<P> Purple bacteria or purple photosynthetic bacteria are proteobacteria that are phototrophic, that is, capable of producing their own food via photosynthesis . They are pigmented with bacteriochlorophyll a or b, together with various carotenoids, which give them colours ranging between purple, red, brown, and orange . They may be divided into two groups--purple sulfur bacteria (Chromatiales, in part) and purple non-sulfur bacteria (Rhodospirillaceae). </P> <P> Photosynthesis takes place at reaction centers on the cell membrane, which is folded into the cell to form sacs, tubes, or sheets, increasing the available surface area . </P> <P> Like most other photosynthetic bacteria, purple bacteria do not produce oxygen (anoxygenic), because the reducing agent (electron donor) involved in photosynthesis is not water . In some, called purple sulfur bacteria, it is either sulfide or elemental sulfur . The others, called purple non-sulfur bacteria (aka PNSB), typically use hydrogen although some may use other compounds in small amounts . At one point these were considered families, but RNA trees show the purple bacteria make up a variety of separate groups, each closer relatives of non-photosynthetic proteobacteria than one another . </P> <P> The reaction centers create a charge separation through a series of favorable redox reactions, after the excitation of the special pigment pair P870 . The reduction of quinones leads to the take up of 2 protons from the cytoplasm . When the quinones are eventually oxidized, they release the protons in the periplasmic side . This builds up a proton motive force that is used by ATP synthase to produce ATP from ADP and phosphate. The ATP is finally used in biosynthesis . </P>

Difference between purple sulfur and purple nonsulfur bacteria
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