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Osmosis and Diffusion Across a Semipermeable Membrane By Kaitlin Boudreaux C00086573

Abstract Osmosis is a form of diffusion across a cell membrane. The rate of osmosis depends on the concentration gradient and the direction of osmosis depends on the tonicity of the solution. To simulate this process, one would make a model cell out of a solution inside a dialysis tube, which is semipermeable, placed in a solution of different concentration than the inside solution. The rate of osmosis is measured by percent of weight change from before model cell is placed in the solution to the weight after the model cell has been in the external solution after a period of fifteen minutes. With a linear increase in molarity, it is predicted that the percent change in weight will also increase linearly; however, a lack of patience increases the possibility of error dramatically, causing the results of this experiment to be meaningless.

Introduction The membrane of a cell is semipermeable, meaning that it allows some products to pass through, but not all. The process by which products in a solution, known as solute, is moved through the membrane is called diffusion. Diffusion is a passive process; it requires no energy input to occur. It allows for the solute of a solution to go from a higher concentration of the solute to a lower concentration to bring the cell to equilibrium. The concentration of a solution, known as molarity(M), is calculated by: 𝐶1 𝑉1 = 𝐶2 𝑉2 Where C1, C2 is the concentration of each solution, respectively, and V1, V2 is the volume of each solution, respectively. A primary example of diffusion is osmosis, or the passing of water through the semipermeable membrane, in place of the solute. This happens when the solute itself cannot pass through the membrane, usually due to size (such as sucrose). It follows the normal diffusion flow pattern of down the concentration gradient; however, there are some consequences to this action, depending on the tonicity of the solution. “Tonicity is related to the concentration of solutes in a solution” (Porthouse, slide 10). When the outside and inside solutions have an equal concentration of solute and water, the inside solution is called isotonic. When the solution inside the cell has a higher concentration of solutes than outside the cell, the solution is said to be hypotonic. If a cell is hypotonic, the water will flow into the cell and can cause the cell to swell and possibly burst. As well as when that inside solution has a lower gradient, the solution is hypertonic. Which causes water to move out of the cell and shrink and possibly go through the process of crenation. Using dialysis tubing filled with varying concentrations of sucrose in baths of water, we can simulate the diffusion process and answer the question: Which concentration of solute will have the optimum rate of osmosis? By definition, the rate is directly affected by the difference in concentrations; therefore, the fastest rate should be the solution with the highest concentration, or at least that the percent change in rate should increase linearly.

Method and Materials Dialysis tubing, transfer pipette, beakers, flasks, water, sugar, weigh boats, scales Using dialysis tubing, we made modeled cells of varying concentrations of sucrose: 0.0M, 0.2M, 0.6M, 0.8M, and 1.0M. To achieve these concentrations, we measured sugar in a weigh boat on a scale measuring mass in grams and mixed each weighed sugar with 20ml of water. To find out how much sugar (in grams) to mix with water to achieve the right solution we use: 𝑚𝑜𝑙𝑒𝑠 𝑔𝑟𝑎𝑚𝑠 𝑔𝑟𝑎𝑚𝑠 ∗ = 𝑙𝑖𝑡𝑒𝑟 𝑚𝑜𝑙𝑒𝑠 𝑙𝑖𝑡𝑒𝑟 After tying off one half of the dialysis tubing, we filled the tubing with 20ml of each solution, respectively and tied the open side of the tubing shut. After making the model cell, we weighed the model cell as our “initial weight,” then put the tubing in a bath of water for 15 minutes which allows for the process of osmosis. After the 15 minutes, the model cell is weighed again for the “final weight.” We use the following equation to find the percent change in weight of our model cell: 𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑤𝑒𝑖𝑔ℎ𝑡 ∗ 100 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑤𝑒𝑖𝑔ℎ𝑡

Results Concentration (M)

Initial Weight (g)

Final Weight (g)

Change in Weight (g)

0.0 0.2 0.6 0.8 1.0

16.64 20.01 22.03 18.62 19.69

16.75 20.46 22.81 22.89 22.69

0.11 0.45 0.78 4.27 3.00

Percent Change in Weight 0.66% 2.24% 3.54% 22.93% 15.24%

Discussion As seen in the graph of % Change in Weight versus Concentration (M) above, the % Change in Weight did not increase linearly. This could be for a number of reasons, but primarily I believe its because when I put the last two solutions in the dialysis tubes, some of the solution spilled. Now, this shouldn’t affect the concentration or the change in weight in any way. So it is possible there was some other thing happened that caused the anomaly in the experiment, such as my nail could have scratched it weird or I tied it too tight, etc. If perhaps the data is accurate and precise, though unlikely, that would indicate that the rate of osmosis is exponentially proportional to the concentration. Suggesting, that if a cell were to be put in a hypertonic solution (as all of these dialysis bags of hypotonic solution were put in) the cell could crenate or lyse at an exponential rate, which doesn’t seem very stable for lifeforms. Therefore, this experiment can neither support nor reject my hypothesis as the data is useless. In the Osmosis and Diffusion Demonstration video, the dialysis bags were compared to the bags in the produce section of a grocery store (Porthouse 2021). To improve on this experiment, perhaps we can expand on this comparison: the way that the bags are connected at the store should be the way the dialysis bags are connected. That way one would only have to tie one side of the tube, reducing the chances of tearing by tying the bag in a weird way by 50%. Realistically, if I were a bit more patient I probably would have had better data.

References Porthouse, K. (2021). BIOL 110: Fundamentals of Biology, week 5: Membrane Transport [Powerpoint]. Porthouse, K. Osmosis and diffusion lab demo_SP21 [Video]. Moodle. https://louisiana.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=a5c5b3f9-86a3-41e0-a418aca401447d59...