Materials & Prep
Prepare the two data displays in the lesson: dry seedling mass changes from 5 g to 35 g while dry soil changes from 1,000 g to 998 g; a 35 g fresh leaf contains 7 g dry matter after its water is removed. Write the photosynthesis equation on the board. Students need paper or notebooks. No special equipment is needed.
Opening
DisplayA seedling's dry mass increases from 5 g to 35 g. During the same period, the dry soil's mass decreases from 1,000 g to 998 g. Students silently calculate both changes and write: “The plant gained grams. The soil lost grams.”
Ask“What could account for the plant's 30-gram increase? Choose a first prediction: soil, water, air, or more than one. Give one observation as evidence.” Accept predictions without correcting them. Students are succeeding by reading data, comparing changes, and using the idea that matter must come from somewhere.
Connect“You just separated an observation from an explanation. The observations are the measured mass changes. Your proposed source is an inference. Today we will test which source can account for the new plant matter.” Keep the class predictions visible for later revision.
Direct instruction
ExplainA plant's mass cannot appear from sunlight. Sunlight supplies energy, but it is not matter. Matter enters the plant mainly as carbon dioxide from air and water through the roots. Small amounts of mineral nutrients enter from soil. When we measure fresh plant mass, much of it is water. When we measure dry mass, the largest increase is usually carbon-rich plant material made using carbon dioxide.
Display6CO2 + 6H2O → C6H12O6 + 6O2
Think aloud“I will not use the equation as a chant. I will count atoms. Six carbon dioxide molecules contain six carbon atoms, and the glucose molecule on the right contains six carbon atoms. Those carbon atoms had to come from the carbon dioxide. The plant uses the carbon to build sugars and then larger materials such as cellulose in stems and leaves. Water also contributes matter, especially to fresh mass and to the hydrogen and oxygen in plant compounds. Oxygen gas is released.”
Confront“The soil lost only 2 grams, but the plant gained 30 grams. Soil cannot be the main source of those 30 grams. It supplies essential minerals, but a small essential input is not the same as the bulk source of plant mass.”
Check for understandingStudents hold up one finger for energy, two for matter, or three for both. Ask about sunlight, carbon dioxide, water, and nitrogen minerals. Expected: sunlight is energy; carbon dioxide and water are matter and reactants; minerals are matter and nutrients. Correct the common error that “air” means oxygen only. The relevant air input for building plant matter is carbon dioxide.
Guided practice
Worked exampleModel a short CER response using the opening data. Claim: “The plant's new dry mass did not mainly come from soil.” Evidence: “The plant gained 30 g, but the dry soil lost only 2 g.” Reasoning: “Because matter is conserved, the remaining mass had to come from other inputs. The photosynthesis equation shows carbon dioxide and water entering the process, and carbon from carbon dioxide becomes part of glucose and plant tissue.” State that this evidence supports the claim but does not mean soil minerals are unnecessary.
Have students work in pairs with this second data set: A fresh leaf has a mass of 35 g. After its water is removed, its dry mass is 7 g. The pair answers: “How many grams were water?” and “What does this show about fresh plant mass?” Expected: 28 g of water; fresh mass can be dominated by water, while dry mass represents the plant material left after water is removed.
AnnotateStudents label each statement O for observation or I for inference: “The dry plant gained 30 g”; “Carbon dioxide supplied carbon atoms”; “The soil lost 2 g”; “Water is part of fresh plant mass.” Review answers aloud. Scaffold students with the frame: “The plant gained g, but the soil lost only g. Therefore, soil could not supply all the mass. The equation supports this because .”
Independent explanation
Students write a CER response to this prompt: “A student says, ‘Plants get their mass from the soil because their roots are in soil.’ Explain why this is incomplete. Use the opening data, the fresh-versus-dry leaf data, and at least one detail from the photosynthesis equation.” Require a claim, two pieces of evidence, and reasoning that connects the evidence to conservation of matter.
CirculateLook for students treating sunlight as matter, calling oxygen the food source, or saying that all plant mass comes from water. Prompt them to identify whether they are discussing fresh mass or dry mass and to trace the six carbon atoms in the equation.
ExtensionStudents add a sentence explaining why a plant can need soil minerals even though soil is not the main source of its mass. Strong responses distinguish “needed for growth” from “largest source of mass.”
Closing
Exit ticket“Explain where the mass of a growing plant comes from. Include: 1) the main source of the carbon in new dry plant material, 2) how water contributes, 3) the role of soil minerals, and 4) why sunlight is not a source of mass. Use one observation or equation detail as evidence.”
Collect the tickets and return to the opening predictions. Students revise one sentence: “I originally predicted . I now revise or support that prediction because .” A complete response identifies carbon dioxide from air as the major source of carbon-rich dry biomass, recognizes water as a major part of fresh mass and a reactant, describes soil minerals as necessary but small inputs, and identifies sunlight as energy rather than matter.