72 Rubisco enzyme is involved in both reductive and oxidative carbon cycles in plants. Following are certain statements regarding them:
A. Sugars are produced in both the cycles.
B. Ferredoxin is reduced only in oxidative carbon cycle.
C. Product of oxidative cycle is one of the substrates of reductive cycle.
D. NADP and ATP are used in both the cycles.
Which one of the following options represents the combination of all correct statements?
(1) A and B   (2) A and D
(3) C and D   (4) B and C

Introduction

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes key steps in both the reductive (Calvin-Benson) and oxidative (photorespiratory/C2) carbon cycles. Understanding differences and connections between these pathways is crucial for comprehending plant carbon metabolism.

Explanation of Each Statement

A. Sugars are produced in both the cycles.

• Incorrect. Sugars are produced in the reductive cycle (Calvin cycle), but the oxidative cycle (photorespiration) primarily salvages carbon and does not result in net sugar synthesis.

B. Ferredoxin is reduced only in oxidative carbon cycle.

• Incorrect. Ferredoxin is mainly reduced in the light reactions of photosynthesis (supporting the Calvin cycle), not exclusively in the oxidative (photorespiratory) cycle.

C. Product of oxidative cycle is one of the substrates of reductive cycle.

• Correct. Photorespiratory (oxidative) cycle produces 3-phosphoglycerate (3-PGA), which is reintegrated as a substrate for the Calvin (reductive) cycle.

D. NADP and ATP are used in both the cycles.

• Correct. Both the Calvin cycle and photorespiration consume ATP and NAD(P)H at different stages—Calvin for carbon reduction, photorespiratory cycle for salvage reactions and conversion steps.

Why Option (3) C and D is Correct

• Statement C highlights direct metabolic connection—product flow from oxidative (C2) to reductive (C3) carbon cycle.

• Statement D accurately reflects shared use of key metabolic cofactors in both cycles.

• Statements A and B are incorrect by standard plant physiology principles.