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
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.