Q.Dark reaction in photosynthesis is called so because
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The Calvin Cycle: Making Sugar from Thin Air
Plants do something that seems almost magical: they take carbon dioxide — a waste gas we breathe out — and turn it into sugar. The Calvin cycle is the biochemical machine that makes this happen. It runs in the stroma of the chloroplast, the fluid-filled space that surrounds the thylakoid membranes where the light reactions happen.
Think of it this way. The light reactions capture sunlight and produce ATP and NADPH — these are like charged batteries and reducing power. The Calvin cycle uses those batteries to "fix" carbon from CO₂ into organic molecules. No light is directly needed for the cycle itself, which is why it's sometimes called the dark reaction (though it usually runs during the day).
The Big Picture: Three Phases
The Calvin cycle has three distinct phases, and every turn of the cycle is about one thing: taking one molecule of CO₂ and attaching it to a five-carbon sugar called RuBP. The entire cycle must turn six times to produce one molecule of glucose.
Phase 1: Carboxylation — CO₂ is attached to RuBP. This is the entry point for carbon into the biosphere.
Phase 2: Reduction — The fixed carbon is reduced using ATP and NADPH, producing a three-carbon sugar called G3P (glyceraldehyde-3-phosphate).
Phase 3: Regeneration — The remaining carbon skeletons are rearranged to rebuild RuBP, so the cycle can continue.
6 CO2+18 ATP+12 NADPH+12 H2O→C6H12O6+18 ADP+18 Pi+12 NADP+
Phase 1: Carboxylation — The Key Step
The enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) grabs a CO₂ molecule and attaches it to RuBP, a five-carbon sugar with two phosphate groups. The result is an unstable six-carbon intermediate that instantly splits into two molecules of 3-phosphoglycerate (3-PGA).
RuBisCO is the most abundant protein on Earth. It is also notoriously slow and inefficient — it can mistakenly grab O₂ instead of CO₂, leading to photorespiration. This is a major reason why C₄ and CAM plants evolved alternative pathways.
Phase 2: Reduction — Making Sugar
Each 3-PGA molecule now gets phosphorylated by ATP (using one ATP per molecule) to form 1,3-bisphosphoglycerate. Then NADPH donates electrons to reduce this to glyceraldehyde-3-phosphate (G3P) — a three-carbon sugar. This is the first stable carbohydrate produced.
For every three CO₂ molecules fixed, six G3P molecules are produced. One G3P leaves the cycle to be used for making glucose or other organic compounds. The other five G3P molecules stay in the cycle.
Phase 3: Regeneration — Rebuilding RuBP
The five remaining G3P molecules (each with 3 carbons, so 15 carbons total) are rearranged through a series of reactions involving several enzymes. They combine and split to form three molecules of RuBP (each with 5 carbons, so 15 carbons total). This regeneration requires three more ATP molecules.
A common mistake is thinking the Calvin cycle produces glucose directly. It produces G3P, which is then used to build glucose and other carbohydrates elsewhere in the cell. One glucose requires two G3P molecules.
Why Six Turns? …
The "dark reaction" is so named because it does not itself depend directly on light energy, not because it happens in darkness.
- The stromal reactions of photosynthesis are not photochemical steps — unlike the membrane-based light reactions, they are not directly driven by light.
- However, they entirely depend on the ATP and NADPH supplied by the light reaction, so they cannot continue indefinitely without light having occurred.
- This is confirmed experimentally: after light is removed, the biosynthetic process continues briefly (using ATP/NADPH already made) and then stops, resuming only when light becomes available again. …
"Dark reaction" refers to the fact that these stromal steps are not themselves photochemical (light-absorbing) reactions, not that they occur in darkness — they still depend indirectly on the ATP and NADPH the light reaction supplies.
Within the chloroplast there is a clear division of labour. The membrane system — the grana and stroma lamellae — traps light energy and synthesises ATP and NADPH; because this is directly driven by light, these are called the light reactions. The stroma, on the other hand, carries out enzymatic reactions that synthesise sugar, and because these reactions are not themselves photochemical steps, they are conventionally called dark reactions or carbon reactions. …
Method 1 — Distinguish what 'dark reaction' actually means
- Recall the biosynthetic (stromal) reactions of photosynthesis are not themselves photochemical steps — they don't directly absorb light.
- Rule out (a) and (d): the reaction is not literally confined to darkness, nor does it run faster at night; it occurs in daylight too, alongside the light reaction.
- Rule out (c): the reaction is not blocked by daylight — plants clearly photosynthesise during the day. …
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.If 6 CO2 molecules entered into the Calvin cycle, number of G-3-P formed, net gain of G-3-P and G-3-P participated in RUBP regeneration are (A) 12, 2 & 10 (B) 2, 12 & 10 (C) 10, 12 & 2 (D) 6, 12 & 10
›Reveal solutionSolution
Six CO2 into the Calvin cycle → 12 G3P formed → 2 net gain (for glucose) → 10 used to regenerate RuBP. Answer: 12, 2 & 10.
Concept and Intuition
The Calvin cycle has three phases: carboxylation, reduction, and regeneration. To restore the 6 RuBP molecules consumed in fixing 6 CO2, the cycle must run enough times that its bookkeeping balances in carbon atoms: 6 CO2+6 RuBP→12 G3P (via 12 molecules of 3-PGA reduced using 12 ATP and 12 NADPH). Of the 12 three-carbon G3P molecules, only 2 (i.e. 6 carbons, matching the 6 CO2 fixed) can leave the cycle as net product without unbalancing the carbon count — these 2 combine to form one glucose. The other 10 G3P (30 carbons) are shuffled through a series of reactions to regenerate the 6 RuBP (each RuBP being a 5-carbon sugar, 6×5=30 carbons), consuming 6 more ATP.
Step-by-Step Solution
- Carboxylation: 6 CO2 + 6 RuBP → 12 molecules of 3-phosphoglyceric acid (3-PGA).
- Reduction: 12 PGA + 12 ATP + 12 NADPH → 12 G3P. …
- AP EAPCET 2026Set ap-2026-05-20-AN1 markMCQQ.Number of ATP and NADP required for every CO2 molecule entering the Calvin cycle. (A) 3 ATP 3 NADP (B) 2 ATP 2 NADP (C) 18 ATP 12 NADP (D) 3 ATP 2 NADP
›Reveal solutionSolution
The Calvin cycle's energy requirement per fixed CO2 is a fixed ratio of 3 ATP to 2 NADPH, which scales to 18 ATP and 12 NADPH for the six CO2 needed to build one glucose molecule.
Concept and Intuition
The Calvin cycle (reductive pentose phosphate pathway) has three phases per CO2 fixed: carboxylation (RuBisCO fixes CO2 onto RuBP, forming two molecules of 3-phosphoglyceric acid), reduction (using ATP and NADPH to convert 3-PGA to glyceraldehyde-3-phosphate), and regeneration (using additional ATP to regenerate RuBP so the cycle can continue). Careful accounting of these steps for a single CO2 shows a fixed requirement of 3 ATP and 2 NADPH per CO2 fixed. Multiplying this by six (since six turns of the cycle, fixing six CO2, are needed to net one six-carbon sugar) gives the commonly quoted total of 18 ATP and 12 NADPH per glucose — but that total is for six CO2, not one, so it does not answer this specific per-CO2 question.
Step-by-Step Solution
- Recall the standard Calvin cycle energetics: 3 ATP and 2 NADPH are consumed per CO2 fixed. …
- AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQQ.Arrange the following intermediate substances of RUBP regeneration in the correct sequence I) Ribulose -5 Phosphate II) Erythrose - 4 - Phosphate III) Fructose - 6 - Phosophate IV) Sedoheptulose - 7 Phosphate V) Ribose - 5 - Phosophate VI) Fructose 1, 6 bisphosphate (A) I, V, IV, VI, II, III (B) II, VI, III, IV, V, I (C) III, II, VI, IV, V, I (D) VI, III, II, IV, V, I
›Reveal solutionSolution
The Calvin cycle's regeneration phase runs Fructose-1,6-bisP to Fructose-6-P to Erythrose-4-P to Sedoheptulose-7-P to Ribose-5-P to Ribulose-5-P (VI, III, II, IV, V, I) — option (D).
Concept and Intuition
The Calvin cycle has three phases: carboxylation (CO2 fixation by RuBisCO), reduction (3-PGA to G3P using ATP/NADPH), and regeneration (rebuilding RuBP from the surplus G3P so the cycle can continue). The regeneration phase is a carefully sequenced set of sugar-phosphate rearrangements, driven by aldolases, phosphatases, and transketolase, that shuffle carbon atoms among 3-, 4-, 5-, 6-, and 7-carbon sugar phosphates until enough 5-carbon ribulose-5-phosphate is produced to be phosphorylated (by ATP) into RuBP.
Step-by-Step Solution
- Two G3P/DHAP molecules condense (aldolase) to give Fructose-1,6-bisphosphate (VI).
- This loses a phosphate (FBPase) to give Fructose-6-phosphate (III).
- Fructose-6-phosphate donates a 2-carbon unit to G3P (transketolase), producing Erythrose-4-phosphate (II) (and xylulose-5-phosphate).
- Erythrose-4-phosphate condenses with DHAP (aldolase) to give sedoheptulose-1,7-bisphosphate, which loses a phosphate to give Sedoheptulose-7-phosphate (IV). …
- AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQQ.Number of ATP and NADPH required to make one triose through calvin cycle. (A) 6 ATP and 9 NADPH (B) 9 ATP and 12 NADPH (C) 18 ATP and 12 NADPH (D) 9 ATP and 6 NADPH
›Reveal solutionSolution
The ATP/NADPH cost of the Calvin cycle scales with the number of CO2 molecules fixed, not directly with 'one triose'.
Concept and Intuition
Fixing one CO2 through the Calvin cycle costs 3 ATP and 2 NADPH. Making one net triose phosphate (G3P) requires 3 turns of the cycle (3 CO2 fixed) because 5 of the 6 carbons produced per 3 turns are recycled to regenerate RuBP, leaving 1 G3P as net output.
Step-by-Step Solution
- One turn of the Calvin cycle (fixing 1 CO2) uses 3 ATP and 2 NADPH.
- To yield one net triose phosphate (3-carbon G3P), 3 CO2 must be fixed → 3 turns.
- Total ATP = 3 × 3 = 9 ATP; total NADPH = 3 × 2 = 6 NADPH. …
- AP EAPCET 2025Set ap-2025-05-20-FN1 markMCQQ.Choose the correct sequence of no.of molecules formed when 6 mol of CO2 enters into calvin cycle pathway. (A) 4 xylulose 5P, 6 Ribulose 5P (B) 4 xylulose 5P, 2 Ribose 5P, 6 Ribulose 5p (C) 2 Ribose 5P, 4 xylulose 5P, 6 Ribulose 5p (D) 4 xylulose 5P, 2 Ribulose 5P, 2 Ribose 5p
›Reveal solutionSolution
The RuBP-regeneration phase of the Calvin cycle, starting from 10 leftover G3P molecules (after 2 leave as net product), passes through 4 xylulose-5-phosphate and 2 ribose-5-phosphate intermediates before yielding 6 ribulose-5-phosphate molecules — option (B).
Concept and Intuition
The Calvin cycle has three phases: carboxylation (CO₂ fixation onto RuBP via RuBisCO), reduction (PGA → G3P using ATP and NADPH), and regeneration (rearranging leftover G3P molecules back into RuBP to keep the cycle turning). The regeneration phase is a complex sugar-rearrangement pathway involving several named phosphate-sugar intermediates.
Step-by-Step Solution
- 6 CO₂ combine with 6 RuBP to give 12 molecules of 3-phosphoglyceric acid (PGA).
- All 12 PGA are reduced (using ATP and NADPH) to 12 molecules of glyceraldehyde-3-phosphate (G3P).
- 2 of these 12 G3P molecules leave the cycle to be used for net synthesis of glucose/other products.
- The remaining 10 G3P molecules undergo a series of transketolase/aldolase-type rearrangements to regenerate RuBP: this pathway proceeds through 4 molecules of xylulose-5-phosphate and 2 molecules of ribose-5-phosphate as intermediates. …
- AP EAPCET 2024Set ap-2024-05-16-FN1 markMCQQ.In Calvin cycle the ratio of trioses formed, net gain and used to regenerate RUBP. (A) 2 : 3 : 2 (B) 10 : 2 : 6 (C) 6 : 1 : 5 (D) 5 : 1 : 6
›Reveal solutionSolution
Six turns of the Calvin cycle yield 12 G3P; 2 are net gain and 10 regenerate RuBP, giving the ratio 6:1:5.
Concept and Intuition
The Calvin cycle (C3 pathway) must run six times to fix 6 CO2 molecules (matching the 6 carbons needed to make one hexose sugar). Each turn fixes 1 CO2 and produces triose phosphates via the reduction phase. Bookkeeping the carbons:
- Total trioses (G3P, 3-carbon each) formed per 6 turns = 12 (since 6 CO2 × 3 turns of the cycle each producing molecules works out such that 12 G3P total are generated).
- Of these 12, only 2 leave the cycle as net gain (used to build glucose/sucrose/starch).
- The remaining 10 are recycled through the regeneration phase to reform 6 molecules of RuBP so the cycle can continue.
Step-by-Step Solution
- Trioses formed per 6 CO2 fixed = 12. …
- AP EAPCET 2024Set ap-2024-05-17-AN1 markMCQQ.Give the ratios between the following compounds in photosynthesis of plants A. Tetroses and sedoheptoloses B. CO2 fixed and net production of G-3-P C. DHAP takes part in regenerative phase and RUBP formed (A) A - 5:1 B - 3:1 C - 2:3 (B) A - 3:2 B - 1:2 C - 3:4 (C) A - 1:5 B - 1:3 C - 3:2 (D) A - 2:4 B - 3:4 C - 3:2
›Reveal solutionSolution
Tests the numeric stoichiometry of the Calvin cycle's regeneration phase — the answer is (A).
Concept and Intuition
To make 1 hexose, the Calvin cycle must turn 6 times (fixing 6 CO2), producing 12 G3P (3-carbon each). Only 2 of those 12 G3P leave the cycle as net product; the other 10 are shuffled through a series of transketolase/aldolase reactions (the same reactions as the pentose phosphate pathway, run in reverse) to regenerate the 6 RuBP (5-carbon each) needed to keep the cycle running. Along the way, intermediates like DHAP, erythrose-4-phosphate (a tetrose) and sedoheptulose-1,7-bisphosphate (a heptulose) appear transiently.
Step-by-Step Solution
- B: CO2 fixed vs net G3P. 6 CO2 are fixed (one per turn) to eventually yield 1 hexose. Of the 12 total G3P formed, only 2 are net export (the rest regenerate RuBP). Ratio =6:2=3:1.
- C: DHAP in regeneration vs RuBP formed. The regeneration network (aldolase and transketolase reactions converting the 10 recycled trioses into 6 pentose phosphates) consumes DHAP in the aldolase steps that build fructose-1,6-bisphosphate and sedoheptulose-1,7-bisphosphate; across the full regeneration of 6 RuBP, DHAP participates 4 times. Ratio =4:6=2:3. …
- AP EAPCET 2023Set ap-2023-05-22-AN1 markMCQQ.'Transketolase' catalyses the following reaction (A) G-3-P + DHAP and Fructose 6 P + G-3-P Reactions (B) Fructose-6 P + G-3-P and Sedoheptulose – 7-P + G-3-P Reactions (C) G-3-P + DHAP and Erythrose -4-P + DHAP Reactions (D) Erythrose-4-P + DHAP and Sedoheptulose – 7-P + G-3-P Reactions
›Reveal solutionSolution
Transketolase catalyses two Calvin-cycle reactions: F6P + G3P and S7P + G3P (transferring 2-carbon units), distinguishing it from aldolase, which instead catalyses the G3P+DHAP and E4P+DHAP reactions.
Concept and Intuition
The Calvin cycle's regeneration phase reshuffles carbon skeletons among 3-, 4-, 5-, 6-, and 7-carbon sugar phosphates to regenerate RuBP. Two different enzymes do this reshuffling: aldolase condenses a 3-carbon unit (DHAP) with an aldose to build 6- or 7-carbon sugars, while transketolase transfers a 2-carbon (glycolaldehyde) unit from a ketose donor to an aldose acceptor.
Step-by-Step Solution
- Transketolase reaction 1: Fructose-6-phosphate (ketose, 6C) + Glyceraldehyde-3-phosphate (aldose, 3C) → Xylulose-5-phosphate (5C) + Erythrose-4-phosphate (4C).
- Transketolase reaction 2: Sedoheptulose-7-phosphate (ketose, 7C) + Glyceraldehyde-3-phosphate (aldose, 3C) → Xylulose-5-phosphate (5C) + Ribose-5-phosphate (5C).
- Both reactions share the same enzyme mechanism — transfer of a 2-carbon unit — and both involve G-3-P as a partner substrate, matching the substrate pairs "F6P + G3P" and "S7P + G3P" in option (B). …
- AP EAPCET 2023Set ap-2023-05-22-FN1 markMCQQ.In plants sugar synthesis takes place in (A) Grana of chloroplast (B) Stroma lamella of chloroplast (C) Mitochondrial matrix (D) Stroma of chloroplast
›Reveal solutionSolution
Sugar (carbohydrate) synthesis via the Calvin cycle happens in the chloroplast's stroma, using ATP/NADPH produced in the thylakoid-based light reactions.
Concept and Intuition
The chloroplast is divided into two functional compartments: the thylakoid membranes (organised into grana), where the light-dependent reactions occur (capturing light energy to produce ATP and NADPH, plus O₂ from water splitting), and the stroma, the fluid matrix surrounding the thylakoids, where the light-independent (dark) reactions — the Calvin cycle — actually fix CO₂ and synthesize sugars using the ATP and NADPH supplied from the thylakoids.
Step-by-Step Solution
- Sugar synthesis refers to the Calvin cycle (carbon fixation and reduction to triose phosphates, eventually sugars).
- This cycle's enzymes, including RuBisCO, are located in the stroma, not the thylakoid/grana membranes. …
- AP EAPCET 2023Set ap-2023-05-23-AN1 markMCQQ.The number of absorbed CO2 and water molecules to produce 180 g of glucose in plants are (A) 264 g and 108 g (B) 264 g and 193 g (C) 271 g and 108 g (D) 193 g and 108 g
›Reveal solutionSolution
This tests stoichiometric calculation from the balanced photosynthesis equation to find the mass of CO2 and water needed to produce 180 g of glucose.
Concept and Intuition
The overall photosynthesis reaction is: 6CO2 + 6H2O → C6H12O6 + 6O2. This balanced equation shows that producing 1 mole of glucose (180 g, since glucose's molar mass = 6(12) + 12(1) + 6(16) = 180 g/mol) requires 6 moles of CO2 and 6 moles of water.
Step-by-Step Solution
- Confirm glucose's molar mass: C6H12O6 = 6(12) + 12(1) + 6(16) = 72 + 12 + 96 = 180 g/mol.
- From the balanced equation, producing 180 g (1 mole) of glucose requires 6 moles of CO2 and 6 moles of H2O.
- Mass of 6 moles CO2 = 6 x 44 g/mol = 264 g.
- Mass of 6 moles H2O = 6 x 18 g/mol = 108 g. …
- AP EAPCET 2021Set ap-2021-09-03-FN1 markMCQQ.Identify the correct sequence of stages of Calvin cycle? (A) Reduction → Carboxylation → Regeneration (B) Carboxylation → Regeneration → Reduction (C) Carboxylation → Reduction → Regeneration (D) Reduction → Regeneration → Carboxylation
›Reveal solutionSolution
This tests the correct sequence of the three phases of the Calvin cycle — carboxylation must come first, followed by reduction, then regeneration.
Concept and Intuition
The Calvin cycle (dark/light-independent reactions of photosynthesis) is the biochemical pathway that fixes atmospheric CO2 into sugar. It logically has to begin by first capturing (fixing) the CO2 onto an acceptor molecule before that carbon can be chemically reduced, and only after the sugar precursor is made can the CO2-acceptor molecule be regenerated to keep the cycle running.
Step-by-Step Solution
- Carboxylation: RuBisCO catalyses the fixation of atmospheric CO2 onto ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar, forming an unstable 6-carbon intermediate that splits into two molecules of 3-phosphoglyceric acid (3-PGA). This is the rate-limiting, defining first step.
- Reduction: 3-PGA is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH generated in the light reactions. This is where the fixed carbon is converted into a usable sugar phosphate. …
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