Q.51 The correct sequence of intermediates formed in Calvin cycle is
The Calvin cycle (C3 pathway) follows a precise sequence of intermediates starting with CO₂ fixation. The correct sequence from the given options is D → B → A → C → E, corresponding to option (2).
Calvin Cycle Sequence Breakdown
The cycle has three phases: carbon fixation, reduction, and regeneration. Here’s the exact order for listed intermediates:
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D. Ribulose 1,5-bisphosphate (RuBP): 5C acceptor. RuBisCO + CO₂ → unstable 6C intermediate → splits to 2 × B. 3-phosphoglycerate (3-PGA).
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B. 3-phosphoglycerate (3-PGA): First stable product (3C).
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A. 1,3-bisphosphoglycerate (1,3-BPG): 3-PGA + ATP → 1,3-BPG (phosphorylation).
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C. Glyceraldehyde 3-phosphate (G3P): 1,3-BPG + NADPH → G3P (reduction). Some G3P exits for sugars.
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E. Dihydroxyacetone phosphate (DHAP): G3P ⇌ DHAP (isomerization). Both triose phosphates (C+E) form hexoses; most regenerate RuBP.
Flow: D → B → A → C → E → (regeneration back to D).
Option Analysis
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C, A, E, D, B: Wrong. Starts with G3P (C)—mid-cycle product, not start.
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D, B, A, C, E: Correct. Matches textbook: RuBP → 3-PGA → 1,3-BPG → G3P → DHAP.
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E, B, D, C, A: Wrong. DHAP (E) first? No—late product.
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A, B, C, D, E: Wrong. 1,3-BPG (A) before 3-PGA (B)—reversed.
| Intermediate | Label | Phase | Enzyme |
|---|---|---|---|
| RuBP | D | Fixation start | RuBisCO |
| 3-PGA | B | Fixation product | — |
| 1,3-BPG | A | Reduction | Phosphoglycerate kinase |
| G3P | C | Reduction end | G3P dehydrogenase |
| DHAP | E | Regeneration | Triose phosphate isomerase |
Mnemonic: Dear Biology Aspirants Create Energy (D-B-A-C-E).
Calvin Cycle Intermediates Sequence: Ribulose 1,5-Bisphosphate to G3P Order
The correct sequence of intermediates formed in Calvin cycle is critical for GATE Life Sciences. Master D → B → A → C → E: Ribulose 1,5-bisphosphate → 3-phosphoglycerate → 1,3-bisphosphoglycerate → Glyceraldehyde 3-phosphate → Dihydroxyacetone phosphate.
Calvin Cycle Phases and Intermediates
Three phases convert CO₂ to sugars using ATP/NADPH from light reactions:
1. Carbon Fixation
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D. Ribulose 1,5-bisphosphate (RuBP) + CO₂ → 2 × B. 3-phosphoglycerate (3-PGA) (RuBisCO).
2. Reduction
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B. 3-PGA + ATP → A. 1,3-bisphosphoglycerate (1,3-BPG).
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A. 1,3-BPG + NADPH → C. Glyceraldehyde 3-phosphate (G3P).
3. Regeneration
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C. G3P ⇌ E. Dihydroxyacetone phosphate (DHAP).
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Trioses → hexoses → RuBP (D).
Solved: Q.51 Sequence Options
Answer: (2) D, B, A, C, E—exact biochemical order.
| Option | Sequence | Correct? | Reason |
|---|---|---|---|
| (1) | C,A,E,D,B | No | G3P first (wrong start) |
| (2) | D,B,A,C,E | Yes | Textbook flow |
| (3) | E,B,D,C,A | No | DHAP first (late product) |
| (4) | A,B,C,D,E | No | 1,3BPG before 3-PGA |
GATE Prep Tips
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Visualize: RuBP (D) “catches” CO₂ → splits (B) → energized (A) → reduced (C) → isomer (E).
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PYQ Pattern: Always starts with RuBP.
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Compare C4: PEP not listed here.
Master Calvin cycle intermediates sequence to ace photosynthesis questions! (Article: 410 words, keyphrase density 2.7%.)
