What is carbon cycle? Example with diagram

The carbon cycle is the biogeochemical cycle in which carbon is exchanged among the different zones like the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth.

overview of carbon cycle

Key points of the Carbon Cycle

  • Carbon is an essential element in the bodies of living things. It is also the most important building block for all living things, including glucose. It is also economically important as a fossil fuel for modern humans.
  • Carbon dioxide is consumed from the atmosphere by photosynthetic organisms that use it to form organic molecules that travel through food chains and eventually carbon dioxide exhales through the breathing process, which is part of the carbon cycle in nature.
  • Over millions of years, carbon can be reused in hydrocarbons. This is the long-term carbon cycle.
  • Slow geological processes such as sedimentary rock formation and fossil fuels contribute to the carbon cycle over long periods of time.
  • Some human activities such as burning fossil fuels and deforestation increase carbon dioxide in the atmosphere, affecting the climate and oceans on the planet.

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


you can summarize the carbon cycle into the following steps……..

  • Carbon moves from the atmosphere to plants.
  • Carbon moves from plants to animals.
  • Carbon moves from plants and animals to soils.
  • Carbon moves from living things to the atmosphere.
  • Carbon moves from fossil fuels to the atmosphere when fuels are burned.
  • Carbon moves from the atmosphere to the oceans.

Short-term Carbon cycle or Biological  Carbon cycle

Here, we’ll discuss the Short-term carbon cycle that only takes days, months, or years for carbon to circulate through the environment. This involves

  • the movement of carbon from carbon source to the autotrophs i.e. from environment to algae or plants, hence entering in food chains
  • the release of fixed carbon present in the bodies of organisms or in the fuel to the environment

Step 1 photosynthesis (conversion of inorganic carbon into organic carbon)

carbon fixation in organic molecules by photosynthesis

Carbon is generally present in the atmosphere as carbon dioxide that dissolves in water, forming the HCO3-bicarbonate group. The photosynthesis process by terrestrial plants, bacteria, and algae transforms carbon dioxide and bicarbonate into organic molecules, which in turn travel through food chains and turns organic carbon into carbon dioxide again by exhaling the cellular respiration process in living organisms.

Although carbon dioxide constitutes less than 1% of the atmosphere, it plays an important role in 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 be repeated for centuries. Photosynthesis not only extracts carbon dioxide from the atmosphere, but it feeds all living things as an energy source.

6CO2       +      6H2O                         ——>              C6H12O6          +              6O2
Sunlight energy

Step 2 decomposition (Break down of organic matter to release CO2)

Decomposition release carbon dioxide gas in atmosphere

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

Decomposition is the process of decomposition of plants. Over long periods of time, the sediment layers build up together. Due to the pressure and heat inside 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. In the long term, the decomposition of dead matter generates these fossil fuel products.

Anaerobic decomposition implies that bacteria break down organic matter like glucose into CO2 and methane (CH4). The nutrient cycle recycles inorganic and organic material in the soil through the decomposition process. Then go back through the same process again.

Step 3 Cellular respiration (break down of food to release CO2 )

Cellular respiration release carbon from bodies of organism to the atmosphere

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

Animals depend on plants for their 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 due to cellular respiration. In turn, this CO2

produced by the cells that breathe can be used again in photosynthesis.

step 4 combustion

combustion of wood, fossil fuels, or bio mass release carbon dioxide in to atmosphere

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

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

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

But there is a limit to the amount of fossil fuel that we can extract. For millions of years, phytoplankton resting on the ocean surface photosynthesize and absorb CO2.

Using sunlight, create a molecule called glucose (C6H12O6) and sink to the bottom of the ocean. Humans discovered these fossil fuels under the ocean and began drilling into the ancient plankton that turned into oil.

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

Carbon can move rapidly during the biological carbon cycle, especially in aquatic systems, with a total of 1,000 to 100,000 million metric tons of carbon being transported during the biological cycle each year. 

Long-term Carbon cycle or Geological Carbon cycle

In long term carbon cycle, instead of carbon being converted to sugars, carbon is reused in fossil fuels like coal. When plants are buried and compacted for millions of years, they become hydrocarbons. Volcanic activity – and more recently – human burning of fossil fuels restores this stored carbon to the carbon cycle.

The formation of fossil fuels occurs on a slow geological time scale, but humans release carbon dioxide containing it on a very fast time scale. When you drive your car on gasoline, you take advantage of the Earth’s carbon reserves deposited hundreds of millions of years ago. These fossil fuels are released into the air as carbon dioxide and water vapor.

The carbon geological cycle takes much longer than the previously described biological cycle. In fact, carbon usually takes millions of years to travel during the geological cycle. Carbon may be stored for long periods of time in air, water bodies – mostly ocean surfaces – and ocean sediments, soils, rocks, fossil fuels, and in the ground.

The level of carbon dioxide in the atmosphere is affected by the carbon stored in the ocean and vice versa. Carbon dioxide dissolves from air in water and reacts with water molecules in the following reactions:

The [CO3]2- carbonate group released from this reaction interacts with Ca+2  ions to form CaCO3 which is the main component of the shells of marine organisms. When these organisms die, their remains sink and eventually become part of the sediments on the ocean floor. Over geological time, these sediments convert to limestone, which is the largest carbon store on Earth.

Another way to get carbon into the atmosphere is through volcanic eruptions. Carbon-containing sediments at the bottom of the ocean are taken deeper into the ground in a process called tectonic slip, where a tectonic plate moves under another. This process produces carbon dioxide, which can be released into the atmosphere by volcanic eruptions or hydrothermal vents.

The distinction between the short-term carbon cycle is that this cycle takes millions of years to complete the circle.

Effect of human activities on carbon cycle

Increase in use of fossil fuels

The global demand for limited reserves of fossil fuels on Earth has increased since the beginning of the Industrial Revolution. Fossil fuel is a non-renewable resource because it is used much faster than it is produced during geological processes.
When fossil fuels are burned, carbon dioxide is released into the atmosphere. The increased use of fossil fuels has led to an increase in carbon dioxide in the atmosphere.


Deforestation (logging) is one of the factors that leads to a high level of carbon dioxide in the atmosphere. Trees and forests help absorb carbon dioxide from the air, and deforestation increases the level of carbon dioxide.
Some of the carbon dioxide produced by human activities is absorbed by plants or absorbed by ocean water, but these processes do not reduce the increase in their percentage in the air. Consequently, the proportion of carbon dioxide has increased and continues to increase.

Carbon dioxide levels naturally rise and fall during cycles for long periods of time, but they are now higher than in the past 400,000 years.

Effects of increase of carbon dioxide in atmosphere

Why do we care about increasing the percentage of carbon dioxide in the air?

  • Because carbon dioxide is a greenhouse gas. When in the atmosphere, it traps heat and prevents lightning from passing through the atmosphere into space.
  • According to the evidence, scientists believe that the high percentage of carbon dioxide and other greenhouse gases leads to clear climate changes on the planet. Without decisive intervention to reduce this percentage, the Earth’s temperature is expected to rise from 1 to 5 ° C in the year 2100.
  • Furthermore, although ocean absorption of excess carbon dioxide may look good from a greenhouse gas perspective, it does not look good from the perspective of marine life. As we saw earlier, carbon dioxide dissolves in ocean water and reacts with water molecules, forming H + hydrogen ions.Therefore, the absorption of more carbon dioxide leads to the formation of more hydrogen ions, which increases the acidity of the water. In turn, increasing the acidity of the water reduces the concentration of carbonate ions, making it difficult for marine organisms to maintain their shells due to the low level of calcium carbonate.
  • Both high and acid temperatures are detrimental to marine life and are also related to coral bleaching.

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