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Diffusion & Osmosis with Data Analysis

Class Laboratory Activity
By: Jeff Lukens (AEJLukens@aol.com)

Type of Activity : Hands-on lab, simulation, inquiry lab, integration of math and science, group learning.

Target Audience :
Most applicable to Advanced or Advanced Placement Biology. Can also be used with general life science or Biology courses.This activity helps students understand the essential principles governing diffusion and osmosis. Concepts such as permeability, equilibrium, water potential, concentration gradients, and plasmolysis are illustrated.

Background Information:
This lab is derived from the AP Biology lab on Diffusion & Osmosis. One segment of the lab is the classic starch/IKI/glucose/dialysis tubing experiment, and the other uses core samples of potato in varying concentrations of sugar-water to measure water potential of the potato cells. Data is collected and analyzed by using the Texas Instruments TI-82 graphing calculator. If TI-82's are unavailable or foreign, data may be analyzed by hand. However, since math technology and science technology should be integrated whenever possible, it is worthwhile to expose students to instruments like graphing calculators. Even though TI-82's seem to be most user-friendly, TI-81's and TI-85's may be used in place of the TI-82's.

Students need to be familiar with the use of electronic balances, and with the basic principles of diffusion & osmosis, although this lab could be used as an introduction to the topics. There would be much more "discovery" that way! Students will be using the graphing calculator as well, so a basic working knowledge of that tool would be helpful. However, if the teacher is familiar with the calculator, students can be taught in just a few minutes how to do what needs to be done for this particular activity. Interest will be high, and extension opportunities will abound.

Preparation time is moderate . Most solutions may be prepared will in advance and stored in the refrigerator. A suggestion is to make a large supply of stock sucrose solution and dilute it as necessary.

Class time needed for this activity will be about two 50 minute periods, then an additional class period to compare, share, and integrate data.

Activity Abstract
Understanding concentration gradients is essential to understand life processes. Any time a cell has something move into or out of it, a gradient is either involved or created in the process. Diffusion, osmosis, passive transport, and active transport are fundamental concepts in Biology. This activity simulates osmosis in the cell. Dialysis tubing is used to simulate an animal cell, and actual plant cells are used to illustrate the process of osmosis in plants.

Materials Needed
Dialysis tubing (8-12" long, soaked in water overnight)
String for tying dialysis tubing
Clear plastic drinking cups
Sharpie Markers for labeling cups
Glucose/starch solution (10g soluble starch/liter of water. Add 150g glucose)
Distilled water
IKI solution (3g KI to 400 ml water. Add 0.6g iodine, stir, store in dark bottles)
Glucose TesTape (Clinistix work too)

1.0-molar sucrose solution (to be used as a stock solution. Use 342g sucrose/liter of water)
To make 250 ml of each molarity, use:
For 1.0-M, use full strength.
For 0.8-M, 200ml stock + 50ml water
For 0.6-M, 150ml stock + 100ml water
For 0.4-M, 100ml stock + 150ml water
For 0.2-M, 50ml stock + 200ml water
HandiWrap (or similar)
Rubber bands
Cork borer (varying diameters for optional exercises)
Sharp knife for cutting produce
Potatoes (sweet potatoes, turnips, jicama, carrots, winter squash, apples, radishes are optional)
Electronic balance (triple beam will work)
TI-82 calculator (or similar)

Procedure (Part I)
1. Fill a plastic cup 2/3 full of distilled water, and add a few squirts of IKI solution to the cup. Test the contents of the cup with TesTape.

2. Tie off one end of a piece of soaked dialysis tubing. This works best if one end of the tube is folded over on itself about 1", then tied with string. Rub the other end between the fingers to open it up. Do this only when ready to fill it.

3. Place the funnel into the open end of the tubing and pour 10-15 ml of the glucose/starch solution into the tube. Repeat the "tying-off" procedure on the open end. Test a small sample of the glucose/starch solution with TesTape to see the results.

4. Place the dialysis bag into the plastic cup and let it stand for 15-20 minutes.

5. Test the liquids in the beaker and in the bag with TesTape.

Have the students chart the initial and final observations, then have them explain (orally or in writing) what happened in this part of the activity. You may want to perform the activity again and switch the location of the solutions.

Procedure (Part II)
1. Each group will need 6 dialysis tubes, unless you want to divide up the molarities among the groups to save on materials. Obviously, the second option saves time.
2. Tie off each bag, open each up, and funnel about 15 ml of each (or selected ones) of the following solutions: distilled water, 0.2 molar, 0.4 molar, 0.6 molar, 0.8 molar, and 1.0 molar. Made each solution from the stock solution of 1.0-M sucrose. For each bag tested, you will need a plastic cup filled 2/3 full of distilled water. Make sure each cup is labeled with the appropriate molarity.
3. Place an empty, dry plastic cup on the electronic balance, tare the balance EACH TIME, blot off the outside of the bag and weigh. Record the initial masses. DO THIS QUICKLY!
4. Immediately place each bag into an appropriately labeled plastic cup with distilled water.
5. Let stand for a predetermined time (15-30 minutes), but make sure that all bags are allowed to stay in the cups for the same amount of time.
6. At the end of the immersion time, remove one bag at a time, blot it off, and re-weigh it. Record the final masses. Calculate the percent change in mass of each bag: Final-Initial/Initial X 100. You will plot percent change vs. molarity in the bag.

Procedure (Part III)
1. Pour about 100 ml of sucrose solution into a plastic cup. Have lab groups each take 2-3 different molarities of solution. These are the same solutions as you used in Part I.

2. With the cork borer, cut 4 cylinders of potato from the center. Make sure there is no skin on the cylinders. Each cylinder should be about an inch long. An option here is to give each group a different fruit/vegetable. Another is to have each group take a different molarity and test several different fruits/vegetables at that molarity.

3. Using a dry beaker on the balance, tare the balance and weigh the 4 cylinders QUICKLY. Record the mass and immediately place the potato into the appropriate cup. Cover the cup with plastic wrap, put a rubber band around the top of the cup, and let is stand overnight.

4. Next day, remove, gently blot, and determine the final mass of the cylinders. Record this mass and calculate the percent change.

Data Analysis
The data can surely be plotted manually on graph paper, however making use of the graphing calculator here is easy, effective, and better lends itself to extending this activity by making predictions and drawing conclusions. Following, you will find basic instructions for viewing a graph of the collected data using the TI-82.

Turn the calculator on. Press "STAT" key. Press "1" for "Edit", you will see columns labeled "L1", "L2", etc. These are the "list" columns.

Arrow the "cursor" so it darkens "L1" in the first list column. Clear out each list by pressing CLEAR, then pressing ENTER. You are now ready to enter data from the lab activity.
Arrow to the 1st entry spot under "L1", and enter 0.00001 (trust me, entering "0" will cause a problem).

Press ENTER, enter 0.2 in the next spot down, 0.4 in the third, then 0.6, 0.8, & 1.0.
Arrow over to the first spot under "L2". Enter the respective percent changes for the sucrose dialysis bags in distilled water.

In "L3", enter the percent changes for the potato cores for their respective sucrose concentrations. If other vegetables/fruits were used, enter their percent changes in further list columns.

Press "2nd", then press "Y=". This puts you into the "STAT PLOT" function. If "1:" is highlighted, press ENTER. Arrow to "On" and press ENTER. Arrow to "Type" and choose the first one (scattergram). Arrow to "X list" and choose "L1". Arrow to "Y list" and choose "L2".

Arrow the "Mark" and choose the first one. Press "2nd" "Y=" again and make sure that the other plots are turned off. If any are on, simply arrow to the correct one, press enter, and turn it off.

To see the graph, press "ZOOM" then hit "9". This automatically makes the graph viewing area (window) appropriate to the data you have in the lists. To see what the data points are, press "TRACE", then use the right and left arrows to see what the data points represent.

If you want to view a graph of the other data in your lists, simply go into STAT PLOT and change your "Y list" to L3, L4, or whatever you want to view. Remember, if you keep your "X list" as L1, then the molarities will be used each time. If you want the points connected, go into STAT PLOT and choose the second in line next to "Type".

A linear regression of the data can be produced as well. Press STAT and arrow to CALC. Press 5 for LinReg. At the flashing cursor, press ENTER. The equation for the line will appear. The "r=" value is how closely the line fits the data. Press the "Y=" key and clear any data that appears by putting the cursor right after the "=" sign and pressing CLEAR. With all of them cleared out, press VARS, then press 5 for "Statistics". Arrow over to "EQ", then press 7 for the RegEQ. The equation appears next to the Y1=. Now press GRAPH and you will see your original plot AND a line that best fits the data.

Press 2nd then WINDOW. For TblMin, enter 0. For "delta"Tbl, enter .1. Press 2nd GRAPH and you will see a table that allows you to predict new percent change values for molarities that were not examined.

Using this form of data analysis, many windows of opportunity will open up for teacher and student alike. Be creative! Be imaginative! Extend the activity and the use of the calculator!

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