The Calvin cycle is the light-independent pathway of photosynthesis that uses CO2 to build carbohydrate molecules in the chloroplast stroma. Students need this cheat sheet because the cycle combines enzyme action, carbon counting, and energy transfers that are easy to mix up. A detailed reference helps connect the cycle to the light reactions and to the overall goal of making sugar from carbon dioxide.
Key Facts
- The Calvin cycle occurs in the stroma of chloroplasts and uses CO2, ATP, and NADPH to produce G3P.
- The three phases of the Calvin cycle are carbon fixation, reduction, and regeneration of RuBP.
- In carbon fixation, rubisco attaches CO2 to RuBP, forming an unstable 6-carbon compound that splits into two 3-PGA molecules.
- For 3 CO2 entering the cycle, the pathway produces 6 molecules of 3-PGA, then 6 molecules of G3P after reduction.
- For every 3 CO2 fixed, 1 G3P exits the cycle while 5 G3P are used to regenerate 3 RuBP molecules.
- The net energy cost to produce 1 G3P is 3 CO2 + 9 ATP + 6 NADPH.
- The net energy cost to produce 1 glucose is 6 CO2 + 18 ATP + 12 NADPH because two G3P can combine to form one glucose.
- Photorespiration happens when rubisco binds O2 instead of CO2, wasting energy and reducing sugar production.
Vocabulary
- Calvin cycle
- A set of enzyme-controlled reactions that uses CO2, ATP, and NADPH to make G3P in the chloroplast stroma.
- Rubisco
- The enzyme that starts carbon fixation by attaching CO2 to RuBP, but it can also bind O2 during photorespiration.
- RuBP
- A 5-carbon molecule called ribulose-1,5-bisphosphate that accepts CO2 at the beginning of the Calvin cycle.
- 3-PGA
- A 3-carbon molecule called 3-phosphoglycerate that forms after the unstable 6-carbon fixation product splits.
- G3P
- A 3-carbon sugar phosphate produced by the Calvin cycle, with some molecules leaving the cycle to build carbohydrates.
- Photorespiration
- A wasteful process that occurs when rubisco uses O2 instead of CO2, decreasing the efficiency of photosynthesis.
Common Mistakes to Avoid
- Calling the Calvin cycle light-dependent is wrong because it does not directly require light, although it depends on ATP and NADPH made by the light reactions.
- Forgetting carbon bookkeeping is wrong because 3 CO2 are needed to produce one net G3P, while the other five G3P molecules regenerate RuBP.
- Saying the Calvin cycle directly makes glucose is wrong because the immediate carbohydrate product is G3P, and two G3P can be used to build one glucose.
- Mixing up ATP and NADPH roles is wrong because ATP supplies energy while NADPH supplies high-energy electrons for reduction.
- Ignoring photorespiration is wrong because rubisco can bind O2, especially under hot or dry conditions, which lowers carbon fixation efficiency.
Practice Questions
- 1 If 6 CO2 enter the Calvin cycle, how many net G3P molecules can be produced?
- 2 How many ATP and NADPH are required to make 2 net G3P molecules?
- 3 During carbon fixation, 3 CO2 combine with 3 RuBP molecules. How many 3-PGA molecules are produced?
- 4 Explain why closing stomata on a hot day can increase photorespiration and reduce sugar production.