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Carbon Cycle

Trace carbon transfers between atmospheric carbon dioxide, living material and fuels.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe the carbon cycle in simple terms, to include — the processes of combustion, respiration and photosynthesis
  • describe the carbon cycle in simple terms, to include — how the carbon cycle regulates the amount of carbon dioxide in the atmosphere (see also 11.1(d))

Outcome: trace whether combustion, respiration and photosynthesis add carbon dioxide to or remove it from the atmosphere. Prerequisite: atoms are conserved in chemical reactions; combustion chemistry remains owned by Energetics/Organic Chemistry.

1. Definition

The carbon cycle is the transfer of carbon among atmospheric, biological and geological stores.

2. Key Ideas

  • combustion and respiration add atmospheric CO₂;
  • photosynthesis removes atmospheric CO₂ into plant material;
  • deforestation can both release stored carbon and reduce future removal.
Retrieval check
Which process uses carbon dioxide to make plant material? Photosynthesis.

3. Detailed Explanations

Required processes in the carbon cycleAn atmospheric carbon dioxide box is connected to plants, living organisms and fuels. Photosynthesis moves carbon dioxide from the atmosphere into plants. Respiration and combustion return carbon dioxide to the atmosphere.Carbon moves between the atmosphere and carbon storesAtmospheric carbon dioxideCO₂Plantscarbon stored in biomassLiving organismsrespiration releases CO₂Fuelscombustion releases CO₂photosynthesisremoves CO₂respirationadds CO₂combustionadds CO₂
Combustion and respiration add carbon dioxide to the atmosphere; photosynthesis removes it and stores carbon in plants.
ProcessCarbon transfer relative to atmosphere
combustionfuel/biomass → atmospheric CO₂
respirationliving material → atmospheric CO₂
photosynthesisatmospheric CO₂ → plant material

Representative equations conserve the carbon atoms:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)

4. Common Mistakes

  • Photosynthesis removes, not releases, atmospheric carbon dioxide.
  • Respiration is not the same process as breathing; cells release energy and carbon dioxide.
  • Carbon does not vanish when fuel burns; it moves to products.
  • A forest can affect atmospheric carbon dioxide even before it is burned because photosynthetic uptake changes.

5. Exam Tips

Always name the starting store, process and destination. An arrow without a direction cannot show a carbon transfer.

6. Worked Examples

Modelled example 1

Model: read the cycle diagram

Core

Problem

Name one atmospheric source and one sink in the diagram.
Study the worked solution

  1. Follow arrow direction

    Method

    Combustion or respiration is a source; photosynthesis is a sink.

    Reason

    Sources point towards atmospheric CO₂; the sink points away.

    Working

    combustion/respiration → atmosphere; atmosphere → photosynthesis.

Guided practice 2

Guided: changed forest context

About 5 min

Problem

A forest is cleared but not burned immediately. What direct carbon-cycle change occurs?

Try this before viewing the solution

Atmospheric change

Hints

Hint 1: identify the changed process
The forest has not burned, so first consider how clearing changes photosynthesis.
View solution step by step

  1. Change the sink

    Method

    Photosynthetic removal decreases.

    Reason

    There is less plant material taking in carbon dioxide.

    Working

    fewer plants → less photosynthesis → less CO₂ removed.

Challenge 3

Transfer: a geological store

Minimal support

Problem

How does burning natural gas move carbon from a long-term store?

Try this before viewing the solution

Carbon transfer

Hints

Hint 1: follow the carbon atom
Locate the carbon before combustion, then locate it in the carbon-containing product.
View complete solution

  1. Trace methane carbon

    Method

    Combustion oxidises methane carbon to atmospheric carbon dioxide.

    Reason

    The equation shows the carbon atom conserved in CO₂.

    Working

    geological CH₄ → combustion → atmospheric CO₂.

7. Mind Stretchers

Independent: Explain two different ways burning cleared forest can increase atmospheric carbon dioxide.

Show answer
Combustion releases carbon stored in biomass as carbon dioxide. Removing the trees also reduces future carbon-dioxide removal by photosynthesis.

8. Quiz

Practise this lesson
Start focused practice, then continue to Greenhouse Effect, Climate and Mitigation.