Diffusion and Osmosis. Two terms to describe things moving in and out of the semi-permeable cell membrane. In case you didn’t know, semi-permeable means only allowing certain things to move through. The cell membrane literally chooses what it wants inside and outside of the cell.
As a group, we wanted to know out of the substances iodine, glucose, and starch which would pass through a semi-permable membrane. For this “membrane” we used dialysis strips which are normally used in the same concept for kidney dialysis. In this experiment, we filled a beaker with water and some iodine solution. Then, we filled a dialysis tube of a glucose and starch mix and tied it off with string. Both of these mixtures were tested with a glucose indicator strip to check where the glucose was. We also recorded the colors of the solutions. The before pictures looked as follows:
We then immersed the bag of starch and glucose in the water and iodine solution. Then, we left it over night to record the results in the morning. Here are pictures of the results:
(The picture on the right is of a glucose strip from the new beaker solution.)
As seen, many physical changes occurred. These, are the factual results:
So, as of right now, you may be a little confused to what is going on here. Don’t worry! I’m going to explain! At first we begun with glucose and starch in the dialysis tubing and water and iodine in the beaker. Many changes happened in that ever night span. First of all, the iodine. Some iodine transferred from the beaker to the bag mixture. How do you know this? The color change. From previous experiment, we know that a polysaccharide (starch) react together when there is a color change from a light color to a dark color. That exactly happened within the bag. Therefor, the iodine entered the tubbing through diffusion and reacted with the starch. Next, the glucose. We were able to record these results based on the glucose test strips. The strips confirmed for us with a change from green to brown that the glucose left the bag and entered the beaker. Lastly, the starch. Like earlier explained, we know that the starch remained in the bag because of it’s reaction with the iodine. But why did the dialysis tubing also test positive for glucose? Research shows that in the process of diffusion, the purpose is to balance both sides of the membrane equally with what can pass through. So, not all the glucose left the bag. Some stayed so the amounts were equal.
Now, you might be asking, why did only these substances transfer? We came to the conclusion that it depends on the size of the molecules. Again, looking back at previous experiments, we know that glucose has smaller molecules than starch being a disaccharide. If the molecules are too big, they can not get in or out.
But, wait! Aren’t we forgetting something???
Alex Blair's Biology Blog :) 