Process of Science: Using Diffusion and Osmosis OBJECTIVES: 1. To understand one way to approach the process of science through an investigation of diffusion and osmosis. 2. To explore how different molecules move through a semi-permeable membrane. 3. To define homeostasis and understand how diffusion and osmosis through semipermeable membranes contribute to this process in cells.. I. BACKGROUND MATERIAL In class we have discussed that science in practice is integrated and includes many components to facilitate discovery and understanding. The standard scientific method is only one way in which science is practiced. Many of the terms utilized by the scientific method are applicable to all approaches. Today in the laboratory we will use diffusion and osmosis to understand these terms. In addition, understanding the processes of diffusion and osmosis will be important for our later discussions of evolutionary adaptations and plant/animal physiology. All cells are separated from their external environment by a lipid membrane. Cell membranes regulate the molecular traffic in and out of the cell. As a result of their structure, membranes are selectively permeable, meaning that certain molecules pass through the membrane unaided while other molecules are blocked or must use special protein pores (holes) to pass through the membrane. In this lab, we will use dialysis tubing to mimic the cell membrane, and a set of solutions to model environmental conditions cells encounter. NOTE: Diffusion and osmosis only operate efficiently over short distances, so a cell is limited in the size it can attain. This is a basic consideration in the evolution of life. TERMS Before coming to lab you should review the terms of the scientific method: observation, question, hypothesis, method, prediction, result and conclusion. In addition, here are a few other terms that are relevant. Diffusion: The random movement of molecules or particles, resulting in the net movement of a substance from a region of high concentration to a region of low concentration. 1
Osmosis: The diffusion of water across a semi-permeable membrane. Solute: A substance that is dissolved in a solution. Solution: A homogeneous, liquid mixture of two or more substances. Semi-permeable Membrane: A membrane that allows some molecules, but not others, to pass through it. Homeostasis: The inherent tendency in an organism to maintain physiological balance. Review of Scientific Method 1. Make observations about the natural world. 2. Ask questions about those observations. 3. Formulate a reasonable testable hypothesis to explain observations. 4. Collect data to test the hypothesis. 5. Evaluate the hypothesis by comparing it to the collected data. II. FORMING HYPOTHESES Observations / Previous Data: All living cells are surrounded by lipid membranes. These lipid membranes are semi-permeable. The type of molecule passing through the membrane is determined by the types of pore proteins present. This semi-permeability establishes concentration gradients across the membrane, altering diffusion and osmosis rates. (NOTE: Dialysis tubing is a membrane that is selective based on the size of solutes, and can be used as a model for a cellular membrane.) Questions that result from the above information: 1) How does the size of a solute affect diffusion through the dialysis tubing? 2) How does the concentration of solute affect the rate of diffusion and osmosis? 3) What happens if one solute can pass through the membrane, but another cannot? Hypotheses to be tested in the lab: 1. Large starch molecules are too large to pass through the semi-permeable membrane, but glucose and iodine are small enough to pass through the membrane. 2. The concentration of solute on either side of the membrane has no affect on the rate of osmosis because it does not affect the movement of water. 2
Experiment: We will utilize solutions of starch, glucose, sucrose, and iodine to test hypotheses 1 and 2, using dialysis tubing as a model of the membrane of cells. Below, we lay out basic techniques that you can use to structure experimental protocols. Results: Record your results and compare them to the hypotheses. Do your data support or reject each hypothesis? Do your results differ from/ agree with your classmates? What inferences can you make from your own results and from compiled results? III METHODS Hypothesis 1: Large starch molecules are too large to pass through the semipermeable membrane, but glucose and iodine are small enough to pass through the membrane. Before you can make predictions about the results of your experiment, you need to know a little chemistry. When glucose or starch is dissolved in water they are colorless (or milky white). Starch and iodine react when in solution and turn the solution blue. Additionally, we can test for the presence of glucose in solutions with urinalysis test strips. In this experiment, you will fill a dialysis tube with starch and glucose and submerge it in a beaker full of water and iodine solution. Think about this experimental design, and make predictions about what results you might expect to see from your experiment, based on the hypothesis you are testing. Be sure to record your observations in Table 2. Write your predictions based on these methods: 3
Table 1. Part I Materials Materials needed: Amount needed per group: Beaker 1 Potassium iodide solution (IKI) A dropper full +/- Glucose Test Strips 3 Dialysis tubing 125mm Glucose/starch solution 10-15ml Clamps 2 Experimental procedure: 1. Fill a beaker about 1/2 full of water and add a dropper full, or so, of Iodinepotassium iodide (IKI) solution, enough to make the solution yellow. 2. Use a glucose test strip to test for the presence of glucose in the beaker. Dip the strip into the solution, remove immediately, and wait 30 seconds before comparing the color with the scale on the bottle. Record the concentration in Table 2. 3. Take one piece of soaked dialysis tubing, fold the end over on itself about 1, and then close with a clamp 4. When you are ready to fill the dialysis tube, rub the open end of the tube between your fingers to open it up. Use a pipette to fill the tube with 10-15ml of the glucose/starch solution. 5. Use one glucose test strip to test for the presence of glucose in the solution you have just put into the dialysis tube. 6. Clamp the open end of the dialysis tube and then place it into the beaker of water and iodine. Monitor the experiment for changes. Once the results are apparent, record the color of the liquids in the beaker and the tube. Repeat the glucose test of the solution in the beaker. Table 2. Data Table for Part I: Color Records of Liquid Contents. Start After 30 min. Beaker Contents Color Beaker Contents Glucose Test Strip Solution In Dialysis Tube Color Dialysis Tube Glucose Test Strip Don t test 4
Evaluation: Compare your results in table 2 to your predictions for this part of the lab. Do these results cause you to reject your original hypothesis? If your data are not in accord with your predictions, how could you revise the hypothesis to match these results, and what new experiment would you use to test the revised hypothesis? Thought questions: 1. If the dialysis tubing makes a good model for the cell membrane, what do the results of your experiment mean for a cell that needs to maintain homeostasis? 2. If the tube represents a cell, what do the contents of the beaker represent, i.e. the solution outside of the dialysis tubing? 3. If a molecule is able to pass out of the dialysis tubing, do you think it would be able to pass the other way? (In other words, is the dialysis tubing a one-way path?) Describe how you could alter the above experiment to determine if the dialysis tubing is semi-permeable one-way or two-ways. Hypothesis 2: The concentration of solute on either side of the membrane has no affect on the rate of osmosis because it does not affect the movement of water. To test this hypothesis, we have provided you with an experimental apparatus that can be used to quantify (measure) changes in volume that result from the process of osmosis. It consists of a glass pipette, a stopper, some dialysis tube, a beaker, and a ring stand. Your TA will demonstrate how to use this set up. How can this apparatus be used to test a hypothesis that has to do with the rate of osmosis? You also have several solutions of sucrose to use to test this hypothesis. See Table 3 for a list of materials available for this experiment. Sucrose (common table sugar) is a molecule composed of one glucose molecule, and one fructose molecule. 5
Table 3. Materials for testing Hypothesis 2 Materials: Amount per group: 0.2 M sucrose solution 20ml 0.4 M sucrose solution 20ml 0.8 M sucrose solution 20ml 1.6 M sucrose solution 20ml Dialysis tubing 125mm Osmosis apparatus 1 Food coloring As needed Experimental procedure: For this experiment, the procedure is not provided, and relies on you to establish it. Discuss with your group, and the class, the hypothesis you are testing, the materials you have available, and develop an experiment that tests the hypothesis. When an experiment has been agreed upon by the class, you can then develop predictions of the experimental results. Describe your experimental protocol, and predictions below. Experimental Procedure: Prediction(s) for experiment 2: Use the space below to record your results. 6
Evaluation: Do the results of this experiment match your predictions and support your hypothesis? Use experimental evidence to support your answers. If the results do not match your predictions, what revised hypothesis would explain what you did see? How could you test this revised hypothesis? Thought Questions: 1. What do the results of your experiment tell you about how the environment might affect a cell s ability to maintain homeostasis? 2. How might the concentration of a solute influence the rate of osmosis? 3. Do the results of your experiment tell you anything about the influence of concentration on the rate of diffusion in general? 4. How might you test whether the concentration of a solute alters its rate of diffusion? 7