Electron transport chain energy capture

The microbes use two general strategies to synthesize ATP respiration and fermentation. A respiring microbe captures the energy released when electrons are transferred from a reduced species in the environment to an oxidized species (Fig. 18.1). The reduced species, the electron donor, sorbs to a complex of redox enzymes, or a series of such complexes, located in the cell membrane. The complex strips from the donor one or more electrons, which cascade through a series of enzymes and coenzymes that make up the electron transport chain to a terminal enzyme complex, also within the cell membrane. 

Although the value of AG should not be memorired, it does indicate the large amoimt of energy released by both reactions. The electron transport chain is a device to capture this energy in a form useful for doing work. 

Many enzymes in the mitochondria, including those of the citric acid cycle and pyruvate dehydrogenase, produce NADH, aU of which can be oxidized in the electron transport chain and in the process, capture energy for ATP synthesis by oxidative phosphorylation. If NADH is produced in the cytoplasm, either the malate shuttle or the a-glycerol phosphate shuttle can transfer the electrons into the mitochondria for delivery to the ETC. Once NADH has been oxidized, the NAD can again be used by enzymes that require it. 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

Corrrect answer = E. When phosphorylation is partially uncoupled from electron flow, one would expect a decrease in the proton gradient across the inner mitochondrial membrane and, hence, impaired ATP synthesis. In an attempt to compensate for this delect in energy capture, metabolism and electron flow to oxygen is increased. This hypermetabolism will be accompanied by elevated body temperature because the energy in fuels is largely wasted, appearing as heat. The electron transport chain will still be inhibited by cyanide. 

Nonetheless, photosynthesis did not evolve immediately at the origin of life. The failure to discover photosynthesis in the domain of Archaea implies that photosynthesis evolved exclusively in the domain of Bacteria. Eukaryotes appropriated through endosymbiosis the basic photosynthetic units that were the products of bacterial evolution. All domains of life do have electron-transport chains in common, however. As we have seen, components such as the ubiquinone-cytochrome c oxidoreductase and cytochrome hf family are present in both respiratory and photosynthetic electron-transport chains. These components were the foundations on which light-energy-capturing systems evolved. 

In Chapter 10 the basic principles of oxidative phosphorylation, the complex mechanism by which modem aerobic cells manufacture ATP, are described. The discussion begins with a review of the electron transport system in which electrons are donated by reduced coenzymes to the electron transport chain (ETC). The ETC is a series of electron carriers in the inner membrane of the mitochondria of eukaryotes and the plasma membrane of aerobic prokaryotes. This is followed by a description of chemiosmosis, the means by which the energy extracted from electron flow is captured and used to synthesize ATP. Chapter 10 ends with a discussion of the formation of toxic oxygen products and the strategies that cells use to protect themselves. 

Electron transport chain (1) A series of compounds that pass electrons to oxygen (the final electron acceptor). (2) A sequence of electron carriers of progressively higher reduction potential in a cell that is linked so that electrons can pass from one carrier to the next. The chain captures some of the energy released by the flow of electrons and uses it to drive the synthesis of ATP. 

Oxidative phosphorylation (1) Process in which the energy of electrons is captured in high-energy bonds as phosphate groups combine with ADP to form ATP. (2) The phosphorylation of ADP to ATP that occurs in conjunction with the transit of electrons down the electron transport chain in the inner mitochondrial membrane. 

Photophosphorylation Phosphorylation of ADP to ATP that depends directly on energy from sunlight. The light energy is captured by a pigment such as chlorophyll and is passed in the form of excited electrons to an electron transport chain the electron transport chain uses energy from... 

Photosystem A structural unit in a cellular membrane that captures light energy and converts a portion of it to chemical energy. The photosynthesis practiced by plants, algae and cyanobacteria involves two types of photosystem, both of which capture energy in the form of high-energy electrons and transduce it via an electron transport chain. 

The chemical energy liberated during oxidation of the Krebs cycle substrates is captured in the electron transport chain and delivered in small packages to yield the energy-rich phosphate bond of ATP. 

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