Light reactions sequence During the light reactions, chemical reactions are occurring simultaneously at photosystem I and photosystem II. On the left, put the reactions starting at photosystem II in order from first to last. Then, on the right, put the reactions starting at photosystem I in order from first to last. Photosystem 11 Photosystem Light energy excites and ejects electrons from photosystem Electrons reduce NADP- to NADPH Hydrogen ions move back to the stroma, producing ATP Light energy excites and ojects elections from photosystem Hydrogen ions move into the thylakoid space MacBook Air