Where Did the Plant’s Mass Come From?
1 sequential days · Grade 7 · Lesson plan
Public Schools scope of work recommends 12–15 days for Cellular Respiration and Photosynthesis (Part 1). These eight days are the current two-lesson application pilot, not the complete district sequence.
Standards and lesson objective
7.LS1.7 — Develop a model using evidence that explains the process of photosynthesis, cellular respiration, and anaerobic respiration in the cycling of matter and flow into and out of organisms.
7.PS3.2 — Develop a model to explain how food is utilized through chemical reactions to form new molecules that support growth, resulting in the release of energy as matter moves through an organism.
Lesson objective
Plan and carry out a controlled investigation, collect and compare plant-growth evidence, and use a model to explain how matter and energy are involved in plant growth.
Evidence of learning
Students explain or model matter and energy using controlled comparisons, recorded lab data, four understanding checks, and a five-question exit ticket for each day. The mastery target is 4 of 5 correct (80%).
The Mystery
Objective: Make an initial claim about where a growing plant gets the matter that becomes new plant material.
Driving question: A small seedling becomes a much larger plant. Where did the extra mass come from?
Learning sequence
- Check-in and prior knowledge: A seedling gains mass as it grows. Which source could supply atoms for its new cells? Discuss why the correct response is Matter from air and water.
- Engage: Compare a seedling with a larger plant. Record your first idea before reading the explanation.
- Article: A seed may be tiny, yet the plant that grows from it can have a much greater mass. Where did the added matter come from? Imagine weighing a seed, its soil (organic matter), and the whole plant over time. A change in plant mass is an observation; an explanation must account for the atoms that entered the plant. Read Aloud is available.
- Video: What Is Photosynthesis? (0:00–3:25). Watch for carbon dioxide, water, and light in the photosynthesis equation. Which inputs contain the atoms that become sugar? Students watch at least one minute of this segment with captions available.
- Probe and practice: Sort soil, air, water, and sunlight (electromagnetic energy) into possible matter inputs and energy inputs. What evidence would test your claim? Students must answer each of four checks correctly to advance.
- Virtual investigation: Where does a seedling get matter as carbon dioxide changes? Run at least two conditions and record a prediction, trial data, and an observation.
- Apply: Imagine a plant growing in a sealed container with measured soil. What measurements would help explain its growth?
Teacher moves
Show a seedling and a mature plant. Ask students to predict the source of the added mass without correcting them yet. Capture initial claims and distinguish matter from energy.
Collect
Initial claim, prediction, and a reason for the prediction.
Watch for
Sunlight (electromagnetic energy) or soil alone becomes most of the plant’s mass.
Exit ticket · benchmark style
Students interpret a scenario, investigation, data table, or model and choose the best explanation. Each item has four options. Review the reasoning after submission.
- Which statement best explains why Plant A gained more mass?Answer: B — Plant A used light energy to make sugars from CO₂ and water.Light supplies energy for photosynthesis. Carbon dioxide and water supply atoms that become sugars and other plant matter.
Bean plants after four weeks Plant Light Water CO₂ Mass change A Yes Yes Yes +18 g B No Yes Yes +2 g CO₂ + H₂O → sugar + O₂
Which change would best improve this photosynthesis model?Answer: A — Add light energy entering the process.The equation lists matter inputs and outputs; light energy is also needed to make sugar.- Which claim is best supported by these measurements?Answer: C — Soil loss alone cannot account for the plant’s mass gain.The plant gained 15 g while dry soil lost only 1 g; matter from air and water must be considered.
Seedling and soil measurements Measurement Start End Plant mass 4 g 19 g Dry soil mass 200 g 199 g A student says: “The plant gets both its matter and energy from sunlight.”
Which response best evaluates the claim?Answer: B — Incorrect, light provides energy; CO₂ and water provide much of the matter.Energy and matter play different roles. Light is an energy input, while carbon dioxide and water supply atoms.A plant grows in a sealed container. A student wants to test where its added carbon comes from.
Which measurement would provide the most relevant evidence?Answer: D — CO₂ concentration in the container before and after growthChanges in carbon dioxide can help trace carbon matter from air into new plant material.