explain that during oxidative phosphorylation: hydrogen atoms split into protons and energetic electrons, energetic electrons release energy as they pass through the electron transport chain (details of carriers are not expected), the released energy

12.2 Respiration – Oxidative Phosphorylation

Oxidative phosphorylation is the final, ATP‑producing stage of aerobic respiration. It occurs on the inner mitochondrial membrane and couples the flow of electrons from the reduced co‑enzymes NADH and FADH2 to the synthesis of ATP by chemiosmosis.

1. What You Must Know for the Cambridge 9700 Exam

  • Hydrogen atoms from NADH and FADH2 are split into protons (H⁺) and energetic electrons (e⁻).

  • Electrons travel through a series of protein complexes (I → II → III → IV) in the inner mitochondrial membrane. The exact names of the carriers are not required for the exam.

  • The energy released when electrons move down the chain is used to pump H⁺ from the matrix into the inter‑membrane space, creating a proton‑motive force (PMF).

  • The PMF drives chemiosmosis: H⁺ flow back through ATP synthase (Complex V) synthesises ATP from ADP + Pi.

  • Oxygen is the final electron acceptor; it combines with electrons and H⁺ to form water.

  • Oxidation of NADH and FADH2 regenerates NAD⁺ and FAD, allowing glycolysis, the link reaction and the Krebs cycle to continue.

  • Approximate ATP yield: ≈2.5 ATP per NADH and ≈1.5 ATP per FADH2.

2. Extension Material (For Enrichment)

2.1 Step‑by‑Step Overview

  1. Entry of electrons – NADH donates two electrons to Complex I (NADH dehydrogenase); FADH2 donates electrons to Complex II (succinate dehydrogenase). Each hydrogen atom contributes one H⁺ to the matrix and one e⁻ to the chain.

  2. Electron transport & energy release – Electrons move:

    Complex I → Coenzyme Q → Complex III → Cytochrome c → Complex IV.

    Energy is released at each transfer.

  3. Proton pumping (creation of the PMF) – The released energy powers active transport of H⁺ from the matrix into the inter‑membrane space at Complexes I, III and IV. The inner membrane is otherwise impermeable to protons.

  4. Oxygen reduction (final electron acceptor) – At Complex IV, electrons, four matrix H⁺ and O₂ combine:

    O₂ + 4e⁻ + 4H⁺ → 2H₂O.

  5. Chemiosmosis & ATP synthesis – The proton‑motive force drives H⁺ back into the matrix through ATP synthase (Complex V). The flow provides the energy for:

    ADP + Pᵢ + H⁺ (energy) → ATP + H₂O.

  6. Regeneration of co‑enzymes – NAD⁺ and FAD are regenerated in the matrix, ready for further cycles of glycolysis, the link reaction and the Krebs cycle.

2.2 Link to Earlier Stages of Respiration

The NADH and FADH2 that feed the electron‑transport chain are produced in:

  • Glycolysis (2 NADH per glucose)

  • Link reaction (1 NADH per pyruvate → 2 NADH per glucose)

  • Krebs cycle (3 NADH + 1 FADH2 per turn → 6 NADH + 2 FADH2 per glucose)

Oxidising these carriers restores NAD⁺ and FAD, allowing those pathways to continue unabated.

2.3 Practical / Experimental Relevance

  • Inhibitors such as rotenone (blocks Complex I) or cyanide (blocks Complex IV) demonstrate the dependence of ATP synthesis on a functional electron‑transport chain.

  • Measuring oxygen consumption with a Clark electrode or a respirometer provides a direct assay of oxidative‑phosphorylation activity.

  • Isolation of mitochondria and the use of uncouplers (e.g., dinitrophenol) illustrate the distinction between electron flow (which continues) and ATP synthesis (which stops when the PMF is collapsed).

3. Summary Tables

3.1 Key Stages

StageWhat HappensResult / Significance
Hydrogen‑atom splitH → H⁺ + e⁻H⁺ stays in matrix; e⁻ enter ETC
Electron transport (Complex I–IV)e⁻ passed through carrier complexes, releasing energyEnergy used to pump H⁺ across the inner membrane
Proton‑motive force (chemiosmosis)H⁺ accumulated in inter‑membrane spaceElectrochemical gradient stores potential energy
Oxygen reductionO₂ + 4e⁻ + 4H⁺ → 2H₂OWater formed; electrons removed from chain
ATP synthesis (ATP synthase)H⁺ flow back through ATP synthaseADP + Pᵢ → ~2.5 ATP (per NADH) or ~1.5 ATP (per FADH₂)

3.2 Quantitative ATP Yield

Reduced co‑enzymeApprox. ATP producedNotes (entry point)
NADH (glycolysis, link reaction, Krebs cycle)≈2.5 ATP per NADHEnters at Complex I
FADH₂ (Krebs cycle)≈1.5 ATP per FADH₂Enters at Complex II (bypasses Complex I)

4. Overall Reaction for Oxidative Phosphorylation

NADH + H⁺ + ½ O₂ → NAD⁺ + H₂O + ~2.5 ATP

FADH₂ + ½ O₂ → FAD + H₂O + ~1.5 ATP

5. Key Points to Remember

  • Hydrogen atoms supply both the protons that build the gradient and the electrons that travel through the ETC.

  • The ETC itself does not synthesise ATP; it creates the proton‑motive force that powers ATP synthase.

  • Oxygen is essential as the final electron acceptor; without it the chain stalls and ATP production stops.

  • The inner mitochondrial membrane’s impermeability to protons forces them to re‑enter the matrix only via ATP synthase (chemiosmosis).

  • Regeneration of NAD⁺ and FAD links oxidative phosphorylation back to glycolysis, the link reaction and the Krebs cycle.

Suggested diagram: Cross‑section of a mitochondrion showing the inner membrane, the four ETC complexes, the proton gradient, ATP synthase, and the role of O₂ as the final electron acceptor.