How I Teach Conservation of Matter in Photosynthesis to Avoid Misconceptions

A tree starts as a tiny seed. If you ask your students where all the "stuff" the tree needs to grow comes from, you might be surprised by their response. I've found that over 95% of my students incorrectly believe most of a growing tree's mass comes from the soil when most of it comes from carbon dioxide in the air. How can we guide students in understanding the actual source of a growing tree's mass and correct this common misconception?

The first step is identifying any additional misconceptions that may hinder students from mastering the photosynthesis concept. For example, many students mistakenly believe that gases have no mass or make their containers lighter. I like to target this misconception at the beginning of the lesson, during the "Engage" phase of a 5E lesson, to determine how many of my students hold this belief. To do this, I present them with the following discrepant scenario:

A student places an empty balloon on a scale. They then take a deep breath and fill the balloon with air from their lungs. The student finds the mass of the balloon a second time. What will they find?

a. The mass of the filled balloon is lighter than the mass of the empty balloon

b. The mass of the filled balloon is heavier than the mass of the empty ballon

c. The mass of the filled balloon is the same as the mass of the empty balloon

I prefer using a multiple-choice format to quickly poll the class and assess the number of students with misconceptions before conducting the demonstration. Many students are surprised that the filled balloon is heavier than the lighter balloon. With this phenomenon in mind, students are primed for the main lesson, where they will discover that most of the mass of a growing tree comes from carbon dioxide gas in the air.

In the "Explore" segment of the lesson, I ask my students to use Legos to build the reactants of photosynthesis, carbon dioxide and water. They then measure the mass of the reactants and rebuild their model using the same pieces to represent the products of photosynthesis, glucose, and oxygen. After finding the mass of the products, they observe that the mass of the reactants and products is equal, demonstrating the conservation of mass. I like to have students use concrete objects, like Legos, when modeling abstract ideas in science class because it helps make the concepts more tangible and easier for students to understand. By physically manipulating objects, students can visualize and comprehend complex scientific processes, like photosynthesis, more effectively.

I then instruct students to complete a graphic organizer to identify the inputs and outputs of the plant system. Creating a visual model helps students realize that a plant appears to gain mass through photosynthesis because it takes in carbon dioxide from the air and water from the environment. The plant releases oxygen, while glucose remains inside the plant and is used to build cellulose, lignins, and other molecules that form plant structures.

During the "Explain/Elaborate" portion of the lesson, I have a group of students present their graphic organizer to the class. The rest of the class can then critique it by agreeing or disagreeing with the group's conclusion about where the mass of a growing tree comes from.

For the "Evaluate" section of the lesson, I have students revisit the driving question of the lesson: What is the source of the atoms that a plant requires to grow? Students' responses inform me whether or not they've overcome the common misconception.

To download a free copy of the graphic organizer I provide students during this lesson, click here.

The full BioDistilled lesson discussed in this post can be purchased here.

A differentiated unit that targets common student misconceptions surrounding photosynthesis, including the lesson discussed in this post is available for purchase here.