Plants Lesson 5 Background Information

Three-dimensional Learning Progression

The three activities in this lesson represent the Explanations Phase of the Plants Unit. This involves modeling and coaching with the goal of helping students develop atomic-molecular scale accounts of biosynthesis that answer the question: where do the atoms come from that make up a plant? Plants are different from animals because animals take in organic materials for food. Most of the atoms in a plant come from CO2 in the air, and a few atoms come from water and minerals in the soil, like the nitrogen from ammonia.

This lesson is about molecules and how all the molecules of a plant are made during biosynthesis. During biosynthesis, plants use the glucose produced by photosynthesis and soil minerals to produce other small organic molecules or monomers (including amino acids, fatty acids, other simple sugars) to construct large organic polymers: primarily proteins, fats, and carbohydrates. Most of the chemical energy stored in the bonds is transferred from monomers to polymers. These polymers are used to construct the plant’s cells and organelles. Thus most of the dry mass of plants originates in CO2 taken in from the air. Coming into this unit, students may incorrectly think that plants either create mass themselves (e.g., through cell division) or build most of their mass using molecules from soil and water. The activities in this lesson help students revise these ideas.

We will consistently focus on the idea that understanding carbon-transforming processes involves answering the Three Questions:

  • The Matter Movement Question: Where are molecules moving? (How do molecules move to the location of the chemical change? )
  • The Matter Change Question: How are atoms in molecules being rearranged into different molecules? (What molecules are carbon atoms in before and after the chemical change? What other molecules are involved?)
  • The Energy Change Question: What is happening to energy? (What forms of energy are involved? What energy transformations take place during the chemical change?)

Matter (the Matter Movement and Matter Change Questions). We find that even students who have learned how to balance chemical equations do not appreciate the meaning of the procedure:

  • Conservation of atoms (the Matter Change Question): The numbers of atoms on the left and right side of a chemical equation have to be the same because they are THE SAME ATOMS! A chemical equation just shows how they are being rearranged into new molecules.
  • Conservation of mass (the Matter Movement Question): ALL the mass of any material is in its atoms (and none of the mass is in the bonds, which are just attractive forces between atoms). So the mass of the products is always the same as the mass of the reactants.

Energy (the Energy Change Question). Chemists, physicists, and biologists have many different conventions for describing and measuring chemical energy. We have a deeper explanation of the conventions used in Carbon TIME units and how they relate to conventions used in different scientific fields on the Carbon TIME website in a document called “Carbon TIME Content Simplifications.” Here are some key points:

  • All bond energies are negative relative to individual atoms. So during a chemical reaction, it always takes energy (the activation energy) to break bonds. Then, energy is released when new bonds are formed.
  • Whether a chemical reaction releases energy or not depends on the total energy of the reactants, compared with the total energy of the products. So energy is released when the total bond energy of the products is lower (i.e., more negative relative to individual atoms) than the energy of the reactants.
  • In systems like our atmosphere, where excess oxygen is always present, the most abundant sources of chemical energy are substances that release energy when they are oxidized (e.g., substances with C-C and C-H bonds).

Our research has consistently showed that these ideas are extremely difficult for students who have not formally studied chemistry. We therefore use the convention of twist ties to identify bonds that release energy when they are oxidized.

The investigations in all units will make use of two essential tools:

  • Digital balances. Students can detect movement of atoms (the Matter Movement Question) by measuring differences in mass. In this activity, students observe a difference in the plants systems.
  • Bromothymol blue (BTB) is an indicator that changes from blue to yellow in response to high levels of CO2. Thus changes in BTB can partially answer the Matter Change Question by detecting whether there is a chemical change that has CO2 as a reactant or product.