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 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 era, 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.^[1]

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 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.

[edit] See also

• C4 carbon fixation
• CAM photosynthesis

[edit] References