Adenosine triphosphate electron transport chain

The energy released in catabolic pathways is used in the electron-transport chain to make molecules of adenosine triphosphate, ATP. ATP, the final result of food catabolism, couples to and drives many otherwise unfavorable reactions. 

Recent work has shown that bacteria, in common with chloroplasts and mitochondria, are able, through the membrane-bound electron transport chain aerobically, or the membrane-bound adenosine triphosphate (ATP) anerobically, to maintain a gradient of electrical potential and pH such that the interior of the bacterial cell is negahve and alkaline. This potential gradient and the electrical equivalent of the pH difference (1 pH unit = 58 mV at 37°C) give a potential difference across the membrane of 100-180 mV, with the inside negative. The membrane is impermeable to protons, whose extmsion creates the potential described. 

The primary process of photosynthesis (in both photosystems) is an electron transfer reaction from the electronically excited chlorophyll molecule to an electron acceptor, which is in most cases a quinone. This primary electron acceptor can then hand over its extra electron to other, lower energy, acceptors in electron transport chains which can be used to build up other molecules needed by the organism (in particular adenosine triphosphate ATP). The complete process of photosynthesis is therefore much... 

Fats, carbohydrates, and proteins are metabolized in the body to yield acetyl CoA, which is further degraded in the citric acid cycle to yield two molecules of CO2 plus a large amount of energy. The energy output of the various steps in the citric acid cycle is coupled to the electron-transport chain, a series of enzyme-catalyzed reactions whose ultimate purpose is to synthesize adenosine triphosphate (ATP). 

The biochemical pathway of both assimilatory and dissimilatory sulfate reduction is illustrated in Figure 1. The details of the dissimilatory reduction pathway are useful for understanding the origin of bacterial stable isotopic fractionations. The overall pathways require the transfer of eight electrons, and proceed through a number of intermediate steps. The reduction of sulfate requires activation by ATP (adenosine triphosphate) to form adenosine phosphosulfate (APS). The enzyme ATP sulfurylase catalyzes this reaction. In dissimilatory reduction, the sulfate moiety of APS is reduced to sulfite (SO3 ) by the enzyme APS reductase, whereas in assimilatory reduction APS is further phosphorylated to phospho-adenosine phosphosulfate (PAPS) before reduction to the oxidation state of sulfite and sulfide. Although the reduction reactions occur in the cell s cytoplasm (i.e., the sulfate enters the cell), the electron transport chain for dissimilatory sulfate reduction occurs in proteins that are peiiplasmic (within the bacterial cell wall). The enzyme hydrogenase... 

Chlorophenols block adenosine triphosphate (ATP) production, without blocking the electron transport chain. They inhibit oxidative phosphorylation, which increases basal metabolic rate and increases body temperature. As body temperature rises, heat-dissipating mechanisms are overcome and metabolism is accelerated. Adenosine diphosphate (ADP) and other substrates accumulate, and stimulate the electron transport chain further. This process demands more oxygen in a futile effort to produce ATP. Oxygen demand quickly surpasses oxygen supply and energy reserves of the body become depleted. 

Energy from fuel oxidation is converted to the high-energy phosphate bonds of adenosine triphosphate (ATP) by the process of oxidative phosphorylation. Most of the energy from oxidation of fuels in the TCA cycle and other pathways is conserved in the form of the reduced electron-accepting coenzymes, NADH and FAD(2H). The electron transport chain oxidizes NADH and FAD(2H), and donates the electrons to O2, which is reduced to H2O (Fig. 21.1). Energy from reduction 0/O2 is used for phosphorylation of adenosine diphosphate (ADP) to ATP by ATP synthase (FgFjATPase). The net yield of oxidative phosphorylation is approximately 2.5 moles of ATP per mole of NADH oxidized, or 1.5 moles of ATP per mole of FAD(2H) oxidized. 

Mitochondria are membranous organelles (Fig. 1-9) of great importance in the energy metabolism of the cell they are the source of most of the adenosine triphosphate (ATP) (Chap. 10) and the site of many metabolic reactions. Specifically, they contain the enzymes of the citric acid cycle (Chap. 11) and the electron transport chain (Chap. 11), which includes the main Oj-utilizing reaction of the cell. A manunalian liver cell contains about 1000 of these organelles about 20% of the cytoplasmic volume is mitochondrial. 

Adenosine triphosphate (ATP) as a universal free energy transmitter undergoes the following turnover reaction ADP + Pi = ATP + H2O. This reaction represents a simplified synthesis of ATP and hydrolysis of ATP, which releases energy utilized in the transport processes. Enzyme-catalyzed reactions, including the electron transport chain and proton translocation, are composed of series of elementary reactions that proceed forward... 

The inner membrane houses the electron transport chain and adenosine triphosphate synthesis. The inner membrane has numerous folds called cristae, which have a folded structure that greatly increases the surface area where ATP synthesis occurs (Figure 11.1(b)). 

The need for energy by the cell regulates the tricarboxylic acid cycle, which acts in concert with the electron transfer chain and the ATPase to produce adenosine triphosphate in the inner mitochondrial membrane. The cell has limited amounts of ATP, adenosine diphosphate (ADP), and adenosine monophosphate (AMP). When ADP levels are higher than ATP, the cell needs energy, and hence NADH is oxidized rapidly and the tricarboxylic acid cycle is accelerated. When the ATP level is higher than ADP, the cell has the energy needed hence, the electron transport chain slows down. 

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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