C3 carbon fixation pathway

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A metabolic pathway where CO2 is converted to 3-phosphogylycerate, the first stable intermediate organic compound containing three carbon atoms.


The C3 carbon fixation pathway is in fact the initial phase of Calvin cycle. It starts with the enzyme rubisco catalyzing the carboxylation of Ribulose-1,5-bisphosphate by CO2 producing a highly unstable 6-carbon intermediate known as 3-keto-2-carboxyarabinitol 1,5-bisphosphate, which splits instantaneously into two molecules of the more stable 3-phosphogylycerate, an organic compound containing three carbon atoms (hence the name C3).

Plants that solely depend to C3 pathway for carbon fixation are faced with the negative effects of photorespiration, such as the wasteful loss of CO2. Photorespiration occurs under conditions of drought, high temperatures and low nitrogen or CO2 concentrations. These conditions cause the stomata to close in an attempt to prevent excessive water loss. The closure of stomata increases O2 levels, and the enzyme rubisco reacts with O2 instead of CO2, consequently losing CO2 instead of fixing CO2.

Word origin: from the intermediate compound, which contains three carbon atoms, hence the name C3.

Compare: C4 carbon fixation pathway, crassulacean acid metabolism.
See also: C3 plant.