Bio Review Notes #21
ELECTRON TRANSPORT SYSTEM
Performance Objectives:
Throughout the Krebs cycle and glycolysis, hydrogen atoms and their electrons are passed from one energy level to the next by a series of electron acceptors, including NAD and cyrochromes. Atmospheric oxygen, the final electron acceptor, combines with hydrogen to make H2O.

Cytochromes: electron acceptors made of protein + porphyrin rings

Electron transport system (cytochrome system):   When glucose breaks down, most of the ATP comes from the electron transport system, but only if O2 is present.
Hydrogen atoms and their electrons are removed by hydrogen acceptors like Nicotinamide Adenine Dinucleotide (NAD), NAD Phosphate (NADP),
and Flavine Adenine Dinucleotide (FAD).
Electrons then pass down the energy levels in the cytochrome chain:
  • At site I:
    • NAD or NADP
    • Flavin MonoNucleotide (FMN)
    • Iron-sulfur proteins (Ferredoxins)
    • Coenzyme Q (ubiquinone)
  • At site II:
    • Cytochrome b
    • Cytochrome c
  • At site III:
    • Cytochrome a
    • Cytochrome a3
    • Oxygen atoms from atmospheric O2
  • In each reaction in this series, one compound is oxidized (loses an electron) and another is reduced (gaining the electron).
    Example: Cytochrome b (reduced form) ——> Cytochrome b (oxidized form); electron is carried to Cytochrome c, which becomes reduced.
  • The reactions are coupled in 3 distinct groups, distributed at different sites on the inner mitochondrial membrane.   At each site, one proton (H+) at a time is forced across the inner mitochondrial membrane.

Chemiosmosis:   The reactions at each site pump protons (H+) into the "outer compartment" between the mitochondrial membranes, forming a proton gradient.   Protons leak back across the inner membrane through ATP synthetase molecules, which use the energy of the gradient to power ATP synthesis. Peter Mitchell first suggested this hypothesis, now known as the chemiosmotic theory.

Oxidative phosphorylation:   The process for making ATP from ADP. Its rate is usually controlled (limited) by the amount of ADP coming from ATP breakdown, thus by the rate of energy use.

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