Diffusion Osmosis Lab Part 1A

Part 1A:

Introduction / Purpose: 

The purpose of this experiment was to demonstrate the concept of selective permeability. When a membrane is selectively permeable, this means that only certain molecules are allowed to pass through. Generally, smaller molecules are able to pass through the membrane with ease, but larger molecules take a longer amount of time, or cannot pass through period. The process by which molecules pass through the membrane is called dialysis. In our experiment we measured to see how selectively permeable the dialysis tubing  is. We filled the dialysis tubing up with a 15% Glucose / 1% Starch solution and let it sit in a cup of water filled with H2O and Iodine. After 30 minutes it was clear that Iodine molecules had passed into the membrane and glucose had escaped.  

Procedure:

1. Obtain a 30-cm piece of 2.5-cm dialysis tubing that has been soaking in water. Tie off one end of the tubing to form a bag.

2. Test the 15% Glucose / 1% Starch solution for the presence of glucose. Your teacher may have you do a Benedict's test or a Cit. Record the results in Table 1.1.

3. Place 15 mL of the 15% Glucose / 1% Starch solution in the bag. Tie off the other end of the bag, leaving sufficient space for the expansion of the contents in the bag. Record the color of the solution in Table 1.1.

4. Fill a 250 mL cup two-thirds full with distilled water. Add approximately 4 mL of Lugol's solution to the distilled water and record the color of the solution in Table 1.1. Test this solution for glucose and record the results in Table 1.1.

5. Immerse the bag in the beaker of solution.

6. Allow your setup tp stand for approximately 30 minutes or until you see a distinct color change in the bag or the beaker. Record the final color of the solution in the bag, and of the solution in the beaker, in Table 1.1.

7. Test the liquid in the beaker and in the bag for the presence of glucose. Record the results in Table 1.1.

Conclusion/Analysis:

Based on the results of the experiment, it is possible to conclude that the dialysis tubing was selectively permeable. Before being immersed in the iodine, when tested, the dialysis bag showed presence of glucose. This was demonstrated with the paper of the Clinistix turning a dark brown color, signaling glucose. On the other hand, there was no glucose presence detected by the Clinistix when dipped into the iodine, since the stick turned mint-green, the color for no glucose. However, when the bag and the iodine were tested a second time for the presence of glucose, after the bag had sat in the iodine for 30 minutes, the Clinistix strip turned lime-green for both the bag and the iodine. This told us that the glucose molecules of the 15% glucose / 1% starch solution were small enough to go through the membrane of the dialysis tubing and mix in with the iodine. However, the lack of starch indicates that these starch molecules were too large in order to be able to pass through the dialysis tubing membrane. Evidence of the lack of starch is the fact that iodine will turn the starch a dark blue color when the two substances are mixed together. However, at the end of the experiment, the solution in the cup remained red, its original color, but the dialysis bag was now a dark blue color. This indicated that the starch molecules had remained inside of the bag and not drained into the cup. It was instead the iodine molecules which were able to pass through the dialysis tubing membrane and mix with the starch molecules inside of the bag, thus turning the bag a dark blue color.