56. Photo-chemically generated ATP is consumed in which one of the following phases of Calvin-Benson cycle?
(1) Only carboxylation
(2) Only regeneration
(3) Carboxylation and reduction
(4) Reduction and regeneration

Introduction

In photosynthesis, the Calvin-Benson cycle uses ATP produced in the light-dependent reactions (“photo-chemically generated ATP”) to drive the fixation and synthesis of sugars. Understanding which phases consume ATP—carboxylation, reduction, or regeneration—is crucial for mastering plant physiology and excelling in exams.

ATP consumption in Calvin-Benson cycle phases

The Calvin-Benson cycle has three phases: carboxylation, reduction, and regeneration. ATP consumption is associated mainly with the reduction and regeneration phases:

• Reduction phase: ATP is used to phosphorylate 3-phosphoglycerate (3-PGA) into 1,3-bisphosphoglycerate, an essential step before the reduction by NADPH to glyceraldehyde-3-phosphate (G3P). NADPH provides the reducing power, but ATP supplies the energy for phosphorylation.

• Regeneration phase: ATP is also required to regenerate ribulose 1,5-bisphosphate (RuBP) from glyceraldehyde-3-phosphate and related intermediates. Phosphoribulokinase uses ATP to phosphorylate ribulose 5-phosphate into RuBP, allowing the cycle to continue.

• Carboxylation phase: No ATP is consumed here. The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) fixes CO₂ to RuBP without ATP input.

Explanation of options

(1) Only carboxylation: Incorrect. Carboxylation involves CO₂ fixation catalyzed by RuBisCO without ATP consumption.

(2) Only regeneration: Incorrect. ATP is consumed in regeneration, but also in reduction. So this is incomplete.

(3) Carboxylation and reduction: Incorrect because carboxylation does not use ATP.

(4) Reduction and regeneration: Correct. Both these phases require ATP to phosphorylate intermediates—key for reducing 3-PGA and regenerating RuBP.

Thus, photochemically generated ATP is consumed in the reduction and regeneration phases of the Calvin-Benson cycle, making option (4) the correct answer.