Decomposers Lesson 4 Background Information

Three-dimensional Learning Progression

This lesson helps students develop atomic-molecular scale explanations for what happens when things decay. In this Lesson, students return to the data they collected from the bread mold investigation as a starting point for explaining what happens when fungi grow, move, and function at an atomic-molecular scale.

This lesson focuses on the use of food—as a source of energy—and on the carbon-transforming process that makes food energy available to decomposer cells—cellular respiration. Every living organism, from the smallest bacteria to the largest tree in the forest, needs to acquire a source of chemical energy, which is found in the C-C and C-H bonds in organic matter. Once organic matter is oxidized, the chemical energy found in the high-energy bonds is made available for cell functions such as movement, chemical work, and transport of materials. Ultimately all of this energy becomes heat. The atoms once tied up in organic molecules are rearranged into inorganic water and carbon dioxide. Unfortunately, many students incorrectly see cellular respiration as the way food is converted into energy to move and function. Students even make these matter-energy conversions at the atomic-molecular scale when they learn about ATP (another organic molecule). Students need to develop an explanation of cellular respiration that conserves both matter and energy, and makes the connection between atomic-molecular transformations and macroscopic observations.

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? How do molecules move away from 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 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).

The two activities in this lesson represent part of the Explanations Phase of the Decomposers Unit. This involves modeling and coaching with the goal of helping students develop atomic-molecular scale accounts of cellular respiration that was one of the drivers of the macroscopic changes they observed in their Bread Molding investigation in the previous lesson.

Key Ideas and Practices for Each Activity

Activity 4.1 is the first part of the Explanations Phase of the instructional model (going down the triangle) for cellular respiration. Students construct molecular models of the chemical change that took place during the investigation to help them develop an atomic-molecular explanation for how decomposers use food to move, breathe, and function. Activity 4.1 also simplifies the full story of what happens to matter during the multi-step process of cellular respiration.

The activity uses a standard but simplified formula for the overall chemical change:

C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O

This incorrectly suggests that some of the oxygen atoms O2 in end up in CO2, which is not actually the case. A more accurate formula to represent the multi-step process would be as follows:

C6H12O6 + 6 O
2 + 6 H2O-> 6 CO2 + 12 H2

Thus, all of the oxygen atoms in O2 (bolded in the equation above) end up in H2O, while the oxygen atoms in CO2 all come from glucose or water.

In practice biochemists often do not try to trace individual H and O atoms through biochemical processes, since the processes always take place in environments where water provides H and O atoms.

In Carbon TIME units we explain that the chemical energy released during cellular respiration is used for cell functions and ultimately converted to heat. In more advanced classes, you may choose to include another intermediate step in this story: the energy released by oxidation of glucose is used to convert ADP (adenosine diphosphate) and phosphate into ATP (adenosine triphosphate), which is the immediate source of energy for cell functioning. Some of your students may believe that ATP is a form of energy and not a form of matter or that the matter in glucose is converted to ATP, so pay particular attention to how students describe ATP when learning about cellular respiration. ATP is matter with chemical energy stored in its bonds.

Activity 4.2 is the second part of the Explanations Phase of the instructional model (going down the triangle) for cellular respiration. Students use the Explanations Tool to construct final explanations of what happens when decomposers oxidize small organic molecules to release energy to move and function, and then release water and carbon dioxide. Ideally, at this phase their explanations will combine evidence from macroscopic-scale observations during the investigation with their new knowledge of chemical change at the atomic-molecular scale.

Key Carbon-Transforming Processes: Cellular Respiration

Content Boundaries and Extensions