Figure 14–54 A comparison of H+ concentrations and the arrangement of ATP synthase in mitochondria and chloroplasts. In both organelles, the ph in the intermembrane space is 7.4, as in the cytoplasm. the ph of the mitochondrial matrix and the ph of the chloroplast stroma are both about 8 (light gray). the ph in the thylakoid space is around 5.5, depending on photosynthetic activity. this results in a high proton-motive force across the thylakoid membrane, consisting largely of the h+ gradient (a high permeability of this membrane to Mg2+ and Cl– ions allows the flow of these ions to dissipate most of the membrane potential).
In contrast to chloroplasts, the h+ gradient across the inner mitochondrial membrane is insufficient for atp production, and mitochondria need a membrane potential to bring the proton-motive force to the same level as in chloroplasts. the arrangement of the mitochondrial atp synthase in rows of dimers along the cristae ridges (see Figure 14–32) next to the respiratory-chain proton pumps may help the flow of protons along the membrane surface toward the atp synthase, as the availability of protons is limiting for atp production. In the chloroplast, the atp synthase is distributed randomly in thylakoid membranes.
ATP-driven proton pump working in reverse to make ATP
Eventually, some bacteria would have developed H+-pumping electron-transport systems that were so efficient that they could harvest more redox energy than they needed to maintain their internal pH. Such cells would probably have generated large electrochemical proton gradients, which they could then use to produce ATP. Protons could leak back into the cell through the ATP-driven H+ pumps, essentially running them in reverse so that they synthesized ATP (stage 3 in Figure 14–55). Because such cells would require much less of the dwindling supply of fermentable nutrients, they would have proliferated at the expense of their neighbors.
By providing an Inexhaustible Source of reducing power, photosynthetic Bacteria Overcame a Major Evolutionary Obstacle
The gradual depletion of nutrients from the environment on the early Earth meant that organisms had to find some alternative source of carbon to make the sugars that serve as the precursors for so many other cell components. Although the CO2 in the atmosphere provides an abundant potential carbon source, to convert it into an organic molecule such as a carbohydrate requires reducing the fixed CO2 with a strong electron donor, such as NADPH, which can generate (CH2O) units from CO2 (see Figure 14–41). Early in cellular evolution, strong reducing agents (electron donors) are thought to have been plentiful. But once an ancestor of ATP synthase began to generate most of the ATP, it would have become imperative for cells to evolve a new way of generating strong reducing agents.
A major evolutionary breakthrough in energy metabolism came with the development of photochemical reaction centers that could use the energy of sunlight to produce molecules such as NADPH. It is thought that this occurred early in the process of cellular evolution in the ancestors of the green sulfur bacteria. Present-day green sulfur bacteria use light energy to transfer hydrogen atoms (as an electron plus a proton) from H2S to NADPH, thereby producing the strong reducing power required for carbon fixation. Because the redox potential of H2S is much lower than that of H2O (–230 mV for H2S compared with +820 mV for H2O), one quantum of light absorbed by the single photosystem in these bacteria is sufficient to generate NADPH via a relatively simple photosynthetic electron-transport chain.
the photosynthetic Electron-transport Chains of Cyanobacteria produced atmospheric Oxygen and permitted New Life-Forms