C4 Photosynthesis

C4 photosynthesis supplements the Calvin–Benson cycle with an initial carbon-fixation step. In mesophyll cells, phosphoenolpyruvate carboxylase fixes bicarbonate onto phosphoenolpyruvate to form the four-carbon compound oxaloacetate. This may be converted to malate or another transportable molecule and moved to bundle-sheath cells. There, decarboxylation releases carbon dioxide near Rubisco, which supplies the Calvin–Benson cycle. The remaining three-carbon compound returns to mesophyll cells, where energy-dependent reactions regenerate phosphoenolpyruvate. The biochemical details vary among C4 species.

By concentrating carbon dioxide near Rubisco, the C4 mechanism suppresses Rubisco's oxygenation reaction and reduces photorespiration. The pathway requires additional energy, but can improve photosynthetic performance and water-use efficiency when heat, strong light, or dryness make photorespiration costly. The advantage depends on environmental conditions; C4 plants are not universally more efficient in every habitat.

C4 photosynthesis is a complex adaptation involving coordinated biochemical and anatomical traits. In most C4 plants, mesophyll and bundle-sheath cells share the pathway's functions; the arrangement is often described in relation to Kranz anatomy. Research reviews describe C4 photosynthesis as having evolved independently in multiple plant lineages, with variations in the anatomical features and biochemical subtypes involved.