pho·to·sys·tem noun \ˈfō-tō-ˌsis-təm\
: either of two photochemical reaction centers consisting chiefly of photosynthetic pigments complexed with protein and occurring in chloroplasts:
a : one that absorbs light with a wavelength of about 700 nanometers —called also photosystem I
b : one that absorbs light with a wavelength of about 680 nanometers —called also photosystem II
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