The Calvin cycle is the set of light-independent reactions that plants, algae, and some bacteria use to turn carbon dioxide into sugar. It takes place in the stroma of chloroplasts, the fluid-filled space around the thylakoids. This cycle matters because it is how carbon from the air enters most food webs.
Even though it does not require light directly, it depends on ATP and NADPH made by the light reactions.
Key Facts
- Overall for one G3P: 3 CO2 + 9 ATP + 6 NADPH -> 1 G3P + 9 ADP + 8 Pi + 6 NADP+
- Carbon fixation: RuBisCO attaches CO2 to RuBP, forming unstable 6-carbon intermediates that split into 3-PGA.
- Reduction: 3-PGA is converted into G3P using ATP and NADPH.
- Regeneration: most G3P is rearranged using ATP to rebuild RuBP so the cycle can continue.
- To make one glucose molecule, the cycle must fix 6 CO2 and use 18 ATP and 12 NADPH.
- For every 3 CO2 fixed, the cycle produces 6 G3P molecules, but only 1 net G3P exits the cycle.
Vocabulary
- Calvin cycle
- A cyclic pathway in the chloroplast stroma that uses CO2, ATP, and NADPH to produce carbohydrate.
- RuBisCO
- The enzyme that fixes carbon dioxide by adding CO2 to RuBP at the start of the Calvin cycle.
- RuBP
- Ribulose bisphosphate is a 5-carbon molecule that accepts CO2 during carbon fixation.
- G3P
- Glyceraldehyde-3-phosphate is a 3-carbon sugar produced by the Calvin cycle and used to build glucose and other carbohydrates.
- Stroma
- The fluid-filled region inside a chloroplast where the Calvin cycle takes place.
Common Mistakes to Avoid
- Calling the Calvin cycle the dark reactions means it only happens at night. This is wrong because the cycle can run in the light as long as ATP, NADPH, and CO2 are available.
- Saying the Calvin cycle directly makes glucose in one turn is incorrect. One net G3P requires 3 turns, and two G3P molecules can be combined to form one glucose.
- Forgetting RuBP regeneration leaves the cycle incomplete. RuBP must be rebuilt so RuBisCO has a CO2 acceptor for the next round of fixation.
- Mixing up ATP and NADPH roles causes errors in energy accounting. ATP supplies energy and phosphate transfers, while NADPH supplies high-energy electrons for reduction.
Practice Questions
- 1 If 9 molecules of ATP are used in the Calvin cycle, how many molecules of CO2 are fixed and how many net G3P molecules are produced?
- 2 A plant cell fixes 12 molecules of CO2 through the Calvin cycle. How many ATP and NADPH molecules are required, and how many glucose molecules could be formed?
- 3 Explain why the Calvin cycle stops when the light reactions stop, even though the Calvin cycle does not directly require light.