How the Size of Molecules Affects their Movement Through Semi-Permeable Materials
This experiment was conducted in order to find scientific proof of diffusion and osmosis occurring, it also sought to find proof of which molecule was larger; glucose or starch. A total of eight controls were conducted using a variety of laboratory tools. Dialysis tubing was used to determine what molecule was larger and also exhibited signs of diffusion and osmosis.
Introduction:
In many previous classroom lessons, it was taught that starch is larger than glucose because it is made out of many more molecules linked together in a long chain. In this experiment, it was actually the case that we had to find proof that starch was larger than glucose. Not only that, but we also had to be able to measure the affects of diffusion and osmosis. The general hypothesis was that the smaller molecule, glucose, would be able to diffuse through the semipermeable membrane of the dialysis tubing while the larger starch molecule would be to big to pass through the membrane. Dialysis tubing is used by doctors to replicate the function of the kidneys. The tubing only works by diffusion and osmosis, it cannot filter. Benedict's reagent is semiquantitative, this means that although it will show it there is or is not glucose is a solution, it will not tell how much. Glucose reacts with Benedict's because the Glucose contains one -CHO group that reacts with the free Cu++ ions in the Benedict's solution. This ends up changing color because of the reaction. Iodine reagent reacts with starch because part of the starch molecule has spots that the iodine reagent fits into when the starch-iodine complex changes shape after being heated above 35 degrees. Iodine reagent contains the element iodine and ions of the element iodine. Iodine reagent only appears blue when the element iodine is in the presence of the iodide ion.
Materials and Methods:
2 eyedroppers 1 beaker water glucose starch 3 inches of dialysis tubing 1 three beam balance ruler 2 four-inch lengths of string
A 3 inch length of dialysis tubing was cut, and rinsed under water until it became soft. It was then tied closed on one end with string and filled with water and tied closed at the opposite end. This package of water and tubing weighed a total of 7.45 grams before the experiment. A beaker was then filled with 50ml of glucose and 50ml of starch. The combined weight of the starch, glucose and beaker was 201.69 grams.
To test for diffusion and osmosis occurring, the packet of tied-closed water-filled dialysis tubing was placed in the beaker containing the 50% glucose, 50% starch solution. This experiment was set to the side for 45 minutes while the control experiments were tested. The first control was with glucose. Two ml of glucose was put in a test tube in a beaker of boiling water with two drops of Benedict's Reagent for one minute. The second control was with starch. Two ml of starch was put in a test tube in a beaker of boiling water with two drops of Benedict's Reagent for one minute. The third control was with glucose. Two ml of glucose was put in a test tube in a beaker of boiling water for one minute. After one minute, the test tube was taken out of the boiling water and placed in a test tube holder. Two drops of iodine were then added to the glucose. The fourth control was with starch. Two ml of starch was put in a test tube in a beaker of boiling water for one minute. After one minute, the test tube was taken out of the boiling water and placed in a test tube holder. Two drops of iodine were then added to the glucose. The fifth control was with water. Two ml of water was put in a test tube in a beaker of boiling water with two drops of Benedict's Reagent for one minute. The sixth control was with water. Two ml of water was put in a test tube in a beaker of boiling water for one minute. After one minute, the test tube was taken out of the boiling water and placed in a test tube holder. Two drops of iodine were then added to the glucose. The seventh control was with glucose and starch. One ml of glucose and one ml of starch was put in a test tube in a beaker of boiling water with two drops of Benedict's Reagent for one minute. The eighth control was with glucose and starch. One ml of glucose and one ml of starch was put in a test tube in a beaker of boiling water for one minute. After one minute, the test tube was taken out of the boiling water and placed in a test tube holder. Two drops of iodine were then added to the glucose and starch solution.
Results:
After the beaker of the 50 ml starch and 50 ml glucose solution had been left for 45 minutes, the packet made from dialysis tubing was removed weighed on a three beam balance. The beaker was weighed next. The package of water and dialysis tubing weighed a total of 7.45 grams. The beaker The combined weight of the starch, glucose and beaker was 201.69 grams. Both measurements of weight were exactly the same as when they were first measured. The dialysis tubing was cut open at one end and an eye dropper was used to put 2ml of it's liquid contents into two test tubes. The test tubes were then placed in a beaker of water and were left until the water was boiling. Two drops of Benedict's was then added to one beaker and that beaker was left in the boiling water for another minute. The other beaker was removed and placed in a test tube holder. Two drops of iodine were placed in the solution.
Solutions color when properly mixed with reagents in this experiment:
Controls:
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Tests:
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Mixture:
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End Color:
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Mixture:
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End Color:
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Glucose & Benedict's
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Brown
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Starch + Glucose & Benedict's
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Brown
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Starch & Benedict's
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Transparent blue
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Starch + Glucose & Iodine
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Dark brown-purple
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Glucose & Iodine
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Transparent yellow
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Starch & Iodine
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Dark brown-purple
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Water & Benedict's
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Transparent blue
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Water & Iodine
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Transparent yellow
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Starch + Glucose & Benedict's
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Brown
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Starch + Glucose & Iodine
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Dark brown-purple
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Discussion/Conclusion:
The reason why the weight of the dialysis tubing full of water and the beaker of water weighed exactly the same after the experiment as when it begun, was because as glucose molecules diffused through the semipermeable membrane of the dialysis tubing trying to get to equilibrium, the water in the dialysis tubing moved out of the tubing package by osmosis to get to equilibrium in the container and the dialysis tubing packet. The test tube that had the liquid from the dialysis tubing packet turned brown when tested for glucose with Benedict's but the septate sample from the same place did not change color when tested with iodine. This means that only the glucose molecules were able to diffuse through the membrane. This allows the conclusion to be made that the glucose molecule was able to diffuse into the tubing because the molecule was small enough to fit through the membrane. Water molecules were inversely able to leave the tubing also because of their size. The starch molecules were to large to be able to get through the tubing without some sort of facilitated diffusion or active transport. Because the tubing in not alive, it is not able to produce ATP to be able to move the larger starch molecule through its membrane and so the starch remained outside the tubing package. This is a perfect reference to why the body must use enzymes such as amylase to break down starch before it can use the molecule, because otherwise, the starch molecule would be to large to be able to diffuse through the cell and the body would do something efficiently rather than waisting time and energy trying to bring the starch molecule inside the cell before it breaks it down. The many controls for this experiment ensure accuracy in the final conclusion by vitrifying that iodine and Benedict's reagent do not react unexpectedly with anything other than what they are suppose to react with.
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