The main objective of our introductory biology sequence is to get students to think critically. To that end, we utilize the investigative approach in the laboratory. Students learn to design their own experiments and learn biology in the context of major processes and concepts. Modern technologies are used as a means to achieve these goals and as a way to expose students to the uses of technology in biology. Computers can easily capture dynamic processes graphically, and additionally, can be used to collect large amounts of data for analysis.
One laboratory exercise that we've developed allows students to examine how plants (sunflower seedlings) take up CO2 over a short time period in a sealed chamber under various light regimes. Carbon dioxide concentration is measured in parts per million (ppm) with a gas analyzer. The analyzer is connected to a computer with an A/D board; software allows CO2 uptake to be graphed on the computer as changes in the atmospheric concentration of the chamber. The setup is sensitive and allows students to quickly perform multiple experiments.
Students are introduced to the techniques and equipment in the first week of the 2-week lab. Our laboratory sections are 2 hours long, which is partly why this lab takes 2 weeks. Teaching Assistants demonstrate the methods which will be used and the students are given the task of designing the experiment to investigate some aspect of photosynthesis before the second week. The investigative approach works well in this lab because students have experience with the methods and equipment before they design their experiment and the Teaching Assistants can provide the necessary background information which students need to ask appropriate questions about the system. For data analysis, students can print graphs as they go, or store data on disk to be analyzed later on a spreadsheet program. A simple comparison of the slopes will give an indication of the rate of carbon dioxide change in each experiment, although more complex analyses can be performed. By changing the light intensity and using different color filters students can examine the rates of carbon dioxide uptake under various experimental conditions.
This laboratory is part of an introductory course, but modifications would allow it to be used in upper level courses. A comparison of shade tolerant and intolerant plants, or C3, C4, and CAM plants could be performed, for example, as well as studies of area-specific CO2 uptake and energy flow analyses.
Although the initial cost of the equipment may be high, the equipment is useful for many other labs. In fact, we use the gas analyzers and data acquisition software for a yeast respiration lab and the gas analyzers alone in an exercise physiology lab. Any type of recording equipment which has an electrical output can be connected to the A/D boards to collect data on the computer, including spectrophotometers, dissolved oxygen meters, pH meters, etc. The computers and associated hardware/software are quite versatile.
In our lab on photosynthesis and respiration in plants students also investigate starch accumulation in seedlings under varying light intensities, and dissolved oxygen changes in aquatic plants under varying light quantity and quality. Because we have only two CO2 setups/lab it is important for the students to have alternate exercises to work on while other groups are using the CO2 uptake apparatus.
The starch accumulation experiment uses radish or Fastplant (Brassica rapa) seedlings grown for 1 week under different light treatments. Students use a standard technique of clearing pigments with hot ethanol and then use an iodine solution to stain the starch. Seedlings are examined under dissecting microscopes to qualitatively compare the amount of starch present in different treatments. The measurement of dissolved oxygen in aquatic plants is used much the same as the CO2 uptake studies. The dissolved oxygen meter is connected to the computer and aquatic plants (e.g., Chlorella sp.) are exposed to varying light intensities and qualities. Lower wattage flood lights are used in place of high pressure sodium lamps, since the aquatic plants are much more sensitive to temperature fluctuations.
It is appropriate to link together aspects of the three experiments and wrap it all up with material from the lecture at the end of the lab. For instance, one could discuss carbon dioxide uptake and production of oxygen under varying light intensities and make predictions for starch accumulation and subsequent growth of plants. By examining multiple aspects of the photosynthetic process, students are better able to integrate the information into an overall picture which includes cellular mechanisms, physiology, and ecology. The other two exercises are not included here, but the entire lab exercise as it appears in our in-house lab manual is available upon request from either author.
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Page last updated 11 May 2000