The Carbon Cycle

This lesson covers the carbon cycle as required by the Edexcel GCSE Combined Science specification (1SC0). You need to describe how carbon is recycled through ecosystems, explain the roles of photosynthesis, respiration, combustion and decomposition in the carbon cycle, and understand the concept of carbon sinks.

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Why Carbon Is Important

Carbon is an essential element for life. It is found in:

• Carbohydrates (e.g. glucose, starch)
• Lipids (fats and oils)
• Proteins (made from amino acids)
• DNA and RNA (nucleic acids)

Carbon atoms are constantly being recycled between the living (biotic) world and the non-living (abiotic) environment. The carbon cycle shows how this happens.

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The Carbon Cycle — Overview

graph TD
    A["CO₂ in the atmosphere"] -->|"Photosynthesis"| B["Carbon in plants (glucose, starch)"]
    B -->|"Feeding"| C["Carbon in animals"]
    B -->|"Respiration"| A
    C -->|"Respiration"| A
    B -->|"Death"| D["Dead organic matter"]
    C -->|"Death / waste"| D
    D -->|"Decomposition (respiration by decomposers)"| A
    D -->|"Fossilisation over millions of years"| E["Fossil fuels (coal, oil, gas)"]
    E -->|"Combustion"| A
    A -->|"Dissolves in oceans"| F["CO₂ dissolved in water"]
    F -->|"Used by aquatic plants / algae"| B

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Processes That Remove CO₂ from the Atmosphere

Process	Description
Photosynthesis	Plants, algae and some bacteria absorb CO₂ and convert it into glucose using light energy. The equation is: CO₂ + H₂O → C₆H₁₂O₆ + O₂
Dissolving in oceans	CO₂ dissolves in seawater, forming carbonic acid. It can then be used by marine organisms to build shells (calcium carbonate)

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Processes That Return CO₂ to the Atmosphere

Process	Description
Respiration	All living organisms (plants, animals, fungi, bacteria) respire, releasing CO₂. The equation is: C₆H₁₂O₆ + O₂ → CO₂ + H₂O
Combustion	Burning fossil fuels (coal, oil, gas) and wood releases CO₂ that was previously locked away
Decomposition	Decomposers (bacteria and fungi) break down dead material. They respire, releasing CO₂

Exam Tip: Decomposition returns CO₂ to the atmosphere because decomposers respire as they break down dead matter. It is the decomposer's respiration that releases the CO₂, not the breakdown itself.

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Carbon Sinks

A carbon sink is a reservoir that absorbs and stores more carbon than it releases. Important carbon sinks include:

Carbon Sink	How it stores carbon
Forests	Trees absorb CO₂ by photosynthesis and store carbon in wood (biomass)
Oceans	Dissolve CO₂; marine organisms incorporate carbon into shells and skeletons
Fossil fuels	Carbon from ancient organisms was buried and compressed over millions of years
Soil	Organic matter (humus) from decomposition stores carbon
Limestone	Calcium carbonate (CaCO₃) in rocks formed from ancient marine shells

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Fossil Fuels and the Carbon Cycle

Millions of years ago, dead organisms were buried under layers of sediment before they could fully decompose:

• Coal formed from ancient plants in swamps
• Oil and natural gas formed from ancient marine organisms

The carbon in these organisms became trapped underground as fossil fuels. When we burn fossil fuels, we release carbon that was locked away for millions of years, increasing atmospheric CO₂.

graph LR
    A["Ancient organisms die"] --> B["Buried under sediment"]
    B --> C["Heat and pressure over millions of years"]
    C --> D["Fossil fuels form (coal, oil, gas)"]
    D -->|"Combustion by humans"| E["CO₂ released to atmosphere"]

Exam Tip: The carbon cycle is normally balanced — CO₂ removed by photosynthesis roughly equals CO₂ added by respiration and decomposition. Burning fossil fuels adds extra CO₂ that was not part of the recent cycle, causing atmospheric CO₂ levels to rise.

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The Carbon Cycle and Ecosystems

In a healthy ecosystem, the carbon cycle is in dynamic equilibrium:

• Carbon moves continuously between the atmosphere, organisms and the soil
• The rate of CO₂ removal (photosynthesis) roughly balances the rate of CO₂ release (respiration + decomposition)
• Human activities (burning fossil fuels, deforestation) are disrupting this balance

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Human Impact on the Carbon Cycle

Activity	Effect on carbon cycle
Burning fossil fuels	Releases stored carbon as CO₂ → increases atmospheric CO₂
Deforestation	Fewer trees to absorb CO₂ by photosynthesis; burning/decaying trees release CO₂
Cement production	Heating limestone (CaCO₃) releases CO₂
Agriculture	Livestock produce methane (a carbon compound); rice paddies release methane

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Balancing the Carbon Cycle

Efforts to restore balance include:

• Reforestation — planting trees to absorb CO₂
• Reducing fossil fuel use — switching to renewable energy (solar, wind, hydroelectric)
• Carbon capture — technologies that trap CO₂ before it enters the atmosphere

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Summary

• The carbon cycle shows how carbon is recycled between the atmosphere, living organisms and the Earth.
• Photosynthesis removes CO₂ from the atmosphere; respiration, combustion and decomposition return it.
• Carbon sinks (forests, oceans, fossil fuels, soil) store carbon for long periods.
• Fossil fuels contain carbon from ancient organisms; burning them releases extra CO₂.
• Human activities are increasing atmospheric CO₂, disrupting the natural balance.
• The carbon cycle depends on decomposers to recycle carbon from dead organisms.

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Worked Example 1: Biomass and Carbon Content

A mature oak tree has a dry biomass of 5,000 kg. Approximately 50% of dry plant biomass is carbon.

Step 1 — Carbon stored.

Carbon = 0.50 x 5,000 = 2,500 kg.

Step 2 — Equivalent CO₂ absorbed.

Using atomic masses: CO₂ relative molecular mass = 44; carbon = 12. So each kg of C corresponds to 44/12 = 3.67 kg of CO₂.

CO₂ absorbed = 2,500 x 3.67 = 9,175 kg of CO₂ locked in the tree over its lifetime.

Step 3 — Fate if the tree is burnt.

Combustion returns all 2,500 kg of carbon to the atmosphere as 9,175 kg of CO₂, plus water vapour. This is why deforestation has a double effect on the carbon cycle: it removes a future sink and releases a present stock.