Animals Lesson 4 Tab 3

This lesson also focuses on the second use of food—as a source of energy—and on the carbon-transforming process that makes food energy available to animal 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 body heat. The atoms once tied up in organic molecules are rearranged into inorganic water and carbon dioxide. Cellular respiration helps to explain why we breathe out CO₂ and water, and why our body temperature stays a toasty 98.6 degrees. Unfortunately, many students incorrectly see cellular respiration as the way we convert food or stored biomass (fat) into energy to move and exercise. 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 (Matter Movement and Matter Change). 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). 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 BSCS website in a document called “Carbon TIME Content Simplifications.” Here are some key points:

• All bond energies are negative relative to individual atoms. 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. 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 the Explanations Phase of the Animals unit. This involves modeling and coaching with the goal of helping students develop atomic-molecular scale accounts of the digestion, biosynthesis, and cellular respiration that were the drivers of the macroscopic changes they observed in their Mealworms Eating investigation in the previous lesson.

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 animals 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 + 6O2 + 6 H₂O --> 6 CO₂ + 12 H₂ O

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.

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.

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 animals 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.

Note: Activities 4.1 and 4.2 focus on the fact that animals remove oxygen atoms in glucose and fat molecules from their bodies through cellular respiration in the form of H₂O and CO₂ molecules. Although the curriculum does not go into this amount of detail, it should be noted that most of the oxygen atoms from fat and glucose are expelled from the body in CO₂ molecules (approximately 84%) and some of the oxygen atoms leave the body in H₂O molecules as well (approximately 16%).

Key Carbon-Transforming Processes: Cellular Respiration