The entry of carbon dioxide from the atmosphere into the biosphere is mediated by the enzyme Rubisco, which catalyzes the carboxylation of ribulose 1,5-bisphosphate (RuBP) as the entry reaction of the Calvin Benson Bassham cycle (CBBC), leading to the formation of 2 molecules of 3-phosphoglyceric acid (3PGA) per CO2 fixed. 3PGA is reduced to triose phosphates at the expense of NADPH + H+ and ATP that are provided by the photosynthetic light reactions. Triose phosphates are the principal products of the CBBC and the precursors for almost any compound in the biosphere.
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