67. Following are certain statements regarding CO₂ assimilation in higher plants:
    A. The action of aldolase enzyme during Calvin-Benson cycle produces fructose 1,6-bisphosphate.
    B. The conversion of glycine to serine takes place in mitochondria during C₂ oxidative photosynthetic carbon cycle.
    C. During C₄, carbon cycle, NAD-malic enzyme releases the CO₂ from the 4-carbon acid, malate yielding a 3 carbon acid, pyruvate.
    D. Malic acid during crassulacean acid metabolism (CAM) is stored in mitochondria during dark and released back to cytosol during day.

Which one of the following combinations of above statements is correct?

(1) A, B and C         (2) A, C and D
(3) B, C and D        (4) A, B and D

Introduction

CO2 assimilation in higher plants operates through C3 (Calvin-Benson), C2 (photorespiratory), C4, and CAM pathways, each featuring unique organellar coordination and biochemical steps crucial for plant productivity and adaptation.

Explanation of Each Statement

A. The action of aldolase enzyme during Calvin-Benson cycle produces fructose 1,6-bisphosphate.

• Correct. The enzyme aldolase catalyzes the condensation of dihydroxyacetone phosphate (DHAP) with glyceraldehyde 3-phosphate to generate fructose 1,6-bisphosphate in the Calvin cycle.

B. The conversion of glycine to serine takes place in mitochondria during C2 oxidative photosynthetic carbon cycle.

• Correct. In photorespiration (C2 cycle), glycine is decarboxylated and deaminated to yield serine in the mitochondria, releasing CO2 and NH3.

C. During C4 carbon cycle, NAD-malic enzyme releases the CO2 from the 4-carbon acid, malate yielding a 3-carbon acid, pyruvate.

• Correct. In NAD-malic enzyme (NAD-ME) type C4 plants, decarboxylation of malate in the bundle sheath cells produces CO2 and pyruvate, the latter returning to the mesophyll.

D. Malic acid during crassulacean acid metabolism (CAM) is stored in mitochondria during dark and released back to cytosol during day.

• Incorrect. In CAM plants, malic acid produced at night is stored in vacuoles (not mitochondria), then released to the cytosol during the day for decarboxylation and conversion to CO2 used in photosynthesis.

Why Option (1) A, B and C is Correct

Statements A, B, and C accurately describe fundamental biochemical reactions and compartmentalization in plant CO2 metabolism. Statement D is wrong regarding the subcellular storage of malic acid during CAM—storage occurs in vacuoles, not mitochondria.