In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Application of a Datalogger in Observing Photosynthesis submitted by:
Martin M. F. Choi,* Pui Shan Wong, and Tak Pong Yiu Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, PRC; *
[email protected] checked by:
Mark Case Emmaus High School, Emmaus, PA 18049
Photosynthesis is the conversion of absorbed photon energy from sunlight into various forms of chemical energy by the chlorophyll of green plants. The overall reaction of photosynthesis involves the oxidation of water to oxygen and the reduction of carbon dioxide to form carbohydrates (1): chlorophyll
x CO2 + y H2O + light → Cx(H2O)y + x O2 Studies of photosynthesis have been well documented (2–9) and the process of photosynthesis is widely taught in basic courses in biology, chemistry, and physics. The application of a datalogger (a computer interfaced to one or more sensors) in biology, chemistry, and physics laboratories of Hong Kong secondary schools is getting popular. The major advantage of a datalogger is that students can simultaneously monitor, in real time, various parameters of an environment, such as temperature, electric current, electric potential, pressure, photons, pH, and oxygen. The primary goal of the experiment described here is to demonstrate the role of light energy in photosynthesis. The principal light-absorbing pigment in all green plants is chlorophyll a. Chlorophyll absorbs photon energy, which is utilized in a photosynthetic “reaction center” to extract electrons from a weak donor such as water; the hydrogen and electrons are then used to reduce carbon in carbon dioxide to carbohydrates (7 ). This paper describes the application of a Pasco CI-6542 dissolved-oxygen sensor to monitor the liberation of dissolved oxygen in the photosynthesis of seaweed. The method is simple, safe, and convenient to use and offers a visual study of photosynthesis. Experimental Procedure The experimental set up is displayed in Figure 1. Seaweed was bought from a local aqua shop and kept in tap water before use. A small bunch of seaweed (~30 g) was cut and put in a 50-mL conical flask. To this flask ca. 0.1 g of sodium hydrogen carbonate1 and 30 mL of tap water were added and stirred. A tungsten lamp with either a 25- or a 60-W light bulb was placed 15 cm from the flask to illuminate the seaweed. A Pasco CI-6542 oxygen sensor 2 was placed in the solution. The dissolved oxygen signal was captured at a rate of 10 samples per second and processed by a ScienceWorkshop 500 interface2 consisting of a data logger, serial cables, a power supply, and control software. The data were logged in a notebook PC for real-time display and processing. In a second experiment, an aquarium pump might
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Figure 1. An experimental setup for photosynthesis of seaweed. (1) seaweed in conical flask; (2) tungsten lamp; (3) oxygen electrode; (4) ScienceWorkshop 500 interface; and (5) notebook PC.
be used to saturate the water with air so that this maximum level can be compared to the levels obtained in the photosynthesis experiment. Results and Discussion The change of dissolved oxygen content in the solution could be observed by turning the lamp on and off at 25- or 60-W power, as shown in Figure 2. Illuminating the seaweed with the light beam increased the dissolved oxygen content in the solution. However, when the lamp was switched off, the increase stopped. It was also found that increasing the power of the lamp increased the rate of oxygen generation from the seaweed. The increases in dissolved oxygen concentration were 0.425 and 1.278 mg L᎑1 in 5 min under illumination with 25- and 60-W light bulbs, respectively, a difference of about a factor of 3. In the past, secondary school students have studied the rate of photosynthesis of seaweed by counting the oxygen bubbles produced per minute. This is an inaccurate method and requires much attention in counting the bubbles. The application of a data logger and an oxygen electrode can provide convenient real-time monitoring of oxygen level, and the results are more accurate and can be processed at a later
Journal of Chemical Education • Vol. 79 No. 8 August 2002 • JChemEd.chem.wisc.edu
In the Classroom
Education Department, Hong Kong SAR, for the loan of the ScienceWorkshop system and a notebook PC. We also express our gratitude to Ed Vitz for valuable suggestions about the manuscript. Notes 1. Sodium hydrogen carbonate was obtained from Farco Chemical Supplies, Beijing, China. 2. The Pasco CI-6542 oxygen sensor and ScienceWorkshop 500 interface were purchased from Pasco Scientific, Roseville, CA (http://www2.pasco.com). The total cost is approximately U.S. $720. Alternative sources for oxygen probes and their accessories are Vernier Software & Technology, Beaverton, OR (http://www.vernier.com) and PP Systems, Haverhill, MA (http://www.ppsystems.com). Figure 2. The change of oxygen level in the volumetric flask. (1) 25-W and (2) 60-W light bulbs were used.
time. The experiment can be completed in 30–45 min. In addition, the apparatus can be modified to provide other experiments associated with variations in oxygen concentration. Acknowledgments We would like to express our sincere thanks to Raymond W.-H. Fong of the Curriculum Development Institute of
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.
Arnon, D. I. Sci. Am. 1960, 203, 104. Smith, J. H. C. J. Chem. Educ. 1949, 26, 631. Bassham, J. A. J. Chem. Educ. 1959, 36, 548. Bassham, J. A. J. Chem. Educ. 1961, 38, 151. Shahn, E. J. Chem. Educ. 1978, 55, 48. Bering, C. L. J. Chem. Educ. 1985, 62, 659. Bishop, M. B.; Bishop, C. B. J. Chem. Educ. 1987, 64, 302. MacDonald, J. J. J. Chem. Educ. 1995, 72, 1113. Barceló, A. R.; Zapata, J. M. J. Chem. Educ. 1996, 73, 1034.
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