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113lecture0706 - BIOLOGY 113 MICROBIOLOGY Lecture 7...

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BIOLOGY 113 - MICROBIOLOGY Lecture 7: Microbial Metabolism - Oxidation-reduction, mechanisms of ATP generation, glycolysis, Krebs cycle The "trick" in maintaining the energy balance required to carry out cellular metabolism is to control the chemical energy released during reactions Many of the reactions of central metabolism are oxidation-reduction reactions, in which electrons are transferred (Tortora et al., Figure 5.8) - Oxidation is removal of electrons from a molecule = In many metabolic reactions, protons (H + ) are also removed = The net effect is to remove a "hydrogen equivalent" - Reduction is the addition of electrons to a molecule; as in oxidations, protons may also be involved - Oxidations are always coupled to reductions; that is, oxidation of one molecule is always associated with reduction of another - Two molecules commonly used by living systems as carriers of electrons are NAD+ and NADP+ = When a molecule is oxidized during metabolism, a pair of electrons (and one proton) may be transferred to one of these coenzymes to yield NADH or NADPH ( Tortora et al. Fig 5.9) = Similarly, reduction of a molecule during metabolism may be coupled to oxidation of NAD(P)H to NAD(P)+ - During catabolic metabolism, energy is released during oxidation of nutrients ATP, formed from phosphorylation of ADP, serves as a convenient carrier of chemical energy; organisms use three mechanisms of phosphorylation to generate ATP from ADP - In substrate-level phosphorylation, ATP is generated when a high-energy phosphate group is directly transferred from another phosphorylated compound - In oxidative phosphorylation, ATP is generated by a chemiosmotic mechanism (discussed below) in which electrons obtained from a donor (frequently NADH) are passed through a controlled series of oxidation-reduction reactions - an electron transport chain - Photophosphorylation (see Tortora et al., Figure 5.23) is similar to oxidative phosphorylation, except that the electrons are obtained from light-activated pigments rather than from a donor molecule The nature of microbial catabolism can be most readily understood from consideration of metabolic
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