In $\text{C}_3$ plants, primary $\text{CO}_2$ fixation is catalysed by RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). RuBisCO adds $\text{CO}_2$ to ribulose-1,5-bisphosphate (RuBP, a $\text{C}_5$ compound) to produce two molecules of 3-phosphoglycerate (3-PGA, a $\text{C}_3$ compound) — hence the name $\text{C}_3$ plants. In $\text{C}_4$ plants, the primary carboxylation enzyme is PEP carboxylase (in mesophyll cells), which is ~60× more efficient at fixing $\text{CO}_2$.