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.
Understanding Calvin Cycle Detailed Reference
Carbon accounting is the best way to make sense of the cycle. Begin by following individual carbon atoms instead of memorizing a list of names. A five-carbon acceptor molecule joins one incoming carbon from carbon dioxide.
The short-lived product breaks into smaller molecules, so no carbon disappears. After three turns, the system has handled three incoming carbons and six three-carbon intermediates. Only one three-carbon product is available as net gain.
The remaining molecules are not wasted. They are the working material needed to rebuild the carbon dioxide acceptor. This is why the pathway is called a cycle rather than a straight assembly line.
The reduction stage changes a relatively oxidized compound into a higher-energy sugar phosphate. ATP first transfers a phosphate group, making the intermediate more reactive. NADPH then donates high-energy electrons and a hydrogen ion.
Those electrons are important because carbohydrate molecules store chemical energy in their carbon-hydrogen bonds. The light reactions supply ATP and NADPH, so photosynthetic sugar production depends on both sets of reactions even though they occur in different parts of the chloroplast. When studying, separate the jobs clearly.
ATP mainly provides usable energy for chemical steps. NADPH provides reducing power, meaning electrons that help build energy-rich organic molecules.
Regeneration is often the hardest phase because it involves rearranging carbon skeletons. Enzymes move groups of carbon atoms between molecules until the original five-carbon acceptor can be made again. This stage uses ATP because rebuilding the acceptor requires energy.
A useful check is to count carbons before moving on. Five three-carbon molecules contain fifteen carbons. Those fifteen carbons can be rearranged into three five-carbon acceptors.
If a diagram seems to create or lose carbon atoms, revisit the arrows and molecule numbers. Students often confuse molecules produced during the cycle with molecules that leave it. Net output means the amount remaining after regeneration has been paid for.
Photorespiration shows a limitation of rubisco, the enzyme that begins carbon fixation. Rubisco can react with oxygen because oxygen resembles carbon dioxide in ways that matter at its active site. This problem becomes more common on hot, dry days.
Plants close stomata to reduce water loss, which lowers the carbon dioxide level inside leaves while oxygen builds up. Photorespiration then consumes energy and releases some previously fixed carbon, lowering productivity. Many grasses, including corn and sugarcane, reduce this loss with a carbon-concentrating strategy called the C four pathway.
Cactus plants use CAM photosynthesis, taking in carbon dioxide mainly at night. These adaptations connect the cycle to crop yields, climate, and water availability.