What is the Calvin Cycle (Dark Reactions of Photosynthesis)?
The dark reactions of photosynthesis, better known as the Calvin cycle, use the chemical energy captured in the light reactions to turn carbon dioxide into sugar. They don't need light directly, but they depend completely on the ATP and NADPH the light reactions supply.
The Calvin cycle is the set of light-independent reactions in the chloroplast stroma that fix CO2 using the enzyme RuBisCO and the ATP/NADPH from the light reactions to build glyceraldehyde-3-phosphate (G3P), the precursor of glucose.
- 1.Carbon Fixation — RuBisCO attaches CO2 to RuBP, forming an unstable 6-carbon compound that splits into two 3-PGA molecules.
- 2.Reduction — ATP and NADPH from the light reactions convert 3-PGA into G3P; one G3P out of every six exits to build glucose.
- 3.Regeneration — The remaining G3P molecules use more ATP to regenerate RuBP, restarting the cycle.
Step-by-step worked examples
To fix 3 CO2 molecules and produce 1 net G3P, how many ATP and NADPH does the Calvin cycle use?
Each CO2 fixed costs 3 ATP and 2 NADPH 3 CO2 × 3 ATP = 9 ATP 3 CO2 × 2 NADPH = 6 NADPH Result: 9 ATP and 6 NADPH are used to make 1 G3P
How many total CO2, ATP, and NADPH are needed to build one glucose molecule (C6H12O6)?
Glucose has 6 carbons, so 2 G3P molecules (3 carbons each) must combine Each G3P requires 3 CO2, 9 ATP, 6 NADPH 2 G3P → 6 CO2, 18 ATP, 12 NADPH Result: 6 CO2, 18 ATP, and 12 NADPH per glucose
A leaf fixes 40 CO2 molecules per second in the Calvin cycle. How many ATP molecules does it consume per second?
ATP cost = 3 ATP per CO2 fixed 40 CO2 × 3 ATP/CO2 = 120 ATP Result: 120 ATP consumed per second
Flashcards
Quick quiz
Q1.What molecule accepts CO2 at the start of the Calvin cycle?
Q2.How many ATP and NADPH are needed to make one glucose molecule via the Calvin cycle?
Q3.Why are the Calvin cycle reactions called 'dark reactions'?
Q4.Which organelle compartment hosts the Calvin cycle?
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Common mistakes
The dark reactions only happen at night. — Correct: They can run any time ATP and NADPH are available, which is normally during the day when light reactions are active.
CO2 is directly converted into glucose in one step. — Correct: CO2 is first fixed into 3-PGA, then reduced to G3P; two G3P molecules combine to form glucose afterward.
The Calvin cycle doesn't need light reactions at all. — Correct: It depends entirely on ATP and NADPH supplied by the light reactions in the thylakoid membrane.
RuBisCO only fixes carbon and never causes inefficiency. — Correct: RuBisCO can also bind O2 instead of CO2 (photorespiration), which wastes energy and reduces efficiency.
FAQ
What is the Calvin cycle?
It's the series of light-independent reactions in the chloroplast stroma that use ATP and NADPH to fix CO2 into G3P, the building block of glucose.
What is the formula/stoichiometry of the Calvin cycle?
To build one glucose molecule: 6 CO2 + 18 ATP + 12 NADPH → C6H12O6 + 18 ADP + 12 NADP+ + 6 H2O (simplified).
How do you calculate ATP and NADPH used in the Calvin cycle?
Multiply the number of CO2 molecules fixed by 3 ATP and 2 NADPH each, since that's the cost per carbon fixed.
Why is the Calvin cycle also called the C3 pathway?
Because the first stable product of carbon fixation, 3-phosphoglycerate (3-PGA), is a 3-carbon molecule.