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# Where does the carbon in a tree go?

Year 9 Earth and space · The carbon cycle

<!-- Speaker note: Students explain photosynthesis and respiration (including decomposition) as opposite flows of carbon between the atmosphere and biosphere, and explain balance between them (AC9S9U03, node 3, a hinge node). About 22 minutes (Engage and Explore 9 min · Explain 7 min · Evaluate 3 min · Typical adds 2 min, Wider about 1 min). -->

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## Today

**We are learning to** photosynthesis moves carbon from the atmosphere into living things; respiration and decomposition move it back. These huge flows nearly balance, so the air's CO₂ stays steady until something upsets them.

- I can explain that photosynthesis moves carbon from the air into living things.
- I can explain that respiration and decomposition move it back.
- I can explain why the air's CO₂ stays steady when the flows balance.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge is representing each process as a directional flow between spheres. The misconception that plants are made from soil is raised in Engage. Board notes: stores in billions of tonnes of carbon (GtC); the air starts at a balanced 280 ppm (before 1800). Photosynthesis takes about 120 GtC a year; respiration and decomposition return about the same; the oceans exchange about 80 each way. Switching a process off is a "what if": with respiration and decomposition off, plants draw the air's CO₂ down within years; with photosynthesis off, it climbs. The model's numbers are rounded and its rules simple (plants take up a little more CO₂ when there is more of it). -->

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## Why it matters

> A big gum tree can weigh many tonnes. Almost all of that mass came out of the air.

<!-- Speaker note: You know the four spheres. This lesson follows carbon between two of them. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**Where does most of the mass of a tree come from?**

- Sunlight
- The soil
- Water alone
- Carbon dioxide from the air (plus water)

<!-- Speaker note: What happens: Carbon dioxide from the air. Photosynthesis joins carbon from CO₂ with water to make sugars, and the tree builds itself from them. Sunlight provides the energy, not the mass. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose the soil. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fearth-9%2Fcarbon-life%2Fstep-2&code=widget%3A+carbon%0Amode%3A+cycle%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/earth-9/carbon-life/?full=step-2#step-2)

<!-- Speaker note: First press **Run 10 years** with everything on. Then switch **Respiration and decomposition** off and run again. Then switch it back on, switch **Photosynthesis** off, and run again. Press **Start again** whenever you like. -->

<!-- Speaker note: Teacher: ask what happened in each run, and why. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Carbon dioxide from the air. Photosynthesis joins carbon from CO₂ with water to make sugars, and the tree builds itself from them. Sunlight provides the energy, not the mass.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Directions

**Which way does each process move carbon?**

Groups: Atmosphere → biosphere · Biosphere → atmosphere

Photosynthesis · A kangaroo breathing out · Fungi rotting a fallen log · A tree respiring at night · Grass growing

* Answer: Photosynthesis → Atmosphere → biosphere; A kangaroo breathing out → Biosphere → atmosphere; Fungi rotting a fallen log → Biosphere → atmosphere; A tree respiring at night → Biosphere → atmosphere; Grass growing → Atmosphere → biosphere

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the balance question is the checkpoint. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Balance

**With every process on, why did the CO₂ in the air stay steady?**

- Carbon leaving the air (photosynthesis, oceans) equalled carbon entering it (respiration, decomposition, oceans)
- The ocean stopped working
- Plants stopped photosynthesising
- Nothing was happening

* Answer: Carbon leaving the air (photosynthesis, oceans) equalled carbon entering it (respiration, decomposition, oceans)

<!-- Speaker note:  -->

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## Clearing a forest

**If a large forest is cleared and burned, what happens to the carbon flows?**

- Nothing changes
- Less photosynthesis takes carbon out of the air, and burning and rotting put more in: CO₂ in the air rises
- The carbon disappears
- CO₂ in the air falls

* Answer: Less photosynthesis takes carbon out of the air, and burning and rotting put more in: CO₂ in the air rises

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: a carbon atom's journey

**Put a carbon atom's journey in order.**

Groups: 1st · 2nd · 3rd · 4th · 5th

CO₂ in the air · Taken in by a grass leaf in photosynthesis · Eaten by a kangaroo, part of its body · The kangaroo dies and fungi decompose it · Respired by the fungi back into the air as CO₂

* Answer: CO₂ in the air → 1st; Taken in by a grass leaf in photosynthesis → 2nd; Eaten by a kangaroo, part of its body → 3rd; The kangaroo dies and fungi decompose it → 4th; Respired by the fungi back into the air as CO₂ → 5th

<!-- Speaker note: What this step is for: Tracing one atom through a food chain connects the carbon cycle to food webs. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: seasons in the air

**Measurements of CO₂ in the air in the Northern Hemisphere dip every northern summer and rise every winter. Why?**

- The sun burns CO₂ in summer
- In summer the huge northern forests photosynthesise more than they respire; in winter respiration and decomposition win
- People burn less fuel in summer
- Trees only respire in winter, when they have lost their leaves

* Answer: In summer the huge northern forests photosynthesise more than they respire; in winter respiration and decomposition win

<!-- Speaker note: What this step is for: Reading a real pattern as evidence of the photosynthesis–respiration balance applies the model. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Photosynthesis:** carbon moves from the **atmosphere** (CO₂) into the **biosphere** (sugars, then wood, leaves and animals).
- **Respiration** (plants, animals, decomposers) and **decomposition** move carbon back to the atmosphere.
- When the flows balance, CO₂ in the air stays steady; change one and the store changes.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Describe how carbon moves between the air and living things.**

* Photosynthesis takes CO₂ from the air into plants; respiration by plants, animals and decomposers returns it to the air as CO₂.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: burning fossil fuels (node 4). Later: the greenhouse effect (Year 10). -->