07 Lab 5 Movement of Molecules

Lab 5: Movement of Molecules 21 5. After 30 minutes, remove the bag from the beaker, blot it dry, and weigh it again. Enter this final weight in the space provided below. Any change in the weight of the bag is due to water entering or leaving by osmosis. Initial weight of bag = ___________g Final weight of bag = ___________g 6. Now, obtain 4 clean green-coded test tubes. Use a clean 3 mL pipette and add 3 mL of the solution from the beaker to each tube. 7. Perform the following four biochemical tests on both the before ( white ) and after ( green ) beaker samples. Record the results in Part C Results Table (3 rd page of Lab Report). a. Benedict's test  Add 3 mL of Benedict's reagent to one “before” tube and one “after” tube. Heat in a boiling water bath for one minute and interpret the results as (+) or (–) for glucose. b. Iodine test  Add 5 – 10 drops of iodine to one “before” tube and one “after” tube. Interpret the results as (+) or (–) for starch. c. Biuret test  Add 10 – 15 drops of Biuret solution to one “before” tube and one “after” tube. Interpret the results as (+) or (–) for protein. d. Silver nitrate test  Add 2 – 3 drops of silver nitrate (AgNO 3 ) to one “before” tube and one “after” tube. AgNO 3 will react chemically with Cl - of NaCl to form a cloudy white precipitate. Interpret the results as (+) or (–) for NaCl. _____________________________________________________________________________________________________________________ D. THE EFFECT OF MOLECULAR WEIGHT ON THE RATE OF DIFFUSION Problem 3: What effect does the molecular weight of a substance have on its rate of diffusion? In this activity, the effect of molecular size on the rate of diffusion through agar will be determined. Three substances of different molecular weights will be compared: • methylene blue - 320 amu • safranin - 351 amu • potassium permanganate - 158 amu Predictions: 1. Which of the 3 substances will diffuse fastest? ________________________________ 2. Which of the 3 substances will diffuse slowest? _______________________________ Procedure: 1. Obtain a Petri dish containing agar. Using a short soda straw, punch three holes in the agar at equal distances and widely separated from one another. If the plug remains in the agar, it can be removed with a toothpick. 2. Carefully, place the tip of the eye dropper into a hole and add one drop of each of the following solutions: potassium permanganate (KMnO 4 ) safranin methylene blue Note the color of each. 3. Wait 30 minutes. 4. After the time has elapsed, turn the dish over to view the bottom . Note that there are colored zones around each hole where the substances have diffused out in all directions. The rate of diffusion will be determined by measuring the diameter of the diffusion circles. Use a ruler to measure each diameter in millimeters. Record this information in the lab report. E. OSMOSIS IN PLANT CELLS Unlike animal cells, plant cells have a non- living cell wall exterior to the plasma membrane. It reinforces the plasma membrane so that it is able to withstand internal pressure without bursting. If a plant cell is immersed into a hypotonic solution,