Even though a very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why?

C₄ plants concentrate CO₂ in bundle sheath cells around Rubisco, creating a high local CO₂/O₂ ratio that virtually eliminates photorespiration and maximizes carbon fixation efficiency

Phosphoenolpyruvate carboxylase (PEPCase) in mesophyll cells fixes atmospheric CO₂ with higher affinity and specificity than Rubisco, ensuring rapid initial capture even at low CO₂ concentrations

Spatial separation of initial fixation and the Calvin cycle enables each pathway to operate near its optimal conditions

• Mesophyll reactions run with abundant PEPCase and minimize oxygenation reactions
• Bundle sheath cells maintain elevated CO₂ supplied by C₄ acids, allowing Rubisco to work at maximal carboxylation rates

Kranz anatomy with tightly packed bundle sheath and mesophyll layers and high vein density supports swift metabolite shuttling and sustained high flux through both fixation steps

Reduced investment in Rubisco protein per unit leaf area lowers nitrogen requirement and allows allocation of resources to photosynthetic electron transport and ATP synthesis

Enhanced ATP generation via cyclic and non-cyclic photophosphorylation meets the higher energy demands of the C₄ cycle, enabling continuous supply of ATP for both PEP regeneration and Calvin-cycle reactions

Adaptation to high light intensity and elevated temperatures allows C₄ species to maintain high rates of photosynthesis when C₃ plants experience thermal inhibition or excess light stress

Water-use efficiency improves because PEPCase enables partial stomatal closure while maintaining high CO₂ uptake, reducing transpirational water loss per CO₂ fixed

Overall, the coordinated biochemical and anatomical specializations of C₄ photosynthesis enable a small population of bundle sheath cells to drive disproportionately high carbon assimilation and productivity