49. Primary acceptor of CO₂ in photosynthesis is
(1) Ribose                                          (2) Ribulose-5-P
(3) Ribulose1,5-bis Phosphate           (4) 3-Phosphoglycerate

The correct answer is (3): Ribulose-1,5-bisphosphate.

In C3 photosynthesis (Calvin cycle), the primary CO₂ acceptor is ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar that reacts with CO₂ in a process catalyzed by Rubisco. This forms two molecules of 3-phosphoglycerate (3-PGA) as the first stable product.​

CO₂ Fixation: Primary Acceptor in Photosynthesis

Introduction

Key phrase: primary CO₂ acceptor photosynthesis ribulose-1,5-bisphosphate RuBP Calvin cycle C3 plants

Photosynthetic carbon fixation starts when atmospheric CO₂ is captured by a specific organic molecule. In C3 plants, this critical molecule is ribulose-1,5-bisphosphate (RuBP), setting the Calvin cycle in motion to generate sugars for plant metabolism.​

Explanation of Each Option

• (1) Ribose

  • Incorrect. Ribose is not involved as the primary CO₂ acceptor in photosynthesis. It is a 5-carbon sugar present in RNA.

• (2) Ribulose-5-P

  • Incorrect. Ribulose-5-phosphate is an intermediate in the pentose phosphate pathway and Calvin cycle, but it is not the molecule that binds CO₂ for fixation.​

• (3) Ribulose-1,5-bis Phosphate

  • Correct. Ribulose-1,5-bisphosphate (RuBP) is the actual molecule that accepts CO₂, forming 3-phosphoglycerate via Rubisco catalysis.​

• (4) 3-Phosphoglycerate

  • Incorrect. 3-phosphoglycerate (3-PGA) is the first stable product made after CO₂ reacts with RuBP, not the acceptor itself.​

Key Facts: CO₂ Acceptor and Calvin Cycle

• RuBP (ribulose-1,5-bisphosphate) combines with CO₂ for carbon fixation in C3 plants.

• Reaction is catalyzed by the enzyme Rubisco, yielding two 3-PGA molecules.

• C4 and CAM plants use phosphoenolpyruvate (PEP) as their initial CO₂ acceptor in specific steps.​