Activity 4.3: Molecular Models for Potatoes Making Food: Photosynthesis (60 min)

Display slide 3 in the PPT.

• Introduce students to the driving question: How does a potato plant make the food it needs to grow and function?
• Connect this question at the macroscopic scale to an unanswered question about the Energy Change Question at the microscopic scale: How do a potato’s’ leaf cells produce the food the potato needs to grow and function?
• Connect this question at the microscopic scale to an unanswered question about the Energy Change Question at the atomic-molecular scale: What chemical change produces food in the potato’s leaf cells?
• Assure students that we will be able to answer several of their unanswered questions by the end of today’s activity.

Display slide 4 of the PPT. Remind students of the results of the Plants in the Light and Dark investigation: The plants in the light changed the BTB solution to blue, which indicated a decrease in CO₂. The plants in the dark changed the BTB solution to yellow, which indicated an increase in CO₂. The carbon must be going to and coming from somewhere around the plants.

• Discuss the diagram on slide 4, which shows that food made in the leaves is needed for both growth and movement. Explain that modeling photosynthesis will help them understand the results of the investigation.
• Display slide 5 of the PPT. Use the animation to support students in connecting the atomic-molecular scale to the macroscopic scale.
• Tell students they will be modeling the change that occurs during cellular respiration at the atomic-molecular scale.

Divide the class into pairs and give each pair a molecular model kit, a set of Forms of Energy Cards, and Molecular Models 11 x 17 Placemat. Pass out one copy of 4.3 Molecular Models for Potato Photosynthesis Worksheet to each student.

• Show students slide 6 to review the bonding of atoms in molecules. Tell students that the rules on this slide are important because they apply to all molecules that they will make in all Carbon TIME units.

Watch the video about molecular models. https://www.youtube.com/watch?v=9nHHTnnvxFQ&list=PL4te0BRLizI4XHj4SiEmGg148vgPNOYe0&index=8

Use slide 7 to show instructions to construct the reactants: carbon dioxide and water. Students can also follow instructions in Part B of their worksheet.

Show slide 8 in the PPT.

• After students get a chance to create the reactant molecules, show students the photo on Slide 8 and explain that the twist ties represent energy, and will represent the transformation of light energy to chemical energy. Have students put 12 twist ties on the reactants side of their poster. Have students put the light energy card under the twist ties. Have students note how many energy units (twist ties) they begin with.
• Have students compare their own molecule with the picture on the slide.
• Slide 9 shows an important message: after students create their reactant molecules, make sure they put away all unused pieces of their molecule kits. This helps reinforce that the matter and energy in the reactants are conserved through the chemical change, and that only the materials from the reactants are used to build the products.

Show slide 10 in the PPT.

• When they have completed their reactants, tell students to complete the table about matter and energy in Part C of their worksheet for the reactants.

Have students continue working in pairs to observe how light, carbon dioxide, and water can be rearranged into the molecules of the products: glucose and oxygen. Tell students that the light energy from the Sun is transformed into chemical energy in the C-C and C-H bonds of the glucose, which is represented with twist ties. Oxygen does not contain any high-energy bonds, so these molecules do not take twist ties.

• Use Slides 11 and the instructions on the worksheet to have students use the same materials they used to form their reactant molecules to form the product molecules. Tell them to place their products in the appropriate box on the product side of the placemat.

Show Slide 12 to let students compare their products to the products in the picture.

• Have students use the chemical energy card to indicate what form the energy is in after the chemical change by putting the card under the glucose molecule.
• Show slide 13 to give students an overview of the entire reaction.

Show slide 14 in the PPT.

• When students have completed their reactants, tell students to complete the table in Part C in their worksheet that explains what they ended
• Have students verify that the number of atoms before and after remained constant: Atoms last forever! Tell students that this means that the number of atoms before and after the reaction does not change.

Show slides 15-19 in the PPT to help students make connections between what is happening in the animation and the molecular models they made.

• For each slide, focus on different atoms and forms of energy and how they change. The animation draws attention to where they atoms begin and end in the reaction.

Show slide 20 in the PPT.

• Complete the “check yourself” questions with the class in Part C.

Tell students that now that they have represented a chemical change using molecular models and in animations, they will represent chemical change by writing the chemical equation.

• Show Slide 21 Slide 21 to guide students through the process of writing a balanced chemical equation for photosynthesis. Tell students that these rules apply to all chemical reactions.
• Tell students to write their equations in Part D of their worksheet.
• Have students write their own chemical equations before comparing them with the one on Slide 22.

Show slide 23 of the 4.3 Molecular Models for Potato Photosynthesis PPT.

• Conclusions: Where do the reactants for photosynthesis come from?
• Predictions: Where do you think photosynthesis occurs?
• On a sheet of paper or a sticky note, have students individually answer the exit ticket questions. Depending on time, you may have students answer both questions, assign students to answer a particular question, or let students choose one question to answer. Collect and review the answers.
• The conclusions question will provide you with information about what your students are taking away from the activity. Student answers to the conclusions question can be used on the Driving Questions Board (if you are using one). The predictions question allows students to begin thinking about the next activity and allows you to assess their current ideas as you prepare for the next activity. Student answers to the predictions question can be used as a lead into the next activity.