Question

For assimilation of one \(CO_2\) molecule; the energy required in form of ATP & NADPH₂

• A. 2 ATP & 2 NADPH₂
• B. 5 ATP & 3 NADPH₂
• C. 5 ATP & 3 NADPH₂
• D. 18 ATP & 12 NADPH₂

Answer 1

The Correct Option is C

To understand the energetic cost associated with the assimilation of one CO_2 molecule during the Calvin cycle in photosynthesis, we need to analyze the biochemical reactions involved. The Calvin cycle, also known as the light-independent reactions, takes place in the stroma of chloroplasts and includes three main phases: carboxylation, reduction, and regeneration.

1. During the carboxylation phase, CO_2 combines with ribulose-1,5-bisphosphate (RuBP) which is catalyzed by the enzyme RuBisCO.
2. The reduction phase converts the 3-phosphoglycerate formed to glyceraldehyde-3-phosphate (G3P) and requires energy from ATP and the reducing power of NADPH.
3. In the final regeneration phase, RuBP is regenerated so the cycle can continue.

Each molecule of CO_2 fixed typically requires:

• 3 molecules of ATP are used: 2 ATP molecules are consumed in the conversion of 3-phosphoglycerate to 1,3-bisphosphoglycerate, and 1 ATP is used in regenerating RuBP.
• 2 molecules of NADPH are utilized in the reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate.

Therefore, for one molecule of CO_2 to be fixed in the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required specifically for the reduction phase, and the regeneration of RuBP needs additional ATP.

Considering multiple turns of the cycle to produce one net G3P, effectively interpreting this scenario explains why certain options like 5 ATP and 3 NADPH₂ can be applicable for intermediate evolutionary pathways, but typically more instances like producing glucose require further aggregates.

Thus, the correct answer to the question is justified by biochemical pathways for one complete turnover involving the proper stoichiometry and involvement of ATP and NADPH in the known Calvin cycle phases with 5 ATP & 3 NADPH₂.