Diffusion & Osmosis Lab

This lab explores the process of osmosis and diffusion in cells, and the impact of tonicity on this process. Plant cells and their reactions to different solute concentrations are observed and recorded in order to create a relationship between diffusion & osmosis and tonicity.

1A - Diffusion

Introduction:

This exercise investigates selective permeability by exploring the diffusion of molecules across dialysis tubing.

Procedure & Methods:

1. Using a 30cm piece of dialysis tubing that has been soaking in water, tie off one end to form a bag. Rub the other end in between fingers to open the bag.
2. Test the 15% glucose/ 1% starch solution for the presence of glucose using a Clinistix strip [glucose strip]. Record the color in Table 1.1.
3. Place 15 mL of the 15% glucose/ 1% starch solution in the bag. Leave enough space for expansion within the bag. Tie off the other end of the bag. Record the color in Table 1.1.
4. Fill a cup ⅔ full with distilled water. Add 4 mL of Lugol’s solution [iodine] to the cup and record the color in Table 1.1. Test this solution for glucose and record color in Table 1.1.
5. Place bag in cup of solution, making sure it is fully submerged.
6. Allow this to stand for 30 minutes, or until a distinct color change is visible. Record final color of solution in bag and solution in cup in Table 1.1.
7. Test the liquid in the bag and the liquid in the cup for the presence of glucose. Record color in Table 1.1.

Data:

Data Analysis and Conclusion:

This exercise demonstrates the concept of selective permeability and allows us to understand that only certain substances can cross a selectively permeable membrane. The dialysis bag in this experiment essentially represents a cell and its selectively permeable membrane. Understanding the concept of selective permeability allows us to gain a better understanding of how cell membranes can regulate diffusion of substances in and out of the cell.

The change in solution color from clear to black in the bag shows that iodine entered the bag. The presence of glucose is evident because the initial glucose test was forest green, and the final was lime green. Forest green indicates the presence of glucose, so this means glucose left the bag during the 30 minutes. Even though the final glucose strip test for the cup did not turn forest green, we can assume that the glucose was present, but in a very small amount. The glucose was evenly split between the bag's contents and the cup’s contents, and since this was such a small amount of glucose, it did not show up on the glucose strip test.

In the beaker, the color stayed copper and turned lime green with the glucose test throughout the experiment. This shows that there really was no significant change in glucose, iodine, or water levels. But comparing this to the final results of the bag, shows that iodine entered the the bag and glucose exited the bag.

Looking at our data, it becomes clear that the dialysis bag only allowed water, glucose, and iodine to pass through. Starch was not able to pass through. Starch was too large of a molecule to pass through the dialysis bag.

1B - Osmosis

Introduction:

This exercise investigates how selective permeability is impacted by solute concentration by exploring osmosis [diffusion of water] across dialysis tubing.

Procedure & Methods:

1. Using six strips of 30cm dialysis tubing, tie a knot on one end of each strip to form six bags.
2. Pour 15-24 mL of the following solutions into separate bags. One bag can contain only one solution.
1. distilled water
2. 0.2 M sucrose
3. 0.4 M sucrose
4. 0.6 M sucrose
5. 0.8 M sucrose
6. 1.0 M sucrose
3. Remove most of the excess air from each bag by drawing the dialysis bag between two fingers. Leave enough space for expansion within the bag. The solution should fill only ⅓ to ½ of the bag of tubing. Tie off the other end of the bag.
4. Rinse the bags gently to remove any sucrose that may have spilled on the outside surface. Carefully blot bags dry.
5. Measure and record the initial mass of each bag [in grams] in Table 1.2.
6. Place bags in six cups filled ⅔ of the way with distilled water. Make sure these cups are labeled with the molarity of the solution in the dialysis bag. The bags should be fully submerged.
7. Let this stand for 30 minutes.
8. After the time has passed, remove dialysis bags from the cups, carefully blot them dry, and measure and record the final mass of the bags in Table 1.2.

Data:

Data Analysis and Conclusion

This exercise, similarly to 1A, represents the behavior of selectively permeable cell membranes. Only this time, osmotic activity [diffusion of water] is monitored instead of monitoring the diffusion of multiple molecules.

Taking a look at our table and graphs, it is evident that our raw data was flawed. As my lab group analyzed our data and compared our exercise with with other lab groups, we realized that our 0.4 M sucrose bag must have been leaking, because the mass difference was too great of a number. The values for other bags also seemed somewhat skewed, but we were unable to pinpoint the reason for their unreliable data. Our wonderful group member Amanda took it upon herself to recreate an ideal graph based on the concept that the relationship between molarity and percent change should be linear. The percent change gradually increases by 4% as the molarity increases. This should be a linear relationship, because in order for there to be equilibrium in the system of cup and dialysis bag, the amount of water and sucrose in the bad and in the cup should match up. As a result, when the molarity of sucrose increases, the more sucrose will leave the bag. This results in a greater percent change.

Cells always aim to achieve equilibrium, and osmosis is a good example of how tonicity can drive diffusion. In Ch 7 I speak a great deal about hypotonic, isotonic, and hypertonic solutions [To see this information, click on 'Ch 7' on the navigation bar at the top of this page]. What is happening here, is we are placing the dialysis bag is in a hypotonic solution. There is less sucrose outside of the bag than there is inside. In order to achieve equilibrium, the sucrose will diffuse out as water will diffuse in. Osmosis, like diffusion, always strives for equilibrium

1C - Water Potential

Introduction:

This exercise investigates water potential [a measure of water’s movement from one area to another] by exploring the pressure potential and solute potential.

Procedure & Methods:

1. Pour 100 mL of each solution in its own labeled cup.

2. Use a cork borer with about 5mm diameter to cut four potato cores of similar length for each cup. Do not include the potato skin on the potato core.

3. Make sure to keep potatoes covered or submerged in water to retain its moisture while you are preparing the rest of the materials for this lab.
4. Determine the mass [grams] of the four potato cores for each cup, and record this in Table 1.4.
5. Put potato cores in their designated cup.

6. Cover with plastic wrap to prevent evaporation.

7. Let this stand over night.
8. Remove potato cores from cups, carefully blot them dry and determine their total mass. Record final mass in Table 1.4.
9. Calculate percent change [as done in exercise 1B].
10. Graph data for percent change.

Data:

Data Analysis and Conclusion:

Our data was very skewed because we conducted this exercise on a Friday. We had no choice but to leave the potatoes be for the entire weekend, and we returned to a frightening mess of potatoes. So once again, Amanda created an ideal graph of how this situation should have turned out. Because we had left our cores submerged in a sucrose solution for such a long time, when we returned to the potatoes we found that they were covered in goop that could not be 'gently blotted' away or removed. As a result, when we weighed our cores for the final mass, the values were off.

Ideally, what we should have seen is that the cores in distilled water and 0.2M sucrose would have gained mass because the cores were in an hypotonic solution [less solute outside of cell] and so water would enter the cell to even out molarity. For the cores in 0.4M sucrose, there should have been no change in mass because this is an isotonic solution [equal amount of solute inside and outside of cell], so the molarity of the solution is the same as the molarity of the potato. And lastly, the 0.6M, 0.8M, ans 1.0M sucrose solutions would have been hypertonic [more solute outside of cell] so water would have left the cores to even out molarity. The cores in these solutions would have lost mass.

In this exercise, we were toying with the concept of water potential. In order to find water potential, we need the molarity of the substance we are finding the water potential of. To do so, we need to determine the point at which there is no net movement of water. No net movement of water means that the potato cores are in an isotonic solution, so the molarity of the cup’s solution is the same as the molarity of the potato. So, we had to look at the graph to see at which molarity the percent change in mass is zero. This would be 0.4 M. This leads us to 1D, where we will be able to calculate the water potential knowing the molarity of the potatoes to be 0.4 M.

1D - Calculation of Water Potential from Experimental Data

Introduction:

This exercise calculates water potential using data collected in exercise 1C.

Method:

1. The solute potential from the previous exercise can be calculated using the following:

2. Knowing the value of the solute potential of the solution and knowing that the pressure potential of the solution is zero, allows us to calculate the solute potential of the solution.

Analysis and Conclusion:

We calculated our Molar Concentration (C) to be 0.4M and the Temperature (T) would be 273 + 22°C = 295°K. These are the variables we will use to complete the calculations.

This is our work, and the answer we got:

We found the water potential value to be -9.806 bars. The practical use of the water potential value is that it allows us to track and understand osmosis; we can predict where the water will move. In this case, we used plant cells to understand water potential. The smaller a value is for water potential, the less water is likely to travel. This can be seen in the case we have before us with the 0.4M potatoes. The higher the value, the more likely it is to travel because the substance is in a hypertonic solution [lower C value].

References:

Lab One Diffusion and Osmosis Lab worksheet

Diana, Amanda. "Mr. Filipek's AP Bio." Mr. Filipek's AP Bio. Amanda Diana, 22 Oct. 2015. Web. 27 Oct. 2015. <http://amandadoesscience.tumblr.com/tagged/do-lab>.