Diffusion and Osmosis

In this exercise, you will be given agar containing an indicator chemical called phenolphthalein. When phenolphthalein is exposed to the normal alkaline conditions in the agar, it will look pink. But when it is exposed to neutral or acidic conditions, it changes from pink to clear.

You will make different size and shaped agar cubes as a model for cells to study the impact of cell size and shape on diffusion rate. To make the first set of cells, measure out and cut a small cube of agar where each side measures one centimeter. Next, measure and cut out a medium cell cube with sides of two centimeters and a large cell of three centimeters on each side.

Knowing the length of the sides of your cube cells, you should now calculate their surface area using this equation. Where lower case a represents the length of the sides. Record these values in the appropriate column in your table.

Then use the same length value and this equation to calculate the volume of each cube and add these to the table. The experimental hypothesis might be that the acid will diffuse completely to the center of the small cell faster than the medium and large cells. The null hypothesis could be that the acid will diffuse to the center of the small and two larger cubes at around the same time.

Add 100 milliliters of 0.1 molar hydrochloric acid to each of the three 400 milliliter beakers to make the diffusion baths. Working in a team, have one experimenter ready with the timer and the second and third experimenters ready to drop each cube into one of the beakers. When the first experimenter says go, simultaneously drop all three cubes into their respective beakers and start the timer.

Observe carefully until one of the cubes becomes completely clear or 10 minutes have passed. Then stop the timer, remove the agar cubes from the beakers and place the cubes into a Petri dish. Make a note of which of the three cell became clear or had the smallest remaining pink area.

Then also note which cell had the most remaining pink agar. Next, in your table, calculate the surface area to volume ratio for each cell. As the cell size increases, does the surface area to volume ratio increase or decrease?

How did this correlate with your observation of the depth of diffusion into your agar cells? If cells rely on diffusion to deliver essential nutrients and molecules to the whole cell, would it be better to have a smaller or larger surface area to volume? Now, with the remaining agar, cut three rectangular shaped blocks of different sizes and record their length, width, and height.

This will test what happens when the shapes of cells are different. Calculate the surface area of your rectangular cells using this formula where length is l, width is w, and height is h. Then, calculate the volume of your rectangles using this formula.

Repeat the experiment by dropping the new shapes into the hydrochloric acid solution for 10 minutes or until one cube becomes completely clear. Then remove the cells shapes from the solutions and observe the depth that the hydrochloric acid diffused into each of these cells. And which shapes have the smallest and largest remaining pink areas not reached by the solute.

Using the surface area and volume data you recorded for your rectangular shapes, calculate the surface area to volume ratio of these cells. How do these values correlate to which cells have the most and least complete diffusion? Did you see a similar or different pattern to the one observed with the cube shaped cells?

Before beginning the experiment, add 250 milliliters of distilled water to each of four one liter beakers. Then label the beakers from one to four and then add 0.5 milliliters of iodine to the first beaker. In this experiment, the experimental hypothesis is that some of the solutes will be able to pass through the dialysis tubing membrane and others will not.

The null hypothesis is that there will be no difference in the ability to diffuse through the dialysis tubing membrane between the solutes. To prepare the dialysis tubing, remove the pieces one at a time from the distilled water bath and tie a tight knot at one end of each tube. These tubes, when filled, will act as model cells with the dialysis tubing acting like the semipermeable membrane.

Add 10 milliliters of starch solution to the first tube and tie off the open end, making sure to leave space in case the tubing expands during the experiment. Then add 10 milliliters of the sodium chloride and dextrose solutions to the second and third pieces of tubing, respectively, and tie off both tubes. Again, leaving space in case of expansion.

After adding 10 milliliters of distilled water and tying off the fourth tube, weigh each of your model cells. Record the initial weight values in grams and the colors of the starting solution in each tube in the appropriate columns of the table. After quickly rinsing the outside with tap water, place each piece of tubing in its corresponding beaker for one hour at room temperature.

At the end of the diffusion period, weigh the tubes again. Then, observe the tubes carefully, noting any color changes. Record all of these data in the table.

Next, to perform a Benedict's Reagent test for simple sugars, make a water bath by adding 250 milliliters of water to a 600 milliliter beaker and placing it onto a hot plate. Then set the plate to high. Label two new glass test tubes as H2O and dextrose, respectively.

And then use a graduated cylinder to transfer one milliliter of solution from the water and dextrose beakers into the corresponding test tubes. Then, add two milliliters of Benedict's Reagent to each tube. When the water boils, place each test tube into the water bath for three to five minutes.

After this time, note the color of the solution in each tube. Then use this key to assess whether the test is positive or negative, and record these data in the appropriate column in the table. First, look at the mass of your four dialysis tube cells at the beginning versus the end of the experiment.

Calculate the change in mass for each of the four cells and plot it onto a bar chart. Which cells did you observe demonstrated the most change? Did the cells also appear visibly different in size?

For the experiment with the starch and iodine indicator, did you notice a color change in the fluid in the artificial cell? How about the water in the beaker? Finally, in the Benedict's Reagent test for dextrose, does it appear that this simple sugar was able to pass through the semipermeable membrane of the cell into the water in the beaker?

Discuss with the class which of the molecules you think could and could not pass through the semipermeable membrane.