Bacteria aerobic prokaryotes from

This hypothesis presumes that early free-living prokaryotes had the enzymatic machinery for oxidative phosphorylation and predicts that their modern prokaryotic descendants must have respiratory chains closely similar to those of modern eukaryotes. They do. Aerobic bacteria carry out NAD-linked electron transfer from substrates to 02, coupled to the phosphorylation of cytosolic ADP. The dehydrogenases are located in the bacterial cytosol and the respiratory chain in the plasma membrane. The electron carriers are similar to some mitochondrial electron carriers (Fig. 19-33). They translocate protons outward across the plasma membrane as electrons are transferred to 02. Bacteria such as Escherichia coli have F0Fi complexes in their plasma membranes the F portion protrudes into the cytosol and catalyzes ATP synthesis from ADP and P, as protons flow back into the cell through the proton channel of F0. 

The striking differences between eukaryotic and prokaryotic cells have led to many speculations about the evolutionary relationship of these two great classes of living organisms. A popular theory is that mitochondria, which are characteristic of most eukaryotes, arose from aerobic bacteria. After cyanobacteria had developed and oxygen... 

All symbiotic hypotheses assumed that the host possessed this primitive and inefficient mechanism of energy generation until the capacity of aerobic respiration was implanted in it by the acquisition of aerobic protomitochondrial symbionts. Those who had opposed this scenario argued that the modern eukaryote is not an anaerobic cell, containing mitochondria, but that an aerobic pathway of the cytoplasm was a primitive feature of the bacteria that evolved into a prokaryote (Raff and Mahler 1972, 1973 Uzzell and Spolsky 1973). They proposed instead that the mitochondria may have evolved from the enclosure of a bacterial plasmid within a metabolically specialized sac. 

Thus both iron—sulfur proteins and SOD will continue to provide us with two metal proteins which span the anaerobic/aerobic and the prokaryote/eukaryote transitions in the development of plants and animals from bacteria and algae. 

The endosymbiotic theory of the origin of eukaryotes states that eukaryotes arose from symbiotic groups of prokaryotic cells. According to this theory, smaller prokaryotes lived within larger prokaryotic cells, eventually evolving into chloroplasts and mitochondria. Chloroplasts are the descendant of photosynthetic prokaryotes and mitochondria are likely to be the descendants of bacteria that were aerobic heterotrophs. Serial endosymbiosis is a sequence of endosymbiotic events. Serial endosymbiosis may also play a role in the progression of life forms to become eukaryotes. 

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