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Dialysis Diffusion/Osmosis Lab
Introduction: Cells can use the principles of osmosis and diffusion to move molecules in and out of the cell without expending energy. Internal environments of cells are affected by the concentrations of fluids surrounding them and the concentrations of their interior environments. Diffusion and osmosis are passive processes and depend upon concentration, solute size and the permeability of membranes between the internal and external parts of the cell. The direction of osmosis (water movement across a selectively permeable membrane) depends on the relative concentrations of the cytoplasm and the cell’s surrounding environment. If the cell has a higher concentration of solutes in the cytoplasm than in its surrounding solution (a cell in fresh water, for example) it is in a hypotonic solution and water will rush into the cell. If the cell has a lower solute concentration in the cytoplasm than in its surrounding solution (a cell in salt water, for example) it is in a hypertonic solution and water will leave the cell. An isotonic solution maintains homeostasis for the cell with concentrations of water/solutes at equilibrium on both sides of the membrane. It will be important to remember these principles when completing this lab.
Purpose: Read through the entire lab. Discuss a purpose with your partner and record it in your notebook.
LAB NOTE: Once you have set up Part A, set up Part B. The tests for Part A and Part B will run concurrently. Record all of your data and answers to questions in your lab notebook. You must copy the data charts into your notebook.
Part A – Osmosis and Diffusion in Living Cells
In Part A of the lab you will determine how plant cells are affected by hypertonic and hypotonic solutions.
You will need:

Potato pieces

Water

Balance
● sugar solution
● beakers
● toothpicks
Procedure:
1.
Copy down the data table below in your notebook:
Part A. Osmosis and Diffusion in Living Cells Data Table
Contents in beaker
Combined mass of Potato core mass (g) initial
20 min
30 min 24 hr mass difference
(24 hr mass – initial mass)
% change in mass
**
Water
Sugar solution
* * % change in mass = mass difference
X 100
initial mass
2.
Obtain 6 pieces of potato slices and 2 beakers.
3.
Use a balance to weigh your potato slices: place a small piece of paper towel on your balance and zero your balance (press the “Z” on the balance). Then, measure the mass of 3 potato slices (all 3 at the same time) by placing the 3 pieces on your scale all at once. Record the mass in the Osmosis and Diffusion in Living Cells
Data Table by the initial mass section for water. Put these slices in a beaker labeled “water”.
4.
Repeat #2 with your other three slices, placing them in the second beaker labeled “sugar solution”.
5.
Fill the first beaker labeled “water” with water and the beaker labeled “sugar” with the sugar solution. Fill each beaker with just enough liquid to cover the potatoes. When you have filled the beakers, mark on the beakers the level of the liquid.
6.
After 20 minutes, remove the potato from the solutions, blot dry and weigh. Record the mass in the Osmosis and Diffusion in Living Cells Data Table under the 20 min section. Return the potato slices to their beakers.
7.
Repeat step #6, but after 30 minutes.
8.
Cover each beaker with plastic wrap and let stand overnight. (Put the beakers on a labeled paper towel on the shelf above the benches.)

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9.
After 24 hours, repeat step #6, but record in the 24 hour section on the Osmosis and Diffusion in Living Cells
Data Table.
10.
Determine the % change in mass and record your results in the Osmosis and Diffusion in Living Cells Data
Table.
** see the equation above to determine the % change in mass**
Part B – Simulating Diffusion and Osmosis
In this experiment you will measure diffusion of small molecules through dialysis tubing – an example of a selectively permeable membrane. Assumption: small solute molecules and water molecules can move freely through the membrane.
Materials:
Sugar Solution
Dialysis Tubing (20cm)
Test Tube Rack
Benedicts Solution
Starch Solution
Pipettes (2)
Iodine Solution
250 ml Beaker
Procedure:
1.
Copy the chart below into your notebook:
Test Tubes (4)
String (dental floss)
Hot Water Bath
Part B. Simulating Diffusion and Osmosis
Mass change in dialysis bag (g) Data Table
Time (min) (initial) 0 10 20 30
Total mass difference
% change in mass** bag mass with sugar and starch solution
2.
Prepare the dialysis tubing as follows:
- rub ends of the tubing between your fingers to open up the tubing
- once open, twist one end of the tubing, fold it over, and tightly tie off with a piece of string (dental floss)
3. In a small beaker, pour 15 mL of sugar solution and 15 mL of starch solution. Use this solution to fill your dialysis
tubing. Leave some space at the top of the tubing: do not over fill the tubing!
3. Twist the other end of the tubing, fold it over, and tightly tie off with a piece of string.
4. Rinse the dialysis bag carefully in the sink and blot it dry.
5. Weigh the bag and record its initial weight in the Mass change in dialysis data table under initial time = 0
6. Submerge the bag in a beaker of water for 10 minutes. Re-weigh the bag, record its weight in the data table under the
10 minute section. Then return it to the beaker for 10 more minutes.
7. Take the bag out and weigh the bag again, record its weight in the table in the 20 minute section.
8. Repeat step 7, but at 30 minutes.
7. Empty the contents of your dialysis bag into a small beaker. Label this beaker “dialysis bag.” Save the solution that
the tubing was in and label it “beaker solution.”
8. Copy the chart below into your notebook:
Test Tube Contents Data Table:
Test Tube # Substance Tested for
Location of
Solution
Beaker
Description of
Color
Presence of
Substance (+ / - )
1
2
3
4
Sugar
Starch
Sugar
Starch
Beaker
Dialysis bag
Dialysis bag
* Presence of Substance: + = positive test - = negative test

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9. Label your four test tubes: #1, #2, #3, #4 (make sure to also write your initials on the test tubes to identify yours
from other groups in the classroom).
10. Take a pipette full of solution from the beaker solution beaker and add it to test tube #1. Do the same for test tube
#2.
11.
Put one pipette full of solution from the dialysis bag beaker into test tube #3. Do the same for test tube #4.
12.
To test for the presence of sugar, add 5 drops of Benedicts Solution into test tube #1 and into test tube #3. Heat both test tubes for 10 minutes in a hot water bath. Record your observations in the Test Tube Contents Data Table in the correct spot. If you have a color change, then your test is positive.
13.
To test for the presence of starch, add 2 drops of Iodine solution to test tube #2 and test tube #4. Record your observations in the Test Tube Contents Data Table in the correct spot. If you have a color change, then your test is positive.
14.
Clean up…rinsing all materials thoroughly.
Conclusions (answer these questions in your notebook):
Part A:
1.
Illustrate your set up with the two different solutions and the potatoes. Show the direction that water flowed. Label the tonicity for each set up (sugar/water).
2.
Were the potatoes in isotonic, hypertonic or hypotonic solutions? How do you know? (Provide data to support your answer)
Part B:
1. Which two substances are being tested for in this experiment?
2. Why should you blot dry your tubing before submerging it in the beaker?
3. Illustrate how diffusion was demonstrated in this experiment; draw your beaker set up with the tubing inside. Show
the direction of flow of the all of the solutions and label completely.
4. To which substance(s) is the dialysis tubing permeable?
5. If the diffusion of molecules through a membrane is dependent upon the size of a molecule, how do the molecules of
starch and sugar compare?
6. Explain how the dialysis tubing acted like a semi-permeable membrane.
6. In what direction was the net movement of fluid, in or out of t he tubing? How do you know (give evidence)?