Chapter Summary

14.1

  • An electron transport chain (ETS) consists of a series of electron carriers that sequentially transfer electrons to the carrier of next-higher reduction potential E (that is, the next-stronger electron acceptor). Electron flow through the ETS begins with an initial electron donor from outside the cell and ultimately transfers all electrons to a terminal electron acceptor that leaves the cell.
  • The reduction potential E for a complete redox reaction must be positive to yield energy for metabolism. The standard reduction potential E°′ assumes all reactant concentrations equal 1 M, at pH 7.
  • Concentrations of electron donors and acceptors in the environment influence the actual reduction potential E experienced by the cell.
  • The ETS is embedded in a membrane that separates two compartments. Two aqueous compartments must be separate to maintain an ion gradient generated by the ETS.
  • The ETS is composed of protein complexes and cofactors. Protein complexes called oxidoreductases include cytochromes and noncytochrome proteins. Cytochromes are colored proteins whose absorbance spectrum shifts when there is a change in redox state.

14.2

  • The ETS complexes generate a proton motive forcep). The force is usually directed inward, driving protons into the cell.
  • The proton potential drives ATP synthase and other functions, such as ion transport and flagellar rotation
  • The proton potential (Δp, measured in millivolts) is composed of the electrical potential (Δψ) and the hydrogen ion chemical gradient (ΔpH): Δp = Δψ – 60ΔpH.
  • Uncouplers are molecules taken up by cells in both protonated and unprotonated forms. Uncouplers can collapse the entire proton potential, thus uncoupling respiration from ATP synthesis.

14.3

  • Electron carriers containing metal ions and/or conjugated double-bonded ring structures are used for electron transfer. Cytochrome c quinol oxidase has two hemes and three copper ions.
  • A substrate dehydrogenase receives a pair of electrons from a particular reduced substrate such as NADH. NADH dehydrogenase (NADH:quinone oxidoreductase) typically has an FMN carrier and nine iron-sulfur clusters.
  • Quinones receive electrons from the substrate dehydrogenase and become reduced to quinols. Typically, a quinol receives 2H+ from the cytoplasm, and then releases 2H+ across the membrane upon transfer of 2e– to an electron acceptor complex.
  • Protons are pumped by substrate dehydrogenases (oxidoreductases) and terminal oxidases. The bacterial cytochrome bo oxidase pumps 4H+ across the membrane, coupled to transfer of 4e– onto O2 to generate 2 H2O.
  • Protons are consumed by combining with the terminal electron acceptor, such as combining with oxygen to make H2O.
  • The number of protons pumped by a bacterial ETS is determined by environmental conditions, such as the concentrations of substrate and terminal electron acceptor.
  • The proton potential drives ATP synthesis through the membrane-embedded F1Fo ATP synthase. Three protons drive each F1Fo cycle, synthesizing one molecule of ATP.
  • A sodium potential (DNa+) can drive ATP synthesis, replacing or supplementing the proton potential for some bacteria and archaea.

14.4

  • Anaerobic terminal electron acceptors, such as nitrogen and sulfur oxyanions, oxidized metal cations, and oxidized organic substrates, accept electrons from a specific reductase complex of an ETS.
  • Nitrate is successively reduced by bacteria to nitrite, nitric oxide, nitrous oxide, and ultimately nitrogen gas. Alternatively, nitrate and nitrite may be reduced to ammonium ion, a product that alkalinizes the environment.
  • Sulfate is successively reduced by bacteria to sulfite, thiosulfate, elemental sulfur, and hydrogen sulfide. Sulfate reducers are especially prevalent in seawater.
  • Oxidized metal ions such as Fe3+ and Mn4+ are reduced by bacteria in soil and aquatic habitats—a form of anaerobic respiration also known as dissimilatory metal reduction.
  • The geochemistry of natural environments is shaped largely by anaerobic bacteria and archaea.

14.5

  • Anaerobic respiration is the use of metals or oxidized anions as electron acceptors for respiration, in the absence of O2.
  • Lithotrophy (chemolithotrophy) is the acquisition of energy by oxidation of inorganic electron donors.
  • Sulfur oxidation includes oxidation of H2S to sulfur or to sulfuric acid by sulfur-oxidizing bacteria. Sulfuric acid production leads to extreme acidification.
  • Iron oxidation often accompanies sulfur oxidation.
  • Nitrogen oxidation includes successive oxidation of ammonia to hydroxylamine, nitrous acid, and nitric acid.
  • Hydrogenotrophy uses hydrogen gas as an electron donor. Hydrogen (H2) has sufficient reducing potential to donate electrons to nearly all biological electron acceptors, including chlorinated organic molecules (through dehalorespiration).
  • Methanogenesis is the oxidation of H2 by CO2, releasing methane. Methanogenesis is performed only by the methanogen group of archaea
  • Methane oxidizers use O2, nitrate, or sulfate to oxidize the methane produced by methanotrophs.

14.6

  • Bacteriorhodopsin is a light-driven proton pump that supplements heterotrophy in haloarchaea. The homolog, proteorhodopsin, is found in marine proteobacteria.
  • The antenna complex of chlorophylls and other photopigments captures light for transfer to the reaction center in chlorophyll-based photosynthesis.
  • Thylakoids are folded membranes within phototrophic bacteria or chloroplasts. The membranes extend the area for chlorophyll light absorption, and they separate two compartments to form a proton gradient.
  • Photosystem I separates electrons from H2S or an organic electron donor. The electrons are ultimately transferred to NADH or NADPH.
  • Photosystem II separates an electron from bacteriochlorophyll. An electron ultimately returns to bacteriochlorophyll through cyclic photophosphorylation.
  • The oxygenic Z pathway in cyanobacteria and chloroplasts includes homologs of photosystems I and II. Eight photons are absorbed and two electron pairs are removed from 2 H2O, ultimately producing O2.
  • Oxygenic photosynthesis generates 3 ATP + 2 NADPH per 2 H2O photolyzed and O2 produced. The ATP and NADPH are used to fix CO2 into biomass.