Plants | Lesson 4 - Explaining How Plants Make Food, Move, and Function

This lesson helps middle school and high school students understand why plants “breathe” (i.e., exchange gases with the air) differently in the light and in the dark and how the mass of plants can come mostly from the air. As they model photosynthesis, they learn how to explain plant gas exchange and growth in a way that follows the key rules about matter and energy—atoms last forever, and energy lasts forever (in chemical changes). As they model cellular respiration, they learn how to explain this carbon-transforming process that makes food energy available to plant cells.

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). 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 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 four 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 photosynthesis and cellular respiration that were the drivers of the macroscopic changes in CO₂ concentration that they observed in their Plants in the Light and Dark 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 plants get energy to move. Plants use the energy released from cellular respiration to grow and function (for biosynthetic processes and other cellular functions) as well as to move. Plants engage in cellular respiration in both the light and the dark. If your students did cellular respiration molecular modeling as part of the Animals or Decomposers
Units and did well on questions about cellular respiration on the pretest, you may want to skip the cellular respiration modeling steps in this Activity.

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 plants use energy to grow, move, and function through cellular respiration. 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.

Activity 4.3 is the first part of the Explanations Phase of the instructional model (going down the triangle) for photosynthesis. Students construct molecular models of the chemical change that explains how the plants give off oxygen in the light.

Activity 4.4 is the second part of the Explanations Phase of the instructional model (going down the triangle) for photosynthesis. Students use the Explanations Tool to construct final explanations of how plants make their own food. Ideally, at this phase their explanations will combine evidence from the macroscopic-scale observations during the investigation with their new knowledge of chemical change at the atomic-molecular scale. The questions in this Activity should also help students notice two important relationships:

• The relationship between photosynthesis and cellular respiration. The chemical reactions are reverses of one another, but the energy transformations are not—from light to chemical energy for photosynthesis and from chemical energy to motion and heat for cellular respiration.
• The relationship between plants and animals. Animals depend on plants for food and oxygen, while in most cases plants don’t need animals to get food or oxygen.

There are multiple pathways from which to choose when teaching the carbon-transforming processes in Lessons 4 and 5. You may choose to go in the order presented here. You may choose to follow an order that makes more sense instructionally to you. Or you may choose to go in an order based on the types of questions your students are asking. Please see the Plants Unit Front Matter or the Student Challenges and Teacher Choices in the Plants Unit document for clarification in making this instructional decision. Remember that if you choose a different order than what is presented here, you will need to be aware of what you have and have not yet taught your students, and you may want to make small adjustments to the activities accordingly.

A note on the chemical change formulas for photosynthesis and cellular respiration. Activities 4.1 and 4.3 simplify the full story of what happens to matter during the multi-step processes of cellular respiration and photosynthesis. For a more detailed account, see http://dqc.crcstl.msu.edu/node/2027.

In Activity 4.1, we use a standard but simplified formula for the overall chemical change occurring in cellular respiration:

C₆H₁₂O₆ + 6 O₂ -> 6 CO₂ + 6 H₂O

This incorrectly suggests that some of the oxygen atoms in O₂ end up in CO₂, which does not happen directly during the multi-step process of cellular respiration. A more accurate formula to represent the multi-step process would be as follows:

C₆H₁₂O₆ + 6 O₂ + 6H₂O → 6 CO₂ + 12 H₂O

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 Activity 4.3, we use a standard but simplified formula for the overall chemical change occurring in photosynthesis:

6 CO₂ + 6 H₂O -> C₆H₁₂O₆ + 6 O₂

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

6 CO₂ + 12 H₂O → C₆H₁₂O₆ + 6 O ₂ + 6 H₂O

Thus, all of the oxygen atoms in O₂ (bolded in the equation above) come from H₂O, while the oxygen atoms in CO₂ all go to glucose or water. Although we ask students to identify C-C and C-H bonds as high in energy, it is important to recognize that releasing most of that energy requires a reaction with oxygen. It is more accurate to say that the chemical system of glucose and oxygen has more potential energy than the chemical system of carbon dioxide and 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.

Our research has consistently shown 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 (C-C and C-H bonds). The products of cellular respiration have only lower-energy C=O and H-O bonds, so the energy released by the oxidation reaction is available for cell movement and function. 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 leaves the plant as heat.

A note on cellular respiration. Students usually do not think about plants doing cellular respiration. They learn that plants do photosynthesis, and often cellular respiration is overlooked. Students may not even wonder how seeds actually sprout when they have no leaves, no chlorophyll, and no way to photosynthesize. Fully grown plants also undergo cellular respiration on a continuous basis. It is easier to detect this process in plants during the night, as well as in winter months, when plants are not also photosynthesizing. During cellular respiration, plants take organic materials and oxidize them, which releases energy and gives off inorganic carbon dioxide and water as wastes. Many students also incorrectly see cellular respiration as the way plants convert food or stored biomass (fat, starch) into energy for movement, cell functions, and growth. 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.

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.

Key Carbon-Transforming Processes: Photosynthesis & Cellular Respiration