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:

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 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.