Diffusion, Osmosis and Cell Membranes

Osmosis

Introduction

Osmosis is a special case of diffusion in which water passes through a membrane, but other larger molecules do not. In this exercise, you will design an experiment in order to learn more about the factors that caused the changes that you saw in the Elodea at the beginning of this lesson.

Materials

Osmosis apparatus (J-shaped glass tubing (2), dialysis membrane, plastic tubing), water, starch, iodine solution, buret clamp, ring stand, iodine solution, 50-mL beakers (2).

Methods

      1. To assemble the osmosis apparatus, first soak a piece of dialysis tubing (a semipermeable membrane) in a small amount of distilled water. Cut the tubing along one side with a pair of scissors to form a flat sheet. Cover one end of the J-shaped glass tubing with the dialysis membrane. Wet the piece of plastic tubing and slide it over the dialysis membrane to hold it in place. Then slide the other piece of glass tubing into the open side of the plastic tubing to complete the unit. Finally, clamp the assembly to a buret clamp that is supported by a ring stand.

      2. Prepare the solution you have been assigned. As an example, to prepare a 5% solution of starch dissolve 5 grams of starch in 95 grams of water. Remember that water has a density of 1 gram per milliliter, so 95 milliliters of water is the same as 95 grams of water. This means that there are now 5 grams of starch in 100 grams of solution, which makes a 5% starch solution.

      3. To make the starch solution, measure both the starch and the water. Add a small portion of the water to the starch in a container to make a paste. Then slowly add the rest of the water until there is a solution with no lumps of starch.

      4. Solutions of other concentrations or with other solutes can be made in the same way. Remember that distilled water is not the same as tap water. Distilled water is pure water, while tap water has minerals and other molecules dissolved in it.

      5. Obtain two small beakers. Measure 10 mL of the starch solution into one of them. Measure 10 Ml of water into the other, and add ten drops of iodine solution. Swirl it to mix, and record the colors of each solution. Pour the contents of one beaker into the other and swirl to mix. Record your observations.

      6. Carefully pour 10 mL of the starch solution into one side of the osmosis apparatus. Measure 10 mL of distilled water into a graduated cylinder, add ten drops of iodine, swirl to mix, and carefully add the water to the other side of the osmosis apparatus. Use a grease pencil to mark the initial levels of the starch solution and the iodine solution in their respective tubes. Record the colors of the solutions.

      7. At five-minute intervals, record any changes in the liquid levels in the osmosis apparatus and any color changes that you observe. Continue until no change is observed. Record the final height of the test solution in the group data table on the board, then clean up your materials.

Observations

Prepare a data table in which to record the changes in the height of the water and the test solutions, and any color changes you may have observed. Be sure to record what happened when the iodine-water and the starch were mixed.

Prepare a data table in which to record the class data for the final heights of the test solutions. Prepare a histogram (bar graph) for the group data.

Conclusions

1. In which column did the liquid level rise, the one with the water or the test solution?

2. Were there any color changes in either column? If so, explain why you think this occurred.

3. Which test solution had the greatest number of starch molecules per milliliter of solution?

4. Based on the class data, which test solution showed the greatest increase in column height?

5. Were the answers in questions 3 and 4 the same or different? Why?

6. Which solution would be most likely to cause the greatest amount of shrinkage in Elodea cells, a 4% starch solution or an 8% starch solution? Explain your answer.