Carbon fixation
Carbon dioxide assimilation (lat. assimulatio "to make similar") refers to the uptake of carbon and oxygen from carbon dioxide (CO2) by living organisms in order to build up organic, endogenous carbon compounds. The first step in this process is the formation of a carboxy group. Since carbon dioxide is bound in the organic substances in this process, it is also referred to as carbon dioxide fixation.
A distinction is made between autotrophic and heterotrophic carbon dioxide assimilation. In autotrophic organisms, carbon dioxide is the only carbon source for building up the body's own building materials. Heterotrophic organisms, on the other hand, mainly use organic carbon compounds as a source of building materials and assimilate carbon dioxide only to a small extent.
Since life on Earth is based on the chemical diversity of carbon compounds, i.e. all living organisms require carbon for their cellular components and metabolism, the ability of autotrophs, namely plants, algae, many bacteria and archaea, to synthesize organic carbon compounds exclusively from inorganic substances is the prerequisite for the existence of heterotrophic life forms that depend on the availability of organic substances.
There are several pathways by which carbon dioxide is assimilated. In autotrophs, the following pathways occur:
- Calvin cycle (alternative names: Calvin-Benson-Bassham cycle, reductive pentose phosphate cycle, dark reaction of photosynthesis).
- Reductive citrate cycle (alternative designation: Arnon-Buchanan cycle)
- Reductive acetyl-CoA pathway (alternative names: Wood-Ljungdahl pathway, CO dehydrogenase/acetyl-CoA synthase pathway).
- 3-hydroxypropionate cycle (alternative designation: 3-hydroxypropionate/malyl-CoA cycle)
- 3-hydroxypropionate/4-hydroxybutyrate cycle
- Dicarboxylate/4-hydroxybutyrate cycle
- (Serin path)
- (Ethylmalonyl-CoA pathway)
While plants and cyanobacteria only use the Calvin cycle, the other pathways are found in different representatives in the kingdoms of bacteria and archaea, which are thus able to grow with carbon dioxide as the only carbon source (autotrophic).
In heterotrophs, carbon dioxide is introduced into metabolism during some anaplerotic reactions, for example, the formation of oxaloacetate from pyruvate and carbon dioxide.
