Modern Biology I laboratory Zarina Sautbayeva
Introduction:
Osmosis is the process by which water is transported into and out of the cells. Osmosis is the spontaneous net movement of water through a partially permeable membrane to an area with higher solute concentration without the input of any energy.
Free water is not bound to the molecules of solute and therefore has free energy and free water molecules that are ready to diffuse. When water molecules are bound to solute molecules, it is much harder for them to move across a semipermeable membrane. The movement of water depends on molarity of a solution on the other side of the semipermeable membrane and not on the size of molecules. Net movement of water across the semipermeable membrane stops when the concentration of solute is equal on both sides of the membrane, and therefore equilibrium is reached.
Depending on the concentration of solutes, the relationship of two different solutions can be expressed as isotonic, hypertonic and hypotonic. Two solutions are isotonic to each other when their concentrations are the same. No net movement of water will occur between these solutions in case if they are separated by the semipermeable membrane.
The solution that is more concentrated compared to another is called hypertonic. The solution that is less concentrated compared to another is called hypotonic. The net movement of water will occur from the hypotonic solution towards the hypertonic solution.
In most organisms, cells live in an isotonic extracellular environment and therefore are able to keep their normal shape and function properly. Animal cells will swell when they are placed in a hypotonic solution. Some cells, such as erythrocytes, will actually burst as water enters them by osmosis. Placing animal cells in a hypertonic solution causes them to shrink as water leaves them by osmosis. It is important that animal cells be maintained in an isotonic medium, which has a solute concentration close to that of the cell cytosol.
Pre-Lab questions:
1. Define osmosis
2. Describe what happens if an animal cell is placed in the vessel containing pure water
3. Describe what happens if an animal cell is placed in the vessel containing the buffer that is hypertonic to the content of the cell
4. Predict what will happen to the mass of the bag with 1M sucrose that is placed in the pure water?
5. Provide definitions for isotonic, hypertonic and hypotonic solutions
6. How do animal cells maintain their normal shape in the organism?
7. Describe how you will prepare 1 L of 0.4 M sucrose (MW 342.30 g/mol). Show your calculations.

Modern Biology I laboratory
Materials:
Zarina Sautbayeva
1. Dialysis tubing (MWCO 6-8kDa) (size for each piece is about 15 cm) (x7)
2. Small beakers (250mL), clean test tubes, tips, pipettes, tube racks, graduated cylinders
3. Deionized water
4. Samples for dialysis: solutions of sucrose with molarity ranging from 0.4M to 1.2M
5. Weighing balance
Laboratory procedure/protocol for each step
Laboratory Procedure:
1. Obtain seven dialyzing strips, each 15 cm, pre-soak for 10 min
2. Obtain prepared by you on the last lab solutions of sucrose from your TAs: a. Distilled water b. 0.4M sucrose c. 0.8M sucrose d. 1.2M sucrose
3. Place 10mL of 0.4M, 0.8M and 1.2M into each bag by pipetting. Label the bags
4. Place 10mL of distilled water into four bags by pipetting. Label the bags
5. Rinse the bags carefully in distilled water and carefully blot the bags with clean paper towels.
6. Weigh the mass of each bag before starting the dialysis and record in Table 1.
7. Place a bag with sucrose solution (0.4M, 0.8M and 1.2M) in each beaker with distilled water (top up to 200 ml).
8. Place a bag with distilled water in a beaker with distilled water (top up to 200 ml).
9. Place a bag with distilled water in each beaker with sucrose solution (0.4M, 0.8M and
1.2M) (top up to 200 ml).
10. Let the bag sit for 45-60 minutes in the beaker
11. After 45-60 minutes remove the bag from the beaker, and carefully blot dry.
12. Weigh each bag and record the mass in the table
13. Calculate the percent change in mass of each bag by using the following formula
% change mass = final mass - initial mass x 100
initial mass

Modern Biology I laboratory
Table 1.
Contents in Bag
(Beaker)
0.4M
(Distilled water)
0.8M
(Distilled water)
1.2M
(Distilled water)
Distilled water
(Distilled water)
Distilled water
(0.4M)
Distilled water
(0.8M)
Distilled water
(1.2M)
Initial Mass
Zarina Sautbayeva
Final Mass Mass Difference %change in
Mass

Modern Biology I laboratory Zarina Sautbayeva
Table 2.
Group 1
Contents in Bag %change in
Mass
0.4M
(Distilled water)
0.8M
(Distilled water)
1.2M
(Distilled water)
Distilled water
(Distilled water)
Distilled water
(0.4M)
Group 2
%change in
Mass
Group 3
%change in
Mass
Group 4
%change in
Mass
Average
%change in
Mass
Distilled water
(0.8M)
Distilled water
(1.2M)
0.4M
(Distilled water)
12. Graph the results for both your individual data and class average on graph (use the graph paper below). The independent variable is on the X axis, and the dependent variable is on the Y axis
13. What is the independent variable in your experiment_______________________
Indicate this variable on the X-axis
14. What is the dependent variable in your experiment_______________________
Indicate this variable on the Y-axis

Modern Biology I laboratory Zarina Sautbayeva
Graph 1. Title_____________________________________

Modern Biology I laboratory
Post-Lab questions:
Zarina Sautbayeva
1. Did you observe the movement of the water across the membrane?
2. Where did water move? Outside of the dialysis bag or into the bag?
3. Did you observe any changes in the mass of the dialysis bags? If the answer is yes, explain why the mass of some dialysis bags changed?
4. Why were the changes in the mass of the bags different?
5. What bag did not change mass? Explain why?
6. Water was hypertonic in relation to sucrose solution. True or False?
7. In order to study the molecules of cells, disruption of cells is often required. Cell disruption allows releasing molecules from within the cell. What solution (isotonic, hypertonic, hypotonic) will you use in order to burst the cell?
8. Osmosis can be used for preserving fruits. Osmotic dehydration techniques result in fruit that can be stored longer than fruit dehydrated by other methods. What solution
(isotonic, hypertonic, hypotonic) will you use in order dehydrate the cell?
9. Hospital patients often need intravenous fluid therapy as a result of altered intake, losses and shifts of fluids within the body. In medicine, isotonic fluids are used for fluid resuscitation in order to increase blood volume. What will happen to body cells if patients will receive pure sterile water intravenously?