Photosynthetic pigments are composed
of a hydrocarbon tail bound to an active site that contains Mg2+.
The several different pigment types absorb light at different wavelengths.
Chlorophyll a (plants and algae) absorb light wavelengths between 425 and
660 nM (violet and red), while bacteriochlorophyll in purple sulfur bacteria
absorb wavelengths between 350 and 880 nm (ultraviolet and infrared).
Pigments are embedded in layers of membranes called thylakoids
(layers of the cell membrane in bacteria, foldings of the inner membrane
of eukaryotic chloroplasts). The fluid interior of the chloroplast
is called the stroma and contains enzymes
for the synthesis reactions of photosynthesis. The light reactions
responsible for ATP generation, called photosystems
I and II, occur in the thylakoids.