General Botany BOTANY 130
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This 14 page Class Notes was uploaded by Etha Kassulke on Thursday September 17, 2015. The Class Notes belongs to BOTANY 130 at University of Wisconsin - Madison taught by Kenneth Cameron in Fall. Since its upload, it has received 122 views. For similar materials see /class/205325/botany-130-university-of-wisconsin-madison in Botany at University of Wisconsin - Madison.
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Date Created: 09/17/15
Topic 7 Diffusion Introduction This exercise explores the physical phenomenon of diffusion and specifically the special case of diffusion of water through a differentially permeable membrane This process is osmosis We will also observe the rates of diffusion into dry seeds as their cell walls imbibe water A secondary goal is to develop cooperative skills working in a group Diffusjgm Diffusion is simply the net movement of substances in a gas or a liquid from areas of higher concentration to areas of lower concentration The movement of substances into and out of cells and indeed into and out of the bodies of organisms is accomplished largely by diffusion The Cell Membrane and Osmosis Cell are islands of order where the processes of life is sequestered from the randomness of the nonliving environment The smallest thing that is truly alive is a cell The structure that serves as the gatekeeper between the living and nonliving world is the cell membrane The cell membrane is a differentially permeable membrane As the name denotes it is differentially permeable to various substances Water for example freely crosses this boundary but sodium and potassium ions will not pass through at all unless special portals are present to allow transport The membrane we will use in these exercises is dialysis tubing a nonliving sheet of cellophane However the principles you will observe here are essentially the same as those for living membranes Osmosis is the diffusion of water across a differentially permeable membrane Because cells exist in aqueous environments control of osmosis is critical to their survival A cell either needs to maintain equal concentrations of water on both sides of its plasma membrane or the cell needs some mechanism for maintaining a difference There are three possible conditions in regards to the concentration of water in the cell relative to its environment 1 Isotonic the solute concentration is the same on both sides of the cell membrane This is the condition of the cells in the human body The uids in our body blood and plasma are isotonic to the cells in our body 2 Hypotonic the solute concentration is lower outside of the cell than inside Under these circumstances a cell is at risk of bursting Some protozoans have contractile vacuoles that expel excess water to prevent bursting If blood cells are placed in distilled water they will burst Many protists all the fungi and all plant cells regulate the inflow of water under these conditions by means of the cell wall While the membrane is in contact with the cell wall the wall will provide a static force that equals the force generated by the tendency of water to move into the cell This not only prevents the cell from bursting but in the case of herbaceous plants also provides support like that provided by an inner tube in a tire 3 Hypertonic The solute concentration is greater outside the cell than inside A cell under these conditions will tend to shrink This often represents an abnormal stressful condition the cells of a wilting plant dehydration due to drinking sea water etc Another physical phenomenon linked to diffusion is imbibition Imbibition is the diffusion of water into a dehydrated substrate having an affinity for water Examples of imbibition include the expansion of seeds doors and sponges when they absorb water Osmosis is not involved in imbibition The following exercises are intended to give you some understanding of the processes of diffusion osmosis and imbibition and to engender your skills in t amwork between you and your lab partners Students will work in teams of four Exercise A Diffusion A1 Heat the motive force of diffusion One member of each team to do set this up Molecules diffuse in a fluid because the molecules of a liquid are in motion While large objects in water are not visibly effected by this motion small objects are The ran om movement of very smal objects due to molecular collisions in a uid is called Brownian motion Procedure Prepare a wet mount of homogenized milk View the preparation at 400x Note the tiny lipid droplets suspended in the aqueous medium T e movement you see is Brownian motion A2 Effect of molecular weight and temperature on the rate of diffusion Two members of each team will do this activity Procedure a Obtain two small test tubes that are half filled with 5 gelatin b To one test tube add 10 drops of 002 molar potassium dichromate K2Cr207 and to the other add 10 drops of 002 molar Janus green B C30H31C1N6 Cork the test tubes Either place both in the tumbler labelled room temperature or into the tumbler labelled 5 degrees C as dictated by the instructions at your work station I 3 d Record the temperature of your room in degrees C using the thermometer on the side bench Record the temperature in the table below e At the next class meeting measure in millimeters the distance each solute has diffused into the gelatin Determine how much time has lapsed since you started the experiment and calculate a rate of movement through the gel for each condition in mm per hour Record these rates below Potassium dichromate Janus Green Room Temperature Refrigerator 5 degrees C f Graph the relationship between temperature and distance discussion activity g Graph the relationship between molecular mass and distance discussion activity Explain the differences in the rates of diffusion among the four tubes Exercise B Differentially Permeable Membranes Each person should View this demonstration Dialysis tubing is differentially permeable This property will be demonstrated by the following activity Procedure Each lab section will do one setup On the side bench is a test tube filled with a starch suspension sealed by a piece of dialysis tubing At the beginning of the period your TA will invert the test tube into a dilute solution of IZKI Observe during the lab and also during the next lab period Is dialysis tubing permeable to iodine Is it permeable to starch What do you suppose causes dialysis tubing to be differential permeability What is it that allows some things to move across but not others Exercise C Osmosis C1 Osmotic pressure Demonstration of an Osmometer Each person to view this demonstration throughout the period If the water concentration differs on either side of a differentially permeable membrane the net movement of water will tend to go from the side with the higher water concentration to the one with the lower This movement however can be stopped by positive pressure The pressure required to stop the movement for a given system is that system s osmotic pressure If that pressure is exceeded water can be forced through the membrane against a concentration gradient that is from the side of lower concentration to the side with the higher This method is used today to desalinate water and is also used in sophisticated home water purification systems The osmometer provides us with a way to measure osmotic pressure Water will move into the compartment or bag and the solution will rise in the tube until equilibrium is reached At this i point the solution in the tube exerts enough positive hydrostatic 2 pressure or quotosmotic pressurequot on the contents of the bag to equal or offset the negative osmotic potential of the sucrose solution In the osmometer on display a dialysis bag containing a 40 sucrose solution to which a red dye has been added has been suspended in a beaker of distilled water The bag is differentially 5 permeable allowing the passage of water but not the passage of sucrose WT E rt As you observe this demonstration record the time and height of the column using the data sheet by osmometer Why does water move into the bag During osmosis is water moving in only one direction that is only into the 3g Explain If the tube was 50 ft tall what would eventually stop the net movement of water Be specific Your understanding of this concept will be evaluated on the next quiz C2 Rates of osmosis Two members of each team of four to do this activity In this experiment you Will investigate the relationship between the rate of osmosis the difference in solute concentrations and temperature You Will place several dialysis bags containing solutions of different sucrose concentrations into beakers containing tap water at different temperatures The rate and direction of water movement can be determined by measuring the change in the mass of the bags Procedure a Obtain three sections of dialysis tubing and place them in a beaker of distilled water b Fold over one end of each tube tie tightly With string and return each tube to the distilled water c Open the untied end of one of one tube and add 7ml of 20 sucrose solution 49 M H20 Fold the end over forcing out excess air and secure it tightly With blue string 1 Follow the same procedure with two other tubes using a 50 sucrose solution 386 M H20 in one and tap water 556 M H20 in the other Use White string for the water and tan string with a black mark for the 50 SUCFOSC e Weigh each tube Record the mass on your data sheet f Initiate the experiment by placing all three dialysis tubes into one 600 ml beaker of water In each lab three groups to use water warmed to 35 degrees C keep beaker in ice chest of warm water and two groups to use ice water g After 1 hour and 15 minutes examine the tubes and record the results regarding their mass in the table below Treatment circle one Cold Hot Concentration at start Sucrose Mass at start Mass at end Rate of change gminute Concentration at end 50 386 M H20 20 49 M H20 Tap Water 556 M H20 h Determine the rate of change of the mass for each condition as follows Mass at the end Mass at start 75 m1nutes Record these values Rate of change grams Tap water Rate of change grams 20 Rate of change grams 50 grams minute i Pool your data with your classmates and calculate average rates of change in the mass of tubes under different treatments and fill in the tables below Discussion activity Concentration at start Sucrose Average change in mass Hot Average change in mass Cold Average change in mass both 50 386 M H20 20 49 M H20 Tap Water 556 M H20 j Graph the relationship between molar concentration and average rate of change in mass for the hot treatments Discussion activity k Graph the relationship between temperature and average rate of change for the 50 386 M H20 treatments Discussion activity What was the purpose of the tube lled with tap water C3 Plasmolysis Typically in healthy plants the solution around the plant s cells is hypotonic with water tending to move into the cell The pressure resulting from the protoplast pushing against the cell wall due to osmosis is called turgor pressure This pressure is important to herbaceous plants Without turgor pressure plants wilt C3a The role of turgor pressure in providing support to herbaceous plants Each person to View this demonstration Your TA will vmler the wilted plant with distilled water Observe its recovery during the period What would happen if she had vmlered the plant with a hypertohic solution csh obeemru39oh or phsmolyxis and remvery ofplanl ceus Mmhnsnfeuntnmmfmnmdnms W 3 Prepare ammmshde ufaleaf Df udeam msnlled mmr Observe the eehdrnerr Bf the cells are skemh a eeu hear the edge Bf a leaf 1 Place apreee Dfussue paperun DUB srde uflhe cavexshp andblmup the mmr whle addmg 40 sucmse suluunn an the ether srde e Observe the leaf eeu slcemhed earher as rt eerrtreers and pulls am mm quots eeu mu Skemh the eeu m thrs cumimnn an the next page e Place a ussue an DUB srae uflhe cavershp In him up the 40 sucmse suluuun wh e addmg drsuneel mterm the ntherslde Observe the eeu as rt reeevers Skemh the same eeu agam Cell original condition Plasmolyzed cell Cell after recovery Exercise D Imbibition and the Diffusion of Water Each person should View this demonstration At the time indicated in this demonstration 10 dry pea seeds were placed in each of six test tubes Tube 1 contains only dry seeds and will serve as a control The other tubes were set up as follows tube 2 was filled with petroleum ether tube 3 with 50 sucrose solution and tube 4 with distilled water A day later a similar series of test tubes was set up Compare the differences between the two sets of tubes and the differences between the tubes within each set Why do the seeds in various solutions swell at different rates Why do the seeds in petroleum ether fail to swell What is imbibition What is osmosis What is the difference between diffusion and osmosis between osmosis and imbibition between imbibition and diffusion Master Data Sheet Section Rate ot Dittusion Average Rate ot Movement a i conriitions S oiute mmLo Hr Average Rate ot Movement room temp mmhour Average Rate ot Movement co mmhour Janus Green Potassium Dictirornate Rate ot Osmosis Concentration Average Change in mass Average Change in mass Average Change in at start Hot gminute Cold gminute mass both gminute 50 20 Distilled Water
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