Introduction

Carbon is one of the most important elements for life on Earth. It is found in all living organisms and forms the backbone of important molecules such as carbohydrates, proteins, fats, and DNA. Carbon is also present in the atmosphere, oceans, rocks, soil, and fossil fuels. Rather than remaining in one place, carbon is constantly moving between these different parts of the Earth.

The carbon cycle describes the continuous movement of carbon through living organisms, the atmosphere, the oceans, and the Earth's crust. This cycle helps maintain the balance of carbon dioxide in the atmosphere, supports life through photosynthesis, and plays an important role in regulating Earth's climate.

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What Is the Carbon Cycle?

The carbon cycle is the continuous movement of carbon between the:

• Atmosphere
• Living organisms
• Oceans
• Soil
• Rocks
• Fossil fuels

Carbon moves through these reservoirs by natural processes such as:

• Photosynthesis
• Respiration
• Feeding
• Decomposition
• Combustion
• Ocean exchange

Unlike energy, which flows in one direction, carbon is continually recycled.

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Figure 1. The carbon cycle continuously moves carbon between the atmosphere, living organisms, oceans, and Earth's crust.

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Carbon Reservoirs

A carbon reservoir is any place where carbon is stored.

Major carbon reservoirs include:

The Atmosphere

Carbon is mainly found as carbon dioxide (CO₂).

Although CO₂ makes up only a small fraction of the atmosphere, it is essential for photosynthesis and influences Earth's climate.

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Living Organisms

Plants, animals, fungi, and microorganisms all contain carbon in their cells.

Carbon forms part of:

• Sugars
• Proteins
• Lipids (fats)
• DNA

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Oceans

The oceans store enormous amounts of carbon.

Carbon is found:

• Dissolved in seawater
• In marine organisms
• In shells made from calcium carbonate

The oceans act as one of Earth's largest carbon reservoirs.

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Soil

Dead plants and animals add carbon to the soil as they decompose.

Healthy soils store large amounts of organic carbon.

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Rocks and Fossil Fuels

Over millions of years, carbon becomes locked inside:

• Limestone
• Coal
• Oil
• Natural gas

These are considered long-term carbon reservoirs.

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Figure 2. Carbon is stored in several major reservoirs throughout Earth's systems.

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Photosynthesis

Plants remove carbon dioxide from the atmosphere during photosynthesis.

Using sunlight, plants convert:

• Carbon dioxide
• Water

into:

• Glucose (food)
• Oxygen

This transfers carbon from the atmosphere into living organisms.

Respiration

All living organisms carry out cellular respiration.

During respiration:

• Glucose is broken down.
• Energy is released for life processes.
• Carbon dioxide is produced.

Plants, animals, fungi, and microorganisms all return carbon dioxide to the atmosphere through respiration.

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Figure 3. Photosynthesis removes carbon dioxide from the atmosphere, while respiration returns it.

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Feeding and Food Webs

Carbon moves through ecosystems whenever organisms eat one another.

For example:

Grass → Rabbit → Fox

• Grass absorbs carbon dioxide during photosynthesis.
• The rabbit obtains carbon by eating the grass.
• The fox obtains carbon by eating the rabbit.

In this way, carbon moves through food chains and food webs.

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Decomposition

When organisms die or produce waste, decomposers break down the organic matter.

Examples of decomposers include:

• Bacteria
• Fungi

During decomposition:

• Carbon is returned to the soil.
• Carbon dioxide is released into the atmosphere through respiration.

Some carbon remains in the soil for long periods.

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Fossil Fuels and Combustion

Millions of years ago, some dead organisms became buried underground.

Over time, heat and pressure transformed this organic matter into:

• Coal
• Oil
• Natural gas

These are called fossil fuels.

When fossil fuels are burned (combustion):

• Carbon combines with oxygen.
• Carbon dioxide is released into the atmosphere.

Because fossil fuels formed over millions of years, burning them releases stored carbon much faster than natural processes can remove it.

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Figure 4. Burning fossil fuels rapidly transfers carbon stored underground back into the atmosphere as carbon dioxide.

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Human Impacts on the Carbon Cycle

Human activities have significantly altered the natural carbon cycle.

Burning Fossil Fuels

Power stations, factories, and vehicles release large amounts of carbon dioxide.

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Deforestation

Cutting down forests reduces the number of plants available to remove carbon dioxide through photosynthesis.

Burning forests also releases stored carbon.

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Agriculture

Some farming practices release carbon from soils into the atmosphere.

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Cement Production

Manufacturing cement releases additional carbon dioxide.

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These activities increase atmospheric carbon dioxide, strengthening the greenhouse effect and contributing to climate change.

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Figure 5. Human activities have increased the amount of carbon dioxide in the atmosphere, altering the natural carbon cycle.

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Why the Carbon Cycle Is Important

The carbon cycle is essential because it:

• Recycles carbon needed for life.
• Supports photosynthesis.
• Helps regulate Earth's climate.
• Maintains the balance of atmospheric carbon dioxide.
• Connects living organisms with Earth's atmosphere, oceans, and rocks.

Without the carbon cycle, ecosystems and life as we know them could not exist.

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Worked Example

Question

Identify whether each process removes carbon dioxide from the atmosphere or adds carbon dioxide to the atmosphere.

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Real-World Connection

Planting trees is one way to help reduce atmospheric carbon dioxide because growing trees absorb CO₂ through photosynthesis and store carbon in their trunks, branches, roots, and leaves. However, reducing emissions from burning fossil fuels is equally important because newly planted forests cannot absorb carbon as quickly as large amounts are released by human activities.

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Did You Know?

The oceans absorb about one-quarter of the carbon dioxide released by human activities each year. This helps slow the increase of atmospheric CO₂, but it also makes seawater more acidic. Ocean acidification can make it difficult for organisms such as corals, oysters, and some plankton to build and maintain their calcium carbonate shells and skeletons.

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Key Terms

Atmosphere – The layer of gases surrounding Earth.

Carbon cycle – The continuous movement of carbon through Earth's atmosphere, living organisms, oceans, soil, and rocks.

Carbon reservoir – A place where carbon is stored.

Carbon dioxide (CO₂) – A gas involved in photosynthesis, respiration, and the greenhouse effect.

Combustion – The burning of a substance, releasing energy and carbon dioxide.

Decomposition – The breakdown of dead organic matter by decomposers.

Fossil fuels – Coal, oil, and natural gas formed from ancient organisms over millions of years.

Photosynthesis – The process by which producers use sunlight to convert carbon dioxide and water into glucose and oxygen.

Respiration – The process by which cells break down glucose to release energy, producing carbon dioxide and water.

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Key Takeaways

• The carbon cycle continuously recycles carbon between Earth's atmosphere, living organisms, oceans, soil, and rocks.
• Photosynthesis removes carbon dioxide from the atmosphere and stores carbon in living organisms.
• Respiration, decomposition, and combustion return carbon dioxide to the atmosphere.
• Carbon is stored in major reservoirs such as the atmosphere, oceans, forests, soils, rocks, and fossil fuels.
• Human activities, especially burning fossil fuels and deforestation, have altered the natural carbon cycle and contributed to climate change.