Diversity in Electron Donors and Acceptors

The basic types of cellular respiration include chemoor-ganotrophic aerobic respiration, chemoorganotrophic anaerobic respiration, and chemolithotrophic respiration. Chemoorganotrophic respiration is a process whereby organic carbon compounds are oxidized to their inorganic constituents (i.e., CO2 and H2O). For this process, a variety of electron-accepting compounds can serve as oxidants (final electron acceptors). The most well-known oxidant is oxygen (O2), but alternatively the final electron acceptor can be NO3, MnO2, FeOOH, fumarate, SO4~,

HCO 3, etc., resulting in chemoorganotrophic anaerobic respiration. Oxidant specificity is determined by enzymes associated with the ETS, such as cytochrome oxidase, that are required to transfer electrons to the final electron acceptor. In chemolithotrophic respiration pathways, the substrate to be oxidized is an inorganic molecule. These electron-donating substrates (and reactions) can include H2 ! H+, H2S ! SO43, NHi ! NO3, and Fe2+ ! Fe3+, with the released electrons transferred to oxygen or some other acceptor. For the cell to use electrons from inorganic substrates, a specific enzyme or enzyme complex (see Enzymatic Processes) must be available to couple the oxidation reaction (removal of electrons) to the ETS (which receives the electrons).

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