Electron Transport Chain and Cellular Respiration Notes (copy)

Electron Transport Chain (ETC) overview

  • Location and purpose

    • In prokaryotes: plasma membrane.

    • In eukaryotes: inner mitochondrial membrane (extensive folding maximizes surface area).

    • Purpose: Transfer high-energy electrons from NADH/FADH2, reduce oxygen, pump protons to generate a gradient, and synthesize ATP.

  • Entry point and electron flow

    • NADH donates electrons to the ETC at the initial entry point (FMN).

    • Electron flow: NADH
      →\rightarrow FMN
      →\rightarrow Q
      →\rightarrow cytochrome b
      →\rightarrow cytochrome c1
      →\rightarrow cytochrome a
      →\rightarrow cytochrome a3 (terminal acceptor path).

    • Energy released with each transfer is used for proton pumping.

  • Proton pumping and gradient formation

    • FMN and other carriers pump protons (H^+}) across the membrane (outside cell in bacteria, intermembrane space in mitochondria).

    • A total of six protons (H^+}) are pumped per NADH entering the chain.

    • This creates a proton gradient: higher H^+} concentration outside, lower inside.

  • The proton-motive force and ATP synthesis

    • Protons re-enter through ATP synthase, a membrane protein complex.

    • The flow of protons through ATP synthase causes it to rotate, driving the synthesis of ATP from ADP and Pi.

    • This yields about three ATP molecules per catalytic cycle in the model shown.

  • The end product of the chain

    • The final electron acceptor is oxygen.

    • Low-energy electrons and protons combine with oxygen to form water:
      O2+4e−+4H+→2H2O\text{O}_2 + 4e^- + 4H^+ \rightarrow 2 \text{H}_2 \text{O}.

  • The big picture of oxidative phosphorylation

    • Energetic electrons from NADH/FADH2 move through a series of redox reactions in membrane proteins.

    • Redox energy pumps protons, creating a gradient.

    • Protons flow back via ATP synthase (chemiosmosis), producing ATP.

  • Overall ATP yield per glucose

    • Substrate-level phosphorylation (glycolysis, Krebs): about 4 ATP.

    • Oxidative phosphorylation: NADH/FADH2 convert to additional ATP.

      • NADH from glycolysis: about 6 ATP.

      • NADH from prep step: about 6 ATP.

      • NADH from Krebs cycle: about 18 ATP.

      • FADH2 from Krebs cycle: about 4 ATP.

    • Total ideal maximum: approximately 38 ATP in prokaryotes, approximately 36 ATP in eukaryotes (due to shuttle costs).

  • Locations by organism type

    • Prokaryotes: Glycolysis, prep step, Krebs cycle in cytoplasm; ETC and chemiosmosis at plasma membrane.

    • Eukaryotes: Glycolysis in cytoplasm; prep step in cytosol; Krebs cycle in mitochondrial matrix; ETC and chemiosmosis in inner mitochondrial membrane.

  • Anaerobic vs. aerobic respiration

    • Aerobic: Final electron acceptor is O2_2; yields most ATP (up to 38_38 in prokaryotes, 36_36 in eukaryotes).

    • Anaerobic: Final electron acceptor is not O2_2; yields less ATP (from 3_3 to 35_35), and the Krebs cycle often does not operate.