Diffusion and Osmosis JJ Introduction Cells are the smallest livin units of an oranism, and it is the cells that carry out most of the basic physioloical functions of an oranism. In order to carry out these functions, a multitude of materials must enter and exit each cell throuh its plasma membrane. Recall that a plasma membrane is basically a bilayer of phospholipids with proteins embedded into the membrane and attached to its surfaces. In lecture you should already have studied various mechanisms by which materials can cross a cellular membrane. A passive transport mechanism is one in which molecules cross the membrane without requirin the expenditure of enery by the cell. Passive transport mechanisms include simple diffusion, facilitated diffusion, and osmosis. Active transport mechanisms are those that require the cell to expend enery to move somethin across the membrane. In this chapter you will study some of the basic concepts underlyin passive transport. Important note for instructors and students: Bein Activity 2 before Activity 1! Activity 2 takes a fairly lon time, and you can work on Activity 1 while you are waitin to complete Activity 2. I. Diffusion Objective 1: Describe the process of diffusion, includin its relationship to kinetic enery. All matter is made of atoms and molecules, and these atoms and molecules are constantly movin. Any object that is in motion (anythin from an atom to an airplane) has kinetic enery. Even in a solid material, such as your laboratory table, enery causes the atoms and molecules to vibrate. In a liquid or a as, the atoms and molecules can vibrate, rotate, and move from one place to another. Thus, in a lass of water each individual H 2 O molecule is vibratin, rotatin, and movin throuhout the lass. Of course, even with a microscope you cannot observe this motion because the water molecules are much too small. 27
If you place a cube of suar into a lass of water, the molecules of suar will dissolve in the water. As water molecules bump into suar molecules, the suar molecules ain kinetic enery and move throuhout the water. Eventually, the molecules of suar will become distributed evenly throuhout the lass, even if you do not stir the liquid. The suar diffuses throuh the water. Diffusion may be defined as the tendency of particles to become distributed evenly throuh a fluid. Diffusion depends upon kinetic enery, and the movement of individual particles is random. However, the term diffusion is only used when there is net movement of particles from a hih concentration to a low concentration. In other words, there must be a concentration radient, a difference in concentration from one place to another, for diffusion to occur. Consider the suar cube shown in Fiure 4-1. At the time the cube is placed in the lass of water, the concentration of suar is hih where the cube is, but the concentration of suar elsewhere in the lass is zero. Over time, molecules of suar diffuse throuhout the lass. Eventually, the concentration of suar will become uniform throuhout the lass. At this point, there is no loner any diffusion because there is no loner a concentration radient. Time Hayden-McNeil, LLC Concentration radient Suar molecules evenly distributed Fiure 4-1. Over time, the molecules in a cube of suar placed in water diffuse until they are evenly distributed throuhout the container. ACTIVITY 1 Get a lass beaker (exact size does not matter, anythin from 250 ml to 500 ml is fine) and fill it halfway with tap water. Set the beaker on your table, and try as hard as you can durin this experiment to avoid disturbin the beaker. Avoid bumpin and banin the table, too! You should find on your table a bottle with tablets of dye. Drop one tablet of dye into the beaker. Observe what happens for a few minutes (remember to avoid disturbin the beaker). In the space below, describe what happens to the dye. 28
Diffusion and Osmosis Repeat the last experiment. This time, however, use two beakers. Fill one beaker with cold water and set it on ice, and heat the other beaker on a hot plate (do not let the water boil). Add a tablet of dye to each beaker at approximately the same time. Before beinnin this experiment, write down what you predict will happen: Bein the experiment and observe what happens for a few minutes. Briefly describe the results in the space below. Did the results support your prediction? QUESTIONS 1. Why does diffusion require kinetic enery? 2. How does the term concentration radient apply to the experiment with the dye? 3. What is the relationship between temperature and kinetic enery? How does this relate to the last activity? 29
II. Osmosis Objective 2: Describe the movement of water accordin to osmosis. Water can diffuse just like any other molecule. Since water is by far the most abundant molecule in livin oranisms, we ive the diffusion of water special treatment. Osmosis is the special name iven to the diffusion of water throuh a selectively permeable membrane. A selectively permeable membrane is one that allows some particles to pass throuh, while other particles cannot pass throuh. You should understand by now that the plasma membrane of a cell is a selectively permeable membrane. Water follows the same rules of diffusion that any other molecule follows: For osmosis to occur, there must be a concentration radient for the water, and there will be net movement of water from the reion of hih concentration to the reion of low concentration. One thin that complicates this situation is the concentration of water in a solution is enerally not iven. Rather, one is usually iven the concentrations of solutes (e.., salts, suars, proteins). Fortunately, it is enerally safe to assume that a solution with a hiher concentration of solutes has a lower concentration of water than a solution with a lower concentration of solutes. Consider two solutions: Solution A contains 10% NaCl dissolved in water, and solution B contains 5% NaCl dissolved in water. Obviously, solution A contains a hiher concentration of NaCl than solution B. Therefore, solution A has a lower concentration of water than solution B. Imaine the two solutions, A and B, are placed into contact with a selectively permeable membrane, which allows passae of water but not salts (Fi. 4-2). The concentration of water is hiher in solution B than it is in solution A, and water will diffuse from solution B to solution A. In other words, water will move by osmosis from solution B to solution A. Solution A 10% NaCl Solution B 5% NaCl Fiure 4-2. A container with two solutions of salt separated by a selectively permeable membrane. The dashed line represents the membrane. Arrows show the direction of osmosis. Some additional terms will help you discuss the process of osmosis. A solution is hypertonic if water tends to move into it from another solution. A solution is hypotonic if water tends to move out of it to another solution. In the example illustrated by Fiure 4-2, solution A is hypertonic and solution B is hypotonic. If two solutions have equal tonicity, so that osmosis does not occur from one to the other, then the solutions are isotonic. 30
Diffusion and Osmosis ACTIVITY 2 For this activity, your entire table will work toether as a roup. Each roup will fill a section of dialysis tubin with a solution; we ll call this the tube solution. Each roup will place the tube into a beaker that contains another solution; we ll call this the beaker solution. Over time, water will move throuh the dialysis tubin. By monitorin the weiht of the tube, you should be able to determine whether osmosis has occurred, and if there was osmosis into the tube or out of the tube. Your instructor will assin each roup to use a combination of tube and beaker solutions iven in the table below: Group Tube Solution Beaker Solution A 10% sucrose distilled water B 20% sucrose distilled water C 30% sucrose distilled water D distilled water distilled water E distilled water 10% sucrose F distilled water 20% sucrose Once you know what solutions your roup will be usin, follow these procedures for your experiment: Obtain a beaker that holds 250 ml. Fill the beaker with 200 ml of your beaker solution (the amount does not need to be exact, just use the marks on the beaker for your measurement). Obtain a piece of dialysis tubin from your instructor. Your instructor should demonstrate how to open the tubin: Hold the tubin under a faucet in one of the sinks and run tap water over the tubin. Rub one end between your thumb and forefiner until you feel the tube start to open (this may take a minute). Open the entire lenth to make a tube. Get a lenth of thread about 10" lon, and fold it over to make a double strand of thread. Fold one end of the tube over about one centimeter, and tie it off tihtly with the thread. You want the tube tied tihtly enouh that solution will not leak out of it. Use a pipette to dispense 5 ml of tube solution into the tube. Fold over the other end of the tube and tie it off tihtly with double-stranded thread. (At this point, the tube should be flaccid; it should not be filled up like a balloon.) Use a paper towel to dry off any liquid on the outside of the tube, then weih the tube to the nearest 0.1 ram. This measurement is the initial weiht of your tube. Enter this measurement into the proper cell of the table on the last pae of this chapter. (Do not worry about the space for the chane at this time.) Assin someone in your roup to keep track of time. The clock in the classroom will be sufficiently accurate, or the timer can use a stopwatch. The timer will bein keepin track of time as the tube is placed in the beaker. So, place the tube in the beaker and bein keepin time. The tube should be more or less completely submered in the beaker solution. At five-minute intervals, remove the tube from the beaker, use a paper towel to ently blot dry any liquid from the outside of the tube, and weih the tube. Record the weiht in the proper cell of the table on the last pae of this chapter. Aain, do not worry about the space for chane at this time. 31
After a total of fifteen minutes has elapsed, make your final recordin of the weiht and end your experiment. When all roups have finished, your instructor will draw a table on the marker board and collect all data from the class. Before you leave class, you should have the weihts for all roups recorded in your data table. Calculate the chane in weiht for each of the six tubes over time. In this experiment, the chane in weiht always refers to the amount of chane from the initial weiht of the tube. Calculate the chane by subtractin the initial weiht from the measure of weiht at each time interval. For example, suppose for Group C the initial weiht of the tube is 5.3, and suppose the weihts at 5, 10, and 15 min are 5.5, 5.8, and 6.0, respectively. Subtract 5.3 from each weiht to et the followin results: initial weiht chane = 0.0, 5 minutes chane = 0.2, 10 minutes chane = 0.5, and 15 minutes chane = 0.7. Note that your initial weiht chane should always be 0.0! Complete this final portion of Activity 2 as homework. You will be tested on this assinment as part of your first lab practical exam. At his or her discretion, your instructor may also wish to rade this assinment. Get a piece of raph paper. If you do not have one handy, you can download a sheet of raph paper as a PDF file at the same Web site where you downloaded this manual. (If you know how to do it, the easiest way to create a nice raph is to do it on a computer. One way is to use Microsoft Excel to create a scatter plot. ) Create a raph that shows the chanes in weiht for each roup as a function of time. The x-axis of the raph will show time, in units of minutes, and the y-axis of the raph will show chane in weiht, in units of rams. The oriin of the x-axis should be at 0 minutes, and the x-axis should continue throuh at least 15 minutes. Your measurements of initial weiht chane all correspond to a time of 0 minutes. It is expected that your data will include some neative values for chane in weiht. Thus your y-axis should be labeled in a way to accommodate both positive and neative numbers. Finally, your raph should distinctly show the six different plots of data from the six roups, and there should be a key to indicate the roup represented by each plot. QUESTIONS 4. Consider the Group A experiment. Is the tube solution hypertonic, hypotonic, or isotonic compared to the beaker solution? 5. What should happen to the weiht of the Group A tube over the time of the experiment? Explain your answer. 32
Diffusion and Osmosis 6. Which tubes, if any, would you expect to lose weiht over time? 7. Identify some thins that could happen durin the experiment that would cause the actual results to differ from the expected results. CLEAN UP If you are not sure what to do with somethin, ask your instructor! Return your Web browser to the A&P home pae. If yours is the last class of the day, turn off your computer and monitor. Wash all of your beakers and place them on the dryin racks to dry. Throw away all trash, includin the used dialysis tubes. Return all solutions to their proper places. Leave your tables clean (wipe them down if necessary) and push in your stools before you leave. 33
Osmosis Data Group Tube/Beaker Initial Weiht 5 Minutes 10 Minutes 15 Minutes A 10% sucrose/h 2 O weiht chane weiht chane weiht chane weiht chane B 20% sucrose/h 2 O weiht chane weiht chane weiht chane weiht chane C 30% sucrose/h 2 O weiht chane weiht chane weiht chane weiht chane D H 2 O/H 2 O weiht chane weiht chane weiht chane weiht chane E H 2 O/10% sucrose weiht chane weiht chane weiht chane weiht chane F H 2 O/20% sucrose weiht chane weiht chane weiht chane weiht chane Calculate each chane in weiht as the weiht at that time, minus the weiht at the initial time. For example, if the weihts of a ba are: 5.1 (initial), 5.3 (5 min), 5.6 (10 min), and 5.7 (15 min), then subtract 5.1 from each weiht so the chanes in weiht are: 0 (initial), 0.2 (5 min), 0.5 (10 min), and 0.6 (15 min). Note that if a dialysis tube loses weiht after the initial weiht, then the chane in weiht will be a neative number. 34