Carbon fixation

Carbon fixation is a process found in autotrophs (organisms that produce their own food), usually driven by photosynthesis, whereby carbon dioxide is changed into organic materials. Carbon fixation can also be carried out by the process of calcification in marine, calcifying organisms such as Emiliania huxleyi.

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

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

• C₃ - plant that uses the Calvin Cycle for the initial steps that incorporate CO₂ into organic matter, forming a 3-carbon compound as the 1^st stable intermediate. Most broadleaf plants and plants in the temperate zones are C3.
• C₄ - plant that prefaces 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 - plant that uses Crassulacean acid metabolism as an adaptation for arid conditions. CO₂ entering the stomata during the night is converted into organic acids, which release CO₂ for the Calvin Cycle during the day, when the stomate is 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 photolithoautotrophic eubacteria of the Chlorobiales and some chemolithoautotrophic 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 photolithoautotrophically grown eubacteria of the genus Chloroflexus and in modified form in some chemolithoautotrophically grown archaebacteria as a way of fixing carbon.