BACKGROUND (continued)

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3 BACKGROUND (continued) A cell must exchange materials with its surroundings, a process controlled by the plasma membrane. Plasma membranes are selectively permeable, regulating the cell s molecular traffic: Small, uncharged polar molecules and small nonpolar molecules, such as N 2, freely pass across the membrane. Hydrophilic substances such as large polar molecules and ions move across the membrane through embedded channel and transport proteins. Water moves across membranes through channel proteins called aquaporins. Different types of cells fair best in different types of solutions: a) ANIMAL CELL fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water. b) PLANT CELL turgid and generally fair best in a hypotonic environment tendency for water uptake is balanced by the elastic wall pushing back on the cell. ARROW INDICATES WATER MOVEMENT WHEN CELL IS FIRST PLACED IN THE SOLUTION!!! In plants, water pressure against the cell wall provides turgor pressure. An isotonic solution in plants generally promotes limp (flaccid) cells. A lack of water in plant cells causes shrinking of the cytoplasm away from the cell wall, a process referred to as plasmolysis.

4 PART A STRUCTURED INQUIRY: Observing Osmosis & Plasmolysis TEACHER DEMONSTRATION Materials List: Celery stick soaked in water, celery stick soaked in saltwater Video Clip: plasmolysis in Elodea cells, Video Clip: plasmolysis in Onion cells Colored Pencils Procedure 1. Observe the celery stick that was soaked in water. Record your observations in the table below. 2. Break the celery stick that was soaked in water. Record your observations in the table below. 3. Observe the celery stick that was soaked in saltwater. Record your observations in the table below. 4. Break the celery stick that was soaked in saltwater. Record your observations in the table below. Observation of Celery Stick Reaction to Breaking Sketch the Movement of H 2 O Soaked in Water Soaked in Saltwater Analysis of Results: Part A 1. When you drink a glass of water, most of it is absorbed by osmosis through cells lining your small intestine. Drinking seawater can actually dehydrate the body. Explain how this might occur. 2. A marine clam is mistakenly added to a freshwater aquarium. What will happen to the clam and why? 3. What hypothesis was this experiment designed to test?

5 Procedure 4. Observe the following video clip: What do the Elodea cells look like before the solution is added? Sketch and record your observations in the table below. What do the Elodea cells look like after the solution is added? Sketch and record your observations in the table below. 5. Observe the following video clip: What do the onion cells look like before the solution is added? Sketch and record your observations in the table below. What do the onion cells look like after the solution is added? Sketch and record your observations in the table below. Elodea Cells Before/After Treatment Onion Cells Before/After Treatment Analysis of Results: Part A 6. On the basis of your observations, explain the processes that are occurring in both the Elodea and onion cells throughout the course of the video animations.

6 PART B STRUCTURED INQUIRY: Observing Diffusion TEACHER DEMONSTRATION Materials List: Dialysis tubing, plastic cup, glucose/starch solution, distilled water, iodine-potassium iodide (IKI) solution, dropping pipet, glucose test strips, funnel. In this activity, you will explore the diffusion of different molecules through dialysis tubing, a semipermeable membrane. You will use glucose test strips to check for the presence of glucose and IKI solution to test for the presence of starch. As you probably know, IKI reacts with starch to give a dark blue, almost black color. When IKI reacts with starch, it becomes a part of the starch molecule and is removed from solution. Procedure 1. Pour ml of distilled water into a plastic cup. Add approximately 4 ml of IKI solution to the water and mix well. Record the initial solution color in Table Dip a glucose test strip into the solution and record the initial glucose test results in Table 1. Use a + symbol to indicate a positive glucose test result and a symbol to indicate a negative glucose test result. Discard the glucose test strip. 3. Obtain a piece of dialysis tubing that has been soaked in water. Touch the dialysis tubing ONLY AT the ends so that the oils from your fingers do not clog the pores in the tubing. The tubing should be soft and pliable. Close one end of the tubing by tying it into a knot. Roll the other end of the tubing between your thumb and index finger to open it. 4. Using a small funnel, pour 15 ml of glucose/starch solution in the dialysis bag. Smooth out the top of the bag, gently running it between your thumb and index finger to expel the air. Tie off the open end of the bag. Leave enough room in the bag to allow for expansion. 5. Record the initial color of the glucose/starch solution in Table Immerse the dialysis bag in the solution in the cup. Make sure it is completely covered by the solution in the cup. 7. Wait 30 minutes. While waiting, complete the following exercise: Use the space provided on the next page to make your predictions and explanations about the movement of molecules in the diagram above.

7 8. After 30 minutes, remove the bag from the cup. Blot it with a paper towel. Cut a slit in the bag large enough to insert a glucose test strip. Fill in the final columns of Table 1. Analysis of Results: Part B 1. Does this activity account for the diffusion of all molecules that you listed in Figure 2? If not, what data could have been collected to show the net diffusion of this molecule or molecules? 2. What does your data tell you about the sizes of the molecules relative to the pore size of the dialysis tubing?

8 PART C STRUCTURED INQUIRY: Modeling Cellular Environments TEACHER DEMONSTRATION Materials List: Balance, graduated cylinder, beaker 1000 ml, funnel, 20 cm dialysis tubing, 1M sucrose solution, 1M salt solution, 1M glucose solution, 5% albumin (protein) solution, distilled water. In this activity, you will construct and simulate model cells in an external environment, to relate solutes passing through a semi-permeable membrane in hypertonic, hypotonic, and isotonic solutions. The pores in dialysis tubing allow some molecules to freely diffuse across the membrane and some to be restricted. In this lab you will use dialysis tubing as a model cell membrane. Procedure 1. Obtain 5 pieces of pre-soaked dialysis tubing. Tie a knot in one end of each piece of tubing. 2. Measure and pour 10 ml of each of the four prepared solutions into separate graduated cylinders. The solutions are: salt, glucose, sucrose, and protein. 3. Open the dialysis tubing and use a funnel to pour 10 ml of prepared solution into the tubing. Tie a knot in the open end to form a model closed cell membrane (similar to a bag). Be sure to leave enough space in the bag for expansion. Minimize air enclosed in the tubing. 4. Fill beakers with about 100 ml of the solutions (water or salt) to be paired with your model cells. See data table for pairings. 5. Determine the initial weight of each cell and record this data in the data table below. 6. Completely immerse the model cells in their pairing solutions in the beaker. Start your timer. 7. Given what you know about solute concentration, predict whether each cell volume will grow, shrink, or remain constant. Record your predictions below: - 8. Allow the cells to soak for 30 minutes. When 30 minutes has passed, remove the model cells from the solution, pat them dry, and determine the final weight of each. Record this data in the data table. 9. Calculate the percent change in weight and record your results in the data table. Modeling Cells Data Table Cell 1 (protein) Cell 2 (sucrose) Cell 3 (water) Cell 4 (glucose) Cell 5 (salt) Cell Weight (g) Paired Extra-Cellular Solution (in beaker) Start Time 0 End Time 30 min. Salt Water Water Salt Water % Change in Mass* * (Final Mass - Initial Mass) x 100 (Initial Mass)

9 Analysis of Results: Part C 1. Examine the initial and final weights of the model cells. What causes the mass of the dialysis bags to change? Was there more or less water in the dialysis bags at the conclusion of the experiment? Explain. 2. From your results, which solutes, if any, diffused across the membrane, and which, if any, were restricted? Explain why you think this occurred. 3. How is dialysis tubing different from a cell membrane? How is it similar? 4. List three variables that could influence the outcome of this experiment. Briefly describe a method of control that could be used for each of these variables.

10 PART D STRUCTURED INQUIRY: Water Potential in Plant Cells TEACHER DEMONSTRATION Materials List: Plastic cups, distilled water, sucrose solutions, cork borer, potato cores, plastic wrap, paper towels, balance. In this activity, you will investigate water potential by immersing potato cores in sucrose solutions and determining the change in mass, if any, of the cores. You will graph your data and use the graph to determine a value for C. Using the experimentally determined value for C, you will then calculate a value for s. Procedure Continue to step 9 after the potato cores have been in the sucrose solution overnight.

11 Analysis of Results: Part D 1. Graph your percent change in mass of potato cores in different solutions. Gridlines are provided on the next page. 2. On your completed graph, find the point where the line of your data crosses the 0 line (x-axis) of the grid. This is the equilibrium point; at this point there is no net gain or loss of water from the potato cells. 3. Read the corresponding value of sucrose molarity for this point. This is the molar concentration of sucrose that produces equilibrium. Below, record this concentration of sucrose as your experimentally determined value for C. Convert ambient temperature from C to K. 4. Review the information on water potential provided in the introduction at the beginning of this lab. Using the formula s = -icrt, calculate the solute potential at equilibrium. Show your calculations in the space below. 5. Using the formula = p + s, give the following:

12 6. Imagine that you are an agri-science consultant to a large corporation that raises 7,000 acres of wheat on desert land adjoining the Mediterranean Sea. Just before the wheat matures, all the wells used for irrigation run dry. The farm manager wants to irrigate the fields with water drawn from the Mediterranean. From previous tests, you know that the average solute potential of root tissue taken from wheat fields is bars. You test the seawater and determine its solute potential to be bars. What will you advise the farm manager and why?

13 PART E STRUCTURED INQUIRY: Surface Area to Volume Ratio in Cells TEACHER DEMONSTRATION Materials List: 3 Phenolphthalein agar cubes: 3 x 3 cm, 2 x 2 cm, and 1 x 1 cm, 1 plastic spoon, 1 plastic cup, 1 metric ruler 6, 100 ml of white vinegar, timer. The agar cubes have been prepared with 1% phenolphthalein, which is a ph indicator. The chart below indicates a color scale of ph for phenolphthalein. The blocks are pink because the agar blocks were soaked in 0.01% sodium hydroxide. Phenolphthalein Color Indicator Chart Color ph Acid or Base Colorless Acidic or slightly neutral Pink to Red Basic Procedure 1. Obtain agar cubes in a plastic cup from your teacher. Be careful not to scratch any surface of the cubes. 2. Using the metric ruler, measure the dimensions of each agar cube and record the measurements in the table below. 3. Place the three cubes carefully in a plastic cup. Add white vinegar (acetic acid) until the cubes are submerged. Using a plastic spoon, keep the cubes submerged for 10 minutes turning them as needed. Be careful not to scratch any surface of the cubes. Be sure to start the timer once the cubes are submerged. 4. As the cubes soak, calculate the surface area, volume, and surface area to volume ratio for each agar cube. Record this data in the table below. Cube Size (length, width, and height of each side in cm) Surface Area (cm 2 ) Volume (cm 3 ) Surface Area/Volume Ratio (cm 2 : cm 3 or 1:cm) Formulas Surface Area = length x width x # of sides Volume = length x width x height Surface Area/Volume Ratio = surface area / volume Extent of Diffusion = (total cube volume - volume of cube that has not changed color) x 100 total cube volume

14 5. After 10 minutes has elapsed, use the spoon to remove the agar cubes and carefully blot them on dry paper towel. DO NOT CUT THE AGAR CUBES UNLESS EXPLICITLY TOLD TO DO SO BY YOUR TEACHER. 6. Using a metric ruler, measure the distance in centimeters (cm) that the white vinegar diffused into each cube (see Figure below). Record this as the distance from the surface in the table below. For each cube, measure the penetration of vinegar into the agar (distance in cm from edge of white edge of cube to edge of pink in the cube). 7. Calculate the rate of diffusion for each cube in centimeters per minute (cm/min). Record your calculations in the table below. 8. Calculate the volume of the portion of each cube which has not changed color (in other words, the portion of the cube that is still pink). Record your calculations in the table below. 9. Calculate the extent of diffusion into each cube as a percent of the total volume. Record your calculations in the table below. 10. Graph the rate of diffusion (cm/min, x-axis) relative to the surface area to cell volume ratio (1/cm, y-axis). 11. Graph the extent of diffusion (x-axis) relative to cell volume and surface area (y-axis). Show all Calculations: Cube Size (length, width, and height of each side in cm) Distance from Surface (cm) Rate of Diffusion (cm/min) Volume Still Pink Extent of Diffusion

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16 Analysis of Results: Part E 1. Examine your data. What dimensions supported the fastest rate of diffusion? Why? 2. What dimensions supported the greatest diffusion percent total volume? Why? 3. Construct a useful graph of the relationship between cell dimension to the extent of diffusion. 4. The size of some human cells is 0.01mm. Using the formulas in this activity, calculate the surface to volume ratio of such a cell (assume 0.01 mm cube). Describe the extent of diffusion into this living cells as compared to the smallest agar cube. Explain.

17 PART F OPEN INQUIRY: Water Potential in Plant Cells STUDENT-DESIGNED INVESTIGATION Suggested Materials List: Plastic cups, graduated cylinders, paper towels, balance, dialysis tubing, red mystery solution, orange mystery solution, yellow mystery solution, green mystery solution, clear mystery solution, blue mystery solution, china markers, masking tape, timers, pipettes. In this activity, you will investigate water potential by immersing potato cores in sucrose solutions and determining the change in mass, if any, of the cores. You will graph your data and use the graph to determine a value for C. Using the experimentally determined value for C, you will then calculate a value for s. Problem: A laboratory assistant prepared solutions of 0.8 M, 0.6 M, 0.4 M, 0.2 M and 0.0 M sucrose, but forgot to label them. After realizing the error, the assistant randomly added color to each of the flasks containing these five unknown solutions as red, orange, yellow, green, blue, and clear. Challenge: 1. Design an experiment, based on the principles of diffusion and osmosis that the assistant could use to determine which of the flasks contains each of the five unknown solutions. Use the following steps when designing your experiment: Describe the background information, including that which was discovered in previous experiments. Include this information in your inquiry lab NB. Define the question. Include this information in your inquiry lab NB. State a testable hypothesis. Include this information in your inquiry lab NB. Describe the experiment design with controls, variables, and observations. Discuss the materials & methodology used to collect data. Include this information in your inquiry lab NB. Sketch the experimental setup. Label appropriate components. Include this information in your inquiry lab NB. Describe the possible results and how they will be interpreted. Include this information in your inquiry lab NB. After the plan is approved by your teacher: 2. Perform your experiment and determine the molar concentration of each of the mystery solutions. Outcomes of the step-by-step procedure should be documented in your inquiry lab NB. This includes recording the calculations of concentrations, etc., as well as the weights and volumes used. 3. Data tables should be included and used throughout your experiment. 4. The results should be recorded (including drawings, photos, data print-outs, etc.). 5. Graphs of the results should be included. 6. The analysis of the results should be recorded. Draw conclusions based on how the results compared to the predictions. 7. An appropriate statistical analysis should be performed on the data. 8. Limitations of the conclusions should be discussed, including thoughts about improving the experiment design, statistical significance and uncontrolled variables. 9. Further study direction should be considered.