Life Science Curriculum 8-12

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Submitted by: Liane Campbell

Position: N/A

Institution: Azusa Pacific University

Title of Experiment: Passive Transport with Dialysis Tubing

Materials Needed:

250 ml beaker (1)
50ml beakers (2)
petri dishes (4)
pipettes (2) ie. medicine droppers
dialysis tubing (approximately 10 cm)
string(approximately 10 inches)
MATERIALS NEEDED FOR ENTIRE CLASS (approximately 25):
salt (NaCl) solution (approximately 1 liter)
starch solution (approximately 1 liter)
silver nitrate (Ag NO3) solution (approximately 200 milliliters)
iodine (I) solution (approximately 200 millililters)
distilled water (approximately 10 liters

Scientific Background of Experiment:

Passive transport occurs when substances diffuse across biological membranes. Diffusion is the spontaneous tendency of molecules to spread apart and move from areas of high concentration to low concentration. The spreading of a drop of food coloring in water is a good example of diffusion. The difference between diffusion and passive transport is that passive transport involves biological membranes, which can be found in virtually all living organisms. Biological membranes are selectively permeable, which means that some substances can pass through the membrane and some cannot. Passive transport does not require energy, distinguishing it from active transport, which does require energy (Campbell, 1990).

The iodine and the silver nitrate used to test the samples from inside and outside the tubing indicate the presence of specific molecules. Iodine turns a deep blue-black color in the presence of starch. Silver nitrate will turn cloudy and will form a slight precipitate when it is exposed to salt. The sample from outside the tubing that was mixed with iodine will show no color change. Granted, the reddish yellow color of the iodine will give the solution a golden tint, but no significant color change will occur. The sample from inside the tubing that was mixed with iodine will show a noteworthy color change. The entire sample will turn a deep blue-black color, indicating the presence of starch. This color will begin at the place where the iodine was dropped and spread throughout the rest of the sample. The sample taken from outside the tubing that was mixed with silver nitrate will show a definite color change. The solution will become cloudy, first localized around the place where the silver nitrate was dropped, but gradually spreading to the rest of the sample. One will be able to observe a slight whitish solid in the sample. This solid is known as a precipitate. The sample taken from inside the tubing will also show the same color change and formation of precipitate.

The different color changes observed in the four samples will tell the student important information about the diffusion of salt molecules and starch molecules through dialysis tubing, a biological membrane that is selectively permeable. The starch and salt solutions were at high concentrations inside the tubing and at low concentrations outside the tubing. Therefore, the natural tendency of these solutions is to move to the outside of the tubing. However, the selectively permeable membrane kept the starch molecules from moving out of the tubing. This was supported by the absence of starch outside the membrane. The salt molecules, however, were able to pass through the membrane. The reason the salt molecules can pass through the membrane and the starch molecules cannot is the relative size of the molecules. Salt molecules are much smaller than starch molecules and can pass through more easily. Starch molecules are made of many glucose molecules attached to each other. Thus, they are quite large molecules in contrast to the relatively small salt molecules. The smaller salt molecules pass through the membrane easily, but the larger starch molecules cannot pass through the membrane. For this reason, salt molecules could be found on both sides of the membrane whereas starch could be found only inside the tubing.

Methodology:

1. The first step of this lab is to prepare the solutions necessary to perform the experiment. The salt solution can be made by adding half of a cup of table salt (113.4 grams) to 1 liter of distilled water and dissolving the salt. The starch solution can be made in the same manner, by mixing a quarter cup of cornstarch (56.7 grams) with 1 liter of water and dissolving. The silver nitrate and the iodine solution can be purchased in aqueous form and should not have to be prepared.

2. Each group of students should obtain a length of dialysis tubing that is approximately 10 cm and prepare it by moistening it with cold tap water and rubbing it until the walls of the tubing separate. One end of the tubing should then be folded up, twisted, and tightly tied off with string.

3. Equal amounts (approximately 10 ml) of the salt and starch solution should be poured into the tubing and the other end tied off. The tubing should now resemble a sausage-shaped bag filled with fluid with both ends tied off.

4. The filled tubing should be placed in a 250 ml beaker that is filled with 220 ml of distilled water and be allowed to sit undisturbed for 20 minutes.

5. After 20 minutes have passed, the students should test the solutions inside and outside the tubing. First, small amounts of the solution from inside the tubing should be poured into two separate petri dishes. Next, small amounts of the solution from outside the tubing should be poured into two other petri dishes. Each group of students should now have 4 petri dishes&emdash;2 with solution from inside the tubing and 2 with solution from outside the tubing. Three drops of iodine solution should be added to one petri dish with solution from inside the tubing and three drops of iodine solution should be added to one petri dish with solution from outside the tubing. Three drops of silver nitrate solution should be added to the two other samples from inside and outside the tubing, respectively.

6. The contents of each petri dish should be agitated slightly and the color changes observed.

Misc. Helpful Information/ Hints/ Suggestions:

Students may have difficulty tying off the dialysis tubing as it can be slippery. Working with a partner for this step can make this process easier. The silver nitrate and the dialysis tubing can be purchased from science suppliers. These companies have a large selection of scientific supplies and supply public, private and home schools with materials for experiments and will mail specimens and chemicals directly to an individual's home. Iodine can be purchased at local drug stores as well as through scientific supply houses.

Scientific Suppliers:

Home Training Tools
2877 Buffalo Horn Dr.
Laurel, MT 59044
406-628-6614
FAX 406-628-6454

Blue Spruce Biological Supply
701 Park St. #E
Castle Rock, CO 80104-3114
303-688-3396

WARD'S Biology
Box 92912
Rochester, NY 14692-9012
1-800-962-2660

REFRENCES:

 

Campbell, Neil A. 1990. Biology. The Benjamin Cummings Publishing Company, Inc.,

Redwood City, CA, pp.163-164

 

Moeller, Rick. "Passive Transport Lab." AP Biology. Loveland, Colorado. 9 Nov. 1998.