Carbon fixation refers to any process through which gaseous carbon dioxide is converted into a solid compound. It refers mostly 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.
Plants:
The Calvin cycle is the most common biological method of carbon fixation. In plants, there are three types of carbon fixation during photosynthesis:
i. C3 plants that use the Calvin cycle for the initial steps that incorporate C02 into organic matter, forming a 3-carbon compound as the first stable. This form of photosynthesis occurs in the majority of terrestrial species of plants. Plants that use this pathway have a carbon isotope signature of -24 to -33%o.
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ii. C4 plants that preface the Calvin cycle with reactions that incorporate CO, into a 4-carbon compound. C4 plants have a distinctive internal leaf anatomy. Tropical grasses, such as sugar cane andmaize are C4 plants, but there are many broadleaf plants that are C4. Overall, 7600 species of terrestrial plants use C4 carbon fixation, representing around 3% of all species. These plants have a carbon isotope signature of -16 to -10 %.
iii. CAM-plants that use Crassulacean acid metabolism as an adaptation for arid conditions. C02 enters through the stomata during the night and is converted into organic acids, which release CO, for use in the Calvin cycle during the day, when the stomata are closed. The jade plant (Crassula ovata) and cacti are typical of CAM plants. Sixteen thousand species of plants use CAM. These plants have a carbon isotope signature of -20 to -10 %c.
In addition to the Calvin cycle, the following alternative pathways are currently known to be used in certain autotrophic microorganisms:
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i. Reverse Krebs cycle (also known as the reverse tricarboxylic acid cycle, the reverse TCA cycle, or the reverse citric acid cycle). The reaction is the Citric acid cycle run in reverse and is used by photolitho-autotrophic bacteria of the Chlorobiales and some chemolitho- autotrophic sulfate-reducing bacteria.
ii. Reductive acetyl CoA Pathway is found in methanogenic archaea and in acetogenic and some sulfate-reducing bacteria as a way of fixing carbon.
iii. 3-Hydroxypropionate Pathway is found in photolitho-autotrophically grown bacteria of the genus Chloroflexus and, in modified form, in some chemolitho- autotrophically grown archaea as a way of fixing carbon.
Heterotrophs:
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Although almost all heterotrophs cannot synthesize complete organic molecules from carbon dioxide, some carbon dioxide is incorporated in their metabolism. Notably private carboxylaseconsumes carbon dioxide (as carbonate ions) as part of gluconeogenesis.
Ecological Significance of carbon fixation :
Photosynthesis is considered the source of all living plants and animals. Photosynthesis may sometimes be thought to be only used in plants but on the contrary all living animals and plants use some sort of photosynthesis to keep living. Animals need oxygen to survive, and plants need the carbon dioxide that the animals produce to keep living. This whole process starts with the oxygen being released from the plants through photosynthesis.
The importance of photosynthesis is that the plant and animals would not be able to survive if photosynthesis did not occur. Animals and plants are both sources of nutrition for humans, which means that if the plants and animals did not have photosynthesis then humans would lose that source of nutrition. This would end the world as we know it because there would be no source of food or anything to nourish the human body.