electron transport chain

• Energy


  The capacity to do work
• Design and Work


  • For energy to do useful work, a suitable design is required
• Chemical Work in Cells


  Cells perform chemical work to drive non-spontaneous reactions (positive ΔG)
• Metabolism


  Involves harnessing energy from nutrients and storing it in molecules like NADH and ATP
• Photosynthesis


  Converts light energy into chemical energy in leaves
• Cellular Respiration


  Uses the energy stored in nutrients to perform chemical work
• Suitable Design for Energy Harnessing


  • A car engine harnesses work from chaotic explosions
• Mitochondria Design


  • Inner membrane ridges (cristae) provide a design to accumulate and use a proton gradient to drive ATP synthesis
• Redox Potential


  The tendency of a molecule to acquire electrons and be reduced
• Positive Redox Potential


  Greater affinity for electrons (e.g., O₂, +0.82V)
• Negative Redox Potential


  Greater propensity to donate electrons (e.g., NADH, -0.34V)
• Electron Flow


  Electrons flow from molecules with a negative redox potential to those with a positive redox potential, releasing energy
• Complex I (NADH Dehydrogenase)


  • Transfers electrons from NADH to ubiquinone (Q), pumping protons (H⁺) across the membrane
  • Consists of 45 proteins
  • NADH + H⁺ + Q → NAD⁺ + QH₂
• Complex II (Succinate Dehydrogenase)

  • Transfers electrons from succinate to ubiquinone without proton pumping
  • Entry point for electrons from FADH₂
  • Succinate + Q → Fumarate + QH₂
• Complex III (Cytochrome b-c1 Complex)

  • Transfers electrons from ubiquinol to cytochrome c, pumping protons
  • Consists of 22 proteins
  • QH₂ + 2 Cyt c (Fe³⁺) → Q + 2 Cyt c (Fe²⁺) + 2 H⁺
• Complex IV (Cytochrome Oxidase)


  • Transfers electrons from cytochrome c to oxygen, forming water and pumping protons
  • Consists of 26 proteins
  • 4 Cyt c (Fe²⁺) + 4 H⁺ + O₂ → 4 Cyt c (Fe³⁺) + 2 H₂O
• Ubiquinone (Coenzyme Q)


  • Mobile electron carrier transporting electrons between complexes I, II, and III
  • Exists in oxidized (Q) and reduced (QH₂) forms
  • Q + 2 H⁺ + 2 e⁻ → QH₂
• Mechanism of Proton Pumping

  1. Each complex exists in three conformations with different affinities for H⁺

  2. High Affinity (A, B): Binds protons

  3. Low Affinity (C): Releases protons

  4. Electron flow induces conformational changes, facilitating proton pumping and creating a proton gradient
• Energy Conversion in the ETC

  • Electrons Flow: Converts potential energy (voltage) into kinetic energy
  • Proton Gradient: Stores energy as a high concentration of protons in the intermembrane space
  • ATP Synthesis: Proton gradient drives ATP synthesis through ATP synthase
• Analogies and Examples


  • Electric Circuit: ETC functions like an electric circuit with electrons flowing from negative to positive potentials
  • Car Engine: Suitable design (mitochondrial cristae) harnesses energy efficiently
  • Cable Bacteria: Electron flow from hydrogen sulfide to oxygen demonstrates redox potential in nature
• Proton-Motive Force (PMF)


  The force exerted by the proton gradient across the inner mitochondrial membrane
• ATP Synthase


  • Enzyme that uses the PMF to synthesize ATP from ADP and inorganic phosphate (Pi)
  • Protons flow through ATP synthase, causing it to rotate and catalyze ATP formation
• ETC Efficiency and Regulation

  • The design of the ETC allows for efficient energy conversion and minimal loss of energy as heat
  • ETC activity is regulated by the availability of substrates (NADH, FADH₂, O₂) and ADP
• Redox Potential determines the direction of electron flow
• Electron Transport Chain works like an electric circuit, converting energy to create a proton gradient
• Proton Gradient is essential for ATP synthesis
• Design in Mitochondria: Cristae increase surface area for efficient energy conversion and storage