Q.15 During photorespiration under low CO2 and high O2 levels, O2 reacts with ribulose
1,5- bisphosphate to yield
(A) one molecule each of 3-phosphoglycerate and 2-phosphoglycolate
(B) two molecules of 3-phosphoglycerate
(C) two molecules of 2-phosphoglycolate
(D) one molecule each of 3-phosphoglycerate and glyoxylate
Photorespiration occurs under low CO2/high O2 when Rubisco’s oxygenase activity dominates, reacting ribulose 1,5-bisphosphate (RuBP) with O2 instead of CO2. This yields one molecule each of 3-phosphoglycerate (3-PGA) and 2-phosphoglycolate (2-PG) per RuBP, initiating a carbon-recovery pathway that consumes energy. In biotechnology, understanding this process optimizes photosynthetic engineering in microbial and plant systems.
Correct Answer
Option (A) correctly identifies the products: one 3-PGA and one 2-PG from single RuBP oxygenation. Rubisco cleaves RuBP after O2 addition at C2, producing an unstable 6-carbon intermediate that splits into the carboxylation-like 3-PGA (C3-C5 fragment) and oxygenation-specific 2-PG (C1-C2 fragment). This 1:1 stoichiometry drives photorespiratory CO2 loss, contrasting Calvin cycle carboxylation’s two 3-PGA molecules.
Option Explanations
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One 3-PGA + one 2-PG (A): Matches exact enzymatic products; 2-PG enters salvage via glycolate oxidase, while 3-PGA feeds Calvin cycle regeneration.
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Two 3-PGA (B): Describes CO2 carboxylation outcome; oxygenation uniquely generates toxic 2-PG, reducing photosynthetic efficiency by ~25%.
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Two 2-PG (C): Incorrect stoichiometry; requires two RuBP molecules for two 2-PG (and two 3-PGA), not single-molecule reaction.
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One 3-PGA + glyoxylate (D): Glyoxylate appears downstream after 2-PG dephosphorylation to glycolate and oxidation; not direct RuBP product.
Biotech Applications
Photorespiration limits C3 crop yields, prompting synthetic biology efforts to bypass 2-PG salvage in algae or bacteria for bioengineering. Jaipur researchers model enzyme kinetics of Rubisco mutants to enhance CO2 fixation, supporting SEO-optimized publications on metabolic pathway optimization.
