ACTIVITY 25: How does light of various wavelengths (different colors) influence photosynthesis?

Transcription

1 The Bronx High School of Science Valerie Reidy, Principal Name Department of Biology J. Donahue, Biology Coordinator Date ACTIVITY 25: How does light of various wavelengths (different colors) influence photosynthesis? Introduction: Green plants make sugar and oxygen from carbon dioxide and water: 6CO H 2 O C 6 H 12 O 6 + 6O 2 + 6H 2 O This is an endergonic reaction in which visible light is the energy source. The effectiveness of different wavelengths (colors) of visible light in driving the photosynthetic reaction has been studied extensively. In today's laboratory exercise, you will work in groups of four to investigate the effect of different wavelengths of light on the rate of photosynthesis. The green aquatic plant Elodea and a fluorescent lamp with a light spectrum similar to natural sunlight will be available for your use. The acid-base indicator, bromthymol, can be used to detect changes in CO 2 concentrations because: CO 2 + H 2 O H 2 CO 3 (carbonic acid) H + + HCO 3 - The release of hydrogen ions into the solution lowers the ph while the removal of hydrogen ions, increases the ph of the solution. Bromthymol changes color between ph 6.0 and ph 7.6 going from yellow to green to blue as the solution becomes more basic. Bromthymol in dilute quantities is not toxic to aquatic plants. How can bromthymol solution readily be changed from blue to green or yellow with nothing more than a soda straw? Materials: 8 small test tubes & rack, 8 small cuvettes with lids (3.5 ml capacity), 1-10 ml graduated cylinder, ml beaker, 1-50 ml flask containing 25 ml of bromthymol blue, elodea, blue, green, yellow and red cellophane squares, scotch tape, aluminum foil, 1 straw, marking pencil, white paper, light, colorimeter & gray DIN adapter, LabPro computer interface with power cord & USB wire, computer, distilled water. Activity 25 Revised March 2005 Page 1 of 5

2 Procedure: Identify the independent and dependent variables: Independent variable Dependent variable Describe your experimental design being sure to include appropriate controls. Predict which color of light will work best for photosynthesis and which color will be least effective. You will use the small test tubes. Fill them about half full with bromthymol yellow. Sprigs of elodea should be approximately the same size and have about the same number of leaves. If the test tubes are placed in front of the lamp as shown in Figure 2, the beaker will act as a heat shield, protecting the elodea from warming by the lamp. Some groups may want to try it without the heat shield. Experimental design Figure 2 After discussing the design with your lab partners and the class, promptly set up the experiment. Use Table 1 below to enter qualitative results. The following scale may be used to record your observations. Color changes will be minimal and require judgment. 1 = degrees of yellow 5,6 = degrees of blue-green 2,3 = degrees of yellow-green 7 = degrees of blue 4 = degrees of green Table 1. Lab group Tube 1 Tube 2 Tube 3 Tube 4 Tube 5 Tube 6 Tube 7 (no light) (blue light) (green light) (yellow light) (red light) (white light) (no plant) Average Activity 25 Revised March 2005 Page 2 of 5

3 Collect quantitative data using the colorimeter. The Colorimeter is a computer sensor, which measures the amount of light absorbed by a solution. When bromthymol changes from yellow to blue, due to the use of carbon dioxide by the plant during photosynthesis, its absorbance of light increases. This increase in absorbance corresponds with an increase in photosynthesis. In the colorimeter (Fig. 1), monochromatic light passes through a cuvette containing a solution sample. Some of the incoming light is absorbed by the solution resulting in light of a lower intensity striking a detector. cuvette detector You will use the colorimeter to measure which wavelength of light caused the most photosynthesis to occur in the test tubes. The greater the color changes of the bromthymol from yellow to blue, the greater the rate of photosynthesis, and the greater the absorbance in the colorimeter. Follow these directions to set up the colorimeter and take absorbance readings: 1. Connect the Colorimeter to the LabPro computer interface. If this has already been done, skip to Step 2. a. Plug the power cord into the outlet and into LabPro. b. Connect the thin, flat end of the USB wire into the side of the keyboard. Connect the square end of the wire into the appropriate slot on the side of the LabPro. c. Connect the black colorimeter, using the short gray DIN adapter wire, to Channel 1 of the LabPro. 2. Prepare the computer for data collection by logging onto the desktop as you normally would. a. Click on: Start, Programs, Science, Vernier software, LoggerPro 3.1. b. Go to file and open the folder Biology with Computers. c. Open the file 07 Photosynthesis 3. Underneath the toolbar, you should see: LabPro is connected on LabPro-USB (left side) and transmittance and absorbance readings (middle). The absorbance reading is prominently displayed on the screen. 4. You are now ready to calibrate the Colorimeter. a. Prepare a blank by filling a cuvette approximately 3/4 full with distilled water. 5. Take Note: To correctly use a Colorimeter cuvette: Activity 25 Revised March 2005 Page 3 of 5

4 All cuvettes should be wiped clean and dry on the outside with a tissue. Handle cuvettes only by the top edge of the ribbed sides. Position the cuvette in the colorimeter so that light shines through the clear sides. 6. Calibrate the Colorimeter. Open the Colorimeter lid. Holding the cuvette by the upper edges, place it in the cuvette slot of the Colorimeter. Close the lid. First Calibration Point Go to the toolbar and click on Experiments. Choose Calibrate CH1: Colorimeter (%T) and then click. Turn the wavelength knob on the Colorimeter to the 0% T position. Type 0. where it says to Enter Value. When the displayed voltage reading for Reading 1 stabilizes, click. Second Calibration Point Turn the knob of the Colorimeter to the Red position (635 nm). Type 100 where it says to Enter Value. When the displayed voltage reading for Reading 2 stabilizes, click, then click. 7. When enough time has elapsed to allow photosynthesis to occur: Remove the elodea from each test tube and pour the bromthymol into a cuvette filling it about 2/3 full. Number the cuvettes near the top or on a lid if available. Place each cuvette in the cuvette slot of the Colorimeter and close the lid. Allow 10 seconds for the readings displayed in the meter to stabilize, then record the absorbance value in Table 2. Remove the cuvette. Table 2 - Absorbance Lab group Cuvette 1 Cuvette 2 Cuvette 3 Cuvette 4 Cuvette 5 Cuvette 6 Cuvette 7 (no light) (blue light) (green light) (yellow light) (red light) (white light) (no plant) Average Activity 25 Revised March 2005 Page 4 of 5

5 PROCESSING THE DATA 1. Go to the toolbar and open a new file. Follow these directions to create a graph In the data table, double click X. i. Type Wavelength for the name, Wl for the short name, and nm for the units. Click done. Double click Y in the data table. i. Type Absorbance for name, abs for short name and leave units blank. Click done. Right click anywhere on the graph. i. Choose Cartesian graph options. ii. Type Absorbance vs. Wavelength for the title. Click on the tab for Axes Options. o For the Y-axis label, type Absorbance.. o For scaling, choose manual from the drop-down menu o For the top number, enter 0.4 o For the bottom number, enter 0 o For the x-axis scaling, choose manual. o For left, enter 400. o For right, enter 700. Click done 2. Enter data into the data table by clicking on a cell and typing. Hitting the tab or enter button moves you to the next cell. Each data point is graphed automatically after you type and hit enter. For wavelength, enter the following lengths for the four colors blue 400 green 500 yellow 580 red 700 For absorbance, enter your recorded data. 3. Review the graph with your lab partners. Save the file to your personal account. Print a copy for each person in your group. If unable to print, you may have to recreate the graph at home on a computer or by hand. The graph answers summary question #5 below. Summary Questions: (answer on a separate sheet of paper) 1) How does light of various wavelengths (different colors) influence photosynthesis? 2) What is the fate of the CO 2 removed from the water by the green plants? 3) Describe the controls used in your experimental design. Explain why each was necessary. 4) List your sources of experimental error. Suggest how each could be eliminated or minimized. 5) Draw a graph showing the effects of increasing wavelengths of light on the CO 2 uptake by the plant. Label the axes appropriately. By doing this you will have developed an action spectrum. Activity 25 Revised March 2005 Page 5 of 5

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