October 15, 2013
Common student misconceptions about photosynthesis and respiration often arise from the inability to perceive these processes directly. Sensors can help students grow in their understanding of these two concepts and in their ability to design experiments. Students will find the design process engaging and the real-time data collection will help your students connect the variables they are manipulating based on the cell process they are studying. The following activity can help you introduce a unit on the characteristics of living things or cell processes.
Start by giving students rocks, minerals, toothpicks, chalk, leaves, paper, and seeds and have them sort the objects into two piles— living and non-living objects (see more details about the objects in the notes section below). Next, have students describe why they classified these objects as they did. This will quickly reveal, misconceptions your students have about living and non-living things.
Now have students make predictions about what the CO2 level would be for these objects over time. Students will likely predict that the non-living objects will not have a CO2 level change. Additional misconceptions students have about photosynthesis and respiration will also be revealed when they make CO2 predictions for the living objects.
Invite the students to collect CO2 data for the individual objects by providing them with sample bottles that are filled between 50 to 75 mL. In the interest of time, one group can collect data on the rocks, a second can collect data from the minerals, a third group can collect data from the other objects. I recommend calibrating the CO2 sensors before data collection and letting the sensor sit in the sample bottle for about 90 seconds before collecting data to let the humidity level inside the bottle come to equilibrium. Data collection is quick, with only 5 to 6 minutes being necessary to see a trend.
Above you’ll see sample CO2 data from rocks and beans. Your students will find that the objects that have no cellular activity will cause no change in the CO2 level, but the beans and the leaves will cause the amount of this gas to change. The students will be surprised to learn that CO2 is produced by both the leaves and the beans— assuming the leaves are not under direct light. Depending on your students’ understanding of photosynthesis, they likely would have expected that both the leaves and the beans should have absorbed CO2. These discrepant events lend to inquiry, where your students will design experiments to determine why the CO2 increased, and will help your students overcome the misconceptions they have.
Finally, ask your students to develop experiments that would provide more insight into why the leaves and seeds produced CO2. Encourage the development of process skills by having each group of students develop their own experiment to solve one of these problems:
• How does light affect the amount of CO2 produced by leaves?
• How does light affect the amount of CO2 produced by seeds?
• Which parts of a plant produce CO2?
• Which parts of a plant produce O2?
• Under what conditions do plants produce CO2?
• Under what conditions do plants produce O2?
• Can leaves and seeds produce O2?
• Use chickpeas or dry pinto beans for the seeds. They should be set in water a few hours before to encourage germination— but dry the sees before giving them to students.
• Use fresh spinach leaves for the leaves.
• Have lamps with CFL light bulbs and aluminum foil on hand to provide students with materials they can use when they design experiments involving light. CFL light bulbs are recommended, as lights that emit infrared wavelengths will give higher CO2 values than they really are.
• Have fresh plants available that can be used to test the roots, stems, and flowers.
• Follow calibration procedure when using the Oxygen Sensor before collecting data.