Investigation 4.2 on Osmosis

Introduction
Membranes are one of the miracles of life. Present in all life forms, membranes regulate the flow of materials in and out of the cell and make sure that the every cell, from the smallest bacterium to the largest multicellular organism like humans, receive exactly what it needs and exactly how much. In most cells, there is a lipid bilayer that has both hydrophilic and hydrophobic regions to regulate the flow of water in and out of a cell. The transfer of water across a membrane is called osmosis, and the lab group and I carried out another experiment to model this.

A Cell's Phoso

On September 25th, Vinay, Shreyan, Vikram and I did an experiment to determine the effects of different solutions on osmosis out of a cell through measuring the rates of weight change in the tubes using dialysis tubes and solutions of different solutes. A dialysis tube is a selectively permeable membrane that will allow water and certain other solutes to flow through. The direction of the flow of the water is determined by the water potential of both solutions, but the flow will always be into the tube because we filled the tube with pure distilled water and the outside of the tube was comprised of water and a certain solute.

The solute potential of the solution outside of the dialysis tube is determined partially by temperature, but also by the ionization constant of each substance. The substance's ionization constant is the number of particles that it breaks up into when put into a solution, so for example, glucose, a sugar that doesn't ionize in a solution, has an ionization constant of 1 because there is one particle in solution for each particle of solute added. Sodium chloride, an ionic compound that ionizes into two particles while in solution, has an ionization constant of 2. This means that in solutions with higher ionization constants, the solute potential is higher.

The question is, though, how would the group observe osmosis if it occurred at such a small scale? We weren't able to actively see the water molecules go in and out of the tubing, and there was no other way to tell if water was entering the tubing. But that is the solution as well as the problem because we weighed the tubing and its contents every 2 minutes, noting the change in weight and attributing this to the flow of water into the dialysis tube. This was how we would quantitatively measure our data.

The purpose of this experiment was to model water potential and the effects of osmosis as it would occur in a cell. We wanted to see what effects different solutes had and also wanted more practice in the lab carrying out specific procedures so that our work could be replicated if it needs be. The group hypothesized that, because sodium chloride has a larger ionization constant, the difference in solute potentials in the solution and in the dialysis tubes would cause osmosis to occur the fastest when the tubing was put into a solution of NaCl.

Procedure
The lab group started our lab by first filling a beaker with deionized, distilled water, which would be the site of our osmosis experiment. Then we obtained a length of moist dialysis tubing. We didn't want dry tubing because the membrane could dry out and affect osmosis. Next we tied a knot on one end of the tubing that we decided was the "bottom" of the length of tubing. From that point on we kept this end towards the floor so as not to spill the contents of the tubing. Then we filled the tubing with 10mL of a 1.0M glucose solution and tied off the "top" of the tube. Because the membrane was selectively permeable, no fluid dripped out of the tubing even though we were moving the tube around.

We brought the tubing back to our experimental area and weighed the tubing and its contents as our base so that we could observe the changes in weight to the tube. Next we placed the tube in distilled water beaker from the first step of the experiment and waited. From that point on, we would remove the tubing, dry off any excess water and weigh the entire system every two minutes until 15 inutes had passed.

After we finished with the glucose tube, we repeated the same procedure but substituted 10mL of 1.0M solution of sucrose and 10mL of a 1.0M solution of NaCl for the glucose solution we had used. We did this to observe the changes in osmosis that occur due to different solutes and to create more experimental groups.

Data/Results

This is the data we directly received from weighing the dialysis tubing every 2 minutes.

	Mass of the Dialysis Tubing in grams
Time	Glucose	Sucrose	NaCl
0 min	11.7	12.0	11.8
2 min	12.2	12.4	12.3
4 min	12.7	12.6	12.6
6 min	13.1	12.8	13.0
8 min	13.3	13.2	13.2
10 min	13.6	13.5	13.6
12 min	13.6	13.8	13.8
15 min	13.7	14.0	13.9

The percent change was calculated using the measurements obtained from weighing the tubing. The initial weight was subtracted from the weight at each interval, then divided by that interval's weight and the whole thing multiplied by 100 to get a percentage.

	Percent Change in Dialysis Tubing Mass (%)
Time	Glucose	Sucrose	NaCl
0 min	0	0	0
2 min	4.27	3.33	4.24
4 min	8.55	5.00	6.78
6 min	11.96	6.67	10.17
8 min	13.68	10.00	11.86
10 min	16.23	12.50	15.25
12 min	16.23	15.00	16.95
15 min	17.09	16.67	17.80

This is a graph of the percent change in mass of the three different experimental tubes.

Conclusion
The results of the experiment were much as the lab group had predicted. We believed that water would flow from the solution into the dialysis tubing and would be seen in an increase of the tubing's weight. This definitely happened, with the glucose tube gaining 2 grams of water, sucrose gaining 2 grams as well, and sodium chloride gaining 2.1 grams. After 15 minutes, each of the tubes had similar percent changes, but as our group had predicted correctly, the greatest percent change occurred with the tube containing the NaCl solution. Whereas sucrose only increased its mass by 16.67% and glucose only saw an increase of 17.09%, the tube with NaCl saw a mass percentage gain of 17.80%!

The experiment was quite successful in hindsight. The concept of water potentials and osmosis that we had learned in class were confirmed by the fact that all three tubes gained mass. This was due to the fact that the solute potential inside the pseudo-"cells" was lower than that outside of the "cell" because the ratio of water to solute was lower inside than outside. Therefore, water rushed in, but since the solute could not diffuse out because of the selectively permeable dialysis tube, the particles were stuck and the tubing gained mass. Our hypothesis about NaCl gaining the most percent change was correct, so the effect of the ionization constant on water and solute potential has quite an effect on osmosis. Though the sodium chloride test did gain more mass by percentage, we had expected the mass to increase quite a bit more drastically. Because the ionization constant was twice that of glucose and sucrose, we believed that the sodium chloride would have somewhere around twice the percentage mass gained, not just a measly 0.71% more than the sucrose. This could be due to experimental error of not weighing our masses correctly, or perhaps as a lab group we overstated the effects of the ionization of sodium chloride on osmosis. For a follow-up experiment, we could use solutions of higher molarity to more clearly demonstrate the effects of osmosis over the relatively short period of time that is 15 minutes. Because the solute potentials would be so much different between higher molarity solutions and pure water, osmosis would occur at a faster rate. Another possible remedy for this problem would be the short amount of time that we tested each experiment. Perhaps the NaCl system had not reached equilibrium and therefore was not as telling of its effects on osmosis than it could be. A different follow up equation would be to test the effects of temperature on osmosis. To calculate solute potential of a solution, one must factor in temperature, so temperature change must have an effect on osmosis.