- The Calvin-Benson cycle, also known as the Calvin cycle or the C3 pathway, is the primary mechanism by which carbon dioxide is fixed into organic molecules during photosynthesis in plants, algae, and cyanobacteria.
- This cycle operates in the stroma of chloroplasts and is a central component of the light-independent reactions (often inaccurately called the “dark reactions”) of photosynthesis. Although it does not directly use light energy, the Calvin-Benson cycle relies on ATP and NADPH generated during the light-dependent reactions to drive the fixation and reduction of carbon dioxide.
- The cycle consists of three major phases: carbon fixation, reduction, and regeneration of the CO₂ acceptor (ribulose-1,5-bisphosphate, or RuBP). In the first phase, carbon fixation, the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the attachment of a carbon dioxide molecule to a five-carbon sugar, RuBP. This results in an unstable six-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound. Because the first stable product is a three-carbon molecule, this pathway is termed the C3 cycle.
- In the reduction phase, each 3-PGA molecule is phosphorylated by ATP and then reduced by NADPH, forming glyceraldehyde-3-phosphate (G3P), another three-carbon sugar. For every three molecules of CO₂ that enter the cycle, six molecules of G3P are produced. However, only one G3P molecule exits the cycle to be used in the synthesis of glucose and other carbohydrates. The remaining five G3P molecules continue through the cycle.
- The final stage is the regeneration of RuBP, in which the five remaining G3P molecules are rearranged through a complex series of reactions involving ATP consumption to regenerate three molecules of RuBP. This regeneration is essential for the cycle to continue, ensuring that CO₂ fixation can occur continuously as long as ATP and NADPH are available from the light reactions.
- Overall, the Calvin-Benson cycle is a biochemical pathway that transforms inorganic carbon into organic molecules, a process fundamental to life on Earth. For every three molecules of CO₂ fixed, the cycle consumes 9 ATP and 6 NADPH. The glucose or other sugars synthesized from G3P are used by the plant for energy or as building blocks for growth. Though discovered in the mid-20th century by Melvin Calvin, Andrew Benson, and James Bassham using radioactive carbon isotopes, the full regulatory complexity and integration of the cycle into broader plant metabolism continues to be a focus of research in plant biology and bioengineering.
