Diffusion, Osmosis and Cell Membranes
The following is a list of instructions for obtaining and preparing the materials needed for the activities in this unit. The list of materials can be found in the student directions. Most of the materials can be obtained from scientific or biological supply companies. Exceptions and alternatives for some materials are noted below.
Osmosis in Elodea Cells
Elodea can be obtained from a local pet store or aquarium supply store. For the 5% salt solution, dissolve 10 grams of sodium chloride (table salt) in 190 mL of distilled water. Adjust the proportions for the 10% solution. Dispense these solutions in clearly labeled 25-mL plastic dropping bottles.
Small plastic aquaria or terraria (approximately 25cmx40cmx25cm) or large plastic food containers (approximately 4 L capacity) can be used. The changes in the water level are most obvious if clear containers are used. Alternatively, 1- or 2-liter beakers can be used. Provide paper cups that are sized so that they may be dipped easily into the larger containers. Again, small beakers may be used instead.
Factors Affecting Diffusion
Polar-coordinate graph paper may be obtained from
a bookstore or office supply company. Cool several liters of water in
a refrigerator overnight, and provide several liters of warm water by
heating it on a hot plate to approximately 40oC.
Prepare 250 mL of 1M HCl by dissolving 22 mL of concentrated
hydrochloric acid into 228 mL of distilled water. Dispense into clearly
labeled 25 mL dropping bottles for student use. Prepare 250 mL of NH4OH
by dissolving 67 mL of concentrated ammonium hydroxide into 183 mL of
distilled water. Again, dispense into clearly labeled small plastic dropping
bottles for ease of use. Make sure that the students wear aprons and goggles
when handling these materials, and instruct them in proper safety considerations.
The tubing for the osmosis apparatus will have to be prepared. Cut two pieces of 1/2 inch tygon tubing (available at aquarium supply stores). Fire polish the ends of a 10 cm piece of 1 cm glass tubing. The students should then be able to assemble the apparatus on their own. Remind them to wet the glass tubing before inserting it into the tygon tubing.
Prepare 1 inch dialysis tubing by cutting it into 3 cm pieces, then slitting them along both sides to make rectangular pieces. The should soak in water for 15-30 minutes before the lab begins.
The students should prepare the starch solutions. Assign concentrations of 0, 2, 5, and 10% to the different groups. Advise the students to mix the starch with a very small amount of water to form a paste, then add the balance of the water to dilute. Alternatively, prepare a concentrated starch solution and have the students dilute it to the proper concentration. This will require some explanation, and may be more difficult for the students to understand. Be sure to use a soluble starch such as potato or corn starch.
Lugol's iodine is available from the supply companies, and works well as an indicator of starch. It is toxic, however, and the students must be careful when using it.
Advise the students to pour the solutions into the tubes very carefully, so as not to trap air bubbles, and to mark the initial liquid levels and the time.
Osmosis and Blood Cells
Whole blood may be obtained from a slaughterhouse or veterinarian. Physiological saline may be purchased or prepared using recipes from The Sourcebook for the Biological Sciences.
The University of Arizona
Department of Biochemistry and Molecular Biophysics
General Biology Program for Secondary Teachers