---
title: "Where does the carbon in a tree go?"
subtitle: "Year 9 Earth and space · The carbon cycle · Lesson 3"
---

# Where does the carbon in a tree go?

**Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}}

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


:::teacher-note

**In one sentence:** 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).

**Students should be able to say:** 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.

**Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet.

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## Engage


:::keep-together

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

☐ Sunlight

☐ The soil

☐ Water alone

☐ Carbon dioxide from the air (plus water)

My prediction, and why:

{{lines-3}}

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:::teacher-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.

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## Explore

Try it on the board in the tutorial. Was your prediction right? What was different?

{{lines-2}}

## Explain: two flows, opposite directions


:::keep-together

### Directions

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

Groups: Atmosphere → biosphere, Biosphere → atmosphere

- Photosynthesis: {{field-15}}
- A kangaroo breathing out: {{field-15}}
- Fungi rotting a fallen log: {{field-15}}
- A tree respiring at night: {{field-15}}
- Grass growing: {{field-15}}

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:::teacher-note

**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

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:::keep-together

### 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

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:::teacher-note

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

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:::keep-together

### 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

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:::teacher-note

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

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:::typical

## Elaborate: a carbon atom's journey


:::keep-together

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

Groups: 1st, 2nd, 3rd, 4th, 5th

- CO₂ in the air: {{field-15}}
- Taken in by a grass leaf in photosynthesis: {{field-15}}
- Eaten by a kangaroo, part of its body: {{field-15}}
- The kangaroo dies and fungi decompose it: {{field-15}}
- Respired by the fungi back into the air as CO₂: {{field-15}}

:::


:::teacher-note

**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

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:::wider

## Elaborate: seasons in the air


:::keep-together

**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

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:::teacher-note

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

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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.

## Exit ticket

:::keep-together

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

{{lines-3}}

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:::teacher-note

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

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