55. Calvin-Benson cycle is divided into three phases, namely carboxylation, reduction and regeneration. The following statements are related to the three phases of Calvin-Benson cycle:
A. The product of light reaction, ATP and NADPH is utilized in the carboxylation phase.
B. Six molecules of 3-phosphoglycerate is converted into six molecules of glyceraldehyde 3-phosphate in the reduction phase.
C. The action of aldolase enzyme for the production of fructose 1, 6- bisphosphate takes place in reduction phase.
D. Formation of seven carbon compound, sedoheptulose-7- phosphate takes place in the regeneration phase.
Which one of the following combinations is correct?
(1) A and C   (2) B and D
(3) A and B   (4) C and D

Introduction

The Calvin-Benson cycle, central to photosynthetic carbon fixation, is divided into three phases: carboxylation, reduction, and regeneration. Each phase involves specific biochemical reactions and enzyme activities vital for converting atmospheric CO₂ into sugars. Accurate understanding of the processes in each phase is essential for biology students and competitive exam preparation.

Explanation of each statement

A. “The product of light reaction, ATP and NADPH is utilized in the carboxylation phase.”

• Incorrect. ATP and NADPH generated in the light reactions are primarily used during the reduction phase of the Calvin cycle, where 3-phosphoglycerate (3-PGA) is reduced to glyceraldehyde-3-phosphate (G3P).

• Carboxylation phase mainly involves ribulose-1,5-bisphosphate (RuBP) and CO₂ fixation by Rubisco to generate 3-PGA without ATP or NADPH consumption.

B. “Six molecules of 3-phosphoglycerate is converted into six molecules of glyceraldehyde 3-phosphate in the reduction phase.”

• Correct. During the reduction phase, ATP and NADPH reduce 3-PGA into G3P. Since three molecules of CO₂ are fixed per cycle turn, six 3-PGA molecules arise and are converted accordingly.

C. “The action of aldolase enzyme for the production of fructose 1,6-bisphosphate takes place in the reduction phase.”

• Incorrect. Aldolase catalyzes a condensation reaction combining triose phosphates (G3P and dihydroxyacetone phosphate) into fructose 1,6-bisphosphate (a hexose phosphate), occurring during the regeneration phase, which rearranges carbon skeletons to regenerate RuBP.

D. “Formation of seven carbon compound, sedoheptulose-7-phosphate takes place in the regeneration phase.”

• Correct. Sedoheptulose-7-phosphate is a characteristic intermediate in the regeneration phase, formed from sugar phosphate rearrangements facilitating the regeneration of RuBP to continue the cycle.

Why option (2) B and D is correct

Both statements B and D accurately describe biochemical events in the reduction and regeneration phases, respectively. Statement A incorrectly attributes ATP/NADPH usage to carboxylation, and statement C misplaces aldolase action in the reduction instead of regeneration phase.

This understanding aligns with standard photosynthesis biochemistry, making option (2) the best choice for the given question.