Statement I: Photorespiration interferes with the successful functioning of Calvin cycle.\ Statemen

Question

Statement I: Photorespiration interferes with the successful functioning of Calvin cycle.\ Statement II: Photorespiration oxidises ribulose-1,5 bisphosphate which is an acceptor of $\mathrm{CO}_{2}$ in Calvin cycle.\

Professor PC Thomas Classes · Answer

(a) : Photorespiration (Photosynthetic carbon oxidative or PCO cycle) is the light dependent process of oxygenation of ribulose-1, 5-bisphosphate (RuBP) and release of $\mathrm{CO}_{2}$ by the photosynthetic organs of a plant. Under normal conditions, in Calvin cycle, carboxylation of RuBP takes place whereas during photorespiration instead of carboxylation, oxygenation of RuBP takes place. This is due to abnormal behaviour of enzyme RuBisCO, which at high temperature (more than $35^{\circ} \mathrm{C}$ ), functions as oxygenase (instead of carboxylase). Instead of fixing $\mathrm{CO}_{2}$, it performs oxygenation of RuBP to produce a 3 -carbon phosphoglyceric acid (PGA) and a 2 -carbon phosphoglycolate. It is the first reaction of photorespiration and can be represented as : 
$ 	ext { RuBP }+\mathrm{O}_{2} \xrightarrow[	ext { oxygenase }]{	ext { RuBP }} \mathrm{PGA}+	ext { Phosphoglycolate } $
 In this way, photorespiration interferes with the successful functioning of Calvin cycle by causing oxygenation of RuBP instead of carboxylation.