53. Which one of the following reactions takes place during the reduction phase of the Calvin- Benson cycle?
(1) Ribulose 1,5-bisphosphate to 3-phosphoglycerate
(2) 1,3-bisphosphoglycerate to glyceraldehyde-3 phosphate
(3) Dihydroxyacetone phosphate to fructose 1,6- bisphosphate
(4) Ribulose 5-phosphate to ribulose1,5 bisphosphate

 Calvin–Benson cycle phases

The Calvin–Benson cycle is divided into three phases: carboxylation (CO₂ fixation), reduction, and regeneration of ribulose 1,5-bisphosphate (RuBP). In the reduction phase, 3-phosphoglycerate (3-PGA) is converted via 1,3-bisphosphoglycerate into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH. Any reaction that makes triose phosphate (G3P) from 3-PGA intermediates belongs to this reduction phase.

Why option (2) is correct

Option (2): 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate is the key reduction step of the Calvin–Benson cycle.

• In this step, 1,3-bisphosphoglycerate (1,3-BPGA) is reduced to glyceraldehyde-3-phosphate (G3P) by the enzyme glyceraldehyde-3-phosphate dehydrogenase, using NADPH as a reducing agent.

• This conversion is literally the “reduction” that defines the reduction phase: electrons from NADPH are transferred to 1,3-BPGA, forming G3P (a triose phosphate).

Therefore, this reaction is correctly placed in the reduction phase and is the right answer.

Why option (1) is wrong

Option (1): Ribulose 1,5-bisphosphate to 3-phosphoglycerate does not occur in the reduction phase; it belongs to the carboxylation (fixation) phase.

• In this step, RuBP reacts with CO₂ in a reaction catalyzed by RuBisCO, producing an unstable 6‑carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).

• This is the CO₂ fixation step, not a reduction step, because there is no NADPH oxidation or reduction of carbon; instead, CO₂ is attached to RuBP.

So, option (1) is a carboxylation-phase reaction, not a reduction-phase reaction.

Why option (3) is wrong

Option (3): Dihydroxyacetone phosphate to fructose 1,6-bisphosphate occurs in the carbohydrate synthesis/regeneration network, not in the core “reduction phase” definition.

• Dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate can combine (via aldolase) to form fructose 1,6-bisphosphate as part of sugar formation and carbon-skeleton rearrangements.

• This happens in the later steps when triose phosphates are converted into hexose phosphates and intermediates used for RuBP regeneration, not in the specific phase where 3-PGA is reduced to G3P.

Thus, option (3) is associated with sugar formation and regeneration, not the primary reduction step.

Why option (4) is wrong

Option (4): Ribulose 5-phosphate to ribulose 1,5-bisphosphate belongs to the regeneration phase, not the reduction phase.

• In this reaction, ribulose 5-phosphate (Ru5P) is phosphorylated by ATP to form ribulose 1,5-bisphosphate (RuBP) via the enzyme phosphoribulokinase.

• This step uses ATP but does not involve NADPH or a redox change in carbon; its purpose is to regenerate the CO₂-acceptor RuBP so the cycle can continue.

Therefore, option (4) is clearly a regeneration-phase reaction and not part of the reduction phase.

In summary, the only reaction that truly characterizes the reduction phase of the Calvin–Benson cycle among the options is the conversion of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate, so option (2) is correct.