Diffusion through Dialysis Blog

For this lab, we did a simulation of diffusion inside cells using dialysis tubing. First, we tied the end of two dialysis tubes. These dialysis tubes are non-living material that have small pores which separate larger molecules from smaller ones.

We then poured soluble starch within 4 cm of the top of one tube, tied the remaining end, and put it into a beaker of water mixed with a few drops of iodine. We repeated the same steps with the second tube, using glucose instead of starch. We then set up a time-lapse and waited patiently for 20 minutes. After a while, the iodine diffused into the tube and reacted with the starch, creating a blue color. The beaker and dialysis tube with glucose didn't show any changes. We then dipped glucose test strips into the beaker. The glucose strip turned green.

Finally, just for fun, we cut the tube, releasing all the contents into the beaker. The entire beaker turned dark blue instantly.

After some more research, I found that the starch and iodine turned blue because the starch contains a polysaccharide known as Amylose. When it reacts with the iodine, the blue color results from changes in electron orbitals. 

Analysis:

1. The Iodine molecules diffused through the membrane, and reacted with the starch. Since the beaker didn't turn blue, that means the starch molecules were too big to diffuse out of the tube. This was validated when we cut the tube open and the whole beaker turned blue.
2. The glucose test strip showed that some glucose diffused through the membrane. 
3. We didn't wait overnight, but if we did, I would imagine that the tubes, because of osmosis, would start to swell due to the concentration of water being lower on the inside.
4. The starch molecules didn't pass through the membrane because they were too large.
5. My hypothesis is that iodine passes through the membrane, reacting with starch. The glucose could pass through the membrane, which is why the glucose test showed a concentration of glucose.
6. I assumed that the two dialysis tubes had the same number and sizes of pores, and that the pores were just big enough to let the glucose through, but not the starch.