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Question 56
Why is the RuBisCo enzyme more appropriately called RUBP carboxylase-oxygenase and what important role does it play in photosynthesis?


Solution

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.
(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 in first 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 frorm phosphoglycolate.
(iii) C4 plants have mechanisms to increase the concentration of CO2 at enzyme site, and increasing the intracellular concentration of CO2. Thus, here RuB is Co acts as carboxylase, minimising the affect of oxygenase.
 


Biology
NCERT Exemplar
Standard XI

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