Osmosis (Solution, Evaporation, Diffusion and Osmosis)
Grade Level		Prospective and Practicing K-8 Teachers; may be adapted for use in elementary classes.
Time		Exercises 1-4 take approximately 2 1/2 hours.
To Ponder		1.	Our cells need a constant supply of oxygen and of water. They are also continuously producing carbon dioxide which needs to be removed. How do these materials get into and out of a cell? Oxygen is absorbed from the air via the alveoli in the lungs. It diffuses into the bloodstream where it combines with hemoglobin in red blood cells. The circulatory system pumps the blood around the body and the oxygen is released in the tissues, where it diffuses through lymph into the cells. The carbon dioxide produced by cells diffuses out of the cells and through lymph fluid into capillaries where it becomes dissolved in the liquid portion of the blood, and eventually is released into the air via the lungs. Water is absorbed into the bloodstream through the gastrointestinal tract, primarily the large intestine and may diffuse out through the capillaries into the lymph.
		2.	How does water pass from the roots of a plant to its highest leaves? Several factors are involved in helping water rise to the top of a tree. The first is higher water pressure in the root. The second is capillary action arising from cohesion among water molecules and adhesion between water and wood. The third is evaporation of water from stomata (pores) in the leaves. The overall process is called transpiration.
		3.	How does water we pour into our gastrointestinal tracts actually get inside our bodies? The intestine is lined with villi and other convolutions which increase surface area and therefore absorptive capacity. Water diffuses from the area of higher concentration in the intestine to an area of lower concentration in the capillaries that line these areas.
Supplies		Per Group (unless otherwise noted) 1 triple beam or other balance 1 100 ml beaker 1 250 ml beaker 1 100 ml graduated cylinder 1 250 ml graduated cylinder 5 50 ml beakers 2 glass stirring rods 4 4-inch strips of 1-inch flat dialysis tubing waxed dental floss, several containers 2 disposable pipettes paper towels table sugar - 5 pounds per class tape bottled water
Objectives		Once you have completed these exercises you should be able to:
		1.	Distinguish between concentrations of solute and solvent.
		2.	Calculate the concentration of and be able to describe solutions.
		3.	Predict net movement of molecules across a semipermeable membrane.
		4.	Define what molecules move during osmosis.
		5.	Use the terms hypotonic, hypertonic, and isotonic.
Background Information		Movement of Matter Movement is a characteristic which we most readily attribute to living organisms. Animals move around, seeking their prey. Vines twist and contort to maintain a grip on the tree trunk on which they are growing. Flowers open and close. Movement, however, is also a characteristic of the non-living world. Differential heating of air produces wind. Differential heating of water produces currents.
Powerful Idea		Movement is a pervasive characteristic of matter. All matter is made of molecules. All molecules are constantly in motion. To repeat, molecules and atoms are constantly in motion throughout the natural world.
To Ponder		1.	What kinds of evidence have you seen so far that supports the idea that molecules are constantly in motion? We have seen perfume or other aromatic substance diffusing through the air, dye diffusing through water, and water diffusing across the semipermeable membranes of the cells in fruits and vegetables. Perhaps the students can add additional examples to this list.
		2.	If you are a coffee or tea drinker, and if you use sugar, you know that sugar will dissolve faster in hot tea or coffee than it will in iced tea or coffee. That is, a given amount of sugar will dissolve faster in a given volume of water at higher temperature than at lower temperature. This is an example of rate of diffusion, which is the amount of solute that goes into solution per unit time .
		3.	Four of the many factors that affect the rate of diffusion are listed below. Thinking back to Lesson 1 and drawing on your everyday knowledge, identify one or more examples you have seen of the effects of each of these factors. temperature, Students may have seen by now that salt and sugar go into solution faster in hot water than in cold water. size of molecules diffusing, the magnitude of the charge(s) on, or polarity of, the diffusing molecule, and In doing the chromatography experiment in Lesson 1, students observed various dyes separated from black ink, and each dye migrated a different distance from the other dyes in a given amount of time. These differences are a function of both the size of and charges on each type of molecule. size of concentration gradient over which diffusion occurs. We haven't carefully observed the effects of different concentration gradients, but we will do that in the lesson that follows.
		4.	Design a new experiment which would allow you to test the effect of temperature on the rate of diffusion. Numerous possibilities exist. For example, supposing students place exactly 25 ml. of water of differing temperatures in each of two 50 ml beakers. In one case they use ice water and in the other they use boiling water. The students let the water sit for ten minutes to become still. Then they add exactly one drop of dye to each container, using the same dropper to obtain similar drop size. They carefully time the diffusion of dye through each liquid until the liquid appears to be the same color throughout. In general, students should set up two different systems Each system should be timed from the time the solute and solvent are combined until diffusion appears to be completed. Finally, the results from each of the two systems should be compared. Students should find that the rate of diffusion increases when the temperature is raised. Many students may think to do an experiment in which solubility and diffusion interact. This may give similar results, but the cleanest results are obtained with the minimum number of variables.
Exercise 1		Effects of a Concentration Gradient
		1.	In this experiment you will collect some data which supports the statement that all molecules are in motion. You will also examine some consequences of this which are critical to life. Even though you will be using a non-living system as a model to collect the data, it is important to remember that what you discover is applicable in living systems.
To Do		2.	Fill a 100 ml beaker with 80 ml bottled water. Obtain four 4" strips of dialysis tubing from your instructor and place them in the bottled water to soften the tubing. When softened, the dialysis tubing will behave like a semipermeable membrane in a living cell.
To Do		3.	Remove the dialysis tubing from the water one piece at a time and tie off one end with waxed dental floss by folding the tubing over the dental floss twice and then tying the dental floss around the fold (see Figure 1). Return each piece of tubing to the 100 ml beaker of bottled water as you complete the task. Figure 1. Tying off dialysis tubing.
Background		4.	The designation used in this lab, e.g., 40% sugar, is a common way to refer to the solute content of a solution. It means that there are 40 grams of sugar per 100 ml of solution (not 40 grams of sugar per 100 ml of water). A 20% sugar solution would contain 20 grams of sugar per 100 ml solution, and so on. For purposes of thinking about osmosis, you can assume that the concentration of water is approximately equal to the difference - that is, about 60% water in a 40% sugar solution.
		5.	You will now prepare two different solutions of sugar. For each solution, use a clean beaker to hold the sugar, a balance to weigh the beaker alone and then the beaker plus sugar, the beaker for initial mixing of the solution, and a graduated cylinder for measuring the final solution volume (this is because volume measurements are much more accurate in a graduated cylinder than in a beaker). Make sure that the pan of the balance is dry and that the balance is properly zeroed before weighing. Use the weights on the scale to balance the weight of material.
To Do		6.	As noted above, to prepare 100 ml of a 40% sugar solution you would weigh out 40 grams of sugar. However, we will need 200 ml of a 40% sugar solution, so you need to double both the sugar content (to 80 gms) and the solution volume (to 200 ml). Thus, . . . a. Weigh a 250 ml beaker and record the weight: __________ gms. b. Add 80 gms to the weight of the beaker to obtain desired weight: __________ gms. c. Set the weights on the scale to the desired weight in (b). d. Add sugar until the beaker plus sugar weigh the desired weight. e. Remove the beaker from the scale and add 150 ml bottled water to the sugar. Stir until dissolved. f. Pour the sugar solution from the beaker into a 250 ml graduated cylinder. g. Add bottled water until the total volume of the solution is 200 ml. h. Pour the 200 ml of solution back into the original beaker. i. Label the beaker 40 % sugar. If all has gone well, you now have precisely 80 gms of sugar mixed with bottled water to give precisely 200 ml of 40% sugar solution.
		7.	In a similar manner, prepare 20 ml of a 10% sugar solution. Since a 10% sugar solution would contain 10 gms of sugar per 100 ml solution, then 20 ml solution (1/5 the volume) would contain how much sugar? __________ gms. a. Weigh a 50 ml beaker and record the weight: __________ gms. b. Add 2 gms to the weight of the beaker to obtain total desired weight: __________ gms. c. Set the weights on the scale to the desired weight in (b). d. Add sugar until the beaker plus sugar weigh the desired weight. e. Remove the beaker from the scale and add 10 ml bottled water to the sugar. Stir until dissolved. f. Pour the sugar solution from the beaker into a 50 ml graduated cylinder. g. Add bottled water until the total volume of the solution is 20 ml. h. Pour the 20 ml of solution back into the original beaker. i. Label the beaker 10 % sugar. If all has gone well, you now have precisely 2 gms of sugar mixed with bottled water to give precisely 20 ml of 10% sugar solution. Take a deep breath and continue.
		8.	Place these solutions inside the four dialysis sacs according to Table 1, following the directions below. Table 1. Contents of dialysis sacs. Sac # Contents 1 10% sugar 2 40% sugar 3 10% sugar 4 40% sugar
Predict		9.	As you are ready for it, remove a piece of dialysis tubing from the water in which it is soaking and blot it dry.
Interpret		10.	Use a clean disposable pipette (Figure 2). Add 5 ml of the 10% sugar solution to sac #1. Once the sac is filled, tie the open end with waxed dental floss in the same manner as you did previously. Tie a label to one end of the sac and call it #1 Set the filled and labeled sac on a paper towel. Repeat this procedure described in steps 9 and 10 with a second piece of tubing, adding 5 ml of a 10% sugar solution, and label #3. Figure 2. Disposable pipette.
		11.	Obtain a clean disposable pipette. Add 5 ml of the 40% sugar solution to sacs #2 and #4 in the manner described in steps 9 and 10 above, placing each one on the paper towel as completed.
Data Collection		12.	Make sure that the pan of the balance is dry and that the balance is properly zeroed before weighing. Use the weights on the scale to obtain the proper adjustment. Weigh each sac to the nearest 0.1 gm. Record the beginning weights in Table 2 below.
		13.	In the table below, record the beginning weights, ending weights, and net change in weight for each of your dialysis sacs. Table 2. Dialysis sacs, weight in grams. Sac# % Solution Beginning Weight Ending Weight Net Change (+ or -) 1 10%       2 40%       3 10%       4 40%
Interpret		14.	Place 40 ml of bottled water in each of two 50 ml beakers and label them Beaker #1 and #2. Place 40 ml of 40% sugar solution in each of two 50 ml beakers and label them Beaker #3 and #4 (Table 3). Table 3. Beaker Contents. Beaker # Contents 1 bottled water 2 bottled water 3 40% sugar 4 40% sugar
		15.	Simultaneously place each filled and weighed dialysis sac into its corresponding 50 ml beaker (Table 4). Note the time: __________ o'clock. You will remove the sacs after 20 minutes of immersion, at __________ o'clock. Table 4. Sac and beaker contents. Sac & Contents Solution in Beaker Sac # Contents Beaker # Contents 1 10% sugar 1 bottled water 2 40% sugar 2 bottled water 3 10% sugar 3 40% sugar 4 40% sugar 4 40% sugar
Predict		16.	While waiting, predict what will happen to each sac. Use '+' to indicate small weight gain, '++' for large weight gain, '-' for small weight loss, '--' for large weight loss, and '0' for no change. sac 1 +Water should diffuse into the sac. and the sac should gain weight. sac 2 ++Even more water should diffuse into this sac and the sac should gain more weight. sac 3 - Water should diffuse out of this sac. and the sac should lose weight. sac 4 0 There should be no net movement of water and no change in weight of the sac Make sure the students think about and make predictions. Two errors are commonly made. The first is to pay attention to the concentrations of sugar (10% and 40%) instead of the concentrations of water (about 90% and 60%). The second is to forget that it is the water that crosses the membranes, NOT the sugar. You may be able to give hints to help the students correct their thinking, but we would avoid outright telling at this stage, even if their prediction is incorrect. Encourage them to draw pictures showing the flow of molecules from one concentration to another. There appears to be considerable value in engaging students in making serious predictions . First, it prompts them to construct a mental model of the situation. Second it requires them to run that mental model in order to anticipate the outcome. Third, when students work in groups they tend to air and compare several different models during this process and frequently manage to reach a consensus model. And fourth, they become vested in the outcome, so that they seem to pay much closer attention to the results than they would otherwise.
To Do		17.	After 20 minutes have passed, remove the sacs from the solutions in which they are sitting. Pat each sac dry with paper towels. Weigh each sac again (removing the label when you weigh it) and enter this Ending Weight into Table 2 above.
Data		18.	In the last column of Table 2, summarize results using the notation described in step 16.
Collection Results		19.	Compare your predictions with your results (Table 5).
Table 5. Your predictions and re sults: Comparison. Sac # Prediction (gain, loss) Result (gain, loss) 1     2     3     4
Interpret		20.	Explain any discrepancies. Discrepancies may arise if students are not accurate when making their solutions, if the sacs are not left in the beakers for the full 20 minutes, if the sacs are not patted dry before weighing, if the labels are not removed before weighing, etc. In this experiment, it is very useful to think about any results that are unexpected and to try to understand why they were obtained. When students' results differ from their predictions, they usually assume that they did something wrong in their experiment. That is, they assume that their mental model is correct and their observations are incorrect, even though in most cases the opposite is true. One of the best ways to help students re-evaluate their mental model is to review the results of the entire class. If most or all of the groups obtained the same results, and those results differ from a group's predictions, then that group is more likely to rethink their prediction itself.
To Do		21.	Finally, compare your results to those obtained by other groups in the class. Are you all in agreement? If not, why not? Is there a logical explanation?
Table 6. Class predictions and results: Comparison. Group Sac 1 Sac 2 Sac 3 Sac 4   Predict Result Predict Result Predict Result Predict Result 1                 2                 3                 4                 5                 6                 7                 8
Exercise 2		Organizing Your Knowledge
Question		1.	When working with sugar dissolved in water, which molecules flow across a semipermeable membrane and which ones do not? The water flows across a semipermeable membrane. The sugar does not. Sugar molecules are too large and too highly charged to pass through the small pores in the membrane.
Interpret		2.	The water molecules that flow across the membrane do so freely in both directions. However, the net or overall flow will be in the direction of the concentration gradient, from high concentration of that molecule to low concentration of that molecule. What concentration gradient (that is, the concentration of what substance) did you have to pay attention to in order to make your predictions accurately? There are many different concentration gradients in a cell. The movement of each different type of molecule is determined by the relative concentrations of that type of molecule only. In this experiment, students had to attend to relative water concentrations. Water moved from an area of high water concentration to an area of low water concentration. Those who paid attention to sugar concentrations made incorrect predictions.
To Do		3.	When two solutions are separated by a semipermeable membrane, how does the solute concentration of the two solutions affect the direction in which water flows? The more solute in a solution, the lower the water concentration in that solution.
Think		4.	During osmosis, does water violate or obey the laws of diffusion? Justify your answer with your data. Water obeys the laws of diffusion. It moves from an area of high water concentration to an area of low water concentration.
Interpret		5.	Make a general statement relating the direction of water flow to the concentration of solute in solutions separated by a semi-permeable membrane. Water concentration is approximately inversely proportional to solute concentration. Thus, high levels of solute correlate with low levels of water. Water is likely to move from an area with low levels of solute to an area with high levels of solute.
Interpret		6.	Does osmosis require living tissue to occur? Explain. Although osmosis does occur in living things, osmosis is independent of life. As we have just seen, osmosis was able to take place in a beaker devoid of life. The essential structure for osmosis to occur is a semipermeable membrane.
Question		7.	Do living things require osmosis? Explain. Living things do require osmosis both to provide for water uptake and to prevent overloading the cells with water. Osmosis is a critical process in maintaining homeostasis or balance within an organism.
Interpret		8.	Use the terms hypotonic, hypertonic and isotonic to describe the various conditions in the experiment you just performed: These terms have not been introduced in this lab. You may choose to introduce them or not, depending on the grade being taught. They are likely to be encountered in a discussion of this topic in any biology text. One easy thing to remember is that hypo means low. That is, a hypotonic solution has lower solute concentration than a hypertonic solution. Isotonic solutions have the same solute concentrations. Notice that all three terms describe relative concentrations, NOT absolute concentrations. For example, the bottled water was hypotonic to all of the solutions. The 40% sugar solution was hypertonic to the 10% solution and the bottled water. The 40% sugar solution in sac #4 was isotonic with the 40% sugar solution in beaker #4. Table 7. Sac and beaker contents.   Sac & Contents Solution in Beaker Case Sac # Contents Beaker # Contents A 1 10% sugar 1 bottled water B 2 40% sugar 2 bottled water C 3 10% sugar 3 40% sugar D 4 10% sugar 4 40% sugar In Case A, the Sac contents were hypertonic to the solution in the beaker. In Case B, the Sac contents were also hypertonic to the solution in the beaker. In Case C, the Sac contents were hypotonic to the solution in the beaker. In Case D, the Sac contents were isotonic to the solution in the beaker.
Question		9.	Describe the sites in which osmosis occurs in human beings within the context of each of the following systems. digestive system a. ascending colon b. transverse colon c. descending colon Water that we drink is absorbed primarily in the large intestine, although some water is also absorbed in the small intestine. The three parts of the large intestine are the ascending, transverse, and descending colon. Another correct way to respond to this question would have been to say a) small intestine, b) large intestine and c) villi. The villi are the main sites for absorption in the intestine. In contrast, the stomach has a viscous lining that prevents absorption from occurring there. urinary system The kidneys remove many parts of the liquid portion of the blood from the circulatory system and then pump back or reabsorb materials that are needed to maintain balance in the body, particularly water circulatory system Diffusion and osmosis are critical processes in the functioning of the circulatory system. Capillaries deliver water and nutrients to every tissue and cell in the body, and they collect waste materials from those cells. The exchanges occur across semipermeable membranes. Oxygen, carbon dioxide and water diffuse freely across the membranes., from areas of higher concentration to areas of lower concentration.
Exercise 3		Knowledge Integration This is a thought experiment to further test the students' understanding of osmosis and diffusion.
Background Information		1.	Suppose you had a U-shaped tube (shown below) that was filled with water as indicated. Let us also suppose that at the bottom of the U-shaped tube there was a semi-permeable membrane that was permeable to both water and blue dye. However, the membrane IS NOT permeable to yellow dye. Figure 3. U-tube with semipermeable membrane.
Predict		2.	Now let us suppose that we add identical concentrations of blue dye to right side of the tube and yellow dye to the left side of the tube. What color will the right and left sides begin to turn as soon as you add the dyes? right side will turn __________ left side will turn __________ The right side will initially turn blue and the left side will initially turn yellow as each dye diffuses through the side of the U-tube into which it was dropped.
		3.	As diffusion begins to occur across the membrane, what will happen to the water levels? a. On the right side? __________ b. On the left side? __________ c. Why do you make these predictions? The blue dye will diffuse across the membrane to the left side of the tube. The solute concentration on the left side will gradually increase as a consequence. Water will diffuse into the higher concentration of solute, thereby raising the water level on the left side and lowering it on the right side.
		4.	After letting enough time pass for the system to equilibrate, what will the system look like? a. What color will the left side be? __________ b. What color will the right side be? __________ c. Why do you make these predictions? The left side of the tube will contain a mix a blue and yellow dye which will appear green to the eye. The right side will remain blue, perhaps a paler blue than it was initially. d. How will the water level on the left side have changed? ______________________________ e. How will the water level on the right side have changed? ______________________________ f. Why do you make these predictions? The water level will remain higher on the left side and lower on the right side, due to the greater concentration of solute on the left.
		5.	Which processes are occurring in this system? Diffusion of dyes through water, diffusion of one dye across a semipermeable membrane, and osmosis of water are all occurring in this system.
		6.	What molecules are moving across the semipermeable membrane in this system? In this case, both the water molecules and the blue dye molecules are crossing the membrane.
Supplementary Resources		Borovoy, Alexander. 1991. Learning about (not by) osmosis. Quantum. V2(2), 48-51. EJ447781 Sestili, Mary Ann. 1974. An investigative laboratory on diffusion and osmosis. American Biology Teacher V36(8), 492-493.
Related AAAS Benchmarks		Chapter 4: THE PHYSICAL WORLD Section D: The Structure of Matter Grade 3-5 Benchmark 1 of 4 Heating and cooling cause changes in the properties of materials. Many kinds of changes occur faster under hotter conditions. Grade 6-9 Benchmark 3 of 7 Atoms and molecules are perpetually in motion. Increased temperature means greater average energy of motion, so most substances expand when heated. In solids, the atoms are closely locked in position and can only vibrate. In liquids, the atoms or molecules have higher energy, are more loosely connected, and can slide past one another; some molecules may get enough energy to escape into a gas. In gases, the atoms or molecules have still more energy and are free of one another except during occasional collisions. Grade 6-8 Benchmark 4 of 7 The temperature and acidity of a solution influence reaction rates. Many substances dissolve in water, which may greatly facilitate reactions between them. Chapter 5: THE LIVING ENVIRONMENT Section C: Cells Grade 9-12 Benchmark 1 of 8 Every cell is covered by a membrane that controls what can enter and leave the cell. In all but quite primitive cells, a complex network of proteins provides organization and shape and, for animal cells, movement.