Lab 1: Diffusion and Osmosis

Lab 1: Diffusion and Osmosis

Laboratory 1, AP Biology 2011

Spurthi Tarugu, Kavinmozhi Caldwell, Chelsea Mbakwe, Radha Dave, Navya Kondeti

Abstract:

The basic principles of Osmosis and Diffusion were tested and examined in this lab. We examined the percent increase of mass and molarity of different concentrations of sucrose in the dialysis bag emerged in distilled water and the potato cores emerged in concentrations of sucrose. The data reinforces the principles of Osmosis and Diffusion, and in a biological context, we can simulate how water and particles move in and out of our own cells.

Introduction:

           Molecules are in constant motion; they tend to move from areas of high concentration, to areas of low concentration. This broad principle is divided into two categories: diffusion and osmosis.

          Diffusion is the random movement of molecules from an area of higher concentration to an area of lower concentration. This is considered a passive form of transportation because it does not require any additional energy to transport the molecules. In the body, carbon dioxide and oxygen can diffuse across cell membranes.

             Osmosis is a special type of diffusion where water moves through a selectively permeable membrane from a region of higher water potential to a region of lower water potential. In our body, water diffuses across cell membranes through osmosis. Water potential is the measure of free energy of water in a solution and is shown with the use of the symbol Ψ. Water potential is affected by two factors: osmotic potential (Ψπ) and pressure potential (Ψp). Osmotic potential is dependent on the solute concentration, and pressure potential which is the energy that forms from exertion of pressure either positive or negative on a solution. The equation to find the sum of water potential is:

Water Potential = Pressure Potential + Osmotic Potential

Ψw = Ψp + Ψπ

              The purpose of this lab is to observe the physical effects of osmosis and diffusion and determine if  it actually takes place. We hypothesize that, because molecules diffuse down a concentration gradient, the mass of the dialysis tubes will increase, and we believe that as the molarity increases, the percent of change in mass will also increase.

Methods:

               Part 1:  

               First, soak 6 dialysis tubing in distilled water until you can open one end. One you get the top of the tube, tie a knot at the opposite end to prevent any liquid from leaking out. Next, fill each tube about half way with a different solution (0.0M sucrose-distilled water, 0.2M sucrose, 0.4M sucrose, 0.6M sucrose, 0.8M sucrose, and 1.00M sucrose). Once you have all 6 tubes filled with the solutions, remove excess air, and tie the tops with strings. Mark the tubes or string with markings so that you can distinguish between the varying concentrations of sucrose. The marking won’t appear on the tube so it is best to color the strings a certain way and write down what color represents what solution on a separate piece of paper. Weigh each bag separately using an electronic balance and record the initial mass on a table (Table 1.1). Next, get a 250 mL beaker and fill it to 200 mL with distilled water (or tap water). Then place all 6 tubes in the beaker,and let them sit there for 20 minutes. After 20 minutes, remove the bags from the beaker, and carefully blot each one to remove excess water. Weigh each one again on an electronic balance and record the masses on a table ( Table 1.1). Calculate percent change and record on table ( Table 1.1) . On a separate table, collect all class data and find the average of all percent changes for all of the different solutions. Look at the Results Section to see what the table layout should look like.

                Part 2:

             First pour 100 mL of each of the solutions (0.0M sucrose-distilled water, 0.2M sucrose, 0.4M sucrose, 0.6M sucrose, 0.8M sucrose, and 1.00M sucrose) into a 250 mL beaker; you will have 6 beakers in total. Make sure you label each beaker with the solution and a group member’s name. Obtain a potato and a cork borer (5 mm in inner diameter). Use the cork borer to cut out 24 potato cylinders. Each cylinder must be 3 cm in length. Remove any potato skin that you may find on the cylinders. Group the 24 potatoes in groups of 4. You should have 6 groups of potato cylinders. Now, measure the mass of each group. Record the initial masses of all the groups in the table ( Table 2.1). Once you have done that you will place each group of potatoes into a different beaker. Cover each beaker with plastic wrap. Let them sit overnight. The next day, record the temperatures of the sucrose solutions in each beaker. Record the temperature in the table ( Table 2.1). Remove one group of potato cylinders, blot with a paper towel to remove excess water, and measure the group’s mass. Record in the table ( Table 2.1) . Do this for the other 5 groups. Calculate the percent change in mass. Record data of percent change in mass onto the table ( Table 2.1). Collect the classes data, and determine class average. Copy this data onto a table (Table 2.2). Look at the Results Section to see what the table layout should look like. Take note that each group in our class was assigned one solution and told to have only one group of potato cylinders due to limited supplies, hence Table 2.1 has data for only one solution.

Results

Table 1.1 — Dialysis Bag — Individual Group Data

Concentration of Sucrose in Dialysis Bag Initial Mass Final Mass Mass difference % change in Mass
0.0M 15.40g 15.04g .36g -2.34%
0.2M 14.72g 15.42g 0.70g 4.76%
0.4M 14.69g 12.32g 2.37g -16.13%
0.6M 17.84g 18.44g 0.60g 3.36%
0.8M 12.38g 14.37g 1.99g 16.07%
1.0M 17.8g 19.85g 2.05g 11.52%

Table 1.2 — Dialysis Bag — Class Group Data

Concentration of Sucrose in Dialysis Bag

Group 1

Group 2

Group 3

Group 4

Group 5

Group 6

Class Avg

0.0M

-2.34%

52.70%

9.50%

22%

7.94%

0.92%

15.02%

0.2M

4.76%

-57%

-4.87%

120%

6.39%

15.01%

14.05%

0.4M

-16.13%

4.40%

-3.96%

12%

11.76%

12.20%

3.38%

0.6M

3.36%

104%

-8.35%

111%

22.55%

9.02%

40.26%

0.8M

16.07%

-27%

-6.98%

62%

9.82%

10.43%

10.72%

1.0M

11.52%

82%

-8.61%

43%

27.43%

10.95%

27.72%

*

Table 2.1 — Potato Core Results — Individual Group Data

Concentration of Sucrose in Beaker Temperature Initial Mass Final Mass Mass difference % of change in Mass
0.0M Sucrose 24 degree Celsius 9.34 g 11.12g 1.78 g 19.06%

Table 2.2 — Potato Core Results — Class Group Data

Contents of beaker Temperature Initial Mass Final Mass Mass difference % of change in Mass
0.0M Sucrose 24 degree Celcius 9.34 g 11.12g 1.78 g 19.06%
0.2M Sucrose 25 degrees -0.71%
0.4M Sucrose 24 degrees -25%
0.6M Sucrose 24 degrees -25.7%
0.8M Sucrose 24 degrees -46.7%
1.0M Sucrose 26 degrees -52.68%

                         
                       Calculations:
    • Ψw = Ψp + Ψπ
    • Ψπ = – iCRT
      • - i = 1 (ionization constant for sucrose)
      • C = 0.2 (osmolarity)
      • R = 0.0831 (pressure constant)
      • T = Celsius + 273 = 24 +273 = 297 degrees (temperature in Kelvin)
      • Ψπ = -1 (0.2)(0.0831)(297) = -4.94
      • Ψπ = -4.94 bars
  • Ψw = Ψp + Ψπ

Ψw= 0 + (-4.94)
Ψw= -4.94 bars

Discussion
      Looking at our graphs and the correlation coefficient (R2), which helps us identify in all our graphs how accurate our best fit lines are to values we measured, it is obvious that our group data as well as the class data did not have a correlation, although the results should have displayed that water and other small particles could travel through the semipermeable membrane from a higher concentration to a lower concentration. The results were all over the place.
1.Describe the relationship between the increase of mass and the molarity of sucrose within the dialysis bags.

                      As the percent change in mass increased, the molarity increased and decreased due to flawed measurements. Because our data isn’t accurate a clear relationship cannot be determined. However, as the molarity of the sucrose within the dialysis bags increases, more water should’ve been diffused into the bag by osmosis causing an increase in mass. This   would’ve performed a direct relationship between the mass and molarity of the sucrose within the dialysis bags if the measurements were accurate.

2. Predict what would happen in an experiment if all the dialysis bags were placed in a 0.6M sucrose solution instead of distilled water. Draw your predicted line on your graph for Table 1.2 and label it “0.6M prediction.”  

              Our data, as well as the class data, for the dialysis tubing section of the lab showed that, because solutions move from areas of high concentration to low concentration, water moved into the tubing, increasing the overall mass. We believe that sucrose is too big of a molecule to pass through the semi-permeable membrane, so if the water to sucrose ration both within the tubing and outside of the tubing is the same, there should be no increase in mass.

3. If a potato is allowed to dehydrate by sitting in the open air, would the water potential of potato cells increase of decrease? Explain by using the water potential formula from the introduction.  

                Water moves from a high concentration to a low concentration. Because the potato is dehydrating, it has a low concentration of water inside, so water would move into the potato, which means it will have a decrease in water potential.

4. If a plant cell has lower water potential than its surrounding environment and if pressure equals 0, is the cell hypertonic or hypotonic to its environment? What would have to happen for the contents of the cell to be isotonic to its environment?  

               The plant cell has a higher solute potential/concentration than its surrounding which means that the surrounding environment has a higher water potential. As a result water from the surrounding will move into the cell by the process of osmosis. Because of the cells lower water potential, compared to its environment, we say this cell is hypotonic which means that the cell has lower osmotic pressure than its surrounding. In order for the contents of the cell become isotonic there would be have to be an equal amount of osmotic pressure between cell and its environment .

5. How could this lab technique be applied in finding out which apples, Macintosh or Delicious are sweeter?

                   Sweetness in apples is determined by how much sucrose or other sugars it contains. In our dialysis tubing section of the lab, we saw that an increase in sucrose molarity resulted in a greater percent increase in mass. Using this same pattern, if we cut slices of both types of apples, immersed them in distilled water, and record the percent of change in mass, than we can compare the results and see which of the two apples has the higher percent of change in mass. That apple would be the sweeter one.

Conclusion

             The purpose of this lab was to describe the physical mechanism of osmosis and diffusion and describe how molar concentration affects diffusion. We have now observed how solutions diffuse in different situations, always from a high concentration to a low concentration, and how molar concentration affect diffusion, as the molarity goes up, more solution is diffused. We hypothesized that because molecules diffuse down a concentration gradient, the mass of the dialysis tubes will increase, and also that as the molarity increases, the percent of change in mass will also increase.  Our data unfortunately did not support our conclusion due to errors. Some of the dialysis tubes decreased in mass. One explanation is that we might not have tied the strings at the top of the tubing tight enough, resulting in solution leaking out. The overall class data for the dialysis tubing, however, does support our hypothesis and we realize our errors.

Literature Cited
“PHSchool – The Biology Place.” Prentice Hall Bridge Page. Pearson Education, June 2007. Web. 12 Sept. 2011. <http://www.phschool.com/science/biology_place/labbench/>

Moulton, Glen E. “Cell Theory, Form, and Function: Fluid Mosaic Model of Membrane Structure and Function — Infoplease.com.” Infoplease: Encyclopedia, Almanac, Atlas, Biographies, Dictionary, Thesaurus. Free Online Reference, Research & Homework Help. — Infoplease.com. Web. 14 Sept. 2011. <http://www.infoplease.com/cig/biology/fluid-mosaic