The electron transport chains of bacteria (prokaryotes) operate in plasma membrane (mitochondria are absent in prokaryotes). Some bacterial electron transport chains resemble the mitochondrial electron transport chain. Paracoccus denitrificans is a gram-negative, facultative anaerobic soil bacterium.
It is a model prokaryote for studies of respiration. When this bacterium grows aerobically, its electron transport chain possesses four complexes that correspond to the mitochondrial chain.
But, when this bacterium grows anaerobically with nitrate as its electron acceptor, the chain is structured quite differently. Since most bacteria grow anaerobically using different variety of electron acceptor substances, the bacterial electron transport chains are frequently very different.
Bacterial electron transport chains vary in their electron carriers (e.g., in their cytochromes) and are usually extensively branched. Electrons often enter at several points and leave through several terminal oxidases. Bacterial electron transport chains are usually shorter and possess lower phosphorus to oxygen (P/O) ratios than mitochondrial transport chain.
Thus bacterial (prokaryotic) and mitochondrial (eukaryotic) electron transport chains differ in details of construction although they operate employing the same fundamental principles. For convenience, a simplified view of the electron transport chain of Escherichia coli is being given here as to show these differences (Fig. 24.7).
Although the electron transport chain of E. coli transports electrons from NADH (NADH is the electron donor) to acceptors and moves protons (H+) across the plasma membrane similar to mitochondrial electron transport chain, it is quite different from the latter in its construction. E. coli transport chain is short, consists of two branches (cytochrome d branch and cytochrome o branch), and a quite different array of cytochromes (e.g., Cyt b558, Cytb562, Cyt d, Cyt o).
Coenzyme Q (ubiquinone) carries electrons and donates them to both branches, but the branches operate under different growth conditions. The cytochrome d branch shows very high affinity for oxygen and operates at low oxygen levels (low aeration) usually when the bacterium is in stationary phase of growth.
This branch is not as efficient as the cytochrome o branch because it does not actively pump protons to periplasmic space.
The cytochrome o branch shows moderately high efficiency for oxygen and operates at high oxygen concentrations (high aeration). This branch operates normally when the bacterium is in log phase of its growth (i.e., growing rapidly), and actively pumps protons (H+) in the periplasmic space.