Name: Date Block Selective Permeability

Transcription

1 LAB Name: Date Block Selective Permeability OBJECTIVES: Observe the selective permeability of an artificial membrane. Observe diffusion of substances across an artificial membrane. Devise a model for the mechanisms by which materials enter and leave this membrane. Understand the importance of selective permeability to the life of the cell. BACKGROUND INFORMATION: Living things, whether they are huge and multicellular (a blue whale), or tiny and unicellular (an amoeba or bacterium) are composed of cells that are enclosed by a membrane. Staying alive requires a flow of materials into and out of cells, and these materials must pass through the membrane out of necessity. In order to build new cellular parts or to fuel cellular processes, nutrients must continuously enter the cell, whereas waste materials as well as other products are continuously being expelled. Much of the movement of molecules into and out of cells occurs by means of diffusion, which is the movement of molecules from a region of higher concentration to a region of lower concentration. Diffusion occurs spontaneously and does not cost the cell anything in terms of expenditure of energy it is basically free. The cell membrane acts as a barrier to some substances while allowing other substances to enter or leave the cell. When a membrane allows a particular substance to pass through it, the membrane is said to be permeable to that substance. When a membrane does not allow a particular substance to pass through it, the membrane is said to be impermeable to that substance. Since the cell membranes of living things allow the passage of some substances but block others, it is said to be selectively permeable. PURPOSE: In this activity, dialysis tubing will be used to simulate the outer membrane of a cell. Dialysis tubing, which is made from a kind of plastic, has one important characteristic of biological membranes: it is selectively permeable. In addition to dialysis tubing, you will be provided with substances that will be used to represent different cellular components, together with reagents and procedures for tracking these substances. MATERIALS: Starch solution 80% Glucose solution Lugol s Iodine solution Benedict s solution Distilled water Dialysis tubing (pre-soaked) String 450-mL (medium) beaker 50-mL (small) beaker Plastic pipette Test tube Test tube rack Hot water bath 2 Graduated cylinders Funnel Marking Tape Sharpie N. Berg, Biology, NNHS Page 1 of 6

2 PART ONE: Getting Started A. Substances and How to Track Them Your lab group is provided with four different substances that are used to represent cellular components: distilled water, starch solution, iodine solution, and glucose. Each substance may or may not be permeable through your membrane. To detect the presence of each substance in order to see whether they have indeed crossed the membrane, you have been provided with the information below. Iodine is an indicator of starch. In solutions by themselves, iodine is reddish-brown and starch is clear. When iodine and starch are together in the same solution, the solution is blue-black. Benedict s Solution is an indicator of glucose. In solutions by themselves, Benedict s is blue and glucose is clear. When Benedict s and glucose are together in the same solution and heated in a hot water bath for 3 minutes, the solution is orange. B. Making Predictions In the chart below, the molecular formula for each of the four substances is given. Using this information, predict which of the following four substances would be able to cross the membrane. Give a reason why or why not. Substance Molecular Formula Starch C 300 H 520 O 260 Prediction: Will it cross the membrane? Reasoning: Why or why not? Glucose C 6 H 12 O 6 Iodine I 3 - Water H 2 O LAB: Selectively Permeable Page 2 of 6

3 PART TWO: Membrane Simulation 1. Measure 200-mL of distilled water into a 450-mL beaker. 2. Using a calibrated pipette, add 3-mL of iodine solution to the distilled water in the beaker. The mixture in the beaker should turn reddish-brown in color. 3. Using a graduated cylinder, measure 5-mL of glucose solution. Pour into a clean 50-mL beaker. 4. Using a graduated cylinder, measure 5-mL of starch solution. Pour into the beaker with the glucose solution. 5. Obtain a 15-cm piece of pre-soaked dialysis tubing and two 12-cm pieces of string. 6. Fold the bottom of one piece of tubing 1-cm up and tie the folded part with a piece of string tightly to create an open bag as shown in Figure 1 below. 7. To open the other end of the tube, rub the end between your fingers until the edges separate. Using a funnel, pour the glucose and starch solution from the 50-mL beaker into the tube. 8. Fold the top of the tubing 1-cm down and tie off the folded part with string tightly to create a closed bag as shown in Figure Rinse off the outside of the bag with water in the sink. Be sure to rinse the string and squeeze any excess water from the string. 10. Carefully and quickly blot the outside of the bag with a paper towel before determining the initial mass of the bag using an electronic balance. Remember to zero the balance before you mass your tube. Record the initial mass of the bag on your data sheet. 11. Place the bag into the 450-mL beaker of distilled water and iodine. 12. Observe the setup. Record the initial colors of the solution in the beaker and the solution in the bag on your data sheet. 13. Wait 15 minutes. Figure 1. Experimental Set up 14. After 15 minutes has passed, observe the setup. Record the final colors of the solution in the beaker and the solution in the bag on your data sheet. LAB: Selectively Permeable Page 3 of 6

4 15. Remove the bag from the solution in the beaker. Carefully and quickly blot the outside of the bag with a paper towel before determining the final mass of the bag using an electronic balance. Remember to zero the balance before you mass your tube. Record the final mass of the bag on your data sheet. 16. At your lab station using a plastic pipette, add 3 FULL squirts of solution from your beaker into a clean test tube. 17. Using a plastic pipette, add 2 FULL squirts of Benedict s solution to the test tube with the solution from your beaker. 18. Place the test tube into a hot water bath. Wait 3 minutes. Record the final color of the solution in the test tube on your data sheet. 19. Clean up your lab station. The solution in the beaker can go down the sink safely. The bag of solution can be discarded directly into the trashcan. Wash and rinse all of your glassware. Return glassware to your lab tray to dry. Return the lab tray to its designated area indicated by your teacher. 20. Complete the Data & Analysis Sheet. LAB: Selectively Permeable Page 4 of 6

5 Name Date Block DATA & ANALYSIS SHEET: Selectively Permeable Lab Data Table 1: Change in Mass Initial Mass (g) Final Mass (g) Data Table 2: Iodine Test for Starch Solution Tested Initial Color (at time = 0 min) Final Color (at time = 15 min) Solution INSIDE the bag Solution OUTSIDE the bag Data Table 3: Benedict s Test for Glucose Solution Tested Initial Color Final Color Solution OUTSIDE the bag Analysis Questions 1. Using the formula below, CALCULATE the percent change in mass for your bag. Show all your work. % Change in Mass = (Final mass Initial mass) x 100 Initial mass 2. Using your observations above about color changes, COMPLETE the chart describing which substances passed through the membrane and in which direction. Substance Starch Impermeable (Yes / No) Permeable (Yes / No) NET Movement? (Into bag / Out of bag / Zero) Glucose Iodine Water LAB: Selectively Permeable Page 5 of 6

6 3. Based on the molecular structure of starch, give a possible REASON as to WHY starch behaved the way it did in our cell simulation experiment. 4. In our cell simulation as well as a real cell, the cellular membrane is described as being selectively permeable. STATE what this means and PROVIDE EVIDENCE from your lab to support this claim. 5. SUGGEST a model of the physical structure of the dialysis tubing that would explain why you got the results you observed. In other words, if you put a piece of dialysis tubing under a microscope, what would you see? DRAW your suggested physical model and WRITE a brief description explaining it below. Viewed under the microscope LAB: Selectively Permeable Page 6 of 6