Topic 16: Respiration

• Electron acceptors: molecules that combine with electrons liberated during metabolic processes
  • can be reduced when they gain electrons
  • electrons are formed from the ionization of a hydrogen atom
• When an electron acceptor picks up an electron, it becomes negatively charged and combines with the positively charged hydrogen ion
  • reduction: a gain of electrons or hydrogen atoms
• In fermentative metabolism, organic molecules act as electron acceptors
• In anaerobic respiration (or oxidative metabolism), inorganic molecules act as electron acceptors
  • some bacteria reduce nitrates to nitrites
  • some reduce further to nitrous oxide or to nitrogen gas
  • nitrate broth is used to determine a bacterium’s ability to reduce nitrates
  • nitrates are detected by the addition of dimethyl-alpha-napthlamine and sulfanilic acid to nitrate broth
  • red color: + results indicating nitrite is present
  • negative test is further checked by addition of zinc
    • red color: nitrate is reduced to nitrite by zinc (not bacterium)
    • if either of those not present, the nitrogen has been reduced to nitrous oxide or nitrogen gas
• In aerobic respiration, O2 is the final electron acceptor
  • In aerobic bacteria, cytochromes carry electrons to O2
  • There are four general classes of bacterial cytochromes
  • Oxidase test is used to determine the presence of cytochrome c
  • Hydrogen atoms combine with oxygen to form hydrogen peroxide (H2O2) which is lethal to the cell
  • enzyme catalase breaks down hydrogen peroxide to water and oxygen - catalase test

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