RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants?
The affinity of RuBiSCo for oxygen and carbon dioxide is determined by the concentration of oxygen and carbon dioxide. RuBisCo is absent in mesophyll cells but present in bundle sheath where the Calvin cycle occurs. C₄ plants have a special mechanism the increases the concentration of CO₂ at enzyme site hence it carries out carboxylation more. Photorespiration is absent in C₄ therefore oxygen does not bind to RuBiSCO and does not carry out oxygenation. Enzyme PEP carboxylase is present in the mesophyll cells and RUBISCO in bundle sheath cells. Mesophyll cells contain chloroplast with stacked chloroplast while chloroplast in bundle sheath cells exhibits unstacked thylakoids. C₄ plants are more efficient at present carbon dioxide concentration. C₄ plants possess low CO₂ compensation points. The mesophyll cells pump more CO₂ for the Calvin cycle. Thus the concentration of CO₂ is high around the site of Rubisco in C₄ plants, thus little or no chance of photorespiration
