In this activity, students work with small construction-paper squares to model the reactions of both photosynthesis and respiration. There is nothing here that you couldn’t tell them and show them, but I think it is more effective to have them actually manipulate the “atoms” to see the stoichiometry (the relationship between the relative amounts of each compound).
Student begin with paper squares representing carbon, hydrogen, and oxygen, and they build water and carbon dioxide molecules. Then they combine the atoms from these two substances to make molecules of glucose. Be sure they know that in addition to the reactants, the energy of sunlight is needed to form the higher energy molecules of glucose. This is photosynthesis. One of the key points here is to see what is left over in this reaction (oxygen).
Then I have students reverse the reaction and break apart the glucose and reform water and carbon dioxide. Be sure to mention that the energy from the sunlight that was absorbed in photosynthesis is released in this reaction. Combining glucose with oxygen releases chemical energy that plants and other living things use to live.
Photosynthesis is widely discussed. I think it is equally important to focus on respiration because there are several misconceptions, or at least a lack of knowledge, about respiration. First, notice that all of the oxygen produced during photosynthesis is needed to make use of the glucose for energy. Any time a plant or animal consumes glucose or plant products made from glucose, recovering the energy in that food uses the oxygen released during photosynthesis. Plants consume oxygen. They use oxygen to carry out cellular activities at night and in their stems and roots.
They only time plants are net producers of oxygen is when they store glucose and when they make more plant. Plant cell walls are made of cellulose, which is made from linked molecules of glucose. Wood is cellulose. The oxygen in our atmosphere was released from water by photosynthesis.
The oil in the Earth, the lumber in our forests, the leaf litter on the ground, these are all places where the chemical energy of photosynthesis is stored. It would require all of the oxygen in our atmosphere to release this energy through respiration or combustion (I like to examine the similarities between combustion and respiration in that they both entail combining with oxygen).
This activity requires the materials here and for you to cut up an ample supply of red, green, and yellow squares of construction paper. This activity works well with students working individually, and it would also work well with students working in pairs or groups. I have included an answer sheet for the six questions on the lab and a grading rubric. This activity could be administered by a substitute teacher, and it would certainly work better being discussed with the students by a teacher with some background knowledge of photosynthesis and respiration.
If you are looking for an enrichment activity related to this topic, you could have students investigate how organisms that do not photosynthesize get their energy. There are chemotrophic organisms near deep sea volcanic vents. There are even some parasitic plants that do not photosynthesize. Another extension would be to look at the other ways in which organisms can make use of the chemical energy from plants. For example, fungi can make use of glucose and other sugars through fermentation. Finally, students would be interested to learn about the hierarchy of trophic levels in which plants are the primary producers. The organisms that feed on photosynthetic organisms are primary consumers, and the organisms that feed on them are secondary consumers. This idea culminates with the idea of a food web that has decomposers returning the nutrients in the highest level consumers back into the system.