Lab 2 Diffusion Osmosis Lab Report v2

1 | P a g e This activity is modified from Carolina Biological Supply Company ® Diffusion and Osmosis Kit (Item# 746410) Lab 2 – Diffusion and Osmosis Objectives • Use dialysis tubing to model diffusion across the cell membrane • Investigate the influence of solute concentration on osmosis Background to Activities A (Diffusion) and B (Osmosis) The cell membrane is a cell's interface with its surroundings. In one sense, this membrane must function as a barrier: it must keep together in one bundle the enzymes, DNA, and metabolic pathways that make life possible. The cell membrane must also function as a gateway: waste products must be discharged through it and essential materials (oxygen, water, etc.) must enter through it. A membrane that allows some molecules to pass through while blocking the passage of others is said to be semipermeable . Molecules pass through the cell membrane either through processes that require the cell to expend energy ( active transport ), or through processes driven by the kinetic (thermal) energy of molecules ( passive transport ). In these lab activities, you will investigate the passage of materials through a semipermeable membrane by passive transport. The membrane you will use, dialysis tubing, is semipermeable because it has submicroscopic holes through it. Molecules are in constant random motion. By chance, a molecule's motion may move it toward the membrane (Figure 1). If it collides with the membrane wall, it rebounds. If its motion takes it toward a pore, it may either pass through the pore, or it may rebound, depending upon the size of the molecule relative to the diameter of the pore. Molecules that are small enough to pass through the pores can pass through in either direction. Notice that on one side of the membrane solute molecules have displaced some of the water molecules. Thus, there is a higher concentration of water molecules on the opposite side of the membrane. More water molecules are available to collide with the membrane on the side having the higher concentration of water. Thus, although water molecules will move in both directions across the membrane, more will move from the side having the higher concentration to the side having the lower concentration. The movement of molecules from areas of higher concentration to areas of lower concentration is called diffusion . The diffusion of water molecules across a semipermeable membrane is termed osmosis . A process that depends upon random motion might seem inefficient, but so many water molecules are involved and they move so fast, that it is estimated that a red blood cell floating in blood plasma gains an amount of water equal to 125 times its own volume every second. It also loses the same amount of water each second, all by osmosis. This occurs because the concentration of solutes in the blood plasma is the same as the concentration of solutes in red blood cells. Solutions that have the same solute concentration are isotonic . If we took a sample of whole blood and added salt to the plasma, increasing its solute concentration, the plasma becomes hypertonic to the solution in the red blood cells, and the cells lose water and shrink. If we add water to the blood plasma, decreasing its solute concentration, the plasma becomes hypotonic to the solution in the red blood cells. The cells gain water, swell, and may even burst.

2 | P a g e This activity is modified from Carolina Biological Supply Company ® Diffusion and Osmosis Kit (Item# 746410) Activity A Diffusion Materials Dialysis tubing, plastic cup, glucose/starch solution, distilled water, iodine potassium iodide (IKI) solution, dropping pipet, glucose test strips, funnel. Caution : IKI solution can irritate the skin, mouth, and eyes, and can stain skin or clothing. Introduction In Activity A, you will explore the diffusion of different molecules through dialysis tubing, a semipermeable membrane. You will use glucose test strips to check for the presence of glucose and IKI solution to test for the presence of starch. IKI reacts with starch to give a dark blue, almost black color. When IKI reacts with starch, it becomes part of the starch molecule and is removed from solution. Procedure 1. Pour 160 mL of distilled water into a plastic cup . Using the plastic pipet, add approximately 4 mL of the iodine potassium iodide (IKI) solution to the water and mix by gently swirling the cup. 2. Record the initial solution color (in the cup) in Table 1 on page 5. 3. Dip the reagent end (the end with the green square) of a glucose test strip into the solution inside the cup. [ Do not touch the reagent end of the strip with your hands .] Wait for about 30 seconds, and record the initial glucose test results of the solution inside the cup in Table 1. Use the + symbol to indicate a positive test result for glucose and the - symbol to indicate a negative result. Any concentration of glucose above 0 mg/dl is considered a positive test result. Use the color chart below to determine the results of the glucose test: 4. Obtain one piece of dialysis tubing that has been soaked in water. The tubing should be soft and pliable. Roll the tubing between your thumb and index finger to open it. Close one end of the tube by tying it off with a piece of string. This will form a bag. 5. Using a small funnel, pour the glucose/starch solution into the dialysis bag. 6. Dip a fresh glucose test strip into the glucose/starch solution inside the dialysis bag. Record both the initial glucose test results and color of the glucose/starch solution in Table 1 on page 5. Discard the used glucose test strip in the trash can. 7. Smooth out the top of the bag, running it between your thumb and index finger to expel the air. Tie off the open end of the bag. Leave enough room in the bag to allow for expansion. 8. Immerse the dialysis bag in the solution in the cup. Make sure that the portion of the bag that contains the glucose/starch solution is completely covered by the solution in the cup at all times. 9. Wait 20 minutes. While waiting, complete the diffusion prediction activity at the top of page 5.

4 | P a g e This activity is modified from Carolina Biological Supply Company ® Diffusion and Osmosis Kit (Item# 746410) Activity B Osmosis Materials Dialysis tubing, plastic cups, distilled water, funnel, sucrose solutions, paper towels, balance, calculator (optional). Introduction In Activity B, you will investigate the influence (if any) of solute concentration on the net movement of water molecules through a semipermeable membrane. The solute you will use is sucrose (cane or table sugar) in the following molar concentrations: 0.0 M (distilled water) 0.2 M 0.4 M 0.6 M 0.8 M 1.0 M Procedure 1. Your instructor will give you two plastic cups labeled with two different concentrations of sucrose. These will be the concentrations of sucrose that you will test. Pour 160 mL of distilled water into each cup. 2. Obtain two pieces of dialysis tubing that has been soaked in water. The tubing should be soft and pliable. Roll the tubing between your thumb and index finger to open it. Close one end of the tube by tying it off with string. This will form a bag. Do this for both pieces of tubing. 3. Using a small funnel, pour one of the sucrose solutions into one bag. Then pour the other sucrose solution into the other bag. [ Remember which bag has which concentration of sucrose ]. Smooth out the top of the bag, running it between your thumb and index finger to expel the air. Tie off the open end of the bag. Leave enough room in the bag to allow for expansion. 4. Dry the bags on paper towels and then determine their initial masses using the electronic scale on your lab bench, and record these initial masses in Table 2 on page 9. 5. Immerse the correct dialysis bag into the correct plastic cup. For example, place the dialysis bag containing 0.2 M sucrose solution into the plastic cup labeled 0.2 M. Make sure that the portion of the bag that contains the sucrose solution is completely covered by the water in the cup at all times. Wait 20 minutes before continuing to the next step. 6. After 20 minutes, remove the bags from their respective cups and dry them on paper towels. Obtain the final masses of each bag individually and record the final masses in Table 2 on page 9. 7. Calculate the change in mass by subtracting the final mass from the initial mass. Record this data in Table 2. 8. Starting on page 7, answer the questions and construct a graph.

5 | P a g e This activity is modified from Carolina Biological Supply Company ® Diffusion and Osmosis Kit (Item# 746410) Lab 2 Diffusion and Osmosis Lab Report Name: _______________________________ DIFFUSION PREDICTION : Indicate on Figure 2 the initial locations (inside or outside of the bag) of all the kinds of molecules that are available for diffusion through the dialysis membrane. Figure 2 For each of the molecules you list on Figure 2, predict in the space below their direction of net (overall) diffusion: into the bag, out of the bag, both into and out of the bag equally, or none (will not diffuse across the dialysis membrane). Give a reason for each prediction. Table 1 Solution Color Glucose Test Results Location Solution Initial Final Initial Final Dialysis Bag Glucose/Starch Cup IKI

7 | P a g e This activity is modified from Carolina Biological Supply Company ® Diffusion and Osmosis Kit (Item# 746410) Name: _______________________________ Lab 2 Diffusion and Osmosis Lab Report Analysis of Results, Activity B: Osmosis 1. Calculate the percent change in mass. Use the following formula. Record the results in Table 2.% Change in Mass = (Change in Mass/Initial Mass) x 100% 2. What does the change in mass indicate? _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ 3. Write a hypothesis that this experiment is designed to test. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ 4. What variable is being tested in this experiment? _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ 5. List one other variable (other than the one listed in your answer to #5) that could influence the outcome of this experiment. Briefly describe the method of control used for this variable. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________- _____________________________________________________________________________

8 | P a g e This activity is modified from Carolina Biological Supply Company ® Diffusion and Osmosis Kit (Item# 746410) Lab 2 Diffusion and Osmosis Lab Report Name: _______________________________ 7. On page 9, construct a line graph of the percent change in mass versus the concentration of sucrose. Title the graph and supply the following information: a. The independent variable is ____________________________________________________. b. The dependent variable is _____________________________________________________. Plot the independent variable on the x-axis, and the dependent variable on the y-axis. 8. On the basis of your data and graph, has this experiment adequately tested the variable you listed under #5? _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ 9. On the basis of your results, write a statement that expresses the relationship of solute concentration and direction of net movement of water molecules in osmosis. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ 10. In which, if any, of the experimental setups were the solutions in the bag and outside of the bag isotonic to each other? _____________________________________________________________________________ 11. When you drink a glass of water, most of it is absorbed by osmosis through cells lining your small intestine. Drinking seawater can actually dehydrate the body. How? _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________

10 | P a g e This activity is modified from Carolina Biological Supply Company ® Diffusion and Osmosis Kit (Item# 746410) Lab 2 Diffusion and Osmosis Lab Report Name: _______________________________