C4 carbon fixation

C4 carbon fixation is a photosynthetic pathway that first incorporates carbon dioxide into four-carbon compounds, then releases it near Rubisco. This concentration of carbon dioxide reduces photorespiration, especially in hot, bright environments.

Phosphoenolpyruvate carboxylase: An enzyme that fixes bicarbonate onto phosphoenolpyruvate to form oxaloacetate. It captures inorganic carbon in mesophyll cells before Rubisco acts.
Rubisco: An enzyme that catalyzes carbon dioxide fixation in the Calvin cycle and can also react with oxygen. C4 plants concentrate carbon dioxide around this enzyme to favor carbon fixation.
C3 carbon fixation: A photosynthetic pathway in which Rubisco directly fixes carbon dioxide into a three-carbon product. Unlike C4 plants, C3 plants lack a preliminary carbon-concentrating cycle.
Maize: A cereal crop whose photosynthesis uses the C4 pathway. Maize is a familiar crop example of C4 carbon fixation.
C4 photosynthesis evolution: The evolutionary emergence and diversification of the C4 carbon-fixation pathway in plants. The pathway evolved independently many times from ancestral photosynthetic systems.
Oxaloacetate: A four-carbon organic acid that is an early product of C4 carbon fixation. It is the first stable carbon product formed after initial fixation.
Photorespiration: A plant process initiated when Rubisco reacts with oxygen instead of carbon dioxide. Reducing this wasteful reaction is the main advantage of C4 carbon concentration.
CAM photosynthesis: A photosynthetic strategy that separates carbon dioxide uptake and its use by Rubisco between night and day. CAM concentrates carbon by timing, rather than C4’s separation between cell types.
Sugarcane: A tall tropical grass cultivated for sugar and biofuel whose photosynthesis uses the C4 pathway. Its high productivity illustrates the pathway’s effectiveness under warm, bright conditions.
Convergent evolution: The independent evolution of similar traits in separate lineages facing comparable conditions. Repeated origins of C4 photosynthesis exemplify this evolutionary pattern.

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