Homework check: The 3 phases of the Calvin Cycle

23.   Three phases of the Calvin Cycle:
Phase 1: Carbon Fixation
•   A CO2 molecule is added to a RuBP (a five-carbon compound) that is already present in the stroma.
•   Carbon fixation: the enzyme rubisco (RuBP carboxylase) catalyzes the exergonic reaction between RuBP and CO2.
•   The unstable compound of RuBP and CO2 is split into two molecules of a 3-carbon compound called PGA (3 phosphoglycerate).  One of these molecules leaves the Calvin cycle to be made into glucose, while the other is used to regenerate RuBP so that the cycle can start over again.

Phase 2: Reduction of PGA
•   ATP generated from the light-dependant reactions is used to raise the energy level of the six PGA molecules (through the addition of a phosphate group from ATP), resulting in a molecule called PGAP.
•   PGAP is converted to G3P in a reduction reaction, using the NADPH that was generated during the light-dependant reactions. (The H from NADPH is affixed onto the PGAP molecule). This reaction occurs six times per G3P leaving the cycle.
•   One G3P leaves the cycle to form a half-molecule of glucose.

Phase 3: Regeneration of RuBP
•   In order to continue the Calvin Cycle, the remaining five G3P molecules, in combination with three phosphates and three ATP, synthesize three energy-rich RuBP molecules.