic, isotonic). Explain how you know. What about cell 3? 4. How could you test for the diffusion of glucose? 5. Based on what you learned from your experiment, how could you determine the solute concentration inside a living cell? Clean-Up Procedure Day 1: Wash and fully dry funnels. Windex your lab station Clean-Up Procedure Day 2: Throw away all cells. Empty all beaker/cup solutions down the sink Remove tape from beakers and cups. Wash and fully dry all beakers, test tubes, plastic cups. Windex your lab station.