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12.3 - Flow of Electrons

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  1. Front
    Redox half-reaction
    Back
    One part of an oxidation-reduction reaction showing either electron loss or electron gain
  2. Front
    Oxidation
    Back
    Loss of electrons from a molecule
  3. Front
    Reduction
    Back
    Gain of electrons by a molecule
  4. Front
    Standard reduction potential (E°’)
    Back
    A measure of a molecule’s tendency to accept electrons under standard biochemical conditions
  5. Front
    Meaning of positive E°’
    Back
    A higher tendency to accept electrons
  6. Front
    Meaning of negative E°’
    Back
    A greater tendency to donate electrons
  7. Front
    NAD⁺ reduction half-reaction
    Back
    NAD⁺ + 2H⁺ + 2e⁻ → NADH + H⁺ with E°’ = −0.315 V
  8. Front
    Oxygen reduction half-reaction
    Back
    ½ O₂ + 2H⁺ + 2e⁻ → H₂O with E°’ = +0.815 V
  9. Front
    Electron donor
    Back
    A molecule that readily gives up electrons
  10. Front
    Electron acceptor
    Back
    A molecule that readily accepts electrons
  11. Front
    Electron flow direction
    Back
    Electrons move from lower or more negative reduction potential to higher or more positive reduction potential
  12. Front
    Electron flow in respiration
    Back
    NADH transfers electrons to O₂
  13. Front
    Reason respiration releases energy
    Back
    Electrons move downhill in reduction potential from NADH to oxygen
  14. Front
    n in redox equations
    Back
    The number of electrons transferred in a reaction
  15. Front
    F in redox equations
    Back
    The Faraday constant representing charge per mole of electrons
  16. Front
    E in redox equations
    Back
    The reduction potential under nonstandard cellular conditions
  17. Front
    Effect of nonstandard conditions
    Back
    Actual concentrations can alter reduction potentials in cells
  18. Front
    Determinant of electron flow
    Back
    Electron flow depends on relative electron affinity of molecules
  19. Front
    Energy release and reduction potential difference
    Back
    Larger differences in reduction potential result in more energy release
  20. Front
    Electron transport chain order principle
    Back
    Carriers are arranged in order of increasing reduction potential
  21. Front
    Reduction potential of NAD⁺/NADH
    Back
    Approximately −0.320 V
  22. Front
    Reduction potential of FMN/FMNH₂
    Back
    Approximately −0.300 V
  23. Front
    Reduction potential of CoQ/CoQH₂
    Back
    Approximately +0.040 V
  24. Front
    Reduction potential of cytochrome b
    Back
    Approximately +0.070 V
  25. Front
    Reduction potential of cytochrome c₁
    Back
    Approximately +0.230 V
  26. Front
    Reduction potential of cytochrome c
    Back
    Approximately +0.254 V
  27. Front
    Reduction potential of cytochrome a
    Back
    Approximately +0.290 V
  28. Front
    Reduction potential of cytochrome a₃
    Back
    Approximately +0.550 V
  29. Front
    Reduction potential of O₂/H₂O
    Back
    Approximately +0.816 V
  30. Front
    Electron ladder
    Back
    A sequence of carriers arranged by increasing reduction potential
  31. Front
    Direction of electron movement in chain
    Back
    From lower to higher reduction potential carriers
  32. Front
    Purpose of ordered carriers
    Back
    Ensures continuous and controlled electron flow
  33. Front
    Stepwise electron transfer
    Back
    Allows gradual energy release and capture instead of a single large release
  34. Front
    ΔG
    Back
    Free-energy change of a reaction
  35. Front
    ΔE
    Back
    Difference in reduction potential between electron donor and acceptor
  36. Front
    Equation relating energy and reduction potential
    Back
    ΔG = −nFΔE
  37. Front
    Standard condition equation
    Back
    ΔG°’ = −nFΔE°’
  38. Front
    Effect of increasing ΔE
    Back
    Larger positive ΔE produces more negative ΔG
  39. Front
    Meaning of negative ΔG
    Back
    A spontaneous reaction that releases energy
  40. Front
    Energy source for ATP synthesis
    Back
    Energy released from electron transfer reactions
  41. Front
    ΔE°’ for NADH oxidation
    Back
    0.815 − (−0.315) = 1.130 V
  42. Front
    ΔG°’ for NADH oxidation
    Back
    Approximately −218 kJ/mol
  43. Front
    Energy required to form ATP
    Back
    Approximately +30.5 kJ/mol
  44. Front
    ATP yield per NADH
    Back
    Approximately 2.5 ATP molecules
  45. Front
    Reason ATP yield is not maximal
    Back
    Energy is lost as heat and used in proton pumping and transport
  46. Front
    Efficiency of oxidative phosphorylation
    Back
    Approximately 35 percent under standard conditions
  47. Front
    Energy comparison
    Back
    NADH oxidation releases far more energy than required to synthesize one ATP
  48. Front
    Transition to respiratory complexes
    Back
    The lecture shifts from thermodynamics to the structure and function of electron transport complexes
  49. Front
    Complex I
    Back
    NADH–CoQ oxidoreductase also called NADH dehydrogenase
  50. Front
    Reaction of Complex I
    Back
    NADH + CoQox → NAD⁺ + CoQred
  51. Front
    Molecular size of Complex I
    Back
    Approximately 1000 kDa with 42 subunits
  52. Front
    Cofactors in Complex I
    Back
    FMN and iron-sulfur (Fe-S) centers
  53. Front
    Electron entry point of Complex I
    Back
    Receives electrons from NADH
  54. Front
    Electron flow in Complex I
    Back
    NADH → FMN → Fe-S centers → CoQ
  55. Front
    Role of FMN in Complex I
    Back
    Accepts electrons from NADH
  56. Front
    Role of Fe-S centers in Complex I
    Back
    Transfer electrons one at a time
  57. Front
    Output of Complex I
    Back
    Reduction of CoQ to CoQH₂
  58. Front
    Proton pumping by Complex I
    Back
    Contributes to formation of proton gradient
  59. Front
    Function of Complex I
    Back
    Transfers electrons from NADH to CoQ and pumps protons
  60. Front
    Complex II
    Back
    Succinate dehydrogenase
  61. Front
    Reaction of Complex II
    Back
    FADH₂ + CoQox → FAD + CoQred
  62. Front
    Role of Complex II
    Back
    Transfers electrons from FADH₂ to CoQ
  63. Front
    Connection of Complex II
    Back
    Part of both Krebs cycle and electron transport chain
  64. Front
    Electron source for Complex II
    Back
    FADH₂ produced in succinate oxidation
  65. Front
    Key difference of Complex II
    Back
    Does not pump protons
  66. Front
    Impact of Complex II on ATP yield
    Back
    Produces less ATP compared to NADH entry
  67. Front
    Function of CoQ
    Back
    Collects electrons from multiple sources including Complex I and II
  68. Front
    Complex III
    Back
    CoQ–cytochrome c reductase
  69. Front
    Reaction of Complex III
    Back
    CoQred + 2 cyt cox → CoQox + 2 cyt cred
  70. Front
    Electron carrier mismatch
    Back
    CoQH₂ carries 2 electrons while cytochrome c carries 1 electron
  71. Front
    Solution to mismatch
    Back
    Complex III splits two electrons into separate transfers
  72. Front
    Semiquinone intermediate
    Back
    A stable intermediate formed during electron transfer in Complex III
  73. Front
    Electron flow in Complex III
    Back
    Transfers electrons from CoQH₂ to cytochrome c
  74. Front
    Proton pumping by Complex III
    Back
    Contributes to proton gradient formation
  75. Front
    Function of Complex III
    Back
    Converts 2-electron transfer into two 1-electron transfers
  76. Front
    Complex IV
    Back
    Cytochrome c oxidase
  77. Front
    Reaction of Complex IV
    Back
    2 cyt cred + ½ O₂ + 2H⁺ → 2 cyt cox + H₂O
  78. Front
    Electron source for Complex IV
    Back
    Cytochrome c
  79. Front
    Final electron acceptor
    Back
    Oxygen (O₂)
  80. Front
    Product of oxygen reduction
    Back
    Water (H₂O)
  81. Front
    Electrons required for full O₂ reduction
    Back
    Four electrons
  82. Front
    Purpose of Complex IV structure
    Back
    Prevents formation of partially reduced toxic oxygen species
  83. Front
    Irreversibility of Complex IV
    Back
    The only irreversible step in the electron transport chain
  84. Front
    Affinity of Complex IV for oxygen
    Back
    Very high, allowing function at low oxygen levels
  85. Front
    Cofactors in Complex IV
    Back
    CuA, CuB, heme a, and heme a₃
  86. Front
    Function of Complex IV
    Back
    Transfers electrons to oxygen and forms water
  87. Front
    Electron transport chain endpoint
    Back
    Oxygen reduction to water
  88. Front
    Proton gradient formation
    Back
    Electron flow drives proton pumping across the inner mitochondrial membrane
  89. Front
    Direction of proton pumping
    Back
    From matrix to intermembrane space
  90. Front
    Matrix conditions
    Back
    Lower proton concentration, more alkaline, more negative
  91. Front
    Intermembrane space conditions
    Back
    Higher proton concentration, more acidic, more positive
  92. Front
    ΔpH component of gradient
    Back
    Difference in proton concentration across membrane
  93. Front
    ΔΨ component of gradient
    Back
    Difference in electrical charge across membrane
  94. Front
    Electrochemical gradient
    Back
    Combined effect of ΔpH and ΔΨ
  95. Front
    Approximate ΔpH
    Back
    About 1 unit with matrix more alkaline
  96. Front
    Approximate ΔΨ
    Back
    About 0.15 V with matrix more negative
  97. Front
    Complexes generating proton gradient
    Back
    Complex I, Complex III, and Complex IV
  98. Front
    Energy storage in gradient
    Back
    Stored as proton motive force
  99. Front
    Role of proton gradient
    Back
    Drives ATP synthesis via ATP synthase
  100. Front
    Indirect ATP production
    Back
    Electron transport chain creates gradient rather than directly producing ATP
  101. Front
    Electron flow sequence from NADH
    Back
    NADH → Complex I → CoQ → Complex III → cytochrome c → Complex IV → O₂
  102. Front
    Electron flow sequence from FADH₂
    Back
    FADH₂ → Complex II → CoQ → Complex III → cytochrome c → Complex IV → O₂
  103. Front
    Complexes that pump protons
    Back
    Complex I, Complex III, and Complex IV
  104. Front
    Complex that does not pump protons
    Back
    Complex II
  105. Front
    Reason NADH yields more ATP
    Back
    Electrons enter at Complex I and contribute to more proton pumping
  106. Front
    Thermodynamic principle of electron flow
    Back
    Electrons move toward more positive reduction potentials
  107. Front
    Relationship between ΔE and ΔG
    Back
    Larger ΔE results in more negative ΔG and greater energy release
  108. Front
    Main principle of oxidative phosphorylation
    Back
    Electron transport generates a proton gradient that powers ATP synthesis

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