Ncert Exercises & Solutions

Why is the RuBisCo enzyme more appropriately called RUBP carboxylase-oxygenase and what important role does it play in photosynthesis?

RuBP carboxylase and oxygenase has dual nature. It has affinity for both CO₂ and O₂ but
has more affinity for CO₂ than O₂. Thus, the concentrations of two determines which of the two will bind to the enzyme.

Consider the following two situations

(i) In a normal condition when CO₂ and O₂ concentrations are normal, it acts as carboxylase and fix CO₂ by combining with ribulose bisphosphate and C₃ cycle operates normally, producing glucose molecule as anfirst product of photosynthesis.

(ii) If O₂ concentration goes up and CO₂ goes down, it starts acting as an oxygenase enzyme and C₂ cycle (photorespiration) starts where RuBP binds with O₂ to form phosphoglycolate.

(iii) C₄ plants have mechanisms to increase the concentration of CO₂ at enzyme site, and
increasing the intracellular concentration of CO₂. Thus, here RuBisCo acts as carboxylase, minimising the affect of oxygenase.

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