66. Following are certain statements regarding C₂, C₃, C₄ and CAM carbon metabolism in plants.
A. In C₃ cycle, one molecule of 3-phosphoglycerate formed during carboxylation phase is utilized for the biosynthesis of sugars, fatty acid and amino acids
B. During C₂ cycle glycine is transported from peroxisome to mitochondria and glycerate is transported from peroxisome to chloroplast
C. The concentration of CO₂ in bundle sheath of C₄ plants is several fold lower than the external atmosphere
D. The stomata of CAM plants open at night
Which one of the following combination of statements is correct?
(1) A and B   (2) B and C
(3) A and D   (4) B and D

Introduction

Plant photosynthesis includes several specialized cycles—C2 (photorespiratory), C3 (Calvin), C4, and CAM—each with unique carbon metabolism and adaptive features. These processes help plants thrive in diverse environments by optimizing carbon fixation, water use, and photorespiratory responses.

Explanation of Each Statement

A. In C3 cycle, one molecule of 3-phosphoglycerate formed during carboxylation phase is utilized for the biosynthesis of sugars, fatty acid and amino acids

• Incorrect. While 3-phosphoglycerate (3-PGA) formed in the carboxylation phase is a central intermediate, a net G3P (not just 3-PGA directly) is the true export product for biosynthesis after the reduction phase; most 3-PGA continues in the cycle for RuBP regeneration.

B. During C2 cycle glycine is transported from peroxisome to mitochondria and glycerate is transported from peroxisome to chloroplast

• Correct. In photorespiration (C2 cycle), glycine moves from the peroxisome to the mitochondrion for decarboxylation and ammonia release, while glycerate formed in the peroxisome is transported back to the chloroplast for integration into the Calvin cycle.

C. The concentration of CO₂ in bundle sheath of C4 plants is several fold lower than the external atmosphere

• Incorrect. In fact, C4 plants concentrate CO₂ in bundle sheath cells to levels much higher than the external atmosphere, which suppresses photorespiration and enhances fixation efficiency.

D. The stomata of CAM plants open at night

• Correct. In CAM plants, stomata open at night to absorb CO₂ and minimize water loss, storing carbon as malate in vacuoles for daytime photosynthesis.

Why Option (4) B and D is Correct

• B and D represent well-established facts about carbon metabolism in C2 (photorespiration transport) and CAM (nocturnal stomatal opening) plants.

• A is conceptually imprecise regarding 3-PGA export in the C3 cycle.

• C is factually incorrect; C4 plants increase, not decrease, bundle sheath CO₂.