Benjamin B. answered • 08/13/20
Teaching Specialist at the University of Texas at Austin
Non-cyclic photophosphorylation will follow the linear pathway in your textbook.
Light hits Photosystem II, exciting electrons and oxidizing H2O into O2 and protons. These electrons then move through membrane protein complexes down their reduction potential gradient. This really just means that each protein complex "wants" electrons more than the one before it.
As this happens, protons (in addition to those resulting from the oxidation of H2O at the beginning) are pumped from the stroma into the thylakoid lumen through the cytochrome complex, creating a pH gradient. Protons then diffuse through ATP synthase to phosphorylate ADP, creating ATP.
Down the line from Photosystem II we find Photosystem I. This can be a little confusing, with 2 coming before 1. They were just named in the order they were discovered.
After electrons hit Photosystem I, they are excited into a high energy state when the complex absorbs energy from light. Normally, the electrons continue on to NADP+ Reductase and NADPH is produced. However, when the ratio of NADPH to NADP+ is too high, NADP+ Reductase can no longer transfer electrons to NADP+. In this case, electrons might be used to form free radicals, atoms with unpaired electrons. Free radicals are very reactive and can cause damage to the chloroplast.
SO.
When the ratio of NADPH to NADP+ gets too high, we enter cyclic electron flow. Electrons move back from Photosystem I through ferredoxin and plastoquinone, then back into the cytochrome complex. The cytochrome complex pumps protons into the thylakoid lumen, contributing to the pH gradient and allowing ATP synthase to produce even more ATP.
