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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