Lab 3: Diffusion and Osmosis - University of Evansville

22Diffusion is the movement of dissolved moleculesfrom a region of high concentration to a region of lowconcentration. This process tends to spread the moleculesout. Keep in mind that diffusion occurs even inthe absence of a membrane.Osmosis is the diffusion of water molecules acrossa selectively permeable membrane. Selectively permeablemembranes allow some substances, includingwater, to pass freely through the membrane, but blockthe passage of others. As in diffusion, the water moleculesmove from a solution with a higher concentrationof water to a solution with a lower concentrationof water.In this laboratory exercise, you will learn aboutsome of the principles of diffusion and osmosis. Alongthe way, you will design and carry out a simple experiment.The data you collect during this experimentwill, when properly analyzed and displayed, allowyou to determine the concentration of an unknownsucrose solution.ProceduresSection A - DiffusionThe movement of substances across the plasmamembrane is important to the survival of the cell.Certain substances must enter the cell to serve as rawmaterials and to provide an energy source. Likewise,waste products must move out of the cell. The moleculesin liquid water are in constant motion, collidingwith each other and with molecules of dissolvedsubstances. This kinetic molecular motion allows diffusionto occur.Think about a shoebox with twenty marbles allstacked against the left side. These marbles representdissolved molecules, with a high concentration at theleft end of the box and a low (zero) concentrationelsewhere in the box. A concentration gradient exists- high at the left end of the box and low everywhereelse. Now think about shaking the box. This is wherethe constant motion of molecules comes in. Marbleswill tend to move down the concentration gradient,that is from left to right in the above example. Whenyou open the box, the marbles are randomly distributed.The concentration of marbles has evened out,and a concentration gradient no longer exists.Temperature can affect the rate of diffusion. Tosee this, place a drop of food coloring into a smallbeaker of room temperature water. Count the numberof seconds required for the dye to be evenly distributedthroughout the beaker. Now repeat the experimentusing a beaker of hot water. Allow the water toboil, remove from the heat, let stand 30 seconds, thenadd the food coloring and record the time requiredfor even distribution. Do the particles move faster orslower in the hot water? Why? (Be sure to includeyour description of this activity and answer thesequestions in your lab report.)Section B - OsmosisLike any molecule, water can be present in high orlow concentrations. While it may at first seem counterintuitiveto imagine one solution with a higher concentrationof water than another solution, this actuallyhappens all the time. Think about a 5% salt solution.By weight this solution is 5% salt and 95% water.Imagine that this solution was placed across a semipermeablemembrane from a distilled (0% salt and100% water) water solution. Water molecules wouldmove down their concentration gradient from the highconcentration to the low concentration, i.e., from the100% solution to the 95% solution, but the membranewould prevent the salt molecules from moving. (SeeFigure 8.12 on page 147 in your text.)The following protocol will allow you to determinethe rate at which distilled water crosses a semipermeablemembrane enclosing a sucrose solution. Thinkabout how you would use this method to design anexperiment to determine the concentration of a sucrosesolution of unknown concentration. Remember thata rate has units such as miles per hour or grams perminute. In this experiment, you will determine therate of osmosis by determining the slope of a line.1. Each lab group should find 6 lengths of dialysistubing, each 5 inches long, at their table. Dialysistubing is a semipermeable membrane that is selectivelyporous and allows some molecules to passthrough it. You also have several beakers which youshould label using the red wax marker in your labdrawer. Do not mark glassware on the white frostedmarking area of the beaker. Marks made there withwax pencil DO NOT come off.