Unit 5 · Cells & Living Systems · Week 2

Where does a plant's mass come from?

A tree can weigh hundreds of kilograms, but the pot it grew in lost almost no soil mass. Where did all that tree mass come from?

SEP: Constructing Explanations & Using Mathematics and Computational Thinking DCI: LS1.C Organization for Matter and Energy Flow in Organisms — photosynthesis CCC: Energy & Matter — matter is conserved; trace where atoms go

NGSS MS-LS1-6 ~50 minutes Learner code: —

Make this lesson work for you:

Start here

How we work this lesson (our expectations)

Wonder out loud — questions are the work
Use evidence, not guesses
Reasoning > being first or fastest
Supports are for everyone — open any you want

30-second check-in self-management

Last week you ended with a prediction: what are the seed's cells doing with water, sunlight, and air? Hold that prediction in mind — today we find out. Before we start, on a scale of 1 (foggy) to 5 (sharp), where is your focus? Take one slow breath and notice.

Do-Now Quick recall

Pull from what you already know — no looking back. Retrieval makes ideas stick.

Week 1 spiral: Name three characteristics that make the seed alive but not the pebble.
Unit 1 spiral: In Unit 1 we learned that matter is conserved — atoms are never created or destroyed, only rearranged. If a plant grows from tiny particles, where did those atoms come from? (Best guess.)
Unit 2 spiral: Sunlight carries energy as electromagnetic waves. Name one thing you know about how that energy behaves when it hits a surface.

The phenomenon

Van Helmont's willow: a scientist planted a 5-pound willow in 200 pounds of soil. Five years later the tree weighed 169 pounds. The soil lost only 2 ounces. Where did 164 pounds of tree come from?

In the 1600s, a scientist named Jan Baptist van Helmont planted a small willow sapling in a pot of soil, watered it carefully, and covered the pot to keep dust out. Five years later, the tree weighed 169 pounds. The soil weighed almost exactly what it had when he started — it had lost only two ounces. Van Helmont thought the plant must be made mostly of water. He was close but not quite right. We now know the plant's mass comes mostly from carbon dioxide — a gas in the air — plus water, combined using the energy of sunlight. The atoms in the air literally became the atoms in the tree. This process is called photosynthesis, and it is the most important chemical transformation on Earth.

Notice & Wonder. Write one thing you notice about van Helmont's data and one thing you wonder. Keep your wonder — we are answering it today.

Build it Three big ideas about where the tree came from

We will build the answer in steps. After each step there is a quick check before we move on.

Step 1 — The word equation for photosynthesis

Photosynthesis is the process by which plant cells use light energy to convert carbon dioxide and water into glucose (a sugar) and oxygen. As a word equation: carbon dioxide + water + light energy → glucose + oxygen. This is the most important chemical equation in biology. The reactants — CO₂ and water — enter the plant (CO₂ through tiny pores in the leaves called stomata; water through the roots). The products — glucose and oxygen — come out. The glucose is the plant's food: stored chemical energy that it can use later for growth, movement, and all cellular work. The oxygen is released into the air — it is the source of all the oxygen we breathe. The key insight about mass: the carbon atoms in CO₂ become the carbon atoms in glucose, which become the carbon atoms in cellulose (the material that wood and plant bodies are made of). The tree's body is made of rearranged air.

Step 2 — Energy transformation in photosynthesis

In Unit 2 we learned that sunlight is electromagnetic energy. In Unit 1 we learned that energy is never created or destroyed — it is transformed. Photosynthesis is one of the most important energy transformations on Earth: light energy (electromagnetic) is converted into chemical energy stored in glucose. That glucose is the plant's fuel. When the plant later needs energy — to grow a new leaf, to move water up a stem, to reproduce — it breaks down the glucose and releases the stored chemical energy. (We will see this in Week 3 with cellular respiration.) Matter is conserved throughout: the atoms that entered as CO₂ and water leave as glucose and oxygen. No atoms are created or destroyed — only rearranged.

Step 3 — Tracing the carbon atoms

The crosscutting concept of Energy and Matter says we can trace atoms through systems. Let's trace the carbon atoms from the air to the tree: (1) CO₂ in the air — each molecule has one carbon atom and two oxygen atoms. (2) Enters the leaf through stomata and is captured by chlorophyll in the plant's cells. (3) The carbon atom is broken off and bonded into a glucose molecule. (4) Glucose is used to build cellulose — long chains of glucose molecules that form the walls of plant cells. Cellulose is what wood, bark, and leaves are made of. (5) Cellulose is the tree's body. The carbon atoms that were in the air yesterday are now in the tree's trunk, branches, and leaves. Van Helmont's 164 pounds of willow came from 164 pounds' worth of carbon atoms that used to be floating in the air as CO₂.

Worked example — mass balance in van Helmont's experiment

If matter is conserved, then the 164 pounds the tree gained had to come from somewhere outside the soil. Where? Let's check:

The soil lost only 2 ounces — clearly not the source of 164 pounds. Ruled out.
The water van Helmont added — water (H₂O) does provide hydrogen atoms and oxygen atoms. But photosynthesis releases most of the oxygen back as O₂, and the hydrogen that stays is a small fraction of the plant's mass. Water is a reactant but not the main source of the plant's carbon-based mass. Ruled out as the main source.
The air (CO₂) — each CO₂ molecule is 27% carbon by mass. A plant in full sun absorbs large volumes of CO₂ every day. Over 5 years, a willow tree absorbs enough CO₂ to account for all of its woody mass. CO₂ is the source.

Mass balance check: the carbon that enters as CO₂ stays in the glucose and cellulose (in the tree). The oxygen from CO₂ is released as O₂. The hydrogen from H₂O bonds into the glucose. The oxygen from H₂O is mostly released as O₂. Total mass in (CO₂ + H₂O) = total mass out (glucose/cellulose in tree + O₂ released). Matter is conserved.

Show me the atom-tracing as a flow diagram I can copy Tier 2 · visual organizer

Copy and complete this flow diagram in your notebook, filling in each blank:

CO₂ in the air → enters leaf through ________ → carbon atom captured by ________ in the cell → carbon bonded into ________ (glucose) → glucose linked into ________ (cellulose) → cellulose becomes the ________ of the tree

Then label which step involves light energy, and which step involves water.

Word bank for the flow diagram: stomata · chlorophyll · glucose · cellulose · wood/body · sunlight · water/H₂O

Your turn Explain van Helmont's data

Use a Claim–Evidence–Reasoning model to answer the phenomenon question. Choose how you respond — all are equally valid.

✍️ Write it 🎨 Draw an atom-tracing diagram and label it 🗣️ Record yourself explaining it

The task: Van Helmont's willow gained 164 pounds. The soil barely changed. Where did the mass come from? Make a claim, give evidence from the data and from photosynthesis, and explain your reasoning using the words "carbon dioxide," "conserved," and "cellulose."

I want sentence starters and a word bank Tier 2 · frames

Claim: The tree's mass came mostly from ________, not from the soil, because ________.

Evidence: Van Helmont's data shows the soil lost only ________, while the tree gained ________. This means the soil cannot be the source. The tree must have gotten its mass from ________.

Reasoning: Photosynthesis converts ________ and ________ using ________ energy into ________. The carbon atoms from CO₂ are conserved — they are rearranged into ________, which makes up the tree's body.

Word bank: photosynthesis · carbon dioxide · CO₂ · glucose · oxygen · sunlight · conserved · atoms · matter · energy · cellulose · reactant · product

Let's make it concrete first Tier 3 · re-teach

Build-a-tree analogy: Imagine building with LEGO bricks (atoms). The tree is a massive LEGO structure. Where did the bricks come from? Not the ground — barely any soil was missing. They floated in from the air. CO₂ carried carbon "bricks." The sun gave the plant the energy to snap those bricks together. The plant snaps CO₂ + H₂O bricks together into glucose bricks, then into cellulose bricks — the wood. You didn't make the bricks. You rearranged them. That is photosynthesis.

Fill in: The tree's mass came from ________ (CO₂ in the air) because the plant used ________ (sunlight) energy to convert those carbon atoms into ________ (glucose), then into ________ (cellulose — the tree's body). The atoms were ________ (conserved), not created.

Say your answer out loud before you write it. One sentence for claim, one for evidence, one for reasoning is enough.

Push my thinking Extension

Calculate the carbon: A mature oak tree contains roughly 100 kg of carbon. Each CO₂ molecule has one carbon atom. At standard conditions, CO₂ has a density of approximately 1.96 g per liter. Carbon makes up about 27% of CO₂ by mass (12 g of C out of 44 g of CO₂). How many liters of CO₂ would need to be absorbed to supply 100 kg of carbon to the oak tree? Show your reasoning step by step, and then connect your answer to why forests matter for climate. (Hint: if trees are taking that much CO₂ out of the air, what happens when forests are cut down?)

Check yourself Did it click?

Success criteria — I can:

write the word equation for photosynthesis and identify what each reactant and product is;
explain that most of a plant's mass comes from carbon dioxide in the air, not from soil;
trace a carbon atom from CO₂ in the air through photosynthesis into the plant's body, using the concept of conservation of matter.

Where does MOST of the mass of a growing tree come from?

Re-teach: why the tree's mass comes from air, not soil or water alone Tier 3

Return to van Helmont's data: The soil lost only 2 ounces. The tree gained 164 pounds. The numbers simply don't add up for soil to be the source. Cross it off your list.

What about water? Water (H₂O) contains hydrogen and oxygen — but no carbon. Plant bodies are built mostly from organic molecules (sugars, cellulose, proteins) that all have a carbon backbone. Water's hydrogen goes into glucose, but the oxygen in water is largely released as O₂. Water is a reactant, but it is not the source of the carbon in the tree's body.

The carbon in CO₂ is the answer: CO₂ is 27% carbon by mass. Carbon is the backbone of all organic molecules — it is what glucose and cellulose are built from. A plant absorbs enormous quantities of CO₂ through its leaves every day. Over 5 years, this is more than enough to account for 164 pounds of wood. The atoms were conserved — they moved from the air into the plant's body through photosynthesis.

Think of it this way: the willow tree is a sculpture made of rearranged air. Now re-read option B — it describes this correctly. Try saying it in your own words before moving on.

Reflect, then show what you know

Reflection metacognition + SEL

At the start of this lesson you had a prediction from Week 1 about what the seed's cells do with water, sunlight, and air. Was your prediction close? What surprised you about photosynthesis? Noticing the gap between what you predicted and what you learned is one of the most powerful learning moves scientists use.

Exit ticket — answer on paper or in your notebook.

New Write the word equation for photosynthesis. Put a star next to the reactant that provides most of the carbon for the plant's mass.
New The carbon atoms now in a tree trunk were once in the air as CO₂. Use the word "conserved" to explain how this is possible.
Spiral — Week 1 A plant is made of cells, grows, uses energy, and responds to light. Connect photosynthesis to at least two characteristics of life from Week 1.
Spiral — Unit 1 In Unit 1 you learned that energy transfers but is never created. Explain how photosynthesis follows this rule — where does the energy come from, and what form does it end up in?
Transfer → Week 3 Photosynthesis stores energy in glucose. A plant in a dark closet has no sunlight. Where does it get energy to stay alive? Predict what happens to that stored glucose.

Educator panel — facilitation, accommodations, answer key

At a glance

Standard: NGSS MS-LS1-6 — Construct a scientific explanation based on evidence for the role of photosynthesis in the cycling of matter and flow of energy into and out of organisms. 3D: SEP Constructing Explanations & Using Mathematics and Computational Thinking · DCI LS1.C Organization for Matter and Energy Flow in Organisms (photosynthesis) · CCC Energy & Matter (matter is conserved; trace atoms through systems). Throughline: Extends Week 1's "what is alive?" to "how do living things use matter and energy?" — and directly answers the Week 1 exit-ticket prediction. Previews Week 3 (cellular respiration) by leaving glucose as an open question.

Materials (optional, fully digital-capable): Student notebook; a plant near the classroom window is a powerful optional prop. The van Helmont scenario is the primary anchor and requires no lab setup.

Facilitation arc (Rosenshine)

Open by asking students to share their Week 1 exit-ticket prediction (what are the seed's cells doing with water, sunlight, and air?) — this activates prior learning and creates personal investment in the phenomenon (4 min).
Do-Now retrieval while you take attendance (3 min).
Reveal the van Helmont data and the SVG; ask notice/wonder before any explanation (2 min).
Build Steps 1–3 with the worked example; stop and check after each step (12 min).
Differentiated CER practice — circulate; ask "what word from your word bank did you use?" to encourage vocabulary precision (14 min).
Self-check + re-teach branch for any A or C responders (5 min).
Brain break (2 min).
Reflection + exit ticket (10 min).

IEP / 504 accommodation cards

Common accommodations mapped to this lesson

Accommodation	How it's already built in / what to do
Text read aloud	Every passage has a Listen button (TTS via Web Speech API); the simplified toggle shortens all passages including the worked example.
Reduced reading load	"Reading: Simplified" swaps in plain-language text throughout; the atom-tracing step is broken into a numbered 5-step list in the simplified version.
Chunking / extended time	Lesson is in three clearly labeled steps; the worked example is set apart visually; progress saves automatically.
Sentence frames / word bank	Tier-2 panel provides a 13-word word bank and frames for all three CER components, mapped explicitly to van Helmont's data.
Concrete analogy / reduced load	Tier-3 LEGO "build-a-tree" analogy provides a spatial, concrete entry point; the cloze frame gives vocabulary scaffolding without reducing the goal.
Alternative response mode	Students may draw an atom-tracing diagram, record a verbal explanation, or write — all three are explicitly accepted.
Behavior / focus (504, ADHD)	Named expectations (PBIS), 30-sec check-in connecting to prior week's prediction, and a mindful breathing brain break.
Multilingual / EL	Simplified text toggle, SVG with labeled data (van Helmont's numbers), atom-tracing flow diagram in Tier-2, and word bank in Tier-2. Home-language reflection allowed.

Exit ticket answer key

carbon dioxide + water + light energy → glucose + oxygen. The star goes next to "carbon dioxide" (CO₂) — it is the source of the carbon that makes up most of the plant's mass.
Matter is conserved means atoms are never created or destroyed — only rearranged. The carbon atoms that were in CO₂ (a gas in the air) were captured by the plant during photosynthesis and rearranged into glucose molecules, which were then used to build cellulose — the material of the tree trunk. The same atoms moved from the air into the tree's body. No new carbon was created; no carbon was destroyed.
Accept any two of the seven; strongest answers connect photosynthesis to: "Uses energy" (photosynthesis captures light energy and stores it in glucose), "Grows and develops" (the glucose is used to build new cells and cellulose, allowing the plant to grow), "Maintains homeostasis" (the plant regulates how much CO₂ it takes in and O₂ it releases through the stomata), "Made of cells" (photosynthesis happens inside the chloroplasts within plant cells).
Energy is never created — it transfers and changes form. In photosynthesis, light energy (electromagnetic, from the sun — from Unit 2) is transformed into chemical energy stored in glucose. The energy did not come from nothing; it came from the sun. It is now stored in the chemical bonds of the glucose molecule. Later, when the plant (or an animal that eats the plant) breaks down the glucose, that stored chemical energy is released again. The energy moved: sun → glucose → work/heat. Never created, never destroyed.
Accept any reasonable prediction: the plant uses the stored glucose for energy (cellular respiration — preview of Week 3); the plant breaks down the glucose; without sunlight the plant can't make more glucose so it uses up its reserves; it might eventually die if it runs out. The key is the prediction connects stored glucose to energy use — they'll discover cellular respiration next week.

Why it's designed this way

The van Helmont experiment is one of the most compelling pieces of historical science for this standard because the data contradicts common sense — students expect soil to be the source of plant mass, and the data definitively rules it out. This is the ideal structure for a phenomenon-driven CCC lesson on matter conservation: the phenomenon creates cognitive dissonance, and the explanation (CO₂ → carbon atoms → cellulose) resolves it through the atom-tracing practice. The worked example uses a mass-balance approach that explicitly teaches the scientific reasoning process, not just the conclusion. The Tier-3 LEGO analogy was chosen because it preserves the abstract concept (atoms as conserved, rearrangeable units) in a concrete spatial form that concrete-operational thinkers can manipulate mentally. The exit-ticket item 5 is intentionally a transfer-forward prediction about cellular respiration — it ensures Week 2 creates the question that Week 3 answers, maintaining the unit's storyline and giving students ownership over the next lesson's inquiry.