What is the Calvin Cycle?
The Calvin cycle is the second stage of photosynthesis, occurring in the chloroplast stroma. It uses ATP and NADPH from the light reactions to fix atmospheric CO2 into organic sugar (G3P), which the plant uses to build glucose.
The Calvin cycle fixes CO2 into G3P through three phases — carbon fixation, reduction, and regeneration of RuBP — powered by ATP and NADPH from the light reactions.
- 1.Carbon Fixation — RuBisCO attaches CO2 to RuBP (5-carbon), 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 (glyceraldehyde-3-phosphate).
- 3.Carbohydrate Output — For every 6 G3P made, 1 leaves the cycle to build glucose and other carbohydrates.
- 4.Regeneration of RuBP — The remaining 5 G3P molecules use more ATP to regenerate RuBP, restarting the cycle.
Try it: interactive calculator
Step-by-step worked examples
How many CO2 molecules and full turns of the Calvin cycle are needed to produce one molecule of glucose (C6H12O6)?
Glucose has 6 carbons, and each turn of the cycle fixes 1 CO2 (1 carbon). Turns needed = 6 carbons / 1 carbon per turn = 6 turns. So 6 CO2 molecules and 6 turns of the Calvin cycle are required to build one glucose molecule.
If 6 turns of the Calvin cycle are needed for 1 glucose, and each turn uses 3 ATP and 2 NADPH, how much ATP and NADPH are needed total?
ATP per turn = 3, turns = 6, total ATP = 3 × 6 = 18 ATP. NADPH per turn = 2, turns = 6, total NADPH = 2 × 6 = 12 NADPH. So producing one glucose molecule costs 18 ATP and 12 NADPH.
A plant cell fixes 15 CO2 molecules in an hour. How many G3P molecules exit the cycle net (usable for sugar synthesis) in that time?
Every 3 CO2 fixed nets 1 G3P leaving the cycle (since 3 turns net 1 G3P for carbohydrate synthesis). G3P exported = 15 CO2 ÷ 3 = 5 G3P molecules. So 5 G3P molecules are available to build sugars.
Flashcards
Quick quiz
Q1.Where does the Calvin cycle take place?
Q2.Which enzyme catalyzes carbon fixation in the Calvin cycle?
Q3.How many turns of the Calvin cycle are needed to make one glucose molecule?
Q4.What does the Calvin cycle use from the light reactions?
The full card deck, worked steps and AI-tutor support for “What is the Calvin Cycle?” are in Notek — study by hand before your exam.
Common mistakes
The Calvin cycle directly requires light to run its reactions. — Correct: The Calvin cycle itself is light-independent, but it depends on ATP/NADPH made in the light reactions.
RuBisCO produces glucose directly. — Correct: RuBisCO fixes CO2 into 3-PGA; glucose is assembled later from G3P outside the cycle.
Each turn of the cycle produces one full glucose molecule. — Correct: Each turn fixes only 1 CO2; 6 turns are needed to net enough carbon for one glucose.
The Calvin cycle happens in the thylakoid membrane. — Correct: It happens in the stroma; the thylakoid membrane hosts the light reactions.
FAQ
What is the Calvin cycle?
It is the second stage of photosynthesis, occurring in the stroma, that fixes CO2 into sugar using ATP and NADPH from the light reactions.
What is the formula for the Calvin cycle?
Net stoichiometry: 3 CO2 + 9 ATP + 6 NADPH → 1 G3P + 9 ADP + 8 Pi + 6 NADP+.
What are examples of Calvin cycle products?
G3P (glyceraldehyde-3-phosphate) is the direct product; it is later used to build glucose, starch, and cellulose.
How do you calculate the ATP needed for the Calvin cycle?
Multiply the number of CO2 molecules fixed by 3, since the cycle uses 3 ATP for every CO2 fixed.