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Why is it that during the light reactions of photosynthesis photosystem 2 comes before photosystem 1? Or why are they named photosystem 1 and 2?
Answer 1:

Photosystems I and II are named such because Photosystem I was actually discovered (and named) before Photosystem II, even though Photosystem II comes before Photosystem I during photosynthesis (i.e., Photosystem II precedes Photosystem I in the electron flow of photophosphorylation). Photosystem I and II evolved independently, and I think that Photosystem II, which uses H2O (water) as the electron donor, actually may have evolved before Photosystem I, namely in cyanobacteria (blue-green algae). If this is true, then PhotosystemII is actually more primitive than Photosystem I. I think Photosystem II is also used in more organisms.


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