This experiment tested the diffusion of small molecules through dialysis tubing, a selectively permeable membrane. We tested the presence of glucose in a 15% glucose/1% starch solution.
Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. Diffusion through a selectively permeable membrane occurs until the solutions have reached dynamic equilibrium: the solute concentrations on both side of the membrane are the same.
First we tested a 15% glucose/1% starch solution for the presence of glucose using Testape and recorded the color of the Testape. We put 15 mL of this solution into dialysis tubing. Then, we filled up a cup 2/3 of the way full with distilled water, added about 4 mL of an Iodine solution to the distilled water and then tested this solution for the presence of glucose and recorded the color. We then put the tied dialysis bag into the solution and let it sit for about half an hour. After half an hour, we took the dialysis bag out and recorded the color of the solution in the bag and in the cup. Finally, we tested each solution again for the presence of glucose and recorded the results in the table.
Dialysis bag after half an hour of being submerged in the water/Iodine solution.
Table 1.1
| Initial Contents | Solution Color | Presence of Glucose | ||||
| Initial | Final | Initial | Final | |||
| Bag | 15%glucose/1%starch | clear | purple | brown | green | |
| Beaker | H20 & IKI | yellow/orange | same | green | green |
Based on the colors observed in the table above, we can conclude that glucose is leaving the bag and the Iodine solution is entering the bag. This means that the concentration of glucose was higher in the bag than it was in the cup. The concentration of the Iodine solution moved from outside the bag to inside the bag to make the concentrations equal. This movement caused the change in color of the solution inside the bag. If we had weighed the bag and cup with the different solutions we could have determined the percent change in mass (if there was one) and had numbers to support the observation that molecules are moving from high to low concentration. Based on our observations, we believe that the Iodine solution molecules were smallest because they were able to get through the membrane of the dialysis tubing and change the color of the solution drastically in just half an hour. The glucose molecules follow the Iodine solution in size, because based on our Testape results, we see that glucose moved out of the dialysis tubing: high to low concentration. The membrane pores and starch molecules are the two largest. The membrane pores were big enough to let solutions in and out. Starch cannot pass through this semipermeable membrane, making them the biggest molecules. If we would have started with glucose and Iodine solution inside the bag, they would have moved out of the bag into the starch solution in an attempt to reach dynamic equilibrium. The large starch molecules would not be able to penetrate the membrane so nothing would enter the bag.
In this experiment, the Iodine solution diffused from a high to low concentration through a selectively permeable membrane, as expected. In our specific experiment, there was too much water in the cup with the Iodine solution so not as much of our Iodine was able to diffuse through the membrane. Also due to this, the color of the water/Iodine solution in the cup was more of a yellow/orange color than a red color, what it should have been.
1B: Osmosis
This experiment tested the relationship between solute concentration and the movement of water through a selectively permeable membrane by the process of osmosis. We were trying to find the net movement of water through a selectively permeable membrane.
Osmosis is the movement of water from a higher to lower water concentration through a selectively permeable membrane. Osmosis moves down the concentration gradient, which is when there is a high concentration of water in one area and it moves to a lower area of water concentration.
First we formed six bags out of dialysis tubing and filled them with 15-25 ML of distilled water, .2M sucrose, .4M sucrose, .6M sucrose, .8M sucrose, and 1.0M sucrose. Then we weighed each bag separately and recorded the weight. We submerged each bag in a separate cup of distilled water and let them sit for 30 minutes. After 30 minutes we took the bags out and re-massed them.
Potato and tools used to core the potato
Potato cores submerged in the different solution concentrations
Group Data:
| Contents | Initial Mass (g) | Final Mass (g) | Mass Difference | Percent Change |
| 0.0 M Water | 29.8 | 30.04 | 0.24 | 0.8 |
| 0.2 M Sucrose | 17.18 | 18.35 | 1.17 | 6.8 |
| 0.4 M Sucrose | 29.6 | 30.66 | 1.06 | 3.6 |
| 0.6 M Sucrose | 12.6 | 13.48 | 0.88 | 6.9 |
| 0.8 M Sucrose | 24.1 | 25.4 | 1.3 | 5.4 |
| 1.0 M Sucrose | 25.4 | 27.5 | 2.1 | 8.3 |
Class Data, Percent Change in Mass:
| Distilled Water | 0.2 M | 0.4M | 0.6M | 0.8M | 1.0M | |
| Group 1 | -0.45 | 2.38 | 5 | 5.86 | 6.06 | 7.93 |
| Group 2 | 5.15 | 8.47 | 8.54 | 10.43 | 9.76 | 17.44 |
| Group 3 | 0 | 4.33 | 8.61 | 14.41 | 12 | 10.05 |
| Group 4 | -2.17 | 1.73 | 3.14 | -6.9 | 7.2 | 11.54 |
| Group 5 | 0.8 | 6.8 | 3.6 | 6.9 | 5.4 | 8.3 |
| Group 6 | 0.89 | 2.5 | 3.1 | 5.3 | 6 | 7.4 |
| Group 7 | 11.4 | 9.8 | 10.7 | 12.3 | 13.7 | 13.4 |
| Group 8 | 2.7 | 3 | 4.9 | 6.2 | 4.3 | 6.1 |
| Group 9 | ||||||
| Group 10 | 0.63 | 2.14 | 4.85 | 8.19 | 8.6 | -2.17 |
| Group 11 | ||||||
| Group 12 | 1.1 | 4.95 | 10.57 | 16.07 | 19.09 | 16.87 |
| Group 13 | ||||||
| Group 14 | ||||||
| CLASS AVG: | 2.01 | 4.61 | 6.30 | 7.88 | 9.21 | 9.69 |
Our data suggests that sucrose molarity determines whether water will move in our out of the cell. The molecules want to move so that concentrations are equal inside and outside of the dialysis bag. We can see that as the molarity of the solution increased, the mass also increased because the water was moving out and the sucrose was moving in. Osmosis is present here. In the group of bags that were in the cup of distilled water, the mass barely changed at all because there was water on either side of the membrane so they did not want to move across their concentration gradient.
Our data shows that water does move through a selectively permeable membrane through the process of osmosis. Though the results are fairly consistent across the board, each bag did not weigh the exact same amount when we started so the percent change can also be somewhat attributed to that.
1C: Water Potential
In this experiment we tested water potential in potato cores placed in different molar concentrations of sucrose. We were trying to calculate how much water moves in and out of a potato cell
Water potential is the likeliness for water to be diffused from one place to another. Solute potential decreases water potential and pressure potential increases water potential that a cell can have. A cell that is lacking water has a higher water potential than a cell that has a abundance of water.
We used a potato core borer to cut 24 potato cylinders and removed and excess skin. We grouped them into fours and weighed them. After that we filled six cups with 2/3 full of distilled water, .2M sucrose, .4M sucrose, .6M sucrose, .8M sucrose, 1.0M sucrose and submerged the potato cores. We covered them with plastic wrap and let them sit overnight. Then the following day, we took the potato cores out and massed them. We then found the percent change in mass of the potato cores.
Group Data:
| Contents | Initial Mass (g) | Final Mass (g) | Mass Difference (g) | Percent Change in Mass | |
| 0.0 M Water | 13.7 | 15.2 | 1.5 | 10.9 | |
| 0.2 M Sucrose | 12.1 | 12.3 | 0.2 | 1.7 | |
| 0.4 M Sucrose | 10.7 | 9 | -1.7 | -15.9 | |
| 0.6 M Sucrose | 13.1 | 9.4 | -3.7 | -28.2 | |
| 0.8 M Sucrose | 7.3 | 5 | -2.3 | -31.5 | |
| 1.0 M Sucrose | 11.2 | 7 | -4.2 | -37.5 |
Class Data, Percent Change in Mass:
| DISTLLED WATER | 0.2 M | 0.4M | 0.6M | 0.8M | 1.0M | |
| Group 1 | 10 | 4.8 | -12 | -32.2 | -36.4 | -36 |
| Group 2 | 12.99 | 1.65 | -12.5 | -29.69 | -38.46 | -34.65 |
| Group 3 | 3.06 | 2.2 | -14.47 | -39.39 | -75 | -70.37 |
| Group 4 | ||||||
| Group 5 | 10.9 | 1.7 | -15.9 | -28.2 | -31.5 | -37.5 |
| Group 6 | 11.59 | 0.22 | -8.6 | -6.2 | -46.52 | -55.31 |
| Group 7 | 10.1 | 2.9 | -12.5 | -27.4 | -40 | -36.5 |
| Group 8 | 9 | 0.9 | -9.4 | -27.1 | -29.1 | -34.3 |
| Group 9 | 13.11 | 3.51 | -12.28 | -31.57 | -36.84 | -33.33 |
| Group 10 | 17.57 | -2.47 | -8.93 | -27.34 | -32 | -37.52 |
| Group 11 | 2.94 | -1.39 | -13.2 | -30.41 | -34.85 | -31.34 |
| Group 12 | 9.24 | -0.28 | 0.46 | -.27.27 | -35.26 | -33.7 |
| Group 13 | ||||||
| Group 14 | ||||||
| Class Avg. | 10.05 | 1.25 | -10.85 | -27.95 | -39.63 | -40.05 |
Based on our data we can conclude that potato cores had a high water potential before we put them into the solutions, meaning that there was more water inside the potato than outside the potato. If potatoes were left to dehydrate, they would have a lower water potential because water would be rushing into the cell since it moves from high to low concentration. A high water potential means that water will flow out of the cell instead of in. If the environment a cell is in has a high water potential, the water will enter the cell and as a result the cell will be hypotonic. We can also conclude that potatoes contain sucrose molecules because when the cores were placed in distilled water, change in mass was positive meaning they took in water.
Our data shows that potatoes do have a high water potential before being placed in a sucrose solution. Our results were consistent with everyone else's results, however, our potato cores weren't a uniform size, so the overall mass was affected. Some cups had very short cores and other cups had larger cores which could account for a very large percent change.
1E: Onion Cell Plasmolysis
Plasmolysis is when cytoplasm of a plant cell separates from the cell wall which is caused by water loss. In other words it is when a plant gets dehydrated to a point when the insides begin to separate resulting in wilting of the plant and then eventually death. Plasmolysis typically will not occur in nature unless under harsh conditions. Plasmolysis works most efficiently when an a plant cell is emerged in a strongly concentration of saline or sugary solution resulting in water loss by osmosis.
Plasmolysis occurs in an onion cell because the large vacuole in the center of the cell contains a solution with lower osmotic pressure than the solution outside of the membrane. Due to this the vacuole loses water and reduces in size. The cell membrane and cell wall start to get further apart which causes the plasma membrane and protoplasm to move to the center of the cell.
Plasmolysis in a red onion cell
Sources:
Chapter 6&7 Powerpoint
http://www.biology-online.org/dictionary/Plasmolysis
Youtube
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