What Is the Carbon Cycle? Photosynthesis, Decomposition, Respiration and Combustion

Carbon Cycle

Carbon cycles from the atmosphere into plants and living things. For example, carbon is a pollutant in the atmosphere as carbon dioxide.

But it’s also the most important building block for all living things including glucose.

Over millions of years, carbon can get re-purposed into hydrocarbons. This is the long-term carbon cycle.

So, carbon takes up various forms: glucose in plants, carbon dioxide in the air and hydrocarbons like coal.

But today, we’ll talk about the short-term carbon cycle that just takes days, months or years for carbon to cycle through the environment.

1. Photosynthesis

Photosynthesis Process

Plants pull in carbon dioxide out of the air through photosynthesis. Even though carbon dioxide makes up less than 1% of the atmosphere, it plays a major role for living things.

With CO2 and H2O in the atmosphere, photosynthesis produces sugars like glucose. This is the plant material that plants synthesize on their own.

If you have the right conditions, this process can repeat for centuries. Not only does photosynthesis pulls carbon dioxide out of the atmosphere, but it’s fuels all living things as a source of energy.

2. Decomposition

Nutrient Cycle

By mostly using sunlight, water and carbon dioxide, plants can grow. In turn, animals consume food for energy using O2 and giving off CO2. Alternatively, they die, decay and decompose repeating for millions of years.

Decomposition is the process of breaking down plants. Over vast periods of time, layers of sediment build on each other. Because of the pressure and heat from within the Earth’s crust, this generates fossil fuels. Much of this happened during the Carboniferous Era.

For example, coal, oil and natural gas (methane) are some of the common fossil fuels. Over the long-term, the decomposition of dead matter generates these fossil fuel products.

Anaerobic decomposition involves bacteria breaking down organic matter such as glucose into CO2 and methane (CH4). The nutrient cycle recycles inorganic and organic material in soil through the process of decomposition. Then, it goes back again through the same process again.

3. Respiration

Cellular Respiration

You and I are both made of carbon. We consume plants. But we also breathe in the air, which has carbon in the form of carbon dioxide.

Animals rely on plants for food, energy and oxygen. Our cells require oxygen to break down the food we consume through cellular respiration.

Once consumed, carbon dioxide is released into the atmosphere because of cell respiration. In turn, this CO2 produced from respiring cells can be used in photosynthesis again.

In other words, the plants use solar energy to break apart that same carbon dioxide in the air. Through photosynthesis, it uses that same carbon for plant material in turn releasing oxygen again.

4. Combustion

Gas Combustion

Our cars use carbon in the form of fossil fuels. And carbon is also a pollutant as carbon dioxide

We extracting fossil fuels, combustion involves burning them to produce energy. But a by-product of combustion is that it releases carbon dioxide back into the atmosphere. And too much CO2 increases the greenhouse effect.

And as we deplete our oil reserves adding CO2 into the air daily, this affects the carbon cycle with an imbalance of oxygen and carbon. Carbon dioxide is one of the greenhouse gases contributing to climate change.

But there is a limit for how much fossil fuels we can extract. Over millions of years, phytoplankton resting on the ocean surface photosynthesizes and takes in CO2.

Using sunlight, it creates a molecule called glucose (C6H12O6) and sinks to bottom of the ocean. Humans discovered these fossil fuels beneath the ocean and started to drill the ancient plankton which became oil.

Long-term carbon cycle summary

Short Term Carbon Cycle

Today, you’ve learned how carbon cycles from the atmosphere and then into plants and living things. But the distinction between the short-term carbon cycle is that this cycle takes millions of years to come full circle.

Instead of carbon converting into sugars, carbon is re-purposed into fossil fuels like coal. When plants are buried and compacted over millions of years, they become hydrocarbons.

When you drive your gas-powered car, you tap into Earth’s carbon reserves deposited hundreds of million years ago. These fossil fuels are released into the air as carbon dioxide and water vapor.

It may stay in the atmosphere for awhile, but eventually plants consume it during photosynthesis. So that same weight from the tank of gasoline gets converted into wood or plant material by photosynthesis.


  1. Can someone explain how respiration, combustion and decomposition is involved in the carbon cycle?

  2. Carbon facts in trillions of tons:
    Atmosphere. 0.85 TT. .0008%
    Plants and Soil. 2.00. TT. .0020%
    Fossil Fuels. 10.00. TT. .0100%
    Oceans. 38.00. TT .0380%
    Carbonaceous Rocks. 100,000. TT. 99.9490%
    There is currently almost 4 times more CO2 dissolved in the oceans than there is in all fossil found on earth. Think about it.

  3. Sorry, but you people missed it by a mile. 99.95% of all carbon on earth has been absorbed by the oceans and is locked up in carbonaceous rock, mountains of it. A further .01% is locked up in fossil fuels bringing the total to 99.96% of all CO2 on earth LOCKED AWAY. The oceans currently have about .038% of the remainder absorbed and that leaves about .002% left for the atmosphere and land plants. Think about it, the oceans are still absorbing carbon and building reefs and locking more and more of carbon up into carbonaceous rocks. Yes, volcanoes are returning some CO2 to the cycle naturally but how did the world get to the 99.96% locked away before man stepped in with burning and cement production. I think maybe you people should be rewriting your thoughts on the carbon cycle and extinction theories.

  4. Does anyone know how freeze thaw (on rocks) affects combustion?
    I really need an answer for my essay in A-level geography.

  5. Thx for giving a good answer…… I searched everywhere but there was no response till I reached here. Thx a lot.

  6. Examples of carbon sinks = Oceans and forests are the biggest
    Examples of carbon sources = Gasoline vehicles and forest fires

  7. Fossil fuels. Its funny how modern day environmentalists push a narrative invented by the OOG’s(original oil gangsters).

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