This lab was performed to determine the basis or criteria for the cell membrane’s selective permeability.
This lab was performed to determine the basis or criteria for the cell membrane’s selective permeability
- We were given a six inch piece of cellophane tubing commonly used for chemical dialysis.
- The tubing was opened up and one end of the tubing was tightly tied with string one inch from the end.
- We dropped 20 drops of a starch solution into the open bag and then filled the rest of the bag up with glucose solution, leaving enough room at the end to tie the top of the tubing.
- The tubing that was now tied at both end, was shaken to ensure that the starch and glucose were mixed.
- Meanwhile, water was poured into a small glass jar along with one to five mL of liquid iodine.
- The tubing was placed into the glass jar.
- After about five minutes, we took one diastick and placed it in the outside iodine solution.
- We noted its reading and noted any other observation.
- The tubing was left inside the jar for 72 hours.
- We again noted the observations and took another glucose test.
- Immediately after placing the tubing in the jar, blue solution started forming in the outside iodine solution.
- After about three minutes, a blue solution began forming inside the bottom of the tubing.
- At first, the solution was a light blue. It slowly rose to the top of the bag and turned into a darker blue.
- The diastick that was placed into the outside solution turned from a pale blue to a light green. According to the chart on the bottle, the solution contained about 1.25% glucose.
- After 72 hours, the bag had turned completely black. The outside solution was very clear. The diastick that was placed in the outside solution this time turned from pale blue to a dark green with some brown spots. According to the chart on the bottle, the solution now contained about 1.5% glucose.
The glucose, prior to coming into contact with the iodine, was a clear/cloudy color. The starch was also a clear color, but it was also thicker than the glucose solution. The iodine was a yellowish color when it made contact with the water. When the glucose and starch were exposed to the iodine, two things immediately occurred.
First, judging by the blue color forming in the outside solution, something was forced out of the bag and into the iodine solution. We know that the iodine dyed this substance and it could be either glucose or starch. We later find out using the diastix, that there is a presence of glucose in the outside solution. Therefore, we can assume the glucose leaked out of the bag and into the iodine solution.
Second, judging by the blue color forming inside the bag, something was also “allowed” through the cellophane and into contact with the starch and glucose. The iodine dyed the two compounds in the bag a black and blue color. Just from these observations, we can assume that iodine leaked into the bag and into the glucose and starch solution.
From these two observations, we can prove our hypothesis correct. The cell membrane does have a selective permeability in that it “decides” what can go out (glucose in this case) and what can come in (iodine in this case).
A few errors that could have been made would have created the same reactions, but would not be the correct reason behind it. One of these errors includes not tying the ends of the tubing tight enough. This would result in glucose leaking out of the end instead of through the cellophane. Another error could be the miscalculation of the amount of glucose and starch to place into the bag. This would result in the glucose levels in the outside iodine solution to be different then it should be. These errors could be easily avoidable with careful measuring and reinforcing ties at the ends of the tubing.
The cell membrane has small holes in its walls that “decide” what substances come in and out of the cell. In this lab, we saw how the miniscule holes in the cellophane tubing allowed glucose to pass out and iodine to enter the bag. The starch was also too thick to pass through.