correspond to successively more positive redox potentials, or to successively decreased potential differences relative to the terminal electron acceptor. in other words, they correspond to successively smaller gibbs free energy changes for the overall redox reaction. individual bacteria use multiple electron transport chains, often simultaneously. bacteria can use a number of different electron donors, a number of different dehydrogenases, a number of different oxidases and reductases, and a number of different electron acceptors. for example, e. coli (when growing aerobically using glucose and oxygen as an energy source) uses two different nadh dehydrogenases and two different quinol oxidases, for a total of four different electron transport chains operating simultaneously. a common feature of all electron transport chains is the presence of a proton pump to create an electrochemical gradient over a membrane. bacterial electron transport chains may contain as many as three proton pumps, like mitochondria, or they may contain two or at least one. electron donors in the current biosphere, the most common electron donors are organic molecules. organisms that use organic molecules as an electron source are called organotrophs. chemoorganotrophs (animals, fungi, protists) and photolithotrophs (plants and algae) constitute the vast majority of all familiar life forms. some prokaryotes can use inorganic matter as an electron source. such an organism is called a (chemo)lithotroph ("rock-eater"). inorganic electron donors include hydrogen, carbon monoxide, ammonia, nitrite, sulfur, sulfide, manganese oxide, and ferrous iron. lithotrophs have been found growing in rock formations thousands of meters below the surface of earth. because of their volume of distribution, lithotrophs may actually outnumber organotrophs and phototrophs in our biosphere. the use of inorganic electron donors such as hydrogen as an energy source is of particular interest in the study of evolution. this type of metabolism must logically have preceded the use of organic molecules and oxygen as an energy source. complexes i and ii bacteria can use several different electron donors. when organic matter is the electron source, the donor may be nadh or succinate, in which case electrons enter the electron transport chain via nadh dehydrogenase (similar to complex i in mitochondria) or succinate dehydrogenase (similar to complex ii). other dehydrogenases may be used to process different energy sources: formate dehydrogenase, lactate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, h2 dehydrogenase (hydrogenase), electron transport chain. some dehydrogenases are also proton pumps, while others funnel electrons into the quinone pool. most dehydrogenases show induced expression in the bacterial cell in response to metabolic needs triggered by the environment in which the cells grow. in the case of lactate dehydrogenase in e. coli, the enzyme is used aerobically and in combination with other dehydrogenases. it is inducible and is expressed when the concentration of dl-lactate in the cell is high. quinone carriers quinones are mobile, lipid-soluble carriers that shuttle electrons (and protons) between large, relatively immobile macromolecular complexes embedded in the membrane. bacteria use ubiquinone (coenzyme q, the same quinone that mitochondria use) and related quinones such as menaquinone (vitamin k2). archaea in the genus sulfolobus use caldariellaquinone. the use of different quinones is due to slight changes in redox potentials caused by changes in structure. the change in redox potentials of these quinones may be suited to changes in the electron acceptors or variations of redox potentials in bacterial complexes. proton pumps a proton pump is any process that creates a proton gradient across a membrane. protons can be physically moved across a membrane, as seen in mitochondrial complexes i and iv. the same effect can be produced by moving electrons in the opposite direction. the result is the disappearance of a proton from the cytoplasm and the appearance of a proton in the periplasm. mitochondrial complex iii uses this second type of proton pump, which is mediated by a quinone (the q cycle). some dehydrogenases are proton pumps, while others are not. most oxidases and reductases are proton pumps, but some are not. cytochrome bc1 is a proton pump found in many, but not all, bacteria (not in e. coli). as the name implies, bacterial bc1 is similar to mitochondrial bc1 (complex iii). cytochrome electron carriers cytochromes are proteins that contain iron. they are found in two very different environments. some cytochromes are water-soluble carriers that shuttle electrons to and from large, immobile macromolecular structures imbedded in the membrane. the mobile cytochrome electron carrier in mitochondria is cytochrome c. bacteria use a number of different mobile cytochrome electron carriers. other cytochromes are found within macromolecules such as complex iii and complex iv. they also function as electron carriers, but in a very different, intramolecular, solid-state environment. electrons may enter an electron transport chain at the level of a mobile cytochrome or quinone carrier. for example, electrons from inorganic electron donors (nitrite, ferrous iron, electron transport chain) enter the electron transport chain at the cytochrome level. when electrons enter at a redox level greater than nadh, the electron transport chain must operate in reverse to produce this necessary, higher-energy molecule. terminal oxidases and reductases when bacteria grow in aerobic environments, the terminal electron acceptor (o2) is reduced to water by an enzyme called an oxidase. when bacteria grow in anaerobic environments, the terminal electron acceptor is reduced by an enzyme called a reductase. in mitochondria the terminal membrane complex (complex iv) is cytochrome oxidase. aerobic bacteria use a number of different terminal oxidases. for example, e. coli (a facultative anaerobe) does not have a cytochrome oxidase or a bc1 complex. under aerobic conditions, it uses two different terminal quinol oxidases (both proton pumps) to reduce oxygen to water. bacterial complex iv can be split into classes according to the molecules act as terminal electron acceptors. class i oxidases are cytochrome oxidases and use oxygen as the terminal electron acceptor. class ii oxidases are quinol oxidases and can use a variety of terminal electron acceptors. both of these classes can be subdivided into categories based on what redox-active components they contain. e.g. heme aa3 class 1 terminal oxidases are much more efficient than class 2 terminal oxidases anaerobic bacteria, which do not use oxygen as a terminal electron acceptor, have terminal reductases individualized to their terminal acceptor. for example, e. coli can use fumarate reductase, nitrate reductase, nitrite reductase, dmso reductase, or trimethylamine-n-oxide reductase, depending on the availability of these acceptors in the environment. most terminal oxidases and reductases are inducible. they are synthesized by the organism as needed, in response to specific environmental conditions. electron acceptors just as there are a number of different electron donors (organic matter in organotrophs, inorganic matter in lithotrophs), there are a number of different electron acceptors, both organic and inorganic. if oxygen is available, it is most often used as the terminal electron acceptor in aerobic bacteria and facultative anaerobes. mostly in anaerobic environments different electron acceptors are used, including nitrate, nitrite, ferric iron, sulfate, carbon dioxide, and small organic molecules such as fumarate. photosynthetic in oxidative phosphorylation, electrons are transferred from an electron donor such as nadh to an acceptor such as o2 through an electron transport chain, releasing energy. in photophosphorylation, the energy of sunlight is used to create a high-energy electron donor which can subsequently reduce oxidized components and couple to atp synthesis via proton translocation by the electron transport chain. photosynthetic electron transport chains, like the mitochondrial chain, can be considered as a special case of the bacterial systems. they use mobile, lipid-soluble quinone carriers (phylloquinone and plastoquinone) and mobile, water-soluble carriers (cytochromes). they also contain a proton pump. the proton pump in all photosynthetic chains resembles mitochondrial complex iii. the commonly-held theory of symbiogenesis proposes that both organelles descended from bacteria.