C3 carbon fixation

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C3 carbon fixation is the most common of three metabolic pathways for carbon fixation in photosynthesis, along with C4 and CAM. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into two molecules of 3-phosphoglycerate through the following reaction:

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

This reaction was first discovered by Melvin Calvin, Andrew Benson and James Bassham in 1950.[1] C3 carbon fixation occurs in all plants as the first step of the Calvin–Benson cycle. (In C4 and CAM plants, carbon dioxide is drawn out of malate and into this reaction rather than directly from the air.)

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, carbon dioxide concentrations are around 200 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 because RuBisCO incorporates more oxygen into RuBP as temperatures increase. This leads to photorespiration (also known as the oxidative photosynthetic carbon cycle, or 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.[3] In dry areas, C3 plants shut their 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. In specificity, C3 plants does not have PEP carboxylase like C4 plants, allowing them to only utilizes 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]

Thanks to a combination of in-silico and synthetic approaches,[5][6] scientists have designed new metabolism pathways which reduces the losses to photorespiration, by more efficiently metabolizing the toxic glycolate produced. This resulted in over 40% increase in biomass production in their model organism (the tobacco plant) in their test conditions. The scientists are optimistic that this optimization can also be implemented in other C3 crops like wheat.[7]


  1. ^ Calvin M (1997). "Forty years of photosynthesis and related activities". Interdisciplinary Science Reviews. 22 (2): 138–148. doi:10.1179/isr.1997.22.2.138.
  2. ^ Hogan CM (2011). McGinley M, Cleveland CJ (eds.). "Respiration". Encyclopedia of Earth. Washington, D.C.: National Council for Science and the Environment.
  3. ^ Raven JA, Edwards D (March 2001). "Roots: evolutionary origins and biogeochemical significance". Journal of Experimental Botany. 52 (Spec Issue): 381–401. doi:10.1093/jexbot/52.suppl_1.381. PMID 11326045.
  4. ^ Alonso-Cantabrana H, von Caemmerer S (May 2016). "Carbon isotope discrimination as a diagnostic tool for C4 photosynthesis in C3-C4 intermediate species". Journal of Experimental Botany. 67 (10): 3109–21. doi:10.1093/jxb/erv555. PMC 4867892. PMID 26862154.
  5. ^ Zhu XG, de Sturler E, Long SP (October 2007). "Optimizing the distribution of resources between enzymes of carbon metabolism can dramatically increase photosynthetic rate: a numerical simulation using an evolutionary algorithm". Plant Physiology. 145 (2): 513–26. doi:10.1104/pp.107.103713. PMC 2048738. PMID 17720759.
  6. ^ Stracquadanio G, Umeton R, Papini A, Lio P, Nicosia G (2010). "Analysis and Optimization of C3 Photosynthetic Carbon Metabolism". 2010 IEEE International Conference on BioInformatics and BioEngineering. Philadelphia, PA, USA: IEEE: 44–51. doi:10.1109/BIBE.2010.17. ISBN 978-1-4244-7494-3.
  7. ^ South PF, Cavanagh AP, Liu HW, Ort DR (January 2019). "Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the field". Science. 363 (6422): eaat9077. doi:10.1126/science.aat9077. PMC 7745124. PMID 30606819.