The Calvin cycle is a series of biochemical reactions that take place in the stroma of chloroplasts in photosynthetic organisms, including plants. It is also known as the light-independent reactions, Calvin-Benson cycle, or dark reactions (though "dark reactions" is misleading because the cycle depends indirectly on light)
Purpose
The Calvin cycle uses ATP and NADPH produced by the light-dependent reactions of photosynthesis to convert carbon dioxide (CO₂) into organic molecules, specifically three-carbon sugars like glyceraldehyde-3-phosphate (G3P), which can then be used to form glucose and other carbohydrates
Main Stages
The Calvin cycle consists of three main stages:
- Carbon Fixation: CO₂ is attached to a five-carbon molecule called ribulose bisphosphate (RuBP) by the enzyme RuBisCO, producing two molecules of 3-phosphoglycerate (3-PGA)
- Reduction: ATP and NADPH are used to convert 3-PGA into G3P, a three-carbon sugar that is a key product of the cycle and can be used to build glucose and other carbohydrates
- Regeneration: Some G3P molecules are used, with the help of ATP, to regenerate RuBP, allowing the cycle to continue and fix more CO₂
Additional Notes
- The cycle fixes carbon from inorganic CO₂ into organic molecules, a process essential for plant growth and energy storage
- Although called light-independent, the Calvin cycle depends on ATP and NADPH generated by light-dependent reactions, so it indirectly requires light
- The enzyme RuBisCO is the key catalyst in the cycle, facilitating the initial carbon fixation step
In summary, the Calvin cycle is the biochemical pathway by which photosynthetic organisms convert atmospheric CO₂ into sugars using energy from ATP and NADPH produced in the light reactions of photosynthesis
