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Diffusion is the flow of molecules from a high concentration region to a low concentration region; the movement comes to a halt until an equilibrium is achieved. At temperatures above absolute zero, diffusion is caused by natural thermal agitation of particles in the absence of additional energy. This mechanism is influenced by a number of variables, including the relative concentration of molecules, temperature, and particle size. Diffusion is important in biological processes for the passage of molecules and ions across the cell membrane.
Osmosis is the mechanism by which solvents diffuse across a semi-permeable membrane in response to a solute gradient (Yaroshchu). In living cells, the selective permeability is evident during the exchange of material with its surrounding. Cell membranes are made of phospholipid bilayer which allows them to distinguish between different materials thereby slowing the movement of some substances and allowing others to pass through. Selective permeability plays a vital role in preventing toxic substances from entering the cell.
It is in light of this property of cell membrane that the lab seeks to investigate the permeability of dialysis bag to starch, glucose, and iodine. According to Ing, Rahman and Kjellstrand, a dialysis tubing can be used in separation and demonstration of the movement of molecules across a semi-permeable membrane. This lab tests the selective permeability of the tubing in two stages; the test for starch and that for reducing sugar.
Ideally, when iodine is introduced to a solution containing starch, the solution should turn either purple or blue-black otherwise it should remain yellow-amber. Moreover, when Benedict’s reagent is added to a solution containing reducing sugar and in the presence of heat, the solution should turn green, yellow, orange, red and brick brown with increasing concentration of the sugar. Otherwise, the solution should remain green.
Research Question. Will glucose, starch, and iodine pass through the membrane of the visking tubing?
Hypothesis. Glucose, starch, and iodine will penetrate the pores of the dialysis tubing.
Materials and Methods
The dialysis bag was prepared by first folding a piece of dialysis tubing that had been soaking in water for some minutes. The folded end was then pleated the “accordion style, ” and the end of the tube clipped securely to form a bag that could not allow the solution to seep through. Next, the opposite end of the bag was opened, and 4ml of 30% glucose was added to the bag. The same amount of starch solution was also added to the bag using a transfer pipette.
The bag was held while closed and then clamped with a dialysis clamp. After securing the end, the content of the bag was mixed by rocking back and forth. The initial color of the dialysis was recorded at this stage. Next, the outer surface of the tubing was rinsed carefully in tap water after which 300 ml was added to a 400 ml beaker. After starting the stir bar, 0.5 to 1 ml of I2KI solution was added to the water until it had turned visibly yellow-amber. The color of the solution was also recorded.
The prepared dialysis bag was gently placed in the beaker containing the iodine solution and was left for 30 minutes after which it was carefully removed and placed in a dry beaker. The final colors of the solution both in the bag and that in the beakers were also recorded. After preparing the dialysis bag with initial solutions, a benedict’s test was performed to check the presence of sugar in the solution.
Before starting the test, three clean test tubes were labeled as follows control, bag, and beaker. About 1ml of distilled water was put in the control tube, and the same amount of the solutions in both the bag and beaker were placed in their respective tubes. After adding 1ml of Benedict’s reagent, the tubes were heated in a water bath for 5 minutes, and each color was recorded in another table. The results in the second table were reviewed and interpreted based either the presence or absence of glucose.
Results of the experiment on permeability of dialysis tubing to I2KI, glucose, and starch
Glucose and starch
Glucose, starch, and Iodine
Color after the test
Is glucose present?
According to the results, the hypothesis was not correct because the tubing was not to the three solutions but two. The solution in the bag formed a purple precipitate owing to the movement of iodine molecules from the beaker to the tubing containing starch. After Benedict’s test, the content of the beaker turned brown indicating the presence of glucose. Similarly, the solution in the bag turned darker brown indicating the presence of reducing sugars as well. Indeed, the glucose passed from the bag to the beaker and therefore the dialysis tubing was only permeable to iodine and glucose and not starch. The fact the solution in the beaker containing iodine remained amber shows that starch did not pass through the membrane.
These results can be used to predict the size of the iodine molecule relative to that of starch. For instance, it is evident that the molecules of glucose and iodine are relatively smaller in size than starch. The fact that the dialysis tubing was not permeable to starch may imply the starch had a larger molecular size. Various precautions were taken to ensure the relevance of the findings; for instance, the dialysis tubing was properly clipped to prevent the solution from seeping through the ends. In addition to that, the apparatus used in the test were handled with caution.
Be that as it may, the findings did not support the hypothesis owing to errors probably caused by some minor mistakes like estimating the quantity of solution used in various stages of the experiment. Another issue that may have affected the test is the lack of adherence to the time needed for the successful complement of the lab. Errors that are associated with flows in the design of the experiment are called systematic errors (Golder). The accuracy of results subject to this type of error cannot be corrected by repeating the measurements but by understanding the nature of the experiment. To avoid this bias, the lab should ensure the right quantity of solution is used in every stage of the trial and strict adherence to time.
The aim of this lab was to test the permeability of dialysis tubing to glucose, starch, and iodine. To do so, a dialysis bag was prepared with the initial solution followed by a benedict’s test, which was performed to investigate the presence of reducing sugar in the solutions. According to the results, the dialysis tubing was only permeable to glucose and iodine but not starch. As can be seen, the pores of dialysis bags are not permeable to all solutes; they exhibit a phenomenon called selective permeability which largely depends on the size of the molecule. Therefore, I reject my hypothesis that glucose, starch, and iodine would pass through the pores of the dialysis tubing.
Golder, W. A. "Systematic errors in clinical studies: A comprehensive survey." Der Ophthalmologe: Zeitschrift der Deutschen Ophthalmologischen Gesellschaft (2017).
Ing, Todd S., Mohamed A. Rahman, and Carl M. Kjellstrand. Dialysis: history, development, and promise. World Scientific, 2012.
Yaroshchuk, Andriy. "“Breakthrough” Osmosis And Unusually High Power Densities In Pressure-Retarded Osmosis In Non-Ideally Semi-Permeable Supported Membranes." Scientific Reports 7.45168 (2017): Web.
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