Demonstrating Osmosis Though a Semipermeable Membrane

DEMONSTRATING OSMOSIS THOUGH A SEMIPERMEABLE MEMBRANE

Introduction: Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. Osmosis is the diffusion of water through a semi-permeable membrane. In order for water and simple nutrients to get into your cells, the nutrients must diffuse through the cell membrane. However, not everything can diffuse through the cell membrane because it is selectively permeable. Therefore, only certain molecules are able to go in and out. Water, oxygen, carbon dioxide, and lipid soluble molecules are the most common molecules to diffuse across a phospholipids membrane. In general, the smaller molecules diffuse freely through the membrane, while the larger molecules do not, although there are exceptions to this generalization. Water will always diffuse across the membrane because it is a small molecule. Dialysis tubing is also selectively permeable because it has very small pores in it. It only allows small molecules to diffuse through the pores, large ones cannot. In contrast to the cell membrane, it does not contain any carrier or channel proteins to help facilitate diffusion. It is commonly used in kidney dialysis machines to help filter wastes out of the blood stream of people with kidney problems. In this lab, you will use dialysis tubing to demonstrate diffusion and osmosis through a semi-permeable membrane.

Pre-Lab Questions: Complete your pre-lab question before beginning this lab.

Materials: Dialysis tubing Balance Indicators for organic 2 pieces of string 3 test tubes macromolecules: Small beaker or jar Hot plate 1. Iodine- starch Glucose solution Pipettes for each solution 2. Benedict’s reagent- Starch solution monosaccharides Protein solution 3. Biuret reagent- protein

Procedures: I. DAY 1- 1. Obtain a piece of dialysis tubing from one of the containers. Carefully run water over the dialysis tubing, rubbing it between your thumb and forefinger to separate the sides. Be careful not to tear the tubing. 2. Once the tubing is open at both ends, tie one end with a piece of string about 1 cm from the end of the tubing. See Figure 1. 3. Carefully pipette 3ml of each solution (to the 3 mark on the pipette), glucose, starch and protein, into your dialysis tubing. Be sure to leave enough room at the top to be able to tie the tubing shut. Do not mix up the pipettes with the stock solutions. Only use the pipettes provided at each stock solution. 4. Begin tying the dialysis bag closed. Before the bag is fully closed, squeeze out as much air as possible from the dialysis tubing, and then fully tie the bag shut. See Figure 2. 5. Gently rinse the dialysis tubing with water, and then dry with a piece of paper towel. Double check to make sure that either end of the dialysis tubing is not leaking by placing the tubing on a fresh piece of paper towel and checking for wet spots. If it is leaking, retie the knot or obtain a new piece of string to tie over the old one. 6. Weigh the tubing and record your data in Table 1. 7. Fill a small beaker or cup full of tap water, then add 5 drops of iodine to the beaker or jar. Add your bag to the beaker leaving the string accessible. Place the jar aside and we will check it the next day. See Figure 3. II. DAY 2- 1. Remove the dialysis tubing from the beaker and gently rinse it with water. 2. Weigh the dialysis tubing and record your data in Table 1. 3. Pipette out 3ml of the water from the beaker into each of the three test tubes. Try to take the water that was near the dialysis tubing. 4. Use the three test tubes to test for each organic macromolecule, glucose, starch and protein. Only use one test tube for each test. a. Test the water using the three indicators and record any color changes that you observe in Table 2. Be careful with the indicators, they will stain your skin and clothes, or may cause burns. b. Be sure to label your test tubes before beginning the tests. c. Use the following amounts of each indicator 1. Iodine- add 3 drops and mix gently 2. Benedict’s reagent- add 6 drops and place the test tube in a hot water bath for 10 to 13 minutes. 3. Biuret reagent- add 4 drops and mix gently d. Record your results in Table 2. e. Compare your results with those from the positive indications on the pre-lab f. Determine which macromolecules passed through the semi-permeable membrane and record you results in Table 2. 5. Wash all the test tubes and beaker with soap and water and properly discard the dialysis tubing into the trash.

Figure 1 Figure 2 Figure 3