Osmosis and Diffusion Lab

Cooler Than Absolute Zero's Osmosis and Diffusion Lab

No sucrose, dialysis bags, or potato cores were harmed in the experimenting of this biology lab.

1.A: Small solute molecules and water molecules can move freely through a selectively permeable membrane but larger molecules will pass through more slowly or not at all. The size of the minute pores of the dialysis tubing determines which substances can pass through the membrane.

The purpose of this section was to measure the amount of diffusion of small molecules through dialysis tubing, which is an example of a selectively permeable membrane.

   

Dialysis definition: The movement of a solute through a selectively permeable membrane.

METHODS -->We placed a dialysis bag of glucose and starch into a small cup of iodine. After waiting 30 minutes, iodine entered the bag turning its color black and glucose exited the bag into the cup.

A dialysis bag glucose and starch submerged in an iodine liquid. The bag turned black.

Our lively discussion: We noticed that glucose was exiting the bag whilst iodine was entering the bag. This is an example of diffusion. The bag was a hypotonic solution due to the fact that iodine was entering the bag. We knew that the glucose was exiting the bag because the bag was turning a blackish color and iodine detects starch-like substance. After testing the liquid with a glucose test strip,  the cup results were positive. This must mean that the molecules of iodine are small enough to pass through the membrane of the dialysis bag.

1.B: 

       In this section of the experiment, our lab group used these thin plastic tubes called dialysis tubing to find the relationship between solute concentration and the movement of water through selectively permeable by a process called osmosis. Osmosis is the tendency of  water to pass through a semipermeable membrane into a solution where the solvent concentration is higher.

A dialysis bag of sucrose being submerged in a beaker or cup of water.

      So in the experiment, we made six bags with the dialysis tubing. In each bag, we added a different solute. For example, Bag #1 had distilled water, Bag #2 had 0.2 M Sucrose, Bag #3 had 0.4 M Sucrose, and so on. After that, we put each of the bags in cups filled with water, completely submerging the bag. Then we waited thirty minutes, and let osmosis take place. Next, we massed each of the bags in order to get our data. Here is a picture of our data chart!
 (insert picture of 1B data chart)

The percent change of the class's average compared to our own data.

Before the experiment, the dialysis bags were in a hypertonic solution. This means that there was less water inside the bag than outside. After the procedure, the environment of the bags turned into a hypotonic solution since the water rushed inside the bag. We gathered this conclusion after discovering that the final mass was larger than the initial mass, meaning that the water entered the dialysis bag.

1.C

      This awesome experiment was all about potatoes! And water potential. :( What is water potential? Well, I'm glad you asked. Water potential is the tendency of water to leave one place in favor of another place. There are two factors that affect water potential. One factor is the addition of solute. This lowers water potential. The other is pressure potential. To raise the water potential, you would need to increase the pressure. In biology, water potential is used to predict the movement of water into or out of plant cells.

In the lower half of this collage, we noticed that as the molarity went up from each beaker, the cores began to float. But the lower the molarity, the core would sink.

Day One: Before we could begin our experiment, we had to obtain potato cylinders. In order to make this, we punctured the potato with a cork borer (a plastic straw) 24 times. After, we made sure that the cylinders did not contain skins by cutting the remaining skin off with a razor blade. What fun! We divided the potato cylinders into groups of four, and then massed each group. During this step, we massed the potato cylinders incorrectly. This made our data invalid, so in order to fix our mistake, we found the average of three other groups' data. Next, we placed each group of cylinders into a cup of sucrose solution. Each cup differed in the molarity of the sucrose. Then, our lovely, god-like biology teacher wrapped the beakers with plastic in order to prevent evaporation. Finally, after a long day's worth of working with potatoes, we left the beakers to sit overnight. 

Our class's average of percent change in mass with the potato cores in beakers of sucrose.

    Side note: During this experiment, we made a quick observation. We saw that some of the potato cores floated while others sank. This is because in some cases the potato cylinders had more water than the molarity of sucrose, causing them to sink. On the other hand, those with less water than the solute floated.

Day Two: We started off our morning by removing the potato cores from the beakers, and then determined their total mass. Then, we filled out the rest of our data table by calculating the mass difference and percent change in mass. This is the moment when we realized our data was severely miscalculated. (put in good chart and bad chart)

    We noticed after analyzing our data that the mass of the solute in some of the beakers decreased. This is because water is moving from a higher water potential to a lower water potential, which means the water is leaving the potato and entering the solute. This makes the mass of the potato cylinders decrease. We discovered that the higher the molarity of sucrose is, the more the mass of the potato decreases. 

We were given a set of data for the same experiment expect the cores were Zucchini. The results were very similar.

1E.

Plasmolysis is the process in which cells try to create equilibrium when placed in a hypertonic solution. Due to the difference in molarity on the inside and outside if the cell, water rushes out in an effort to create a balance. When the onions were placed in the 15% NaCl solution, the onion shriveled because of this effort to make equilibrium. Compared to the onion that was prepared with just water on the slide, this onion was smaller. This is due to the difference in the level of water in each onion. The cells in the water stayed at equilibrium, or at most took in water, because the molarity of NaCl was greater or the same inside the cell. The opposite was true for the 15% NaCl mixture, thus causing water to rush out of the cell.

Here is a photo of a plasmolyzed plant cell. As you can see, it is lacking water, due to the hypertonic environment.