The Calvin cycle represents the second phase of photosynthesis, during which carbon dioxide is transformed into glucose.

Also referred to as the light-independent phase or dark phase, the Calvin cycle is a vital component of photosynthesis—the process through which plants convert sunlight into chemical energy. This cycle takes place in the stroma of chloroplasts, utilizing ATP and NADPH generated during the light-dependent reactions to convert carbon dioxide into glucose.

The Calvin cycle involves three primary stages: carbon fixation, reduction, and regeneration of the starting molecule.

1. Carbon Fixation: In the initial stage, carbon dioxide ($CO_2$) from the atmosphere is attached to a five-carbon sugar called ribulose bisphosphate (RuBP) by the enzyme RuBisCO. This reaction produces a six-carbon compound that promptly splits into two three-carbon molecules known as 3-phosphoglycerate ($3-PGA$).

2. Reduction: During the second stage, these three-carbon compounds ($3-PGA$) are reduced to glyceraldehyde-3-phosphate (G3P) through the utilization of energy from ATP and electrons from NADPH. Some of the G3P molecules are converted into glucose and other organic compounds, while the remainder is directed toward the third stage of the cycle.

3. Regeneration: In the final stage, RuBP is regenerated. The leftover G3P molecules are rearranged and combined, using additional energy from ATP, to reform the initial five-carbon RuBP. This regeneration is crucial, as it enables the cycle to continue, allowing for the fixation of more carbon dioxide and the production of additional glucose.

The Calvin cycle is a complex process that demands significant energy and resources. Nevertheless, it is essential for life on Earth, serving as the primary pathway for carbon fixation—the conversion of inorganic carbon into organic compounds. This process not only supplies the energy that plants require for growth and reproduction but also forms the foundation of the food chain, supporting all animal life.