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Cultivation of Microorganisms : Sources of Metabolic Energy

المؤلف:  Stefan Riedel, Jeffery A. Hobden, Steve Miller, Stephen A. Morse, Timothy A. Mietzner, Barbara Detrick, Thomas G. Mitchell, Judy A. Sakanari, Peter Hotez, Rojelio Mejia

المصدر:  Jawetz, Melnick, & Adelberg’s Medical Microbiology

الجزء والصفحة:  28e , p69-70

2026-05-05

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 The three major mechanisms for generating metabolic energy are fermentation, respiration, and photosynthesis. At least one of these mechanisms must be used if an organism is to grow. Fermentation The formation of ATP in fermentation is not coupled to the transfer of electrons. Fermentation is characterized by substrate phosphorylation, an enzymatic process in which a pyrophosphate bond is donated directly to adenosine diphosphate (ADP) by a phosphorylated metabolic intermediate. The phosphorylated intermediates are formed by metabolic rearrangement of a fermentable substrate such as glucose, lactose, or arginine. Because fermentations are not accompanied by a change in the overall oxidation–reduction state of the fermentable substrate, the elemental composition of the products of fermentation must be identical to those of the substrates. For example, fermentation of a molecule of glucose (C6H12O6 ) by the Embden-Meyerhof pathway yields a net gain of two pyrophosphate bonds in ATP and produces two molecules of lactic acid (C3H6O3 ).

Respiration

 Respiration is analogous to the coupling of an energy-dependent process to the discharge of a battery. Chemical reduction of an oxidant (electron acceptor) through a specific series of electron carriers in the membrane establishes the proton motive force across the bacterial membrane. The reductant (electron donor) may be organic or inorganic (eg, lactic acid serves as a reductant for some organisms, and hydrogen gas is a reductant for other organisms). Gaseous oxygen (O2 ) is the oxidant most commonly used by aerobic bacteria, but alter native oxidants that are used by some organisms include carbon dioxide (CO2 ), sulfate (SO4²⁻), and nitrate (NO³⁻).

Photosynthesis

 Photosynthesis is like respiration in that the reduction of an oxidant via a specific series of electron carriers establishes the proton motive force. The difference in the two processes is that in photosynthesis, the reductant and the oxidant are created photochemically by light energy absorbed by pigments in the membrane; thus, photosynthesis can continue only if there is a source of light energy. Plants and some bacteria can invest a substantial amount of light energy in making water a reductant for carbon dioxide. Oxygen is evolved in this process, and organic matter is produced. Respiration, the energetically favorable oxidation of organic matter by an electron acceptor such as oxygen, can provide photosynthetic organ isms with energy in the absence of light.