Where Does a Tree's Mass Come From? Air, Water, and the Mystery of Plant Growth
Shared by A STEM Educator
Students will develop and use models to explain that plants use carbon dioxide and water to produce sugars (biomass) through photosynthesis, citing evidence that plant matter comes primarily from atmospheric CO₂ rather than soil or light, aligned to NGSS 5-PS3-1 and 5-LS1-1.
Lesson Overview
Students investigate the puzzling phenomenon that plants gain most of their mass from air — not soil — by comparing dried versus fresh plant weights, testing soil mass before and after growth, and analyzing gas exchange data from hydroponic vertical farms and reforestation projects.
Materials
- Small potted bean seedlings (pre-grown 2–3 weeks)
- Digital scale (0.1 g precision)
- Small cups of dry potting soil (pre-weighed and labeled)
- Oven or food dehydrator (teacher use only — to dry plant samples)
- Fresh vs. dried leaf samples in labeled bags
- Baking soda + vinegar in sealed bags (CO₂ demonstration)
- Hydroponic vertical farm data cards (printed)
- Reforestation biomass data table (printed)
- Student investigation handouts
- Four-quadrant note sheets
- Markers and sticky notes
Scaffolded Task Progression
Your teacher has pre-weighed a cup of dry potting soil at g. A bean seedling was grown in that soil for 3 weeks with only water added. The soil (removed and dried) now weighs g. The bean plant weighs g.
Record the data in the table below and answer: Could the soil alone explain where the plant's mass came from?
| Before (g) | After (g) | Change (g) | |
|---|---|---|---|
| Soil mass | _____ | ||
| Plant mass | _____ |
Does the soil mass lost equal the plant mass gained? What does this tell you?
Your teacher provides two labeled bags: a fresh leaf ( g) and the same type of leaf dried overnight ( g).
1. What is the mass difference? _____ g
2. What substance made up most of the fresh leaf's mass?
3. In a hydroponic vertical farm, plants grow with roots in water and NO soil. If water makes up most of a leaf's mass, where does the water come from in a hydroponic system?
4. But water alone can't explain ALL the solid material in the dried leaf ( g). What else might the plant have used to build that solid material?
[ Interactive graph / diagram — available in the full lesson ]
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