Lab Activity No. 3
Cells, Diffusion, Osmosis and Biological Membranes
General Biology Lab
A. Objectives: Upon completion of this lab activity, you should be able to:
1. Define and correctly use the following terms: solute, solvent, semipermeable (differential permeability), concentration, diffusion, osmosis, tonicity, hypertonic, hypotonic, isotonic, turgor pressure and plasmolysis.
2. Describe the effects of hypertonic, hypotonic, and isotonic solutions on cells (plant and animal).
3. Describe an approach to determining the concentration of solutes in a cell.
4. Interpret data from a differential permeability study.
5. Prepare and interpret a graph that summarizes data collected from an experiment.
B. Introduction: 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 semipermeable 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 semipermeable 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 their cellophane walls and enter the surrounding fluid. Then, the machine purifies this fluid.
C. Osmosis in a Mock Cell: A Model Using Dialysis Tubing (Modified from Glick et al., The Process of Science: Seven
Studies of Life; Perry and Morton, Laboratory Manual.)
In this activity, you will make a series of mock cells using pieces of dialysis tubing. This tubing, which is semipermeable, will simulate the plasma membrane of a cell.
Materials:
 Four, 400 ml beakers
 Distilled water
 25% sucrose soluton
 10% sucrose solution
 Dialysis tubing
 String
 Ringstand with funnel
 Graduated cylinder
 Balance
1. Label four, 400-ml beakers “1” to “4” with pieces of white tape and a permanent marker.
2. Add approximately 150 ml of distilled water to beakers 1, 2, and 3. Use the gradients on the side of the beakers to estimate the volume.
3. Add approximately 150 ml of 25% sucrose to beaker 4.
4. Cut four pieces of dialysis tubing, each approximately 16 cm long.
5. Tie one end of the tube to form a bag. To do this, fold the last 1/2cm of the end of the bag back on itself, twist it, and tie it securely with string.
6. Place the bag on the clean tray and thoroughly wet the bag with distilled water. Allow the bag to soak for 1 to 2 minutes.
7. Open the end of the bag by gently rubbing it between your thumb and fingers. Use a long plastic pipette to open the bag.
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8. Slip the open end of the bag over the narrow end of a funnel that is held by a ring stand.
9. Use a graduated cylinder to measure out 10 ml of each of the following solutions. Add each solution SLOWLY to the designated bag. Remember which bag is which by placing them in a beaker with the corresponding number written on it. 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 distilled water
10. As each bag is filled, force out excess air by gently squeezing from the top end of the tube, just above the liquid.
11. 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 .
12. Squeeze your bag gently to check for leaks.
If a leak is detected, you will need to retie the bag .
Trim excess string and rinse the bag with a small amount of distilled water.
13. 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 data table in the report. Remember to tare the balance prior to weighing each bag.
14. After weighing all 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.
15. 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.
16. After 15 minutes, remove the bags and allow them to drip for a second or two, and weigh each one to the nearest
0.1g. Record the weights in the appropriate spaces in the data table in the report.
17. Return the bags to their respective beakers immediately after weighing and begin timing the next 15 minute time interval.
18. Repeat steps 16 and 17 at 30, 45, and 60 minutes (These times are relative to the start of the experiment.).
19. 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.
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Time
(Min.)
Osmosis Data Table:
Record the 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. 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.
Changes in Weights Over Time for Four Mock Cells with Different Cell/Environment Concentrations
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
(D. Water/25% Sucrose)
Weight (g) Change
0 0 0 0 0
15
30
45
60
* The first term indicated the contents of the mock cell and the second term indicates the solution surrounding the mock cell (i.e. the solution in the beaker).
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Graph of Mock Cell Data:
Prepare a LINE graph (NOT a bar graph) of the data in the previous table that shows the CHANGES (see the
“Change” column in the previous table) in mock cell weight at 15 minute intervals. Make certain that your group states an appropriate title, axis labels, and correctly graph the data. Remember this graph will contain four sets of data, one for each bag. Be certain to provide a key (legend) to facilitate line identification.
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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, not necessarily the results of your experiments.
!!Remember, sucrose can't pass through the pores in the membrane but water can (concept of
"semipermeable")!!
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. e. A mock cell is filled with 15% sucrose solution. On the basis of this information, determine whether each of the following statements is True or False .
____________ If the cell was placed in an isotonic solution, the weight of the cell would not be expected to change.
____________ If the cell was placed in a 25% sucrose solution, the cell’s weight would be expected to increase.
____________ A 10% sucrose solution would be hypotonic to the contents of the cell. f. Movement of what substance caused the mock cells to lose or gain weight? _____________________ g. What specific term is used to describe this movement? _______________________
D. Turgor Pressure and Plasmolysis 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 turgor pressure . Such cells 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, then the cell will lose water, causing it to become “plasmolyzed” (This process is called plasmolysis ).
Materials:
 Potatoes
 Rulers
 Knife and cutting board
 Balance
 Distilled water
 Two large test tubes
 Saturated salt solution
1. Cut two strips of potato, each approximately 7cm long, 1.5cm wide, and 1.0 cm thick. Designate them as "1" and
"2". Make certain there is no peel on the strips.
2. After placing aluminum foil on the balance pan, weigh each of the strips to the nearest 0.1g and record the weights in the data table below.
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3. Observe how the strips look and feel before and after soaking in the solutions. Record your observations in the data tables below.
4. Using pieces of white tape and a permanent marker, designate two test large tubes as "1" and "2", and place each potato strip into the appropriate tube.
5. Fill tube 1 (to the top) with distilled water.
6. Fill tube 2 (to the top) with a saturated salt solution.
7. After approximately one hour, remove the strips and reweigh them. Record the weights in the designated table in the report.
8. Discard the potato strips in the trash, wash all glassware, rinse it, and return it to the designated areas.
Data Tables: a. The Weight Changes of Potato Slices Exposed to Distilled Water or Salt Water
Potato Slice Potato Slice
Initial Wt. (g) Final Wt. (g) Difference (g)
Treatment
Distilled water _________ ________ ______
Saltwater _________ ________ ______ b. Observations of Potato Slices Before and After Exposure to Distilled Water or Salt Water
Treatment Observations
Slice at Beginning of Experiment__________________________________________________________
Slice in Distilled Water __________________________________________________________
Slice in Saltwater __________________________________________________________
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Questions:
1. Your roommate bought lettuce and celery this afternoon for a dinner party later in the evening. However, he/she forgot the vegetables in the trunk of the car and when he/she finally remember to get them, they were limp. “Don’t panic” you said. “I can use my Biology 104 knowledge to solve the problem!” Explain how you would restore the crispness to the vegetables using the principles of this activity (Make certain you use appropriate technical terminology 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!
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E. Review Questions
1. Correctly define the terms from the following list: solute, solvent, semipermeable (differential permeability), and concentration.
2. What physical feature of the dialysis tubing results in the property of differential permeability?
3. Consider the differential permeability experiment. To what substances was the membrane permeable? Explain your response by discussing how you interpreted the data.
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