Oxidative phosphorylation is the final and most important step of cellular respiration, where the cell makes most of its ATP (energy). It happens in the inner membrane of the mitochondria.
Here’s a explanation of how it works step by step:
- Electron Donors (NADH and FADH₂):
- During earlier stages of metabolism (like the Krebs cycle and glycolysis), molecules called NADH and FADH₂ are formed.
- These carry high-energy electrons to the electron transport chain (ETC).
- Electron Transport Chain:
- The electrons from NADH and FADH₂ pass through a series of protein complexes in the inner mitochondrial membrane.
- As they move along the chain, energy is released.
- Proton Pumping:
- The released energy is used to pump hydrogen ions (H⁺) from the mitochondrial matrix into the intermembrane space.
- This creates a proton gradient — meaning there are more H⁺ ions outside the inner membrane than inside.
- ATP Synthesis (Chemiosmosis):
- The H⁺ ions naturally want to flow back into the matrix through a special enzyme called ATP synthase.
- As they pass through ATP synthase, the energy from this flow is used to combine ADP and phosphate (Pi) to form ATP.
- Role of Oxygen:
- At the end of the electron transport chain, the electrons combine with oxygen and hydrogen ions to form water.
- This is why oxygen is essential — without it, the chain stops, and no ATP is produced.
In short:
Oxidative phosphorylation generates ATP by using energy from electrons carried by NADH and FADH₂.
The process combines two parts —
- Electron transport chain: creates a proton gradient, and
- ATP synthase: uses that gradient to make ATP.
This method produces about 34 ATP molecules per glucose, making it the main source of cellular energy in aerobic organisms.