Monday, October 21, 2013

Diffusion and Osmosis Lab

 
Purpose: 
The purpose of 1A was to find which substances the dialysis bag was permeable to. We were testing the concepts of diffusion and osmosis. We controlled the placement of each solution, which changed the color of the substance in the bag. The dependent variable was the color of the substance inside the dialysis bag, and the independent variable was where each substance was placed. We specifically wanted to determine if the glucose, starch, IKI, and water were permeable to the bag. 
 
For 1C, the purpose was to find the percent change in mass of the potato cores when placed in different solutions. This experiment was testing the concept of the difference between hypertonic, hypotonic, and isotonic solutions. The independent variable was the different substances that the potato cores were placed in, and the dependent variable was the mass of the potato cores. We wanted to see if the potato cores would gain or lose mass in each solution.
 
In 1E, we were also testing hypertonic, hypotonic, and isotonic solutions. We were testing how each solution would effect the diffusion and cellular contents of the onion. The independent variable was the solution that the onion was placed in, and the dependent variable was the effect that the solution had on the onion. 



Introduction: 
In experiment 1A, we were testing the permeability of the dialysis bag. To be selectively permeable means to allow only certain substances through the plasma membrane. The concept of diffusion is that when a solution has a higher concentration of a substance than its solvent, the substance will spread out evenly throughout the solution. Osmosis is when water diffuses across a membrane from a region with lower solute concentration (more water) to a region with higher solute concentration (less water) to evenly balance out the concentration of water on both sides. IKI is iodine. 
 
For experiment 1C, to calculate the percent change in mass, we used the formula: final mass minus initial mass divided by initial mass multiplied by one hundred. The percent change in mass means how much mass was gained or lost through the experiment. A hypertonic solution would be a solution that has less water in it than the cell does, meaning that the water would diffuse out of the cell, causing it to shrivel. A hypotonic solution is one where there is more water outside of the cell, causing the water to diffuse into the cell, and the cell would swell. In an isotonic solution, the concentration of water is the same both inside and outside of the cell. Diffusion still occurs in an isotonic solution, but at the same rate both going in and out. 
 
Experiment 1E was basically testing the same concepts as experiment 1C, but this time instead of looking for percent change in mass, we were looking at the cellular contents. The cellular contents are composed of the organelles inside the cell, the cell wall, and the plasma membrane. 


Methods: 
For 1A, we were testing diffusion so we filled a dialysis bag with 15% glucose and 1% starch solution then tests it for glucose. Next, we took a cup and filled it 2/3 with water and then the rest of the way with IKI. Then we tested the solution in the cup for glucose. Next, we but the dialysis bag into the IKI and water solution and let it sit for about 30 minutes or until the bag went through a significant color change. Lastly, we tested the the IKI and water for glucose and the 15% glucose and 1% starch solution.
 
              
                                

For 1C, we were testing water potential, first off, we had to core the potato. We had to get 4 cores, of the same length, for each of the six cups of sucrose that we were testing them in. We then had to determine the mass of the potato cores. Next, we had to cover the cups with plastic wrap to make sure there is no evaporation. We had to let it stand overnight, then take the mass again of the potato cores and figure out the percent change in mass. 
 
For 1E, we didn't actually do the experiment, but the point of it was to test onion cell plasmolysis. Onion cells were observed in hypotonic, isotonic, and hypertonic solutions.  


Data: 
For 1A, we were able to test for the presence of glucose in and outside of the bag both before and after the experiment was run.  The original solution colors and glucose presence are shown.

For 1C, we have a chart with the initial and final mass of potato cores. By using the formula below, we were able to find the percent change in mass. 


 
For 1E, we researched to find pictures of the status of onion cells in different solutions. The first shows the behavior of a cell in a hypotonic, isotonic, and hypertonic solution. The second picture shows onion cells in isotonic and hypertonic solutions. The top half shows central vacuoles that are in no state of expansion or water loss, while the bottom half shows shriveled vacuoles that have lost too much water to their environment.
 



Graphs and charts:
1A: Below is the graph of the percent change in mass for potato cores at each solution. If the percent change in mass is positive, that means the solution is hypotonic. This is because there is a lower concentration of sugar outside the potato cells and water will rush in.  When the change in mass is negative, the outside solution is hypertonic. This is because the surrounding solution has a higher concentration of sugar, and water will leave the cells. The point where the change in mass is zero, around .22 or .24 M for the potato, is when the solution has the same molars as the cells. This would be an isotonic solution, and there would be no net mass change.



Discussion:
For 1A, we began with a glucose and starch solution inside the dialysis bag, and an IKI solution outside the bag.  As the experiment progressed, the interior of the bag turned blue. This blue coloration is an effect of reactions between IKI and starch, so we knew that IKI had diffused into the bag. Since the outside of the bag did not change color, we also determined that starch had not been able to diffuse through the bag; if it had, the exterior solution would have also turned blue. The second piece of data is the presence of glucose. Initially, the interior of the bag contained a solution with glucose, and the exterior did not. After the experiment was finished, we tested for glucose on the exterior of the bag, and the reading came out positive. Because of this, we were able to determine that glucose was able to diffuse through the bag. Going into this experiment, we did not know what the dialysis bag would be permeable to. However, in the duration of our trial, we were able to observe a significant color change, and knew that our results would show diffusion of at least one substance. Also, the fact that starch could not diffuse through the bag coincided with common sense; looking at all of the molecules present in this experiment, starch was the largest.
 
For 1C, we gathered percent changes in mass for potato cores in different solutions. As the molarity of the solution increased, the percent change in mass decreased. With a 0 M solution, the change in mass was 17.3%. It is positive because of the effects of a hypotonic solution. With a 0.4 M solution and above, the change in mass ranged from -15.7% to -42.6%. This occurs because of the effects of a hypertonic solution. This means that somewhere in between 0.2 and 0.4 M solutions is the molarity of the potato. The graph of percent change in mass versus solution molarity would have to reach zero at some point between these values, meaning that there would be no net mass change and the solution would be isotonic. Though we did not predict this actual point, we assumed that this would occur. Some point between a 0 M and 1 M solution, we thought the concentration of starch in a potato would be passed.
 
For 1E, though we did not actually complete this experiment, the diagrams and pictures above lined up with our knowledge of solutions.  When the onion cell was in an isotonic solution, no change in size was observed, and the flow of water into and out of the cell were equal. When the onion cell was in a hypertonic solution, water exited the cell and the vacuole became plasmolyzed. When the onion cell was in a hypotonic solution, it took in water from the environment and the vacuole swelled.  This went along with our previous knowledge of the effects of a solution on a cell.
 
 
Conclusion:
For 1A, our conclusion is that the IKI can pass through the selectively permeable membrane, but the membrane is not permeable to starch. We can tell this because when starch reacts with IKI that is when it becomes blue. Glucose, on the other hand, was able to get through the membrane; we started with glucose inside the bag, and we ended with glucose outside the bag .
 
For 1C, the conclusion that we came to was that the mass change in cells will depend on the type of solution. For hypotonic solutions, cells will gain mass as water rushes into the cells. For hypertonic solutions, the cells will lose mass as water is lost to the environment. In an isotonic solution, water will move in and out of the cell at equal rates, and there will be no net mass change.
 
For 1E, the conclusion is that if a cell is in a hypertonic solution, then the water exits the cell. This makes the central vacuole collapse, which causes the cell to go through plasmolysis. There is more salt on the outside, meaning more water wants to go out. For hypotonic the water enters the cell, and becomes turgid and when it enters the cell th vacuole will expand. For a hypotonic cell, there is more salt on the inside of the cell, so the water wants to go in. For an isotonic solution, the water would enter and leave the cell at equal rates, which makes it flaccid. For the salt, the concentrations will be equal on both the inside and the outside of the cell.