9. Ribulose bisphosphate carboxylase (Rubisco) catalyzes both carboxylation and oxygenation of ribulose-1,5bisphosphate. The latter reaction initiates a physiological process known as ‘photorespiration’. The following are certain statements on photorespiration:
(A) The active sites on Rubisco for carboxylation and oxygenation are different.
(B) One of the steps in photorespiration is conversion of glycine to serine.
(C) 50% of carbon lost in chloroplast due to oxygenation is recovered through photorespiration.
(D) The pathway of photo respiration involves chloroplast, peroxisorne and mitochondria.
Which one of the following combinations of above statements is correct?
(1) A and C           (2) A and D
(3) B and D          (4) C and D

Introduction

Photorespiration is a vital plant process initiated by Rubisco’s oxygenase activity on ribulose-1,5-bisphosphate, leading to a metabolic pathway that recycles carbon and mitigates damage from oxygenation. Understanding the enzymatic and cellular steps aids in comprehending plant metabolic efficiency.

Explanation of Each Statement

(A) The active sites on Rubisco for carboxylation and oxygenation are different.

• Incorrect. Rubisco has a single active site that can catalyze both carboxylation and oxygenation reactions.

(B) One of the steps in photorespiration is conversion of glycine to serine.

• Correct. This conversion occurs in mitochondria and is a key step in recycling carbon during photorespiration.

(C) 50% of carbon lost in chloroplast due to oxygenation is recovered through photorespiration.

• Incorrect. Around 75% of carbon lost due to oxygenation can be recovered, not just 50%.

(D) The pathway of photorespiration involves chloroplast, peroxisome, and mitochondria.

• Correct. Photorespiration is a multi-organelle process involving metabolite exchange among these three organelles.

Why Option (3) B and D Is Correct

Statements B and D accurately describe fundamental aspects of photorespiration: the glycine to serine conversion and the involvement of key organelles. Statements A and C reflect common misconceptions about Rubisco’s mechanism and photorespiratory carbon recovery.

This insight is essential for students and researchers studying plant physiology and photosynthetic carbon metabolism.