RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants?

In C₄ plants, bundle sheath cells maintain very high CO₂ concentration around RuBisCO, raising the CO₂/O₂ ratio and thus favoring carboxylation over oxygenation

Phosphoenolpyruvate carboxylase (PEPCase) in mesophyll cells initially fixes CO₂ into four-carbon acids, which efficiently shuttle and release CO₂ inside bundle sheath cells, ensuring Rubisco rarely encounters O₂

The spatial separation of CO₂ fixation (mesophyll) and the Calvin cycle (bundle sheath) creates a CO₂ “pump” that:

• Dramatically elevates local CO₂ partial pressure at the Rubisco active site

• Minimizes O₂ access and thus suppresses the oxygenase reaction

Kranz anatomy and high vein density optimize metabolite flux and gas diffusion by:

• Positioning bundle sheath cells deep within the leaf where O₂ levels are lower

• Reducing the intercellular air space around Rubisco, further limiting O₂ concentration

By reducing photorespiration, C₄ plants allocate more ATP and NADPH toward carboxylation and sugar synthesis rather than wasteful oxygenation pathways

Under high light and temperature, Rubisco’s carboxylation efficiency in C₄ plants remains high because:

• Elevated temperature increases O₂ solubility less than CO₂ in the confined bundle sheath environment

• The CO₂ pump compensates for any temperature-induced decline in Rubisco specificity

Overall, the biochemical CO₂-concentrating mechanism and specialized leaf anatomy of C₄ plants ensure Rubisco predominantly catalyzes carboxylation, boosting photosynthetic productivity