C3 carbon fixation

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Calvin–Benson cycle

C3 carbon fixation is the most common of three

through the following reaction:

CO2 + H2O + RuBP → (2) 3-phosphoglycerate

This reaction was first discovered by


Cross section of a C3 plant, specifically of an Arabidopsis thaliana leaf. Vascular bundles shown. Drawing based on microscopic images courtesy of Cambridge University Plant Sciences Department.

Plants that survive solely on C3 fixation (C3 plants) tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate,

ppm or higher,[2] and groundwater is plentiful. The C3 plants, originating during Mesozoic and Paleozoic eras, predate the C4
plants and still represent approximately 95% of Earth's plant biomass, including important food crops such as rice, wheat, soybeans and barley.

C3 plants cannot grow in very hot areas at today's atmospheric CO2 level (significantly depleted during hundreds of millions of years from above 5000 ppm) because

C2 photosynthesis
), which leads to a net loss of carbon and nitrogen from the plant and can therefore limit growth.

C3 plants lose up to 97% of the water taken up through their roots by transpiration.

stomata to reduce water loss, but this stops CO2 from entering the leaves and therefore reduces the concentration of CO2 in the leaves. This lowers the CO2:O2 ratio and therefore also increases photorespiration. C4 and CAM
plants have adaptations that allow them to survive in hot and dry areas, and they can therefore out-compete C3 plants in these areas.

The isotopic signature of C3 plants shows higher degree of 13C depletion than the C4 plants, due to variation in fractionation of carbon isotopes in oxygenic photosynthesis across plant types. Specifically, C3 plants do not have PEP carboxylase like C4 plants, allowing them to only utilize ribulose-1,5-bisphosphate carboxylase (Rubisco) to fix CO2 through the Calvin cycle. The enzyme Rubisco largely discriminates against carbon isotopes, evolving to only bind to 12C isotope compared to 13C (the heavier isotope), attributing to why there's a low 13C depletion seen in C3 plants compared to C4 plants especially since the C4 pathway uses PEP carboxylase in addition to Rubisco.[4]


Not all C3 carbon fixation pathways operate at the same efficiency.


Bamboos and the related rice have an improved C3 efficiency. This improvement might be due to its ability to recapture CO2 produced during photorespiration, a behavior termed "carbon refixation". These plants achieve refixation by growing chloroplast extensions called "stromules" around the stroma in mesophyll cells, so that any photorespired CO2 from the mitochondria has to pass through the RuBisCO-filled chloroplast.[5]

Refixation is also performed by a wide variety of plants. The common approach involving growing a bigger

C2 photosynthesis.[6]

Synthetic glycolate pathway

C3 carbon fixation is prone to

glycolate metabolism would help significantly to reduce photorespiration.[7][8]

Instead of optimizing specific enzymes on the PR pathway for glycolate degradation, South et al. decided to bypass PR altogether. In 2019, they transferred

glycerate pathway produced a smaller improvement of 13%. They are now working on moving this optimization into other C3 crops like wheat.[9]


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  2. ^ Hogan CM (2011). "Respiration". In McGinley M, Cleveland CJ (eds.). Encyclopedia of Earth. Washington, D.C.: National Council for Science and the Environment.
  3. PMID 11326045
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