C3 carbon fixation
C3 carbon fixation is one 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 3-phosphoglycerate through the following reaction:
- CO2 + H2O + RuBP → (2) 3-phosphoglycerate
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, 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. C3 plants lose 97% of the water taken up through their roots to transpiration. Examples include rice 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. 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.
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