A form of chlorophyll found in photosynthetic bacteria, notably the purple and green bacteria. There are several types, designated a to g. For example, bacteriochlorophyll a and bacteriochlorophyll b are structurally similar to the chlorophyll a and chlorophyll b found in plants, but absorb light maximally in different parts of the spectrum. The purple bacteria contain either of these two types of bacteriochlorophyll, depending on the species, located in specialized membrane systems (chromatophores) in the form of sheets, tubes, or vesicles arising inside the cell from the plasma membrane. In green bacteria the photosynthetic apparatus is contained in cylindrical structures (chlorosomes) underlying the plasma membrane. Compare bacteriorhodopsin.