Introduction

Carbon is one of the most important elements on Earth. It forms the backbone of all living organisms and is found in the air, oceans, rocks, soil, and fossil fuels. Although the total amount of carbon on Earth changes very little, carbon is constantly moving between these different parts of the planet in a process known as the carbon cycle.

The carbon cycle is essential because it recycles carbon, making it continuously available for living organisms. Without this cycle, plants would eventually run out of carbon dioxide for photosynthesis, animals would lack food and oxygen, and Earth's climate would become unstable.

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What Is the Carbon Cycle?

The carbon cycle is the continuous movement of carbon between:

• the atmosphere
• living organisms
• oceans
• soils
• rocks
• fossil fuels

Carbon changes form as it moves through these reservoirs, but the total amount of carbon on Earth remains nearly constant.

Definition:
The carbon cycle is the natural process by which carbon moves between Earth's atmosphere, biosphere, hydrosphere, and geosphere.

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Major Carbon Reservoirs

A carbon reservoir is any place where carbon is stored for a period of time.

The largest reservoirs include:

Some reservoirs store carbon for only days or years, while others, such as sedimentary rocks, can store carbon for millions of years.

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Carbon in the Atmosphere

Carbon in the atmosphere is found mainly as carbon dioxide (CO₂).

Although carbon dioxide makes up only about 0.04% of the atmosphere, it is essential because it:

• supplies carbon for photosynthesis
• helps regulate Earth's temperature through the greenhouse effect
• forms part of the global carbon cycle

Atmospheric carbon dioxide is constantly exchanged with plants, oceans, and living organisms.

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Photosynthesis

Plants, algae, and some bacteria remove carbon dioxide from the atmosphere during photosynthesis.

During this process:

• carbon dioxide enters leaves,
• water is absorbed through the roots,
• sunlight provides energy,
• glucose is produced,
• oxygen is released.

This transfers carbon from the atmosphere into living organisms.

What Happens During Photosynthesis?

• Plants absorb carbon dioxide from the air.
• Carbon atoms become part of glucose molecules.
• Glucose is used to build:
  • cellulose
  • starch
  • proteins (after combining with nitrogen)
  • fats
• Animals obtain this carbon by eating plants or other animals.

Photosynthesis is the main way carbon enters the food chain.

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Respiration

Living organisms release carbon back into the atmosphere through respiration.

During respiration:

• glucose is broken down,
• energy is released,
• carbon dioxide is produced,
• carbon dioxide returns to the atmosphere.

Both plants and animals respire continuously.

This means that:

• photosynthesis removes CO₂,
• respiration returns CO₂.

Together, these processes keep carbon moving through ecosystems.

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Decomposition

When plants and animals die, decomposers such as bacteria and fungi break down their remains.

During decomposition:

• carbon stored in dead organisms is released,
• some returns to the atmosphere as carbon dioxide,
• some becomes part of the soil,
• some may eventually form fossil fuels over millions of years.

Without decomposers, carbon would remain locked in dead material.

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Combustion

When fossil fuels or wood are burned, carbon stored within them combines with oxygen to form carbon dioxide.

Examples include:

• burning coal
• burning petrol
• burning natural gas
• forest fires

Combustion transfers carbon rapidly from long-term storage back into the atmosphere.

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Carbon Exchange with the Oceans

The oceans store enormous amounts of carbon.

Carbon dioxide continuously moves between:

• the atmosphere
• ocean water

Marine organisms also absorb carbon to build:

• shells
• coral skeletons
• marine sediments

Some of this carbon eventually becomes limestone and other sedimentary rocks, storing carbon for millions of years.

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

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Human Activities and the Carbon Cycle

Human activities have altered the natural carbon cycle.

Major activities include:

Burning Fossil Fuels

• coal
• oil
• natural gas

These release carbon that has been stored underground for millions of years.

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Deforestation

Trees remove carbon dioxide through photosynthesis.

When forests are cut down:

• fewer trees absorb carbon dioxide,
• stored carbon is released if wood is burned or decomposes,
• atmospheric carbon dioxide increases.

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Agriculture

Certain farming practices increase carbon emissions through:

• soil disturbance
• livestock producing methane
• burning vegetation

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Industry

Manufacturing processes such as cement production also release significant amounts of carbon dioxide.

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Why Is the Carbon Cycle Important?

The carbon cycle supports life by:

• providing carbon for living organisms,
• supplying carbon dioxide for photosynthesis,
• recycling nutrients,
• maintaining food webs,
• regulating Earth's climate.

Without carbon cycling:

• plants would eventually run out of carbon dioxide,
• food chains would collapse,
• Earth's climate would become unstable.

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Worked Example

A student observes the following sequence:

Plant → Rabbit → Fox → Respiration

Question

How does carbon move through this sequence?

Solution

1. The plant absorbs carbon dioxide through photosynthesis.
2. Carbon becomes part of the plant's tissues.
3. The rabbit eats the plant.
4. The fox eats the rabbit.
5. Both animals respire and release carbon dioxide back into the atmosphere.

This demonstrates how carbon cycles through living organisms before returning to the atmosphere.

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Real-World Connection

Forests are often called "carbon sinks" because they absorb large amounts of atmospheric carbon dioxide. Tropical rainforests such as the Amazon help slow climate change by storing billions of tonnes of carbon in trees and soils. Protecting forests and restoring degraded ecosystems are important strategies for maintaining a balanced carbon cycle and reducing the impacts of global warming.

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Did You Know?

The world's oceans contain around 50 times more carbon than the atmosphere. Tiny marine organisms called phytoplankton absorb enormous amounts of carbon dioxide through photosynthesis. When they die, some sink to the ocean floor, where their carbon may be stored in sediments for thousands to millions of years.

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Key Terms

• Carbon cycle — the continuous movement of carbon through Earth's systems.
• Carbon reservoir — a place where carbon is stored.
• Photosynthesis — the process by which plants use sunlight to convert carbon dioxide and water into glucose and oxygen.
• Respiration — the process by which organisms release energy from glucose, producing carbon dioxide.
• Decomposition — the breakdown of dead organisms by decomposers.
• Combustion — the burning of fuels, releasing carbon dioxide.
• Carbon sink — a reservoir that absorbs more carbon than it releases.

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Key Takeaways

• Carbon continuously cycles between the atmosphere, living organisms, oceans, soils, rocks, and fossil fuels.
• Photosynthesis removes carbon dioxide from the atmosphere and transfers carbon into living organisms.
• Respiration, decomposition, and combustion return carbon dioxide to the atmosphere.
• Major carbon reservoirs include the atmosphere, oceans, living organisms, soils, rocks, and fossil fuels.
• Human activities such as burning fossil fuels and deforestation have increased atmospheric carbon dioxide, affecting Earth's climate.
• A balanced carbon cycle is essential for sustaining life and regulating the global climate.