Q.41 C3 plants utilize 18 molecules of ATP to synthesize one molecule of glucose from CO 2 . How many
molecules of ATP equivalents are used by C 4 plants to synthesize one molecule of glucose from
CO2 ?
C4 plants require 30 molecules of ATP equivalents to synthesize one molecule of glucose from CO₂. This contrasts with the 18 ATP used by C3 plants due to the additional energy demands of the C4 pathway’s CO₂-concentrating mechanism. The calculation accounts for both the Hatch-Slack cycle and Calvin cycle costs.
C3 Pathway Basics
C3 plants fix 6 CO₂ molecules into one glucose via the Calvin cycle alone. Each CO₂ requires 3 ATP (2 for RuBP regeneration, 1 for reduction), totaling 18 ATP and 12 NADPH for glucose. No extra ATP is needed for CO₂ concentration.
C4 Pathway Breakdown
C4 plants perform initial CO₂ fixation in mesophyll cells using PEP carboxylase, forming a 4-carbon compound shuttled to bundle sheath cells. Per CO₂: C4 cycle uses 2 ATP (PEP regeneration from pyruvate), Calvin cycle uses 3 ATP, totaling 5 ATP per CO₂. For 6 CO₂ (one glucose): 6×5=30 ATP, plus 12 NADPH.
ATP Equivalents Explained
The query specifies “ATP equivalents,” referring directly to ATP molecules (not NADPH conversions). C4’s extra 12 ATP (2 per CO₂ × 6) powers metabolite shuttling, minimizing photorespiration in hot environments despite higher costs.
Common Options Analysis
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12 ATP: Matches C3 NADPH count, ignores total energy.
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18 ATP: Correct for C3 Calvin cycle only.
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24 ATP: Incorrect; confuses double glucose or partial sums.
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30 ATP: Correct for C4 complete pathway.
C4 plants ATP equivalents for glucose synthesis from CO2 represent a key CSIR NET Life Sciences concept, highlighting photosynthetic efficiency trade-offs. While C3 plants require just 18 ATP molecules to fix 6 CO₂ into one glucose, C4 plants demand 30 ATP equivalents due to their CO₂-concentrating mechanism.
Why C4 Costs More ATP
The C4 pathway (Hatch-Slack) adds 2 ATP per CO₂ for PEP regeneration: pyruvate + ATP + Pi → PEP + AMP + PPi (costs 2 ATP equivalents). Bundle sheath Calvin cycle then uses standard 3 ATP per CO₂. Total: 5 ATP/CO₂ × 6 = 30 ATP for glucose synthesis from CO2.
CSIR NET Exam Relevance
Questions often test this exact difference: “C3 plants utilize 18 molecules of ATP… how many for C4?” Answer: 30. Extra 12 ATP enables hot/dry adaptation by suppressing photorespiration, though energetically costlier under ideal conditions.
