6. In which one of the following subcellular organelles in serine synthesized during the oxidative photosynthetic carbon (C2) pathway?
(1) Chloroplast
(2) Mitochondria
(3) Peroxisome
(4) Rough endoplasmic reticulum

Introduction

The C2 oxidative photosynthetic pathway, or photorespiration, involves a series of enzymatic reactions across multiple organelles. One crucial step is the synthesis of serine from glycine, which occurs in a specific subcellular location that regulates carbon recycling and nitrogen metabolism.

Explanation of Each Option

(1) Chloroplast

• Chloroplasts are the site of carbon fixation but do not synthesize serine directly during photorespiration.

(2) Mitochondria

• Correct. In mitochondria, two molecules of glycine are converted enzymatically into one molecule of serine, CO₂, and NH₃ via the glycine decarboxylase complex and serine hydroxymethyltransferase enzymes.

(3) Peroxisome

• Peroxisomes mediate the conversion of glycolate to glyoxylate but do not synthesize serine.

(4) Rough endoplasmic reticulum

• The rough ER is involved primarily in protein synthesis, not in photorespiratory metabolite conversions.

Why Mitochondria Are the Site for Serine Synthesis

Mitochondria house key enzymes of the C2 cycle needed for decarboxylation of glycine and synthesis of serine, a vital amino acid for further metabolic pathways. This compartmentalization supports efficient processing during photorespiration.

This insight is important for students and researchers studying plant physiology, cellular metabolism, and photosynthetic carbon processing.