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

Published by Patrick Mutisya · 8 days ago

Cambridge A-Level Biology 9700 – Respiration: Oxidative Phosphorylation

Respiration – Oxidative Phosphorylation

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

Key Concepts

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

  • Energetic electrons are transferred through a series of carriers in the electron transport chain (ETC).

  • As electrons move down the ETC, they release energy.

  • The released energy is used to pump protons from the mitochondrial matrix to the inter‑membrane space, creating an electrochemical gradient.

  • Protons flow back into the matrix through ATP synthase by facilitated diffusion.

  • The energy from this proton flow drives the synthesis of ATP from ADP and inorganic phosphate (Pi).

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

Step‑by‑Step Overview

  1. Entry of electrons: NADH and FADH2 donate electrons to the ETC. Each hydrogen atom contributes one proton (H⁺) to the matrix and one electron (e⁻) to the chain.

  2. Electron transport: Electrons pass through a series of carrier proteins (Complex I, II, III, IV). The exact carriers need not be memorised; the important point is that energy is released at each transfer.

  3. Proton pumping: The energy released is used to pump H⁺ from the matrix into the inter‑membrane space, establishing a proton motive force.

  4. Oxygen as the final acceptor: At Complex IV, electrons combine with O₂ and H⁺ to produce H₂O:

    \$\text{O}2 + 4\text{e}^- + 4\text{H}^+ \rightarrow 2\text{H}2\text{O}\$

  5. ATP synthesis: Protons flow back into the matrix through ATP synthase. The flow provides the energy for:

    \$\text{ADP} + \text{P}_i + \text{energy} \rightarrow \text{ATP}\$

Summary Table

StageWhat HappensResult
Hydrogen atom splitH → H⁺ + e⁻Protons remain in matrix; electrons enter ETC
Electron transporte⁻ pass through carriers, releasing energyEnergy used to pump H⁺ across membrane
Proton gradient formationH⁺ accumulated in inter‑membrane spaceElectrochemical gradient (proton motive force)
Oxygen reductionO₂ + 4e⁻ + 4H⁺ → 2H₂OWater formed; electrons removed from chain
ATP synthesisH⁺ flow back via ATP synthaseADP + Pi → ATP

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

Key Points to Remember

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

  • The ETC does not directly make ATP; it creates the proton gradient that powers ATP synthase.

  • Oxygen’s role is essential – without it, the chain backs up and ATP production stops.

  • The overall reaction of oxidative phosphorylation can be expressed as:

    \$\text{NADH} + \text{H}^+ + \frac{1}{2}\text{O}2 \rightarrow \text{NAD}^+ + \text{H}2\text{O} + \text{\overline{2}.5 ATP}\$

    (approximate ATP yield per NADH; similar for FADH₂ with \overline{1}.5 ATP).