This experiment was conducted to investigate the selective permeability of dialysis tubing. The permeability of the tubing to glucose, starch and iodine (potassium iodide) was tested. The dialysis tubing was clipped to form a bag so that glucose and starch was fed into the bag through the other end, and was also clipped to avoid the seeping of the solution. Water with several drops of iodine added to it until it was visibly yellow-amber was added to a 400ml beaker. The bag was then placed in the beaker, which was stirred with a magnetic stirrer. It was left there for 30 minutes. It was seen that the color of the solution in the bag changed to blue-black color, this showed that iodine was able to pass through the membrane into the bag. The solution in the beaker became pale yellow-amber, this showed that starch didn’t pass through the membrane into the beaker. To confirm the presence of glucose in the beaker and also the bag, a Benedict test was performed on the solutions including tap water (control) too. The beaker solution turned into light brown color after Benedict solution was added to it and suspended in water bath for 10 minutes. The bag solution also changed to brown color, while tap water remained blue. This experiment showed that dialysis tubing is selective in its permeability to molecules. It was permeable to glucose and iodine but not starch.
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PURPOSE: The purpose of the experiment was to test the permeability of dialysis tubing to glucose, starch and iodine.
Living cells need to obtain nutrients from their environment and get rid of waste materials to their surroundings. This exchange of materials between the cell and its surroundings is crucial to its existence. Cells have membranes composed of a phospholipid bilayer embedded with proteins. This cell membrane can distinguish between different substances, slowing or hindering the movement of other substances and allowing others to pass through readily. This property of the cell is known as selective permeability (Ramlingam, 2008).
Selective permeability is a property of a cell membrane that allows it to control which molecules can pass (moving into and out of the cell) through the pores of the membrane. Selective permeable membranes only allows small molecules such as glucose, amino acids to readily pass through, and inhibits larger molecules like protein, starch, from passing through it.
The dialysis tubing is a semi-permeable membrane tubing used in separation techniques and demonstration of diffusion, osmosis, and movement of molecules across a restrictive membrane (Todd, 2012). It separates dissolved substances of different molecular sizes in a solution, and some of the substances may readily pass through the pores of the membrane while others are excluded. The dialysis tubing is made up of cellulose fibers. This is shaped in a flat tube.
In this experiment, the selective permeability of dialysis tubing to glucose, starch and iodine (potassium iodide) will be tested. This experiment consists of two tests; the test for starch and the test for reducing sugar. When iodine (potassium iodide) is added to a solution in which starch is present, the solution turns blue-black or purple otherwise it remains yellow-amber. And when Benedict’s reagent is added to a solution in which reducing sugar is present and it is heated in a water bath, the solution turns green, yellow, orange, red, and then brick red or brown (with high concentration of sugar present). Otherwise, the solution remains blue.
Will glucose, starch and iodine (potassium iodide) readily pass through the pores of the dialysis tubing?
Glucose, starch and iodine (potassium iodide) will readily pass through the membrane of the dialysis tubing.
The solution in the bag and the beaker will both turn blue-black due to the presence of iodine and starch; the presence of glucose in the bag and beaker will be investigated using Benedict test.
- Benedict’s reagent
- Iodine (Potassium Iodide)
1) 250 ml of tap water was added to a beaker. Several droppers of Iodine (Potassium Iodide) solution was added to the water until it was visibly yellow-amber in color. The color was then recorded.
2) The dialysis tubing was soaked in water for a few minutes until it began to open. One end of the bag was folded and clipped in order to secure it so that no solution seeped through.
3) The other end of the tubing was opened so that it forms a bag and 4ml of glucose and 3ml of starch was fed into it. The bag was also closed and its content was mixed. The color of the solution was then recorded.
4) The outside of the bag was rinsed in tap water.
5) The magnetic stirrer and then the bag was placed in the beaker. The other end of the bag was made to hang over the edge of the beaker.
6) The bag was left in the beaker for about 30 minutes, as the beaker was being stirred.
7) After 30 minutes, the bag was carefully removed and made to stand in a dry beaker. The final color of the solutions was recorded.
8) Benedict test was performed to test for the presence of reducing sugar in the solution in the bag, beaker and tap water (serves as control).
- a) 3 test tubes were labelled control, bag and beaker.
- b) 2 ml of water was added to the control test tube. 2 ml of the bag solution was added to the bag test tube and 2 ml of the beaker solution was added to the beaker test tube.
- c) 2 ml of Benedict’s reagent was added to each test tube and was suspended in a boiling water bath for 10 minutes. The color change was recorded.
|Solution Source||Original Contents||Original Color||Final Color||Color after Benedict’s test|
|Bag||Starch and Glucose||Colorless||Blue-black||Brown|
|Beaker||Water and Iodine||Yellow-amber||Pale yellow-amber||Brown|
The solution in the bag turned blue-black in color owing to the movement of molecules of iodine from the beaker to the bag which contains starch. The solution in the beaker turned brown after Benedict’s test. This indicated the presence of glucose in the beaker. This means that the tubing was permeable to both glucose and iodine but not starch. It is known that starch didn’t pass because the solution in the beaker which contains iodine didn’t turn blue-black in color, but remained yellow-amber.
1) How can you explain your results?
From the results of the experiment represented in a tabular form above, the hypothesis suggested before carrying out the experiment turned out to be incorrect. The dialysis tubing was not permeable to all the three solutions- glucose, starch and Iodine (Potassium Iodide). Rather, the tubing was permeable to glucose and iodine but not starch. This could be known from the color change in the solutions in the beaker and the bag. The tubing was permeable to iodine and so the content of the bag turned blue-black in color indicating the presence of starch. Glucose also readily passed through the pores of the membrane. After performing Benedict’s test on the solutions, the bag’s solution as well as the beaker’s solution turned brown in color. This shows the presence of reducing sugar in both solutions, meaning that glucose passed into the beaker from the bag.
From the results of this experiment, it is obvious that glucose and iodine (potassium iodide) has smaller molecular size than starch. Because starch had larger molecular size, the dialysis tubing was not permeable to it (it didn’t allow it to readily pass through the pores of its membrane).
3) What colors would you expect if the experiment started with glucose and iodine (potassium iodide) inside the bag and starch in the beaker? Explain
* The solution in the bag will remain yellow-amber in color at the end of the experiment.
* The solution in the beaker will turn blue-black in color at the end of the experiment.
* After performing benedict test, both solutions will turn brown in color.
The solution in the bag remained yellow-amber in color at the end of the experiment because the dialysis tubing is not permeable to starch and so starch didn’t pass through from the beaker into the bag.
The solution in the beaker turned blue-black in color at the end of the experiment because iodine passed from the bag into the beaker through the membrane.
After performing Benedict’s test on the bag and beaker solution, both solutions turned brown in color because the tubing was permeable to glucose, so glucose readily passed from the bag into the beaker through the membrane.
- It was ensured that the right quantity of solutions was used in every part of the experiment.
- It was also ensured that the time required for the successful complement of the experiment was adhered to.
- It was ensured that all apparatus used were handled with caution.
- And also, the dialysis tubing was clipped well on both ends to secure it so that no solution seeped through.
It was concluded that the dialysis tubing doesn’t allow all kinds of substances to pass readily through the pores of its membrane. This means that it is selective in its permeability to substances. The dialysis tubing was permeable to glucose and iodine but not to starch. Starch was excluded because it has a larger molecular size than glucose and iodine.
Ramlingam, S. T. (2008). Modern Biology. Onitsha: African First Publishers.
Todd, I. S. (2012). Dialysis: History, Development and Promise. World Scientific Publishing Co Pte Ltd.