Figure 3.

Simplified C4 and C3 pathways. (a) C4 pathway. (b) C3 pathway. Red numbers indicate the enzymes involved in the reaction: 1, carbonic anhydrase; 2, phosphoenolpyruvate carboxylase; 3, NADP malate dehydrogenase; 4, NADP malic enzyme; 5, ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco); 6, pyruvate, orthophosphate dikinase. The C4 pathway increases the CO₂ concentration in bundle sheath cells, minimizing the competition with O₂ for the Rubisco catalytic site, thus avoiding photorespiration. Glycerate-3-P, glycerate 3-phosphate; PEP, phosphoenolpyruvate.