Cells, Diffusion, Osmosis, and Biological Membranes

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1 Cells, Diffusion, Osmosis, and Biological Membranes A. Objectives Upon completion of this lab activity, you should be able to: 1. Define and correctly use the following terms: solute, solvent, selectively permeable (semipermeable or differentially permeable), concentration, diffusion, osmosis, tonicity, hypertonic, hypotonic, isotonic, turgor pressure, and plasmolysis. 2. Describe the effects of hypertonic, hypotonic, and isotonic solutions on cells. 3. Interpret data from a differential permeability study. 4. Prepare and interpret a graph that summarizes data collected from an experiment. B. Osmosis in a Mock (Simulated) Cell: A Model Using Dialysis Tubing. (Modified from Glick et al., The Process of Science: Seven Studies of Life; and Perry and Morton, Laboratory Manual for General Biology.) When a person suffers from diminished kidney function, it is possible to remove waste materials from the blood with a kidney dialysis machine. Dialysis combines a selectively permeable membrane with the process of diffusion to separate substances of various sizes. The machine passes blood from an artery of the body to tubes made of special cellophane before returning it to a vein. The cellophane tubes are selectively permeable and submerged in a fluid that is similar to blood but free of wastes. As blood flows within the tubes, concentrated wastes in the blood diffuse outward through the cellophane walls and enter the surrounding fluid. In this activity, you will make a series of mock cells using pieces of dialysis tubing. This tubing, which is selectively permeable, will simulate the plasma (cell) membrane of a cell. The tubing is permeable to water, but not to solutes. Materials Available: Four, 400 ml beakers White labeling tape Permanent marker Distilled water 25% sucrose solution 10% sucrose solution Dialysis tubing String Tray Ring stand with funnel Graduated cylinder Clock, watch, or timer Balance Aluminum foil The procedures for the activity are described below: 1. Cut approximately 50 cm of string and place it in distilled water to soak. You will use this string to tie your dialysis bags. 2. Label four, 400-ml beakers 1 through 4 with pieces of white tape and a permanent marker. 3. Add approximately 150 ml of distilled water to beakers 1, 2, and 3. Use the graduations on the side of the beakers to estimate the volume. 4. Add approximately 150 ml of 25% sucrose to beaker 4. Page 1

2 5. Cut four pieces of dialysis tubing, each approximately 16 cm long. Place each piece of tubing on one of four labeled paper towels (i.e. labeled 1, 2, 3, or 4) to avoid confusion. It is imperative that you do not confuse which bag is which! 6. Tie one end of the tube to form a bag. To do this, fold the last few cm of the end of the bag back on itself, twist it, and tie it securely with string you have soaking.. 7. Place the bag on a clean tray and thoroughly wet the bag with distilled water. Allow the bag to soak for 1 to 2 minutes. 8. Open the end of the bag by gently rubbing it between your thumb and fingers. Use a long plastic pipette to open the rest of the bag. 9. Slip the open end of the bag over the narrow end of a funnel that is held by a ring stand. 10..Use a graduated cylinder to measure out 10 ml of each of the following solutions. Add each solution SLOWLY to the designated bag. Be sure to rinse the graduated cylinder and funnel with distilled water between measuring each of the sugar solutions. Bag 1 Contents: 10 ml of distilled water Bag 2 Contents: 10 ml of 10% sucrose Bag 3 Contents: 10 ml of 25% sucrose Bag 4 Contents: 10 ml of 10% sucrose Remember, sucrose can't pass through the pores in the membrane but water can (the membrane is "selectively permeable"). 11. As each bag is filled, force out excess air by gently squeezing from the top end of the tube, just above the liquid. 12. Fold the end of the bag and tie it securely with another piece of string so that it is limp (i.e., leave some empty room at the top of the bag). The bag must be limp after it is tied. 13. Squeeze your bag gently to check for leaks. If a leak is detected, you will need to retie the bag. Trim the excess string and rinse the bag with a small amount of distilled water. 14. Gently blot the bag on a paper towel or Kimwipe to remove excess fluid. 15. Place a sheet of aluminum foil on the pan of the balance, weigh each bag to the nearest 0.1 g, and record its weight in the column marked "0 min" in the appropriate data table found in the DATA AND ANALYSIS section of this packet. Remember to tare the balance prior to weighing each bag. 16. After weighing all of the bags, place each one in the correspondingly numbered beaker. Put all the bags into the beakers at approximately the same time! Make certain each bag is submerged in the fluid in its beaker. 17. Record the starting time above the "0 min" column in the data table. You will need to reweigh the bags in 15 minutes, so I suggest that someone in your group set his/her watch alarm. 18. After 15 minutes, remove the bags, gently blot them on a paper towel or Kimwipe, and weigh each one to the nearest 0.1 g. Record the weights in the appropriate spaces in the data table. 19. Return the bags to their respective beakers immediately after weighing and begin timing the next 15 minute time interval. Page 2

3 20. Repeat steps 18 and 19 at 30, 45, and 60 minutes (these times are relative to the start of the experiment). 21. At the end of the experiment, take the bags to the sink, cut them open, pour the contents down the drain, and discard the bags in the trash can. Pour the contents of the beakers down the drain, rinse them with water, and dry them with paper towels. Return all equipment to the designated areas. C. Osmosis in Plant Cells Plant cells are surrounded by a rigid cell wall that is composed primarily of a complex arrangement of glucose molecules called cellulose. Normally, the solute concentration within the cell is greater than that of the external environment. Consequently, water moves into the cell, creating what is called turgor pressure. Such cells have a firm consistency and are said to be "turgid." Many non-woody plants (such as beans and peas) maintain their rigidity and erect stance via this pressure. However, if a cell is exposed to an environment with a more concentrated solute concentration than that inside the cell, then the cell will lose water, causing the cell (plasma) membrane to pull away from the outer cell wall as the cell shrinks. This process is called plasmolysis. Note: you may assume that any diffusion of solutes into or out of the potato cells is negligible compared to the movement of water into or out of the cells. Materials Available: Potatoes Ruler Knife and cutting board White labeling tape Permanent marker Large test tubes Distilled water Saturated salt solution (salt water) Clock, watch, or timer Balance Aluminum foil The procedures for the activity are described below: 1. Cut two strips of potato, each approximately 7 cm long, 1.5 cm wide, and 1.0 cm thick. Designate them as "1" and "2". (Your strips of potato should be about the same size and must fit into the test tube described in step 3.) 2. After placing aluminum foil on the balance pan, weigh each of the strips to the nearest 0.1 g and record the weights in the designated part of the data table. 3. Using pieces of white tape and a permanent maker, designate two test large tubes as "1" and "2", and place each potato strip into the appropriate tube. 4. Fill tube 1 (to the top) with distilled water. 5. Fill tube 2 (to the top) with a saturated salt solution (salt water). 6. After approximately one hour, remove the strips and reweigh them. Record the weights in the designated part of the appropriate data table in the DATA AND ANALYSIS section of this packet. Page 3

4 7. Obtain a piece of fresh potato for comparison, and observe how the strips look and feel before and after soaking in the solutions. Record your observations in the designated area of the data table. 8. Discard the potato strips in the trash, wash all glassware, rinse it, and return it to the designated areas. D. Review Questions discuss these with your group after completing the DATA AND ANALYSIS section of this packet (pages 5-8) 1. Define the terms from the following list: solute, solvent, selectively permeable (semipermeable or differentially permeable), and concentration. 2. What physical feature of the dialysis tubing results in the property of selective (semi- or differential) permeability? 4. Interpret the data for each of the four mock cell (bag)/beaker combinations (Section B) using appropriate terms from the following list: tonicity, concentration gradient, osmosis, hypertonic, isotonic, hypotonic, diffusion, and selectively permeable (semipermeable or differentially permeable) membrane. 5. Interpret the data from the plant cell experiment (Section C) using appropriate terms from the following list: turgor pressure, plasmolysis, hypertonic, hypotonic, and osmosis. 6. Consider the selective permeability (mock cell) experiment (Section B). To what substances was the membrane permeable? Why were only some of the types of molecules able to move across the membrane? 7. A common expression used in the explanation of osmosis is water follows salt (or solute). Do the data from each experiment (Section B and Section C) support this expression? If not, how could you explain the discrepancy. 8. The rate of diffusion of a substance through a membrane is called flux. The fundamental equation for flux (which is based on Fick s first law) is flux = P x C. (Or, flux = permeability x concentration difference.) That is, the rate of diffusion (flux) of a substance through a membrane is equal to the permeability of the membrane to that substance multiplied by the difference in concentration of that substance on both sides of the membrane. How does this equation help explain the results of Section B with respect to the diffusion of water (osmosis) and the lack of diffusion of sucrose? Page 4

5 DATA AND ANALYSIS (to be detached from this packet and turned in to the instructor when completed) Your name Other group member names (a maximum of four members per group): B. Osmosis in a Mock (Simulated) Cell: A Model Using Dialysis Tubing 1. Data Table (Note: Record weight changes as differences (+ or -) for each reading from time 0. For example, if the weight of the bag at time 0 was 25 g and the weight of the bag 15 minutes later was 23 g, then the Change would be -2 g. If the weight at 15 minutes later (i.e. 30 minutes after the beginning of the experiment) was 21 g, then the Change would be -4 g. Compare back to the initial weight each time.) Changes in Weight Over Time for Four Mock Cells with Different Cell/Environment Concentrations Time (Min.) MOCK CELL 1 (D. Water/D. Water)* Weight (g) Change MOCK CELL 2 (10% Sucrose/D. Water) Weight (g) Change MOCK CELL 3 (25% Sucrose/D. Water) Weight (g) Change MOCK CELL 4 (10% Sucrose/25% Sucrose) Weight (g) Change *The first solution listed indicates the contents of the mock cell and the second solution listed indicates the environmental solution surrounding the mock cell (i.e., the solution in the beaker). 2. Graphical Presentation of Mock Cell Data On the grid that is located at the end of this handout, prepare a LINE graph (NOT a bar graph) of the data in the previous table that shows the CHANGES (from the initial weight; see the Change column in the previous table) in the mock cells weight at 15 minute intervals. Make certain that your group provides an appropriate title, axis labels and units of measurement, and correctly graphs the data points you obtained. Remember, this graph will contain four sets of data, one for each mock cell. Be certain to provide a key (legend) to facilitate line identification. Page 5

6 3. Questions In each of the following four statements, fill in the blanks with the correct term: hypotonic, hypertonic, or isotonic. Base your response on conceptual knowledge (as discussed in lecture), not necessarily the results of your experiments. Again, remember that sucrose can't pass through the pores in the membrane but water can (the membrane is "selectively permeable"). a. The solution in beaker 1 was to mock cell 1. b. The solution in beaker 2 was to mock cell 2. c. The solution in beaker 3 was to mock cell 3. d. The solution in beaker 4 was to mock cell 4. A mock cell is filled with 15% sucrose solution. On the basis of this information, determine whether each of statements e. through g. is True or False. e. If the cell was placed in an isotonic solution, the weight of the cell would not be expected to change. f. If the cell was placed in a 25% sucrose solution, the cell s weight would be expected to increase. g. A 10% sucrose solution would be hypotonic to the contents of the cell. h. Movement of what substance caused the mock cells to lose or gain weight? i. What specific term is used to describe this movement? C. Osmosis in Plant Cells 1. Data Table a. The Weight Changes of Potato Slices Exposed to Distilled Water or Salt Water Treatment Potato Slice Potato Slice Initial Wt. (g) Final Wt. (g) Difference (g) Distilled Water Salt Water Page 6

7 b. Observations of Potato Slices Before and After Exposure to Distilled Water or Salt Water Treatment Observations Control Slice** Slice in Distilled Water Slice in Salt Water **Note: remember to obtain a control slice (i.e. a fresh potato slice) just before you are ready to make your observations. Briefly explain your results using vocabulary from this lab activity: 2. Questions a. Your roommate bought lettuce and celery this afternoon for a dinner party later in the evening. However, she left the vegetables in the trunk of the car and when she finally remembered to get them, they were limp. Don t panic you said, I can use my biology knowledge to solve the problem! Explain how you could restore the crispness of the vegetables using the principles of this lab activity. (Make certain you use the appropriate technical terminology from this lab activity to justify why your treatment would restore crispness.) Note: You don t have time to buy more vegetables, and your roommate insists that the menu can t be changed! Page 7

8 Page 8

9 BIOL& 251 Osmosis Lab Assessment Rubric Student: Points: ASSESSMENT Points possible 1 Legible and complete mock cell data table 10 2 Accurate, detailed answers to questions Item 3, p Legible and complete potato data table 6 4 Detailed observations on potato samples 5 5 Potato Observation Questions Accurate explanation of observations on potato samples (2 points) Use of vocabulary (3 points), at least 3 of o osmosis, high low [water] o hypotonic 5 o hypertonic o solute o solvent (water) o turgor pressure o plasmolysis o referred to weight table 6 Dinner Party Questions Accurate explanation (2 points) Use of vocabulary (3 points), at least 3 of o osmosis, high low [water] o hypotonic 5 o hypertonic o solute o solvent (water) o turgor pressure 7 Graph Axes clearly labeled with appropriate units of measurement (2 points) Legend clearly differentiates between experimental groups (2 points) 10 Data from table accurately graphed as directed (2 points) Appropriate scales for axes indicates planning and accuracy (2 points) Legible (2 points) TOTAL of 50

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