Photosynthesis is a two‑stage process. First, light‑dependent reactions harness solar energy to generate ATP and NADPH. Second, the light‑independent reactions—also called the Calvin cycle—use those molecules to fix carbon dioxide into carbohydrates.

TL;DR

The Calvin cycle, named after Nobel laureate Melvin C. Calvin, comprises four sequential reactions: carbon fixation, reduction, carbohydrate synthesis, and regeneration of the starting enzyme. Although it can occur in the dark, it runs during daylight because it relies on ATP and NADPH produced by the light reactions.

Overview of the Calvin Cycle

Within the chloroplast stroma, the Calvin cycle transforms the energy captured in ATP and the reducing power of NADPH into glucose and other sugars. It couples the oxidation of NADPH and the hydrolysis of ATP with the reduction of CO₂, thereby producing a stable carbohydrate that fuels plant growth.

Key Reactants and Products

Essential inputs: CO₂, ATP, and NADPH. Outputs: glucose, ADP, and NADP⁺. The latter two are recycled back into the light‑dependent reactions, completing the photosynthetic cycle.

Four Steps of the Calvin Cycle

1. Carbon Fixation: CO₂ attaches to the five‑carbon sugar ribulose‑1,5‑bisphosphate (RuBP) via the enzyme RuBisCO, forming a fleeting six‑carbon intermediate that splits into two molecules of 3‑phosphoglycerate (3‑PGA).
2. Reduction: ATP donates a phosphate group to 3‑PGA, while NADPH provides electrons and a hydrogen ion, converting 3‑PGA into the energy‑rich sugar glyceraldehyde‑3‑phosphate (G3P).
3. Carbohydrate Formation: Two G3P molecules combine to produce one molecule of glucose or other sugars that serve as plant food.
4. Regeneration of RuBP: The remaining G3P molecules are phosphorylated by ATP and reshaped back into RuBP, allowing the cycle to repeat.

Even though the Calvin cycle can proceed without direct light, it is fundamentally light‑dependent in plants because it consumes ATP and NADPH produced by the light reactions. The continuous exchange between the two stages ensures a steady supply of carbohydrates and a renewable pool of energy carriers.

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