C3 carbon fixation is a metabolic pathway for carbon fixation in photosynthesis. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into 3-phosphoglycerate through the following reaction:
- CO2 + RuBP → (2) 3-phosphoglycerate
This reaction occurs in all plants as the first step of the Calvin-Benson cycle. In C4 plants, carbon dioxide is drawn out of malate and into this reaction rather than directly from the air.
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 ground water 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 hot areas because RuBisCO incorporates more oxygen into RuBP as temperatures increase. This leads to photorespiration, 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, outcompete C3 plants.
The isotopic signature of C3 plants shows higher degree of 13C depletion than the C4 plants.