Q.34 The number of ATP molecules required for the complete assimilation of one
molecule of CO2 in Calvin cycle is _____.
The number of ATP molecules required for complete assimilation of one CO2 molecule in the Calvin cycle is 3.
This process occurs in C3 plants during the light-independent reactions of photosynthesis, where CO2 is fixed into organic sugars using energy from the light reactions. The Calvin cycle consists of three phases: carboxylation, reduction, and regeneration, each contributing to the ATP count.
Calvin Cycle Phases
Carboxylation phase. CO2 reacts with RuBP via rubisco to form two 3-PGA molecules, requiring no ATP.
Reduction phase. Each 3-PGA converts to G3P using 2 ATP (for phosphorylation to 1,3-bisphosphoglycerate) and 2 NADPH (for reduction).
Regeneration phase. Five-sixths of G3P molecules rearrange to regenerate three RuBP, consuming 1 ATP per CO2 fixed.
ATP Calculation
For one CO2:
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Reduction: 2 ATP
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Regeneration: 1 ATP
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Total: 3 ATP (plus 2 NADPH, though the question specifies ATP only).
This matches standard stoichiometry for C3 pathway; C4 plants require more due to initial fixation.
Common Options Explained
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2 ATP: Covers only reduction; ignores regeneration. Incorrect.
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3 ATP: Correct total for all phases.
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18 ATP: For six CO2 (one glucose); not per CO2.
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Others (e.g., 1 or 5): Mismatch phase requirements.
The ATP molecules required for complete assimilation of one CO2 molecule in Calvin cycle is a key concept in photosynthesis for CSIR NET Life Sciences aspirants. This light-independent process fixes atmospheric CO2 into sugars, powering plant growth and the global carbon cycle. Understanding the exact energy investment—3 ATP per CO2—helps master biochemistry and plant physiology topics.
Why 3 ATP per CO2?
The Calvin cycle processes one CO2 through three stages, demanding precise ATP use from light reactions. Rubisco fixes CO2 to RuBP, yielding two 3-PGA (0 ATP). Reduction phosphorylates and reduces these to G3P (2 ATP). Regeneration recycles intermediates to RuBP (1 ATP). Total: 3 ATP, ensuring cycle continuity.
For glucose (six CO2), scale to 18 ATP and 12 NADPH, but per CO2 remains 3 ATP and 2 NADPH in C3 plants.
CSIR NET Exam Insights
Competitive exams test this stoichiometry via MCQs. Options like 2 ATP (reduction only) or 18 ATP (full glucose) trap students. Focus: “complete assimilation” means full cycle integration, not just fixation.
Key phrase integration: Searches for “ATP molecules required for one CO2 assimilation Calvin cycle” spike during exam season—optimize with phase breakdowns for top rankings.
