Three Phases of the Calvin Cycle: Carboxylation, Reduction, Regeneration
Biology · Photosynthesis in Higher Plants · NEET
The Calvin cycle (dark reaction) has three phases in a fixed order: (1) Carboxylation - CO2 is added to the 5-carbon RuBP by RuBisCO to make two molecules of 3-carbon PGA; (2) Reduction - PGA is turned into sugar (G3P/PGAL) using ATP and NADPH from the light reaction; (3) Regeneration - RuBP is remade using ATP so the cycle can repeat. Memory hook: "Cars Run Recycled" = Carboxylation, Reduction, Regeneration.
The Calvin cycle runs in the stroma in three fixed phases - carboxylation (CO2 + RuBP to 3-PGA via RuBisCO), reduction (3-PGA to sugar G3P using 2 ATP + 2 NADPH), and regeneration (RuBP rebuilt using 1 ATP). Per CO2 the cycle spends 3 ATP and 2 NADPH; one glucose needs 6 turns = 18 ATP and 12 NADPH.
Your doubts, answered
What are the three phases of the Calvin cycle in the correct order?
The order is fixed: Carboxylation first, then Reduction, then Regeneration. In carboxylation, CO2 joins RuBP (5C) using the enzyme RuBisCO to form 2 molecules of 3-PGA (3C). In reduction, 3-PGA is converted to the sugar G3P (also called PGAL, a triose phosphate) using ATP and NADPH. In regeneration, most of the sugar is used to rebuild RuBP so the cycle can accept fresh CO2. Remember: Carboxylation -> Reduction -> Regeneration (Cars Run Recycled).
Which phase of the Calvin cycle uses ATP and which uses NADPH?
NADPH is used only in the reduction phase. ATP is used in BOTH the reduction and the regeneration phases. For every 1 CO2 fixed, the cycle uses 3 ATP and 2 NADPH: 2 ATP + 2 NADPH are spent in reduction, and 1 more ATP is spent in regeneration. Carboxylation itself does not use ATP or NADPH - it only needs the RuBisCO enzyme.
What is the difference between the reduction phase and the regeneration phase?
In the reduction phase, 3-PGA is chemically reduced (given electrons/hydrogen) to become the sugar G3P, and this is where the actual food/carbohydrate is formed. In the regeneration phase, no new sugar leaves the cycle; instead the CO2 acceptor RuBP is rebuilt from the remaining sugar molecules using ATP, so the cycle can run again. Reduction makes the product; regeneration keeps the cycle going.
Does the Calvin cycle need light directly?
No, the Calvin cycle does not use light directly, which is why it is called the dark reaction or biosynthetic phase. But it depends on the light reaction, because it needs the ATP and NADPH made there. So the Calvin cycle can only run when light reaction products are available - it usually stops in the dark once ATP and NADPH run out. NEET often tests this indirect dependence.
How many turns of the Calvin cycle make one glucose, and how much ATP/NADPH?
One turn fixes one CO2, so 6 turns are needed to make one glucose (a 6-carbon sugar). Total for 1 glucose = 6 x 3 = 18 ATP and 6 x 2 = 12 NADPH. This is a very common NEET number question, so memorise: 3 ATP + 2 NADPH per CO2, and 18 ATP + 12 NADPH per glucose.
⚠️ The NEET trap ✗ NADPH is used in all three phases of the Calvin cycle, so 3 ATP and 3 NADPH are needed per CO2. ✓ For every CO2 fixed, only 3 ATP and 2 NADPH are used. NADPH is used ONLY in the reduction phase; ATP is used in reduction and regeneration. Carboxylation uses neither. 🧠 NEET 2024 asked exactly this: per CO2 it is 3 ATP and 2 NADPH, NOT 3 and 3. The '2 NADPH' is the trap answer students miss.
Real NEET questions
NEET 2024
How many molecules of ATP and NADPH are required for every molecule of CO2 fixed in the Calvin cycle?
A · 2 molecules of ATP and 2 molecules of NADPH
B · 3 molecules of ATP and 3 molecules of NADPH
C · 3 molecules of ATP and 2 molecules of NADPH ✓
D · 2 molecules of ATP and 3 molecules of NADPH
Solution: Per CO2 fixed, the Calvin cycle uses 3 ATP and 2 NADPH: 2 ATP + 2 NADPH in the reduction phase and 1 extra ATP in the regeneration phase. So the answer is 3 ATP and 2 NADPH.
NEET 2023
How many ATP and NADPH2 are required for the synthesis of one molecule of Glucose during Calvin cycle?
A · 18 ATP and 12 NADPH2 ✓
B · 18 ATP and 16 NADPH2
C · 12 ATP and 16 NADPH2
D · 12 ATP and 12 NADPH2
Solution: One glucose needs 6 turns (6 CO2). Since each CO2 needs 3 ATP and 2 NADPH, total = 6 x 3 = 18 ATP and 6 x 2 = 12 NADPH. Hence 18 ATP and 12 NADPH2.
NEET 2026
The enzyme required for carboxylation in the Calvin cycle is
A · Hexokinase
B · PEP carboxylase
C · RuBP carboxylase - oxygenase ✓
D · Carboxypeptidase
Solution: The carboxylation phase of the Calvin cycle is catalysed by RuBP carboxylase-oxygenase (RuBisCO), which fixes CO2 onto RuBP to give two molecules of 3-PGA. PEP carboxylase is the C4 enzyme, so it is a distractor.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the first stable product of the Calvin cycle?
The first stable product is 3-PGA (3-phosphoglyceric acid), a 3-carbon compound formed in the carboxylation phase. Because the first product has 3 carbons, plants using only this pathway are called C3 plants.
Where in the chloroplast does the Calvin cycle occur?
The Calvin cycle occurs in the stroma of the chloroplast, the fluid part surrounding the thylakoids. The light reaction that supplies its ATP and NADPH happens on the thylakoid membranes.
Why is regeneration important in the Calvin cycle?
Regeneration rebuilds the CO2 acceptor RuBP using ATP. Without it, RuBP would run out after one round and the cycle could not fix new CO2, so photosynthesis would stop even if CO2 and light were available.
What is the difference between G3P and 3-PGA?
3-PGA is the first product of carboxylation and is an acid. G3P (also called PGAL or triose phosphate) is formed later, in the reduction phase, after 3-PGA is reduced using ATP and NADPH. G3P is the actual sugar that builds glucose and rebuilds RuBP.
