Q.22 Which one is TRUE for Calvin cycle?
(A) Glycerol 3-phosphate is generated in this cycle
(B) CO2 is not consumed in this cycle
(C) This is a reductive pentose phosphate cycle
(D) Ribose 5-phosphate is a carboxylation substrate in this cycle
The correct answer is (C) This is a reductive pentose phosphate cycle.
Option Analysis
The Calvin cycle, also known as the C3 cycle, fixes CO2 into carbohydrates in chloroplast stroma using ATP and NADPH from light reactions.
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(A) Glycerol 3-phosphate is generated: False. Glyceraldehyde 3-phosphate (G3P) forms in reduction; dihydroxyacetone phosphate (DHAP) interconverts with it, but glycerol 3-phosphate occurs in lipid synthesis, not directly here.
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(B) CO2 is not consumed: False. CO2 fixes to RuBP in carboxylation by RuBisCO, producing 3-PGA.
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(C) This is a reductive pentose phosphate cycle: True. Involves pentose phosphates like RuBP (C5); reductive as NADPH reduces 3-PGA to G3P.
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(D) Ribose 5-phosphate is a carboxylation substrate: False. RuBP carboxylated by RuBisCO; R5P appears in regeneration, converted to Ru5P.
Calvin cycle reductive pentose phosphate pathway drives CO2 fixation in photosynthesis, key for CSIR NET exams. This light-independent process converts atmospheric CO2 to sugars using ATP and NADPH.
Cycle Overview
Three phases: carboxylation (RuBisCO fixes CO2 to RuBP, yields 3-PGA), reduction (3-PGA to G3P via ATP/NADPH), regeneration (G3P reforms RuBP via pentose phosphates). Named reductive pentose phosphate cycle for C5 sugars and reduction steps.
Common MCQ Traps
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G3P (not glycerol 3-phosphate) exports for glucose.
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CO2 essential, 6 CO2 for one glucose (18 ATP, 12 NADPH).
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R5P intermediate, not carboxylation acceptor.
Master Calvin cycle reductive pentose phosphate facts for exams.
