Modeling in a Science Classroom

Students objective is to know the chemical formula of all reactants and products involved in the photosynthesis equation.  If students know these components and understand the relationship between photosynthesis and cellular respiration, they can analyze how energy is stored, released, and transferred within and between systems.  

The Lesson

Background: The topic of photosynthesis is a fundamental concept in biology, chemistry, and earth science. Educational studies have found that despite classroom presentations, most students retain their naïve idea that a plant’s mass is mostly derived from the soil, and not from the air. To call students’ attention to this misconception, at the beginning of this lesson we will provide a surprising experimental result so that students will confront their mental mistake. Next, we will help students better envision photosynthesis by modeling where the atoms come from in this important process that produces food for the planet. Using models, students will utilize the atoms from carbon dioxide to build glucose. Additionally there is a follow-up activity where the students can build both cellulose and starch from the same glucose molecules to demonstrate how glucose becomes incorporated into the roots, shoots and wood—the structures of the plants we see around us!

Pt. I:  What % of a plant weight comes from soil?  Take a vote online: poll everywhere or  Students will then read about Van Helmont's experiment and consider why air is overlooked as source of plant weight versus soil components.  Less 0.1% of the plant’s mass came from the soil. Many students believe that plants are mostly made from the materials in the ground, instead from the molecules in the air. They often choose an answer like 20 % or 40%. So now the students will see the class results and the common misunderstanding. During the next pause, the students will be asked to discuss why most people have a difficult time believing that the matter of a tree comes from the air. In the discussion, students may mention that CO2 is a gas and it can’t be seen. Also, even if students can accept that air molecules are utilized by plants, it can be difficult from them to accept that the gas molecules can be compacted into a solid, a solid as heavy and large as a massive tree trunk.

Pt. II:  Build molecules. Each brick is an atom. Three different colors of bricks represent H, C and O. Molecules can be built from the bricks when the atom bricks joined together in defined shapes, with the correct formulas. The black bricks, which are carbon atoms, are first located in the carbon dioxide and then become locked up in the glucose molecules. Students follow a map to build glucose molecules.  It is often good to provide a fundamental overview so that if students forget all the details, they will still know the basic principle very well. Also when students do an activity with their hands like this, it helps their brains experience the concept in a different way, making it more memorable.

Pt. III:  Follow the maps.   Many students think of photosynthesis as a plant’s way of “breathing” in the same way animals are breathe in one gas and release another. This misconception is strengthened by diagrams that show the cycling of gases on planet. They show that plants are taking in CO2 and breathing out O2 and that animals are doing just the opposite, breathing in O2 and breathing out CO2. So the question for the pause is “Do plants need oxygen?”  (Scaffolding) Students recall that plant cells also contain mitochondria. So while plant cells may produce their own food as glucose molecules, they ultimately need to get the energy out of the glucose. Therefore cells, whether plant or animal, burn glucose and burning requires oxygen. This process is named cellular respiration. The point is made here that trees and other green plants on our planet make lots and lots more glucose molecules than they need to burn for themselves. Our planet has benefited from excessive photosynthesis. In closing, we admire plants for all they do for us, and come back to the original question. So what do plants get from soil? Plant roots bring in dissolved minerals along with the water molecules. Minerals such as nitrogen, phosphorous, and potassium are necessary for building proteins and DNA molecules.

Pt. IV:  Groups can combine their glucose molecules with other tables by showing the hydrolysis reactions which maps are provided.  This will also show students how although molecules contain the same elements, structural differences affect whether animals can use certain molecules or not, i.e. cellulose is not digestible in humans due to the orientation of H and O molecules.

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