Q.During photosynthetic CO2 fixation certain plants, produce PGA as first stable product.
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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? …
Plants that form a 3-carbon acid (PGA) as the first product of CO2 fixation follow the Calvin cycle directly, and the details of this pathway were worked out by a specific scientist. …
Plants producing PGA first are C3 plants; the pathway (Calvin cycle) was reported by Melvin Calvin; carboxylation is by RuBisCO; the primary CO2 acceptor is RuBP.
- In many plants the first stable product of CO2 fixation is 3-phosphoglyceric acid (PGA), a 3-carbon compound - such plants are called C3 plants (and the pathway is the C3 or Calvin cycle).
- The Calvin cycle (the path of carbon fixation) was worked out and reported by Melvin Calvin (with his co-workers), for which he received the Nobel Prize. …
- CBSE 2025Set botany-hz1 markMCQQ.Primary CO2 acceptor in C3 plants is:(a) PEP(b) Oxaloacetate(c) 3-PGA(d) RuBP
›Reveal solutionSolution
The primary CO2 acceptor in C3 plants is RuBP (ribulose-1,5-bisphosphate); its carboxylation by RuBisCO yields 3-PGA as the first stable product of the Calvin (C3) cycle.
C3 plants (the majority of plants) fix atmospheric CO2 directly through the Calvin cycle, which has three stages:
- Carboxylation: CO2 is fixed by combining with the 5-carbon compound RuBP (ribulose-1,5-bisphosphate), catalysed by the enzyme RuBisCO (ribulose bisphosphate carboxylase-oxygenase). This is the rate-limiting and most crucial step.
- This reaction produces an unstable 6-carbon intermediate that immediately splits into two molecules of the 3-carbon compound 3-phosphoglycerate (3-PGA) — the first stable product of the cycle (this is why the pathway is called the C3 pathway).
- Reduction: 3-PGA is reduced to G3P (glyceraldehyde-3-phosphate) using ATP and NADPH from the light reactions.
- Regeneration: Most G3P is used to regenerate RuBP so the cycle can continue; a fraction is used to synthesise glucose/starch. …
- CBSE 2024Set ANN1 markQ.Fill in the blank : The dark reaction of photosynthesis takes place at ________ of the chloroplast.
›Reveal solutionSolution
The dark reaction (Calvin cycle / carbon-fixation reaction) of photosynthesis occurs in the stroma of the chloroplast.
A chloroplast has two main compartments: the membranous thylakoid system (where the light reaction occurs) and the stroma, the enzyme-rich fluid matrix surrounding the thylakoids. The dark reaction (also called the biosynthetic or carbon-fixation phase, i.e. the Calvin cycle) does not directly need light and takes …
- CBSE 2024Set HALF_YEARLY1 markQ.State whether True or False: The first product of the Calvin cycle is 3-phosphoglyceric acid.
›Reveal solutionSolution
This statement is True: RuBisCO fixes CO2 onto RuBP to directly yield two molecules of 3-phosphoglyceric acid (3-PGA), the first stable product of the Calvin (C3) cycle.
The Calvin cycle (also called the C3 cycle, since its first product has three carbons) begins with carboxylation: the enzyme RuBisCO (ribulose bisphosphate carboxylase-oxygenase) catalyses the fixation of one molecule of atmospheric CO2 onto ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar. This forms an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglyceric acid (3-PGA), each containing three carbon atoms. This is why plant …
- CBSE 2023Set ANN1 markQ.Fill in the blank : The site of dark reaction in photosynthesis is ________ part of chloroplast.
›Reveal solutionSolution
The dark reaction (carbon-fixation / Calvin cycle) of photosynthesis occurs in the colourless matrix of the chloroplast called the stroma.
A chloroplast has two main functional compartments:
- The grana (stacks of thylakoid membranes) — the site of the light reaction, where light energy is trapped and converted to ATP and NADPH, and water is split (photolysis). …
- CBSE 2023Set ANNUAL1 markMCQQ.The CO2 acceptor in C3 plants is :(a) PEP(b) RuBP(c) RMP(d) PGA
›Reveal solutionSolution
RuBP is the CO2 acceptor in C3 plants.
In C3 (Calvin) cycle, CO2 is fixed onto the 5-carbon acceptor RuBP (ribulose-1,5-bisphosphate) by the enzyme RuBisCO, forming two molecules of 3-carbon PGA. Hence RuBP is the primary CO2 acceptor in C3 plants (PEP is the …
- CBSE 2022Set ANN1 markQ.Observe the first pair and fill the blank. Light reaction : Grana Dark reaction : _____
›Reveal solutionSolution
Just as the light reaction of photosynthesis occurs in the grana (thylakoid membranes) of the chloroplast, the dark reaction (carbon-fixation phase) occurs in the stroma.
The chloroplast has two main functional compartments: the membrane-bound grana (stacks of thylakoids), where the light-dependent reactions — trapping light energy, splitting water, and producing ATP and NADPH — occur; and the stroma, the fluid matri …
- CBSE 2022Set TERM11 markMCQQ.Which among the following is not a step in Calvin cycle?(a) Carboxylation(b) Reduction(c) Photophosphorylation(d) Regeneration
›Reveal solutionSolution
The Calvin cycle's three phases are carboxylation, reduction, and regeneration; photophosphorylation belongs to the light reaction.
The Calvin cycle (C3 cycle, the biosynthetic/dark phase of photosynthesis) proceeds in three stages:
- Carboxylation -- CO2 is fixed onto RuBP by the enzyme RuBisCO, forming two molecules of 3-PGA.
- Reduction -- 3-PGA is reduced to G3P using the ATP and NADPH generated by the light reaction.
- Regeneration -- most G3P is used to regenerate RuBP so the cycle can continue, while a fraction is used to synthesise glucose/starch. …
- CBSE 2022Set ANNUAL1 markMCQQ.First product of the Calvin Cycle is:(a) 3-Phosphoglyceric Acid(b) Oxaloacetic Acid(c) Triose phosphate(d) Phosphoenolpyruvate
›Reveal solutionSolution
Carboxylation of RuBP by RuBisCO produces two molecules of 3-phosphoglyceric acid (3-PGA), the first stable product of the Calvin cycle.
The Calvin cycle (C3 pathway) begins with carboxylation: the enzyme RuBisCO fixes one molecule of atmospheric CO2 onto the 5-carbon sugar ribulose-1,5-bisphosphate (RuBP). This forms an unstable 6-carbon intermediate that immediately breaks down into two molecules of the 3-carbon compound 3-phosphoglyceric acid (3-PGA) — hence the name 'C3 pathway'. 3-PGA is then reduced (using ATP and NADPH from the light reaction) to glyceraldehyde-3-phosphate, some of which regenerates RuBP …
- CBSE 2018Set ANNUAL1 markQ.Match the following. Column A item: 'Stroma'. Choose its correct match from Column B:(1) Casein(2) Water vascular system(3) Fixation of CO2(4) Bentham and Hooker(5) Cockroach
›Reveal solutionSolution
'Stroma' matches 'Fixation of CO2' — the stroma is the site of the Calvin cycle (dark reactions), where atmospheric CO2 is fixed into organic compounds.
A chloroplast has two main functional compartments: the thylakoid membranes (stacked as grana), where the light reactions occur (light absorption, water splitting, ATP and NADPH generation), and the stroma, the protein-rich fluid matrix surrounding the thylakoids. The stroma contains the enzymes (including RuBisCO) needed for the Calvin cycle (C3 cycle), in which CO2 from the atmosphere is fixed onto RuBP and, using …
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