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Forms of Carbon in the Carbon Cycle
Photoautotrophs take carbon dioxide and turn it into organic compounds.

Photoautotrophs take carbon dioxide and turn it into organic compounds

Frank Krahmer, Getty Images

Carbon exists in several forms as it moves through the carbon cycle.

Carbon in the Non-Living Environment


The non-living environment includes substances that never were alive as well as carbon-bearing materials that remain after organisms die. Carbon is found in the non-living part of the hydrosphere, atmosphere, and geosphere as:

  • carbonate (CaCO3) rocks: limestone and coral
  • dead organic matter, such as humus in soil
  • fossil fuels from dead organic matter (coal, oil, natural gas)
  • carbon dioxide (CO2) in the air
  • carbon dioxide dissolved in water to form HCO3

How Carbon Enters Living Matter

Carbon enters living matter through autotrophs, which are organisms capable of making their own nutrients from inorganic materials.

  • Photoautotrophs are responsible for most of the conversion of carbon into organic nutrients. Photoautotrophs, primarily plants and algae, use light from the sun, carbon dioxide, and water to make organic carbon compounds (e.g., glucose).
  • Chemoautotrophs are bacteria and archaea that convert carbon from carbon dioxide into an organic form, but they get the energy for the reaction through oxidation of molecules rather than from sunlight.

How Carbon Is Returned to the Non-Living Environment

Carbon returns to the atmosphere and hydrosphere through:

  • burning (as elemental carbon and several carbon compounds)
  • respiration by plants and animals (as carbon dioxide, CO2)
  • decay (as carbon dioxide if oxygen is present or as methane, CH4, if oxygen is not present)

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