C3 carbon fixation

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Calvin cycle

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:

6 CO₂ + 6 RuBP → 12 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 must be in areas with high concentrations of carbon dioxide because RuBisCO often incorporates an oxygen molecule into the RuBP, instead of a carbon dioxide molecule. This breaks the RuBP into a three-carbon sugar that can remain in the Calvin cycle, and two molecules of glycolate, which is oxidized into carbon dioxide, wasting the cell's energy. High concentrations of carbon dioxide lowers the chance that RuBisCO incorporates an oxygen molecule. C4 and CAM plants have adaptations that allow it survive in areas where the plant cannot take in a lot of carbon dioxide.

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