Purpose
1A: The purpose of this experiment is to understand the nature of selectively permeable membranes. This is accomplished by observing how different molecules such as water, glucose, starch, and iodine interact with a selectively permeable membrane.
1B: The purpose of this experiment is to understand the relationship between solute concentration and the movement of water through a selectively permeable membrane. This can be done by testing the osmosis of water using solutions with different molarities of sucrose.
1C: The purpose of this experiment is to understand water potential and what influences change in water potential. As well as understand the components of the water potential equation and how its used.
1D: The purpose of this experiment is to understand how to calculate water potential.
1E: The purpose of this experiment is to understand plasmolysis and the effect of concentrated solutions on plant cells. This can be done by putting onion cells in different concentrated solutions.
Introduction
1A: Diffusion is the movement of a solute across a membrane. Diffusion only occurs when a salute travels down its concentration gradient. A concentration gradient is the difference in concentration of a solute. To be more specific, dialysis is the diffusion of a solute across a selectively permeable membrane. A selectively permeable membrane is a membrane in which only some materials can pass through. An example of this would be the plasma membrane of a eukaryotic cell. Water molecules and other small substances are able to pass through a selectively permeable membrane freely, but larger molecules are most likely unable to diffuse
Part 1B: Osmosis is the movement of water through a membrane. An isotonic solution includes two solutions that have the same concentration of solutes, therefore there will be no net charge on the water in either solution. When two solutions have different concentrations of solute, the one with less solute is hypertonic and the one with more solute is hypotonic. When you put these two solutions together separated by a selectively permeable membrane, the water will move from the hypotonic solution to the hypertonic solution.
1C: Water potential is the movement of water in or out of cells. The movement of the water is done by the process of Osmosis (movement of water through a membrane). The two components that make up water potential are pressure potential (physicial pressure) and solute potential. Formula is Water potential = pressure potential + solute potential. The potential of pure water is equal to zero. Water always moves from high to low areas of water potential. Water potential can be a positive or negative value, or it can be zero. Pressure influences the movement of water. With an increase of pressure there will be more positive value, where on the other hand decrease in pressure there will be a more negative value. As for solute, if there is an increase water potential will be more negative. When water leaves the cell it will shrink, If water is added it will grow. In animal cells too much water will cause the cell to swell or burst. But in plant cells the cell will only grow, this is because of the plant cell which holds the cell together. Once the water potential on the inside of the cell and on the outside of the cell are equal, then it will have reached dynamic equilibrium, where there is no water movement.
1D: Water potential equals the ionization constant plus molar concentration plus the pressure constant plus tempurature (in Kelvin). Water potential shows water's ability to move from one location to another.
1E: Plasmolysis is the shrinking of the cytoplasm of a plant cell in response to diffusion of water out of the cell when in a hypertonic solution. In a hypertonic solution, cells lose water because there is a greater concentration of solute outside the cell. In an isotonic solution, there is no net water movement because the solute concentration is the same inside and outside the cell. In a hypotonic solution, the cell gains water because there is a lower solute concentration outside the cell.
Methods
1A: First, we soaked about 20 cm of dialysis tubing in water in order to make it easier to work with. Then, we we formed a bag out of the tubing and put a solution made of 15% glucose and 1% starch inside of the bag. We recorded the color of each solution. We then tested the glucose/starch solution for the presence of glucose. Next, we filled 3/4 of a cup with iodine solution (Lugol's solution) and filled the rest with water. We then tested this solution for the presence of glucose. Next, we put the bag of glucose/starch solution into the cup of iodine/water solution, and proceeded to wait for approximately 30 minutes. Once the time was up, we recorded the color of each solution and tested for the presence if glucose in each as well.
1B: First, we filled dyalisis tubing with either water or solutions with different molarities of sucrose. We tied off the ends and left room for water to enter. Then, we recorded the weight of each bag and placed each one in a cup of water. We let them sit for 30 minutes and then took each bag out and weighed them. Finally, we calculated the change in mass for each bag.
1C: First, we poured the five different molarities of sucrose into their designated beakers as well as the distilled water into its own beaker. Next using a cork borer we made 24 potato cylinders (4 per beaker). Then we weighed the potato cylinders 4 at a time and recorded the intial mass. After weighing them we placed the potato cylinders into the beakers (4 per beaker). Following placing the potato cylinders into their beakers, we let them soak in the solution overnight with a sheet of plastic wrap covering the top of the beakers (this prevents evaporation). The day after we took the potatoes out of the beakers, using a paper towel to dab the excess solution off of the cylinders and we weighed them once more to find the final mass. Then we found the percent change in mass (intial mass/final mass).
1D:
1E: First, put a piece of an onion on a slide and observe it under a microscope. Sketch a picture of the image. Then, add several drops of NaCl solution to the slide and record what occurs. Next, add water to the slide and observe and record what happens.
1A
1C
1E
Flaccid Cells
Discussion
1A: The change in color from colorless to black/blue in the glucose/starch solution obviously indicates that iodine passed through the membrane, because iodine changes color when it comes into contact with starch. Beacause we didn't see any color change from red to black/blue in the iodine solution, it must mean that starch was unable to pass through the membrane. Also, because the iodine tested negative for glucose before the glucose/starch bag was put in, and positive for glucose after it was put in, it must indicate that glucose was able to pass through the membrane into the iodine solution. All of this information indicates one thing: the dialysis tubing acted as a selectively permeable membrane. Because glucose was able to pass through the membrane, we must conlude that it is the smallest molecule (besides water) present in this experiment. Iodine was able to pass through the membrane as well. Starch however, has to be the largest of the molecules. It was unable to pass through the dialysis tubing because its size was bigger than that of the pores in the membrane. Pore size directly affects the diffusion of molecules, then. If a substance is larger than the pore size, then it will not be able to diffuse across the membrane. If the molecule is smaller than the pore size, then is will be able to diffuse across the membrane.
1B: Overall, as the molarity of sucrose goes up, the amount of water that diffused into the bag goes up. The bag put into distilled water had the lowest change in mass, because it was isotonic, so water should stay at equillibrium in the two solutions. Then, as the molarity of sucrose went up, more water diffused into the bag because the solution outside of the bag was hypotonic. The solution with 1.0 M sucrose has the highest change in mass because it had the most water diffuse into it. This occurred because of how hypertonic the outside solution was compared to the solution inside the bag. The only error in our data was with the solution of 0.4 sucrose, because it didn't follow the trend; it had a lower change in mass than it should have. Other than that, the trends of our data and the class data are similar, except that our changes in mass of the higher molarity sucrose solutions are significantly higher. The data trend turned out as we expected, because this simulated diffusion in cells, which when placed in a hypotonic solution, take in water.
1C: In our experiement we found that for the distilled water beaker cylinders gained 1.5g of mass with a +17.2% change in mass. At 0.2M sucrose there was a 0.34g gain in mass. As for 0.4-1.0M sucrose solution there was an overall trend of loss in mass and a negative percent change. At 0.4M we recorded a mass change of -1.53g and -16.7% change in mass. The masses would continue to drop from this point on, until we reached a mass change of -3.08g and a percent change of -33.67% for 1.0M sucrose. From less then 0.2M Sucrose we found a postive mass gain. As for 0.4M sucrose and higher molaritites we found a decrease in mass change. This was on behalf of there being enough solute added to the potato cylinders to cause movement of the water from the potatoes. Since water always moves from high water potential to low water potential. Meaning that there was a high water potential within the potatoes and a low potential of water outside of the potatoes. The mass changed because the water was removed from the potatoes, which decreased the mass. When 0.2M or less of sucrose was added there was not enough solute added to change the water potential.
1D: In this experiment, we found water potential. We determined that if you multiply the ionization constant, molar concentration, pressure constant, and tempurature, then you get water potential.
1E: In this experiment we found that when we added the drops of 15% NaCl the onion cell shrunk in size. This is on behalf of plasmolysis. The 15% NaCl was a hypotonic solution. We learned in lab 1D that water potential always moves from high to low. So in this case the onion cell had more water potential then the NaCl solution. The added drops caused the water with in the cell to vacate, then shrinking the cell in size. When we added a lot of water to the onion the cell increased in size. The onion increased in size because after adding the NaCl the water potential with in the cell was less then the solution the cell was in, making for a hypertonic solution and a turid onion cell. Since water potential once again moves from high to low concentration, once the water that had a higher water potential was added the water poured into the onion cell until it reached equillibrium. If the onion cell had been placed in an isotonic solution the cell would have remained the same size since the solution that the cell was in would have the same water potential. So in summary, when NaCl was added to the onion the cell shrunk in size and lost water, after the water was added to the cell the onion gained water and increased in size.
Conclusion
1A: Only molecules that are smaller than membrane pore size are able to diffuse through a selectively permeable membrane. Molecules larger than the pore size of the membrane are unable to diffuse through a selectively permeable membrane.
1B: The more hypertonic the inside solution is, more water diffuses in. An isotonic solution will have little or no change in the amount of water inside.
1C: At 0.2M or above there isn't enough solute to cause a change in water potential. That's why from 0.4-1.0M they lost mass since there was enough solute to cause the water potential to change, causing water to leave the potato, then lowering the mass of the potatoes.
1D: In order to calculate water potential, you multiply ionization constant, molar concentration, pressure constant, an temperature in Kelvin.
1E: When adding NaCl to the onion cell the cell decreased in size, because it was in a hypotonic solution. When adding the water afterwards the onion cell increased in size because it was now in a hypertonic solution. This is on behalf of plasmolysis where water always moves from high to low concentration.
References
Campbell Biology, 9th Edition
Diffusion and Osmosis Lab, College Board