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. Key Ideas and Practices for Each Activity Activity 5.1 is the first part of the Explanations Phase of the instructional model (going down the triangle) for Lesson 5. Students trace the process, on a poster of a plant, of the chemical change that took place during the investigation to help them develop an atomic-molecular explanation for how plants gain mass. Activity 5.2 is a 2-turtle activity appropriate for advanced middle school or high school students and classes. If you decide not to teach Activity 5.2, you can move directly from Activity 5.1 to Activity 5.3. In Activity 5.2, students model the chemical changes of biosynthesis using paper molecules. This activity introduces and uses the vocabulary of polymer and monomer, as well as the names of specific monomers. The modeling focuses on the building of polymers inside the cells. Plants rearrange the atoms of glucose, and soil minerals (especially nitrogen in ammonia) to first build small organic molecules (monomers): amino acids, glucose, fatty acids, and glycerol (this step is not included in the tracing in Activity 5.1). The energy that is stored in the C-C and C-H bonds of the glucose molecules is conserved and passed along from the glucose molecules to the small organic molecules. These small organic molecules are then used to build large organic molecules (proteins, carbohydrates, and fats), which are called polymers. Activity 5.3 is the second part of the Explanations Phase of the instructional model (going down the triangle) for Lesson 5. Students use the Explanations Tool to construct final explanations for biosynthesis. 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. By this point in the unit, the students will have completed at least one of each of the process tools: Expressing Ideas and Questions, Predictions and Planning, Evidence-Based Arguments, and the three Explanations Tools for photosynthesis, cellular respiration, and biosynthesis. 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 mass and weight: Grams and kilograms in the SI (metric) system are units of mass—the amount of matter in a system. On the other hand, pounds and ounces in the English system are units of weight—the force of gravity on a particular mass. As long as gravity doesn’t change, these units are interconvertible: The force of gravity on a 1 kg mass is about 2.205 pounds. Since most American students are more familiar with the English units of weight, we sometimes use “weight” and “weight,” especially when encouraging students to express their own ideas. When referring to measurements in grams, we use “mass” as both a verb and a noun. Key Carbon-Transforming Processes: Biosynthesis Content Boundaries and Extensions
