Published by Patrick Mutisya · 14 days ago
Oxidative phosphorylation is the final stage of cellular respiration and occurs in the inner mitochondrial membrane. It couples the flow of electrons from reduced co‑enzymes to the synthesis of ATP.
Splitting of hydrogen atoms:
Reduced co‑enzymes (NADH, FADH2) donate hydrogen atoms. Each hydrogen atom is separated into a proton (H⁺) and an energetic electron (e⁻).
Electron transport chain (ETC):
The energetic electrons are passed through a series of membrane‑embedded carriers. As the electrons move “downhill”, they release energy. (The specific carriers need not be memorised.)
Proton pumping:
The energy released by the electrons is used to pump protons from the mitochondrial matrix across the inner membrane into the inter‑membrane space, creating an electrochemical gradient (proton‑motive force).
Return of protons through ATP synthase:
Protons flow back into the matrix through the enzyme ATP synthase by facilitated diffusion. The movement of protons drives conformational changes in ATP synthase that provide the energy needed to combine ADP and Pi into ATP.
Final electron acceptor – oxygen:
At the end of the chain, electrons are transferred to molecular oxygen (O₂). Oxygen combines with the electrons and the protons that have been pumped back to form water (H₂O).
\$\text{NADH} + \text{H}^+ + \tfrac{1}{2}\text{O}2 \rightarrow \text{NAD}^+ + \text{H}2\text{O} + \text{energy for ATP synthesis}\$
| Step | What Happens | Energy Outcome |
|---|---|---|
| 1. Hydrogen atom split | H → H⁺ + e⁻ (from NADH/FADH₂) | Stores potential energy in electrons |
| 2. Electron transport | e⁻ pass through carriers | Energy released → pumps H⁺ |
| 3. Proton gradient formation | H⁺ accumulated in inter‑membrane space | Creates electrochemical gradient |
| 4. ATP synthesis | H⁺ flow back via ATP synthase | ADP + Pi → ATP |
| 5. Oxygen as final acceptor | e⁻ + H⁺ + O₂ → H₂O | Completes electron flow, prevents backup |