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Carbon fixation refers to any process through which gaseous carbon dioxide is converted into a solid compound. It mostly refers to the processes found in autotrophs (organisms that produce their own food), usually driven by photosynthesis, whereby carbon dioxide is changed into sugars. Carbon fixation can also be carried out by the process of calcification in marine, calcifying organisms such as Emiliania huxleyi and also by heterotrophic organisms in some circumstances.




The Calvin Cycle is the most common biological method of carbon fixation.

In plants, there are three types of carbon fixation during photosynthesis:

  • C3-plants that use the Calvin Cycle for the initial steps that incorporate CO2 into organic matter, forming a 3-carbon compound as the first stable intermediate. Most broadleaf plants and plants in the temperate zones are C3.
  • C4-plants that preface the Calvin Cycle with reactions that incorporate CO2 into 4-carbon compound. C4 plants have a distinctive leaf anatomy. This pathway is found mostly in hot regions with intense sunlight. Tropical grasses, such as sugar cane and maize are C4 plants, but there are many broadleaf plants that are C4.
  • CAM-plants that use Crassulacean acid metabolism as an adaptation for arid conditions. CO2 enters through the stomata during the night and is converted into organic acids, which release CO2 for use in the Calvin Cycle during the day, when the stomata are closed. The jade plant (Crassula ovata) and cactus species are typical of CAM plants.


In addition to the Calvin cycle, the following alternative pathways are currently known to be used in certain autotrophic microorganisms:

  • Reverse Krebs cycle (also known as the reverse tricarboxylic acid cycle, the reverse TCA cycle, or the reverse citric acid cycle). The reaction is basically the Citric acid cycle run in reverse and is used by photolitho-autotrophic eubacteria of the Chlorobiales and some chemolitho-autotrophic sulfate-reducing bacteria.
  • Reductive acetyl CoA Pathway is found in methanogenic archaebacteria and in acetogenic and some sulfate-reducing eubacteria as a way of fixing carbon.
  • 3-Hydroxypropionate Pathway is found in photolitho-autotrophically grown eubacteria of the genus Chloroflexus and in modified form in some chemolitho-autotrophically grown archaebacteria as a way of fixing carbon.



  1. ^ Science 1942 Carbon Dioxide Utilization In Animal Tissues Dr. E. A. Evans

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