C3 carbon fixation

Calvin–Benson cycle

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 + 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.

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,^[1] 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.^[2] 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, out-compete C₃ plants.

The isotopic signature of C₃ plants shows higher degree of ¹³C depletion than the C₄ plants.

See also

• C2 photosynthesis
• C₄ carbon fixation
• CAM photosynthesis

References