Unit 6 · Ecosystems & Our Footprint · Week 2
Where does matter go when things eat and die?
A massive fallen log lay on a forest floor last spring. By this fall, it has shrunk to half its size and is covered in mushrooms. Where is it going — and where are those atoms ending up?
Start here
How we work this lesson (our expectations)
- Track the atoms — ask "where did they go next?" at every step
- Distinguish matter (stuff) from energy (ability to do work)
- Value the weird and invisible — decomposers are unsung heroes
- Supports are for everyone — open any panel you want, any time
30-second check-in self-management
Before we start, take one slow breath. Notice if your brain is still holding ideas from Week 1 (food webs, energy pyramids). That is good — today we add the missing piece: what happens to matter (the physical stuff) after it is eaten or when something dies. Rate your focus 1–5. If you are at a 1 or 2, the simplified reading and Listen button are here for you.
Do-Now Quick recall
Three questions — one from Week 1, one from earlier this year. No looking back.
- W1 spiral In a food chain, energy flows from producers to consumers. At each step about 90% is lost as heat. So where does all the original solar energy end up?
- Unit 1 spiral Conservation of matter: when a log burns, where does all its matter go? (Hint: Unit 1 Week 4.)
- Look at the last bite of food you ate today. Name one element or type of molecule you think was in it. Do you think that same atom has ever been in something else before you ate it?
The phenomenon
A large tree fell in a forest last spring. By autumn, it is visibly smaller — crumbling, soft, laced with white fungal threads, dotted with mushrooms, crawling with pill bugs and beetles. By next spring, it may be a low mound of dark soil. Where is it going? The log is made of atoms — carbon, hydrogen, oxygen, and others. Those atoms do not disappear. Decomposers — fungi, bacteria, and small invertebrates — break the wood apart, using some of its chemical energy for their own life processes (releasing heat) and returning the matter to simpler forms: carbon dioxide rises into the air, mineral nutrients sink into the soil. A nearby seedling then pulls those same carbon atoms back in through photosynthesis and roots. The atoms in the log become new living things — nothing is destroyed. This is matter cycling, and it is the crucial contrast to last week: energy flows one way and leaves the ecosystem as heat; matter cycles and stays in the system forever. Our question: where does the matter from a dead organism go — and how does it become part of life again?
A fallen log gets smaller over time. Fungi and bacteria (decomposers) break it down. The atoms in the log go two places: some float away as CO₂ gas (carbon dioxide), some sink into the soil as nutrients. Then a nearby plant pulls those atoms back up through its roots and from the air. The atoms are not destroyed — they just moved. Matter cycles; it never disappears. This is different from energy (which flows one way and is lost as heat). Where do the atoms in a dead organism go, and how do they become part of something alive again?
Notice & Wonder. Write one thing you notice about where the atoms go and one thing you wonder about how they get there.
Build it Decomposers, cycles, and the contrast with energy
Three connected ideas. After each step there is a quick check before we continue.
Step 1 — Decomposers are matter recyclers
Decomposers — including fungi, bacteria, and detritivores (like pill bugs and earthworms) — break down dead organisms and waste into simpler molecules. They are not just "rotters"; they are the essential recyclers of every ecosystem. Without decomposers, dead matter would pile up, and the nutrients locked inside would never be released for new life. Decomposers do get energy from the material they break down (they are consumers in the food web), but their most important ecosystem role is releasing matter back into the abiotic (non-living) environment. This connects to Unit 5: plant roots take up those released nutrients from the soil and pull CO₂ from the air — feeding photosynthesis and closing the cycle.
Decomposers (fungi, bacteria, earthworms) break down dead organisms. They get energy from doing this, but their main job for the ecosystem is releasing nutrients back into the soil and CO₂ back into the air. Plants then use those nutrients and CO₂ to grow. Without decomposers, nutrients would be locked in dead bodies forever and nothing new could grow.
Step 2 — Matter cycles; energy does not
Here is the crucial contrast from Week 1. Energy flows through an ecosystem in one direction — it enters as sunlight, moves through the food web, and exits as heat. It does not cycle back. Matter is different: the atoms in a log were once in soil, before that in ancient organisms, before that in rocks. When the log decomposes, those atoms go back to the soil and air, ready to be built into new life again. Carbon, nitrogen, and other elements cycle through the living and non-living parts of an ecosystem endlessly. This is called biogeochemical cycling. The key NGSS crosscutting concept: energy and matter behave differently in ecosystems — energy flows, matter cycles. This also revisits Unit 1's conservation of matter: the atoms in the log are never created or destroyed, only rearranged.
Energy goes one way: from sun → producers → consumers → heat. It does not come back. Matter is different: atoms move in a cycle. Carbon from a fallen log goes into the air as CO₂, is absorbed by a plant, eaten by a caterpillar, and so on forever. Same atoms, different bodies. This is matter cycling. Key contrast: energy flows one way and leaves; matter cycles and stays.
Step 3 — Following carbon: a short version of the carbon cycle
Carbon is the backbone of all living matter. Follow one carbon atom through a simplified ecosystem loop: (1) CO₂ in the air is absorbed by a plant during photosynthesis and built into sugar. (2) A caterpillar eats the plant and uses that carbon in its own body. (3) The caterpillar dies; decomposers break it down and release CO₂ back to the air (and carbon compounds into the soil). (4) The CO₂ is absorbed by a new plant. The atom has completed a cycle. Nutrients like nitrogen and phosphorus cycle similarly through soil and organisms. The take-away: every carbon atom in your body has been in thousands of other organisms over millions of years. Nothing is lost; everything is borrowed.
Carbon cycles like this: plant absorbs CO₂ from air → builds it into sugar → caterpillar eats sugar → caterpillar dies → decomposer breaks it down → CO₂ returns to air → new plant absorbs it → repeat. The same carbon atoms have been in millions of living things. You are borrowing atoms that have been everywhere. Nothing is lost — atoms just move to new locations.
Worked example — tracking a carbon atom from log to seedling (watch how an expert reasons)
Start: a carbon atom is locked in a cellulose molecule in the fallen log.
Step 1 — Decomposer breaks it down: a fungus uses enzymes to break the cellulose into glucose. The fungus uses that glucose for its own cellular respiration — combining it with oxygen, releasing energy (as heat and ATP), and releasing CO₂ and water. Our carbon atom is now in a CO₂ molecule floating in the air above the log.
Step 2 — Plant captures it: a seedling growing from the log's rich soil pulls that CO₂ through its leaves. In photosynthesis, it combines CO₂ with water, using light energy, to make glucose — and our carbon atom is now locked in the seedling's tissue.
Step 3 — Note the energy contrast: the carbon atom cycled (log → CO₂ → seedling). The energy did NOT cycle: the energy stored in the log's cellulose was released as heat during decomposition and left the ecosystem. The seedling's energy came from new sunlight, not from recycled energy. Matter cycled; energy did not.
Show me the carbon cycle as a diagram I can trace Tier 2 · visual
Draw this cycle as a circle with four stops:
Stop 1: CO₂ in the air → (photosynthesis) → Stop 2: Sugar in a plant's leaves
Stop 2: Sugar in leaves → (eaten) → Stop 3: Carbon in a consumer's body
Stop 3: Consumer dies → (decomposition / cellular respiration) → Stop 4: CO₂ back in the air
Stop 4: CO₂ in air → (photosynthesis again) → back to Stop 1
Label which processes move carbon and which release energy. Notice: energy is NOT on this loop — only matter is.
Your turn Track the atoms
Trace matter through a decomposition scenario. Choose how you respond — all are equally valid.
The task: Choose ONE carbon atom from the fallen log. Trace its complete journey: where it is in the log, how decomposers move it, where it ends up in the short term (soil? air?), and how it could become part of a new plant. Include: (1) what happens to the MATTER; (2) what happens to the ENERGY; (3) how this is the same or different from the food-web energy we traced in Week 1.
I want sentence starters Tier 2 · frames
The carbon atom in the log starts as part of ________.
When the decomposer breaks it down, the carbon atom moves to ________ in the form of ________.
The energy in the log is ________ (cycled / released as heat / stored), while the matter is ________ (cycled / released as heat / destroyed).
This is different from energy flow because ________.
Word bank: carbon · decomposer · CO₂ · photosynthesis · nutrient · soil · cycle · energy · heat · matter · conserved · released · cellular respiration
Let's make it concrete first Tier 3 · re-teach
Analogy: Imagine a LEGO house (the log). When it falls apart, do the LEGO bricks disappear? No — they scatter across the floor (the soil and air). Someone picks them up and builds something new (a plant). The LEGO bricks are the atoms — they are always there, just in different buildings. The "energy" used to put the bricks together? That came from the builder's effort (sunlight), and when the house falls apart, that effort is gone — it doesn't get recycled back. Only the bricks (matter) cycle; the energy doesn't.
Fill in: The atoms in the log are like LEGO ____. When the log decomposes, the atoms ____ (disappear / scatter). A new plant picks them up and ____ them into new tissue. The energy that was in the log is released as ____, not recycled.
Now say your answer out loud before writing it. Two sentences is enough.
Push my thinking Extension
The carbon cycle is part of a larger system. Research the nitrogen cycle: how does nitrogen move from the atmosphere into soil, into plants, into animals, and back? What role do nitrogen-fixing bacteria play that is analogous to what decomposers do for carbon? Then answer: what would happen to ecosystems if all nitrogen-fixing bacteria suddenly disappeared? Use the idea of biogeochemical cycling to support your argument.
Check yourself Did it click?
- explain that decomposers break down dead matter and release atoms back to the environment;
- contrast matter cycling (atoms go in a loop) with energy flow (one-way, leaves as heat);
- trace a carbon atom from a dead organism through decomposition to a new living thing.
A student says: "When an organism dies, all its matter is used up and gone." What should you say?
Re-teach: matter cycles; energy does not Tier 3
Two things to hold at the same time:
Matter (atoms): atoms are conserved — they are never created or destroyed. When an organism dies, its atoms are broken down by decomposers and released into the environment (CO₂ into air, nutrients into soil). New organisms pick up those atoms. The same carbon atom cycles through thousands of organisms over millions of years. Matter goes in a loop.
Energy: energy entered the ecosystem as sunlight and was transferred through the food web. At every step, some was lost as heat — that heat left the ecosystem and did NOT cycle back. Decomposers also release heat when they break down organic matter. The energy that was in the dead organism does NOT return to producers; producers get their energy from new sunlight every day. Energy flows one way and leaves.
LEGO analogy: the bricks (matter) are reused in new buildings; the effort (energy) to put the bricks together cannot be recovered — it is gone. Re-read option B and look at the carbon-cycle diagram in the Tier-2 support.
Reflect, then show what you know
Reflection metacognition + SEL
The atoms in your body have been in other living things — including things that lived millions of years ago. Does that change how you think about yourself as part of the ecosystem? There is no wrong answer; this kind of reflection is called environmental identity, and it's the basis of a lot of environmental decision-making.
Exit ticket — answer on paper or in your notebook. (2 new · 2 spiral · 1 transfer)
- New Explain the role of decomposers in an ecosystem in two or three sentences. What would happen if they all disappeared?
- New Describe the key difference between how energy moves through an ecosystem and how matter moves through an ecosystem.
- Spiral — W1 In a food chain, energy is lost as heat at each step. After decomposition, is that energy recovered and put back into the food web? Explain.
- Spiral — Unit 1 Conservation of matter (Unit 1) says atoms are never created or destroyed. How does the decomposition of a fallen log support this principle?
- Transfer → W3 If all the decomposers in a forest disappeared tomorrow, how would that change the populations of producers (plants) over the next year? Predict and explain.
Educator panel — facilitation, accommodations, answer key
At a glance
Standard: NGSS MS-LS2-3 — Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem. 3D: SEP Developing & Using Models · DCI LS2.B Cycle of Matter and Energy Transfer in Ecosystems · CCC Energy & Matter + Systems & System Models. Key contrast with W1: this lesson establishes the energy-vs-matter distinction that is the conceptual backbone of the entire unit. Students must hold both ideas simultaneously: energy leaves ecosystems (as heat), matter stays in ecosystems (cycling). This distinction is explicitly referenced in W3 ("the connections are energy AND matter pathways"), W4 ("humans redirect energy AND matter flows at large scale"), and W5 (the year capstone).
Facilitation arc (Rosenshine)
- Do-Now retrieval (3 min) — item 1 checks W1 energy retention; item 2 fires the Unit 1 conservation-of-matter schema; item 3 surfaces the personal connection.
- Phenomenon SVG + notice/wonder (3 min) — ask: "Where did the log go? Can matter just disappear?" Surface the misconception early.
- Build Steps 1–3 with the class (12 min) — after Step 2, explicitly stop and ask: "So energy and matter behave differently in an ecosystem — can someone describe both at once?" This is the hard conceptual move.
- Differentiated practice (15 min) — circulate and listen for "energy cycles" language; redirect to re-teach panel.
- Self-check + re-teach branch (5 min).
- Carbon-atom 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); simplified toggle shortens it. |
| Reduced reading load | "Reading: Simplified" swaps in shorter plain-language text throughout. |
| Chunking / extended time | Three-step build section with progress auto-save; task is one carbon atom, not the whole cycle. |
| Sentence frames / word bank | Tier-2 panel provides frames and a 13-term word bank. |
| Concrete analogy / reduced load | Tier-3 provides the LEGO-house analogy and a guided cloze with sentence starters. |
| Alternative response mode | Draw the cycle with arrows, write a CER, or narrate a carbon atom's journey — all accepted. |
| Behavior / focus (504, ADHD) | Named expectations, 30-sec check-in, the carbon-atom movement brain break. |
| Multilingual / EL | Simplified text, SVG with labeled arrows, word bank, home-language reflection allowed. |
Exit ticket answer key
- Decomposers break down dead organic matter and release the atoms back into the environment as nutrients in the soil and CO₂ in the air. Without decomposers, nutrients would remain locked in dead organisms, the soil would become depleted, plants could not grow, and the entire food web would eventually collapse.
- Energy flows in one direction — entering as sunlight, moving through the food web, and leaving the ecosystem as heat at every step. It does not cycle back. Matter (atoms) moves in a cycle — the same atoms cycle endlessly through living organisms, soil, water, and air. Matter is conserved; it is never created or destroyed.
- No, the energy is not recovered. When decomposers break down the dead organism, the chemical energy stored in its molecules is released as heat — it leaves the ecosystem. Producers do not get their energy from that heat; they get it from new sunlight every day. Energy flows one way and exits; it does not recycle.
- The atoms that made up the log are still present after decomposition — they moved into CO₂ gas in the air and into nutrient compounds in the soil. If you could collect all the CO₂ and nutrients released and measure their mass, it would equal the original mass of the wood (minus any water) that decomposed. No mass was lost, only rearranged — conservation of matter at work.
- (Transfer preview for W3.) Without decomposers, dead organic matter would accumulate and nutrients would not be released back into the soil. Over time, soil nutrient levels would drop sharply. Plants (producers) would be unable to grow well because they depend on the nutrients released by decomposers to build their tissues. Producer populations would decline, which would then cascade through the entire food web: fewer producers means fewer primary consumers, then fewer secondary consumers, etc. The entire ecosystem would eventually collapse from the bottom up.
Why it's designed this way
The matter-vs-energy contrast is one of the most conceptually demanding ideas in MS-LS2 and is consistently identified in the research literature as a persistent misconception (e.g., students who correctly understand that energy flows one-way often believe matter also flows one-way and "disappears" at the top; Driver et al. 1994; Leach et al. 1996). This lesson addresses the misconception directly and early by building the contrast explicitly in Step 2 and the worked example, then surfacing it in the self-check and re-teach panel. The LEGO analogy in Tier-3 is deliberately separated from the energy analogy used in W1 ($100 bill passed along) so students can hold the two models side by side — bricks cycle; the building effort doesn't. The carbon-atom brain break uses embodied movement to trace the abstract concept of a cycling atom, encoding it multi-modally (Paivio, 1986; Mayer, 2009). The exit ticket item 5 (transfer) deliberately inverts the decomposer scenario to check systems thinking: what happens when you remove this component? This previews W3's keystone-species cascade and requires students to apply the matter-cycling model in a new direction.