Step 2: Calvin Cycle

The second stage of photosynthesis, which takes place in the stroma of the chloroplast, can occur without the presence of sunlight. In this stage, known as the Calvin Cycle, carbon molecules from CO₂ are fixed into glucose (C₆H₁₂O₂). The reactions of the Calvin Cycle is as follows:

1. A five-carbon sugar molecule called ribulose bisphosphate, or RuBP, is the acceptor that binds CO₂ dissolved in the stroma. This process, called CO₂ fixation, is catalyzed by the enzyme RuBP carboxylase, forming an unstable six-carbon molecule. This molecule quickly breaks down to give two molecules of the three-carbon 3-phosphoglycerate (3PG), also called phosphoglyceric acid (PGA).

2. The two 3PG molecules are converted into glyceraldehyde 3-phosphate (G3P, a.k.a. phosphoglyceraldehyde, PGAL) molecules, a three-carbon sugar phosphate, by adding a high-energy phosphate group from ATP, then breaking the phosphate bond and adding hydrogen from NADH + H⁺.

3. Three turns of the cycle, using three molecules of CO₂, produces six molecules of G3P. However, only one of the six molecules exits the cycle as an output, while the remaining five enter a complex process that regenerates more RuBP to continue the cycle. Two molecules of G3P, produced by a total of six turns of the cycle, combine to form one molecule of glucose.

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