-ˈtü noun
Etymology: phot- + system
: either of two photochemical reaction centers occurring in chloroplasts that consist chiefly of photosynthetic pigments (as chlorophyll) complexed with protein which absorb light energy and convert it to chemical energy in the form of high-energy electrons
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