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Semi-Permeable Membranes
An "Egg"cellent Model (No Yolk!)

From the Spring 1996 sciencelines

Yes, biologists are interested in particles, too!

Submitted by Pat Franzen, teacher at Madison Junior High School, District 203, Naperville IL

Background: In all living organisms, vital fluids and particles of solids too small to see must be able to cross membranes. These same membranes must be capable of excluding other materials. As organisms metabolize food, grow, and perform all the functions of life, semi-permeable membranes provide access to and protection from countless particles of nature. Many students (and some math teachers) have considerable difficulty conceptualizing the process of particles too small to see being allowed or obstructed passage through the walls of capillaries, intestinal villi, cell membranes, and other "biological fences." This model provides a concrete example.

Grade Level: This activity is adapted from Science Plus, an STS middle school curriculum developed by Holt, Reinhart, and Winston. It could easily be modified for any grade level.

Materials: For each lab partnership:

Procedure: 1. Shake 5 ML cornstarch and 5 ML dextrose in test tube to mix. Add to 100 ML HOT water. Stir well
2. Crack egg in half, saving large end containing air sac. (I did this for the students while they set up their experiment. I salvage the "innards' in a clean container and made lots of souffles!") Do not break eggs in advance as membranes may dehydrate and crack. Trust me!
3. Using a straight pin, CAREFULLY pierce the large end of the egg shell. Do not break the top end of the air sac membrane. (Yes, the kids really can do this! If the pin does not work, have the tiny screw driver available. Prior sanding the end of the egg also helped.)
4. Using clean eye dropper, place 5 - 10 ML of water into the egg shell. Float the shell "boat" in the sugar/cornstarch mixture.
5. After 15 minutes, use clean eye dropper to divide water in the shell and place equally into the small test tube.
6. Test one half of the liquid with iodine. (A blue/purple color indicates starch.)
7. Add 8 drops of Benedict's solution to the other half of the "egg water."
CAUTION: Benedict's solution is a corrosive. Goggles should be in place! Heat the liquid in a hot water bath - GENTLY! A yellow/orange/red color indicates a positive reaction for sugar. (This may take a few minutes.)

I demonstrated positive reactions for the class using mixture.

Teacher Note: Due to time constraints, I had students complete steps 1-5 on one day and stopper the tubes. We tested and discussed on the next day. The only problem was in labeling and storing 100+ test tubes overnight.

Discussion Questions:

1. In which test tube did a positive reaction occur? (Sugar)

2. What property may have contributed to a positive reaction with the sugar but not the starch? (sugar molecules may be smaller and, therefore, able to pass through the membrane. Starch molecules may be too large to pass through. Different molecules, though microscopic, may differ greatly in size.)

3. What may be the purpose of the air sac membrane? (Let air in for the embryo to utilize. Allow exchange of oxygen and carbon dioxide. Keep moisture in the egg so the embryo will not dehydrate.)

4. Name some of the other membranes that organisms, including man, use to allow some substances in while keeping other materials out. (Plant cell walls, vacuoles, animal cell membranes, intestinal villi, capillaries, etc.)

5. Man metabolizes starch and sugar. How can we absorb starch into our systems if the molecules are too large to pass across membranes? (Springboard for discussion into digestion! Watch for nextmonth's activity!)

6. If you tested for starch and found a positive reaction, what is you justification? (Pierced shell membrane provided a "door" for the larger particles.)

7. How would you justify no reaction to sugar? (Human error. Failure to follow directions, not heatingsolution long enough.)