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Demonstration of Different Biological Events: Diffusion and Osmosis

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Homeostasis, the tendency of an organism to maintain a stable, constant environment, is a fundamental characteristic of all living systems. Diffusion and osmosis are two biological processes that promote homeostasis. Diffusion is the ability of a substance to dissipate to equilibrium in a media (Lange 2015). Osmosis is the flow of water across a differentially permeable membrane (Marieb and Mitchell 2011). Diffusion and osmosis facilitate movement in and out of cells and are also vital in kidney regulation. The function of osmosis in kidneys allows the body to retain water when low (Nguyen 2015). The objective of the diffusion lab was to explore the effects of physical properties of physical properties of a compound (such as molecular shape) on the diffusion rate; while using the scientific method to construct a hypothesis and organize an experimental design. The objective of the osmosis lab was to examine the movement of water and solutes through dialysis sacs in relation to the surrounding solution. For the experiment on diffusion, it was hypothesized that the substance with the lowest molecular weight will diffuse the farthest in the agar solution within the 90 minute time period. Just as it requires less force and energy to move a five pound weight as opposed to a ten pound weight, less energy and force is required to move smaller, lighter molecules. Therefore, it would be logical that the solution with the lowest molecular weight (Potassium Permanganate) would diffuse the quickest and the farthest. The hypothesis for the osmosis lab was that the sacs with solution contained in them placed in distilled water will take on water and weigh more after the experiment since the beakers depict hypotonic solutions. Meanwhile, the sac placed in a solution identical to its contents will exhibit no weight change, depicting an isotonic solution.

Materials & Methods

The procedure of the diffusion lab first required reading the lab manual to learn the procedure. Then a straw was used to remove equally sized circular holes in the agar plate, and a grease pencil was used to label the bottom of the plate with the abbreviations of the solutions: CR, JG, MB, PP, and UK (Congo Red Dye, Jangus Green B Dye, Methylene Blue Dye, Potassium Permanganate Dye, and Unknown Dye). Next, one drop of each dye was placed in the appropriate circle using a dropper. The agar plate was covered and then measurements were taken at 30 minutes, 60 minutes, and 80 minutes (due to time constraints) of the radius of each dye. From the results, a prediction was then made of what the unknown solution was (Lange 2015). The osmosis lab started by reading through the experiments and developing a hypothesis for each part. Then, the materials needed for the experiment were obtained: four dialysis sacs, a small funnel, a 25-ml graduated cylinder, a wax marking pencil, fine twine, and four 250 ml beakers. Then, the wax pencil was used to label the beakers one to four. Beakers one, three and four were half filled with distilled water, and beaker two was half filled with 20% glucose solution. Next, each dialysis sac was prepared individually by filling each with 15 ml of the specified solution (20% glucose solution for sacs one and two, 10% NaCl solution for sac 3, and 30% sucrose solution for sac 4). Once filled, the air was pressed out, and the open end was folded over and tied with fine twine. Each sac was rolled on a paper towel to dry, then weighed and recorded. After a period of 45 minutes, the sacs were dried and the final weight was taken (Marieb and Mitchell 2011).


The results of the diffusion lab are depicted numerically in the table below; showing the radius of diffusion for each given substance over time. The substances had a higher diffusion rate in the beginning of the experiment, when they were at a higher concentration. Once they spread through the gelatinous agar and approached equilibrium, the diffusion rates slowed across the board. Also, the substances with lower molecular weights diffused faster than the substances with higher molecular weights. Molecules or ions of substances tend to move from a region of higher to lower concentration in diffusion, which is movement in response to a concentration gradient, and generally, smaller molecules flow down their concentration gradients faster than large ones do (Starr and Taggart, 2004).

The osmosis lab results are shown in the data table. The movement of water is depicted by the data, as the sacs changed weight. It can be concluded that sacs one, three and four were placed in hypotonic solutions. In numbers, sac two also appears to be hypotonic, since it took on water. However, due to background knowledge, it is known that the solution is isotonic and that the change in weight must have been a result of an experimental error. Due to time constraints, the Benedict’s test and AgNO3 test were not conducted, so no conclusions may be drawn from them.

Discussion/ Conclusion

From the diffusion lab, the major conclusion drawn is that the molecular weight and diffusion rate of a solution have an inverse relationship. As the molecular weight decreases, the diffusion rate increases, and vice versa. Using this pattern, and the distances measured in the experiment, the unknown solution is believed to be Crystal Violet. In relationship to biological systems, this shows that solutions with lower molecular weights will spread throughout the body at a faster rate. For instance, if someone is sick and takes fast-acting antibiotics, it is likely that that given medicine has a low molecular weight so that it may diffuse through the body rapidly and work its way throughout the immune system. Although the hypothesis that the substance with the lowest molecular weight will diffuse the farthest was correct, some errors in the data may be present. The measurements may be inaccurate due to the fact that some of the dye got on the edge of the agar gel, and there is a possibility that the drops of each solution were not entirely equal.

The hypothesis for the osmosis lab was correct for sacs one, three and four, as they took on water weight. However, the results for sac two were not what was expected. Since the concentration and solutions inside and outside the sac were identical, it was predicted that there would be no weight change in the sac. The error in this may be a result of the sac not being tied tight enough, or even an issue with the scale usage, since the equipment was older, and difficult to use. Another issue is that the experiment was cut short about 15 minutes. In relationship to biological systems, this depicts how osmosis is used by cells to balance the concentration of solutions inside and outside the cell when necessary, and to promote homeostasis by recognition within the kidneys that a body is lacking water. At which point, osmosis allows the body to retain water.

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