We can speculate that early ETSs served as a means of nonfermentative regeneration of the NAD+ required for the operation of glycolysis (Figures 4 and 6). Alternatively, ETSs could have originally served as a means of ATP generation (or, at least, proton pumping) in bacteria obtaining energy from sources other than organic chemicals, including the 'light-eating phototrophs and the inorganic-mineral-consuming lithotrophs. Aerobic respiration also can serve as a means of oxygen scavenging (see The Significance of O2 for Biology), with oxygen originally functioning more as a harmful nuisance rather than an exceptional ETS final electron acceptor. Whatever the reason for their origin, horizontal transfer of ETS genes has been extensive among prokaryotes, implying plenty of opportunity for mixing and matching of utilities.
With evolutionary optimization, ETS inefficiencies would decline, resulting in greater ETS contribution to a bacterium's membrane proton gradient. The ATP-powered proton pump that had been responsible for proton-gradient maintenance consequently could be used to augment ATP production as a proton-gradient-dependent ATP synthase. Running in reverse (Figure 2), this early ATP synthase need not have been highly efficient to still marginally increase total ATP available to the cell. The sophisticated cellular respiration that we observe today thus plausibly evolved from extraneous and inefficient mechanisms which tolerated high degrees of evolutionary exploration. Ultimately, many bacteria have taken the economic lifestyle of cellular respiration to a relative extreme: for obligately aerobic bacterial species (where 'aerobic' is more a description of their requirement of an ETS than of oxygen), not only is cellular respiration available for ATP generation, it is compulsory.
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