Comment: "Water does just slip through the membrane along with other small, polar molecules such as ethanol and urea. Larger or charged water-soluble substances will require a protein carrier. The fatty acyl residues don't repel water, but they don't attract it either. It is true that being polar, the water is more attracted to other water molecules on the same side of the membrane than to the hydrocarbon chains that are in the bilayer. When the water molecules are 'buzzing' around, some of them just slip through. Protein pores and carriers are not involved and in fact, water will diffuse across an artificial lipid bilayer, which doesn't even have proteins. It won't diffuse as quickly as if the lipid bilayer were not there, but it will diffuse rather easily. Now you mention water soluble substances, which would refer to polar and charged molecules. Some water soluble substances do require protein channels or even carriers (called pumps if they require metabolic energy). Larger polar molecules have a harder time getting through and charged molecules have an even harder time getting through, even if they are small (i.e., ions). The reason for the ions having trouble getting through has to do with the fact that they become hydrated (surrounded by water molecules whose oppositely charged ends link to the charged ion). So you can't look at the ion by itself (which may seem small), but the whole hydration sphere which is likely to be large. Ions generally require membrane proteins to get through as do larger polar molecules."
—Bob Goodman, Hunter College High School, New York City. 11/3/99
Equipment and Supply Modifications
Tip: "For dialysis tubing, I have used the cheapest plastic sandwich baggies. They work pretty well."
—Jo Ann Burman, Andress High School, El Paso, Texas. 9/9/99
Tips: "Two tips that I would like to pass along. I'm a (fairly) new teacher and these last two discoveries have made things a little easier. First, I buy spray starch, like you would use for ironing. Gone are the days of heating starch to dissolve it. Just keep spraying the starch into a flask of water until you think there is enough. I never measure and it works great! Second, for a quickie Benedict's test result, stick it in the microwave for about two to five seconds; again the results are great. Way better than test strips. I hope I have shaved at least 10 minutes off of your lab prep time. Every minute counts!"
—Sara Sagmeister, Park Ridge, Illinois. 10/6/99
Tip: "Try using purple onion for the lab—it works—just make sure they have some pigmented cells. The differences with dH20 and the salt solution are great. If you don't want to mix the salt solution and have a saltwater aquarium, just get a small beaker of water from the tank. It works well."
—Bobbie Hinson, Providence Day School, Charlotte, North Carolina. 10/14/99
Tip: "I use a plant called Rhoeo discolor instead of onion to do the last part of the lab. Its leaves are purple on the underside and the purple epidermal layer easily peels away from the rest of the leaf. It's very easy to grow (seems to thrive on skillful neglect!) in a pot on a windowsill. It should be available at a garden center. It's a very common plant—used for ground cover around North Texas—I wish I could tell you the common name; it reminds me of wandering Jew, but with larger, less hairy leaves."
—Marcia Sloan, Cleburne High School, Cleburne, Texas. 10/15/99
Question: "I went to the pharmacy yesterday to buy glucose test strips, and there were many possibilities. I didn't find either of the brands listed in the Teacher's Guide. Does the brand matter? Do you buy the urine glucose test strips, or the blood glucose test strips? They are very expensive, and I didn't want to buy the wrong kind."
Answer 1: "Ask your pharmacist to save out-dated urine test strips for you. Often they will give you a price break if you tell them you are a teacher, and if the test strips have expired."
—Franklin M. Bell, St. Mary's Hall, San Antonio, Texas. 11/3/99
Answer 2: "I always get my glucose test strips from Frey Scientific. They work fine for dialysis testing and don't show a false positive in the presence of iodine. I also use them for simulated urinalysis and get much better results than the pharmaceutical dipsticks."
—Joni Driscoll, NW Cabarrus High, Concord, North Carolina. 10/10/99
Tip: "Make a note for next year: use Benedict's solution instead of the test strips to test for the presence of sugars outside of the bag. Just add some of the liquid in the beaker to a test tube that has an equal amount of Benedict's. Mix and heat gently in a water bath for 5 to 10 minutes. You should get a positive test—blue Benedict's will turn rusty (orange/red) if glucose is present. I did the lab last week and got great results by using the Benedict's."
—Carmen Austin, Wharton High School, Tampa, Florida. 10/5/99
Question: "Is there some alternative for the core borer method of getting potato samples?"
Answer 1: "Instead of using a cork borer for making cylinders of potato, try using a French fry cutter to make many uniform pieces of raw potato at one time. One of my students thought of that!"
—Marcia Fischer, Desert Mountain High School, Scottsdale, Arizona. 10/24/00
Answer 2: "... If you don't have a French fry maker, you can just slice them yourself. I gave up on the cork borer years ago, and simply take a good kitchen knife and cut the potatoes into roughly uniform 'sticks' of the approximate size called for. This works, and I consider this one of the more reliable labs. It also reinforces my conclusion (which I'm always telling the kids) that learning how to cook is great training for the lab—many of the skills transfer over!"
—Leslie Haines, Walter Williams High School, Burlington, North Carolina. 10/28/00
Answer 3: "I have found that it is very difficult to do this lab as it is written. I use thin slices of potato instead of the cores, and try to end up with about 20 g. of potato. Once I started doing it this way it works like a charm. I think this is for two reasons: (1) You have a much larger mass of potato and so you have much smaller percentage of errors in massing; (2) you don't have as many little pieces of potato dropping off and reducing the mass."
—Ed McDaniels, Grandville High School, Grandville, Michigan. 10/19/00
Tip: "I used a French mandolin to slice potato sticks for the 1C lab and got great results (R squared = 0.996). For one of my teams I used the 'waffle cut' technique and produced 'waffle chips,' which I then 'punched' with a small dish to produce uniformly sized 'waffle discs.' The result was that the discs in the hypotonic solutions expanded and the discs in the hypertonic solutions contracted in an exaggerated manner only to be borne out by the data. It seems that the increased surface area was responsible."
—Harry Padden, Washington Twp High School, Sewell, New Jersey. 11/17/00
Trouble Shooting and Cleanup
Tip: "While running the osmosis/diffusion lab today, my students made an interesting discovery. The iodine solution reacted with the glucose test strips (Carolina Biological osmosis lab replacement kit) and turned a color indicating a positive glucose reaction. The students wanted to know how could they determine if glucose diffused out of the dialysis bag since the iodine in the beaker solution already gave them a positive test. Likewise, at the end of the experiment when the iodine had diffused into the dialysis bag, they wanted to know how they could detect that glucose had left (as indicated by a lessened color reaction with the glucose test strips). We ended up running a dialysis bag without iodine so we could detect the glucose by itself, but if you follow the lab as written, you might need to consider this problem."
—Jeff Smith, Indiana Academy, Muncie, Indiana. 10/5/99
Conducting Lab Using Probes and Computers/Calculators
Question: "Does anyone have a 'simpler' or modified osmosis lab procedure? One that can be done in a single day?"
Answer 1: "Use a large, cork borer and a large (long) potato. Using a twisting motion, insert the cork borer all the way into the potato lengthwise. Since you must do this to several potatoes, the idea is to use the same size borer on each potato. You must not go all the way through (or even close to through) the potato. If you pull out the borer, the cylindrical piece of potato will stay attached to the potato. However, if you place a scalpel in the back end of the potato borer and twist both the borer and the scalpel together, the cylinder of potato will twist and finally snap. Pull out the borer and, just like popping a bottle of champagne, the cylinder of potato will come out. You now have a hole in the potato, and if you repeat this with several potatoes, you will have UNIFORM size holes in the potatoes. Rinse each out with tap water to get the excess starch grains, which were released from this rough procedure, out of the holes. The potatoes are now ready. Fill each up with a different concentration of sucrose: 0, 0.2, 0.4, 0.6, 0.8, and 1.0. Using a #2 stopper with an inverted pipette, gently twist the cork into the potato. Don't force it too hard or the potato will tear (and thus be useless). You should get an air tight connection. I usually put them in a 600 mL beaker and tape the cork down securely. Attach a Vernier gas-pressure sensor (CBL or computer based) to the pipette and measure the pressure change over a 20 to 30 minute period. The slope (change in pressure over change in time) is a measure of the rate of osmosis. By graphing concentration vs. slope you can determine the concentration at which the slope is 0 (i.e., where the water potential of the potato cells equals the water potential of the sucrose solution). I haven't tried this without probes, but you can also hook it up with very narrow bore graduated pipettes. The procedure has some hitches that I am still trying to work out. I can't say it works cleanly all the time, but it doesn't take 24 hours. I have some ideas for cleaning it up."
—Bob Goodman, Hunter College High School, New York City. 10/23/00
Alternative Lab Ideas
Egg Osmosis Lab
Question: "I have heard of teachers using eggs to demonstrate osmotic principles. Does anyone have any lab activities or demos dealing with this?"
Answer 1: "I have a wonderful reference to this lab from the Journal of College Science Teaching, Nov. 1985. I believe that this is a publication of NSTA? It is called 'Osmosis and the Marvelous Membrane' and deals with using decalcified eggs to demonstrate osmosis. I have my kids decalcify eggs in vinegar for 48 hours, and then I give them four unknown solutions (distilled, .5M sucrose, 1M sucrose, and 2M sucrose). They mass their eggs, put in solutions, and mass them again every 10 or 15 minutes for 1.5 hours. The lab works great! It also will work in a 45-minute period if the kids come back at lunch or later to mass them after class. Then they plot percentage change in mass vs. time. They also have to calculate the molarity of the egg; it usually comes out about .8M.
The article I referenced above recommends using glucose solutions, but I have found that sucrose works as well and is a whole lot cheaper. The article also says that NaCl solutions give odd results, possibly due to the salt ions modifying the membrane somehow.
Just be sure to have extra eggs on hand as there is always that one student who ends up with egg on his hand. I placed three dozen eggs in a gallon of vinegar overnight, and replaced the vinegar the next day. The eggs were ready to go on day three."
—Franklin Bell, St Mary's Hall, San Antonio, Texas. 10/20/99
Answer 2: "Another side trip with the eggs—once you have finished the salt or sugar treatments—is to place them in different types of dye overnight:
- methylene blue
- Rit dye
- food coloring
Each has a different diffusion rate (diffuse to different depths within the egg)—boil and slice in half to see the differences. Not recommended for eating, however!"
— Pam Tidswell, Rancocas Valley Regional High School, Mt. Holly, New Jersey. 10/19/99
Answer 3: "For 30 years I have used the egg lab as a super demonstration or as an individual activity. It drives home the action of our own cells with a familiar animal cell the students can see. Really simplistic directions follow. You add the chemistry, pressures, etc. Soak/submerge a raw hen's egg in white vinegar (cheapest store variety works best) for 24 to 48 hours to remove the calcium carbonate shell. The shell will evidence corrosion immediately with many small bubbles forming around surface—this allows time for a good discussion of basic chemistry and the actions of acids and metal compounds. The membrane may be covered by soluble calcium salts at the end of this time—wash gently to remove, which will allow you to see translucent membrane. At this point you may realize the need for preparation of some backup eggs! Pat dry and mass. You may want to take some other measurements such as circumference, volume by water displacement, etc. Place the egg in a known volume of distilled water (150 mL) in a clean 250 mL beaker. Again collect whatever data you feel appropriate or have students design their own lab (an opportunity for constructivism and the 3P's). Wait 24 hours—overnight. Carefully remove egg; pat dry and mass gain is water. Compare to volume lost in beaker. Osmosis through selectively permeable membrane. You can have students break egg into a petri dish; evaluate consistency of white. An alternative or companion lab is to take a second egg. Remove shell and mass. Place in 100 percent white Karo syrup (liquid fructose). Allow to sit overnight. Remove egg, wash quickly, dry, and mass. Compare new volume in beaker and mass lost. If students are careful, you should notice some layering due to density differences. Results here are fairly dramatic and can be reversed by placing the egg into distilled water. You and your students can make this lab as involved or simple as you choose. It is also a good place to review the structure of the amniotic egg."
—Donna M. Gilbertson, Beloit Memorial High School, Beloit Wisconsin. 10/18/99