The first living cells on Earth may have consumed geochemically produced organic molecules and generated their ATP by fermentation. Because oxygen was not yet present in the atmosphere, such anaerobic fermentation reactions would have dumped organic acids—such as lactic or formic acids, for example—into the
Figure 14–53 Evolution of photosynthetic reaction centers.
pigments involved in light-harvesting are colored green; those involved in the central photochemical events are colored red.
the primitive photochemical reaction center of purple bacteria contains two related protein subunits, L and M, that  bind the pigments involved in the central process of photosynthesis, including a special pair of chlorophyll molecules. Electrons are fed into the excited chlorophylls by a cytochrome.  Lh1 is a bacterial antenna complex.
photosystem II contains the D1 and D2 proteins, which are homologous to the L and M subunits in (a). the excited p680 chlorophyll in the special pair withdraws electrons from water held by the manganese cluster. LhC-II is the light-harvesting complex that feeds energy into the core antenna proteins. (C) photosystem I contains the psa a and psa B proteins, each of which is equivalent to a fusion of the D1 or D2 protein to a core antenna protein of photosystem II. the loosely bound plastocyanin (pC) feeds electrons into the excited chlorophyll pair. as indicated, in photosystem I, electrons are passed from a bound quinone (Q) through a series of three iron–sulfur centers (red circles). (Modified from K. rhee, E. Morris,
J.
Barber and W. Kühlbrandt, Nature 396:283–286, 1998; and W. Kühlbrandt, Nature 411:896–899, 2001. With permission from Macmillan publishers Ltd.) matrix pH 7.9 intermembrane space pH 7.4 thylakoid space pH 5.5 intermembrane space pH 7.4 environment (see Figure 2–47). Perhaps such acids lowered the pH of the environment, favoring the survival of cells that evolved transmembrane proteins that could pump H+ out of the cytosol, thereby preventing the cell from becoming too acidic (stage 1 in Figure 14–55). One of these pumps may have used the energy available from ATP hydrolysis to eject H+ from the cell; such a proton pump could have been the ancestor of present-day ATP synthases.
As the Earth’s supply of geochemically produced nutrients began to dwindle, organisms that could find a way to pump H+ without consuming ATP would have been at an advantage: they could save the small amounts of ATP they derived from the fermentation of increasingly scarce foodstuffs to fuel other important activities. This need to conserve resources might have led to the evolution of electron-transport proteins that allowed cells to use the movement of electrons between molecules of different redox potentials as a source of energy for pumping H+ across the plasma membrane (stage 2 in Figure 14–55). Some of these cells might have used the nonfermentable organic acids that neighboring cells had excreted as waste to provide the electrons needed to feed this electron-transport system. Some present-day bacteria grow on formic acid, for example, using the small amount of redox energy derived from the transfer of electrons from formic acid to fumarate to pump H+.
Figure 14–55 How ATP synthesis by chemiosmosis might have evolved in stages. the first stage could have involved the evolution of an atpase that pumped protons out of the cell using the energy of atp hydrolysis. Stage 2 could have involved the evolution of a different proton pump, driven by an electron-transport chain. Stage 3 would then have linked these two systems together to generate a primitive atp synthase that used the protons pumped by the electron-transport chain to synthesize atp. an early bacterium with this final system would have had a selective advantage over bacteria with neither of the systems or only one.