LAB: DIFFUSION ACROSS A SELECTIVELY PERMEABLE MEMBRANE NAME: PERIOD: DATE: Building Background Knowledge: 1) SELECTIVELY PERMEABLE MEMBRANE: Every cell is surrounded by a selectively permeable membrane that regulates what gets into and out of the cell. A selectively permeable membrane allows some types of molecules to move across the membrane and prevents other types of molecules from doing so. For example, oxygen can cross the selectively permeable cell membrane, but large molecules like proteins and DNA cannot cross the cell membrane. 1a. Why might it be useful for small molecules like oxygen to be able to move cross the cell membrane? 1b. Why might it be useful for the selectively permeable cell membrane to prevent large molecules from moving across the cell membrane? 2) DIFFUSION: When molecules move from an area of higher concentration to an area of lower concentration, it is called diffusion. This happens because ALL molecules in a liquid or gas state are constantly moving randomly. As the molecules move, they collide with each other, which causes them to spread out. Diffusion does not take any extra energy to happen because the molecules are already in motion. Diffusion continues until the solution reaches dynamic equilibrium. This means that, even though the molecules continue to move randomly, there is no overall change in their concentration. 2a. Please watch the demo of food coloring diffusing into water. Draw a picture that shows what is happening at the molecular level. Please label the picture using the words: diffuse, high concentration, low concentration, and dynamic equilibrium. 3) OSMOSIS: Water molecules diffuse, just like any other molecule -- they move from areas of higher concentration of water (meaning lower concentration of other dissolved substances) to areas of lower concentration of water (meaning higher concentration or other dissolved substances.) When water molecules diffuse, it is called osmosis. 1
Experimental Design: Today you will investigate an artificial (or man-made) selectively permeable membrane, which provides a simplified model of the cell membrane. Understanding how the artificial membrane works will help us understand how real cell membranes work! Testable Question: How does the size of a molecule affect its ability to diffuse across a selectively permeable membrane? Independent Variable: Dependent Variable: Hypothesis: Which of the following molecules do you think will diffuse across the selectively permeable membrane? Molecule Will it cross the Why or why not? membrane? Iodine (I 3 - ) Water (H 2 O) Glucose (C 6 H 12 O 6 ) Starch (polysaccharide made up of many molecules of glucose) To test your predictions, you will put solutions of starch and glucose in a bag made of the artificial membrane and put the bag in a beaker of iodine solution. You will allow time for the substances to diffuse across the membrane and then test which of the substances have crossed the membrane. The Initial State diagram shows the locations of each type of molecule or ion at the beginning of the experiment. Based on your hypothesis, predict where each type of molecule or ion will be found after diffusion. In the Final State diagram, write the letter for each type of molecule or ion in the places where you think it will be found at the end of the experiment. Initial State Beaker Final State Synthetic membrane bag W I W G S Water Key W Water I Iodine G Glucose S Starch I believe that this will be the outcome because 2
Measuring our Dependent Variable: o To test whether iodine or starch have crossed the synthetic membrane, you will look for color change. A solution of iodine is tan and a solution of starch is milky white; when iodine and starch are together in the same solution, they react and the solution turns purple, dark blue or black. o To test whether glucose has crossed the synthetic membrane, you will use a glucose test strip. These are thin slips of paper that contain a chemical that will change color from yellow to green in the presence of glucose. o If water can cross the synthetic membrane, it means water is passing into the bag or out of the bag. You will measure this by weighing the bag before and after diffusion to see if the bag gains or loses mass. Check your understanding: For each substance, indicate how you will know whether it crossed the synthetic membrane. What observation will be different, depending on whether or not each substance crossed the membrane? Substance Iodine If this substance crossed the membrane, we would see Expected Observation If this substance did not cross the membrane we would see Starch Glucose Water Procedure: 1) Gather materials: a piece of pre-soaked dialysis tubing (our artificial membrane), two green clamps, and a beaker. (Ensure that your beaker is clean by rinsing it thoroughly in warm water before beginning.) 2) Seal one end of the dialysis tubing with the clamp. Leave about a half centimeter at the bottom to ensure that you get a good seal. Be careful here if it leaks, you will need to start over! 3) Open the other end of the tube by rubbing the end between your fingers until the edges separate. 4) Add 5 ml of glucose solution and 5 ml of starch solution to the tube. Be careful not to mix the graduated cylinders! 5) Use the other clamp to seal the top end of the dialysis tubing. Leave at least a half centimeter again to ensure a good seal. (Try to get rid of most of the air in the bag before you seal it so that your bag does not float in the beaker.) 6) Check to make sure there are no leaks. Rinse the finished bag thoroughly in fresh water. Be sure to clean any glucose or starch solution off of the outside. Dry the bag by gently blotting it with a paper towel. 7) Take the mass of the finished bag and record it in your data table in the initial state row. 8) Add 200 ml of water to your beaker. Then, add 20 drops of the iodine solution to the water in your beaker. (Caution: Iodine will stain skin and clothes and can be harmful to your eyes. Wear goggles and be careful not to splash or spill it. Aprons are available to protect your clothes, but are optional.) 9) Complete the rest of the initial state row of your data table. 10) Put the bag in the beaker and note the start time: (30 minutes later will be ) 11) Wait for 30 minutes. Do NOT stir, shake or move the bag or beaker during this time. 12) While you wait, design investigation #2 and then work on the osmosis & diffusion practice problems. 13) Make observations about the final state of the bag and beaker and record them in your data table. (Be sure to blot the bag dry with a paper towel before taking its mass.) 3
DATA & OBSERVATIONS: In the bag In the beaker Mass of Bag (g) (Assume any change in mass is due Color Glucose? Color Glucose? to movement of water.) Initial State Yes No Final State Yes CONCLUSIONS: Please use the table and diagram below to ORGANIZE your thinking for your conclusions. Molecule Iodine (I 3 - ) Did this molecule cross the membrane? How do you know? Water (H 2 O) Glucose (C 6 H 12 O 6 ) Starch (polysaccharide made up of many molecules of glucose) Initial State Beaker Final State Synthetic membrane bag W I W G S Water Key W Water I Iodine G Glucose S Starch 4
Design a Follow Up Investigation: In your first investigation, you investigated how the size of a molecule affects its ability to diffuse across a selectively permeable membrane. Design a follow-up investigation that would build on this investigation. Do not investigate additional sizes of molecules that is too similar. Instead, choose a variable that would add more to your understanding of osmosis and diffusion in cells. Start by Brainstorming: What are some questions you still have about how substances, like nutrients and waste, move in and out of cells? Testable Question: Independent Variable: Dependent Variable: Controlled Variables: Control Group: Procedure (Include a Diagram): What would the results of this investigation ADD to your understanding of osmosis & diffusion in cells? 5
OSMOSIS & DIFFUSION PRACTICE PROBLEMS: For each problem please draw a diagram with arrows showing how osmosis and/or diffusion would occur. Be sure to show the movement of BOTH water and the substance that is dissolved in the water. Then, please label the solution hypertonic, hypotonic or isotonic. 1. You place a cell containing 3% concentration sodium into a solution containing a 5% concentration of sodium. 2. You place a cell containing no carbon dioxide into a solution containing a 1% concentration of carbon dioxide. 3. You place a cell containing starch into a solution containing no starch. 4. You place a cell containing 0.5% calcium into a solution containing 0.5% calcium. 6
HONORS EXTENSION (OPTIONAL FOR REGULAR BIOLOGY): A model is a simplified representation of a complex biological structure or process. A model focuses on a few key features in order to help us understand a biological structure. Because a model is simpler than the biological structure it represents, a model does not demonstrate all the features of the actual biological structure. For example, the artificial membrane in your experiment today is a model of the selectively permeable cell membrane, but it shows a much simpler kind of selective permeability than an actual cell membrane. The diagram below provides another model of the cell membrane. It gives some idea of the complexity of the selective permeability of an actual cell membrane. The diagram shows that a cell membrane contains proteins that aid in the transport of biologically important ions and molecules across the cell membrane. O 2, Na+, and glucose each diffuse across the cell membrane from a region of higher concentration to a region of lower concentration. Specific protein channels assist in the diffusion of Na+ and of glucose, a process known as passive transport. A third type of membrane protein pumps Na+ and K+ across the cell membrane, in the OPPOSITE direction than they would normally move by diffusion. This protein pump uses energy from ATP to move the molecules against the expected direction. (http://hyperphysics.phy-astr.gsu.edu/hbase/biology/imgbio/lipbitran.gif) Fluid outside the cell Cell membrane has proteins and a double layer of phospholipids. Watery cytosol inside the cell 1. What does this diagram add to your understanding of the cell membrane, beyond what you could have learned from the experiment alone? (Be specific about at least 2 things!) 2. Which feature of cell membranes was demonstrated by your experiment with the synthetic membrane, but is NOT shown in the diagram? (Hint: Think about starch.) 3. This fourth type of transport shown in the diagram is called active transport. How is it different from diffusion or passive transport? 7
4. Explain why active transport is possible in living cells, but not in an artificial membrane. 5. Why might a cell need to use active transport? 6. There is one more major type of transport in cells. Please read page 207 in the text. Draw a diagram that explains this type of transport, and complete the sentence below. A cell would use this type of transport when 8