Electron transport chain proton-motive force

Aminoglycosides must traverse the plasma membrane and, in the case of gramnegative bacteria, the outer membrane to gain access to the target ribosomes. Transport across the plasma membrane has been shown to require the proton motive force, and mutants deficient in electron transport chain components fail to transport aminoglycosides and are consequently resistant. ... 

The electron-transport chain is slowed because the inner mitochondrial membrane is hyperpolarized. Without ADP to accept the energy of the proton-motive force, the membrane becomes polarized to such an extent that protons can no longer be pumped. The excess H2O2 is probably due to the fact the superoxide radical is present in higher concentration because the oxygen can no longer be effectively reduced. 

Figure 3. Diagram of a section through the cell wall of Acidithiobacillus ferrooxidans modified from Blake et al. (1992) showing the relationship between iron oxidation and pyrite dissolution. OM =outer membrane, P = periplasm, IM = inner or (cytoplasmic) membrane, cty = cytochrome, pmf = proton motive force. Passage of a proton (driven by proton motive force) into the cell catalyzes the conversion of ADP to ATP. Ferrous iron binds to a component of the electron transport chain, probably a cytochrome c, and is oxidized. The electrons are passed to a terminal reductase where they are combined with O2 and to form water, preventing acidification of the cytoplasm. Ferric iron can either oxidize pyrite (e.g. within the ore body) or form nanocrystalline iron oxyhydroxide minerals (often in surrounding groundwater or streams).

Inhibition of the electron-transport chain also inhibits ATP synthesis because the proton-motive force can no longer be generated. 

The light reactions of photosynthesis closely resemble the events tive phosphorylation, In both, the flow of high-energy electrons through an electron-transport chain generates a proton-motive force. This force A4T synthesis through the action of an ATP synthase. In photosynthesis the electrons are also used directly to reduce NADP to NAD PH. 

As will be discussed in Chapter 18, the electron-transport chain oxidizes the NADH and FADH2 formed in the citric acid cycle. The transfer of electrons from these carriers to O2, the ultimate electron acceptor, leads to the generation of a proton gradient across the inner mitochondrial membrane. This proton-motive force then powers the generation of ATP the net... 

The higher the proton motive force, the larger the proton gradient, so the slower the rate of proton efflux from the matrix. Consequently, a high proton gradient slows the flux of electrons along the electron transport chain. 

In eukaryotes, oxidative phosphorylation occurs in mitochondria, while photophosphorylation occurs in chloroplasts to produce ATP. Oxidative phosphorylation involves the reduction of O2 to H2O with electrons donated by NADH and FADH2 in all aerobic organisms. After, carbon fuels (nutrients) are oxidized in the citric acid cycle, electrons with electron-motive force is converted into a proton-motive force. Photophosphorylation involves the oxidation of H2O to O2, with NADP as electron acceptor. Therefore, the oxidation and the phosphorylation of ADP are coupled by a proton gradient across the membrane. In both organelles, mitochondria and chloroplast electron transport chains pump protons across a membrane from a low proton concentration region to one of high concentration. The protons flow back from intermembrane to the matrix in mitochondria, and from thylakoid to stroma in chloroplast through ATP synthase to drive the synthesis of adenosine triphosphate. Therefore, the adenosine triphosphate is produced within the matrix of mitochondria and within the stroma of chloroplast. 

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