The photosystems probably evolved independently, with photosystem I evolving first. This energized electron pair move to a higher energy level and are transferred to an acceptor molecule. P 680 replaces its electrons by extracting them from a water molecule. As electrons move along the chain ATP is formed. P 700 becomes oxidized.
Electrons go from the reaction center to the electron acceptor downhill back to the reaction center.
ATP is generated in this process. Carbon is available as carbon dioxide. A reaction pathway known as the Calvin or Calvin-Benson Cycle and is like the Krebs Cycle because by the end of each turn of the cycle, the starting compound has been regenerated. It is a food and forms in the grana. PGAL is also usable directly as a building material and can form chlorophyll, NADP, or Ribulose 5-P or any of the enzymes taking part in carbon dioxide fixation.