The Electron Transport System (ETS) is the metabolic pathway through which electrons pass from one carrier to another. It is located on the inner mitochondrial membrane.

The primary function of ETS is to release and utilise the energy stored in NADH+H⁺ and FADH₂. These molecules are oxidised through the electron transport system, and the electrons are passed on to O₂, resulting in the formation of H₂O. In this process, O₂ acts as the ultimate acceptor of electrons and gets reduced to water.

As electrons move through the system, they release enough energy that is trapped to synthesise ATP. This process is called oxidative phosphorylation.

The energy from the electron transport system is utilised in synthesising ATP with the help of ATP synthase (complex V). This complex consists of two major components: F₁ and F₀. The F₁ headpiece is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. F₀ is an integral membrane protein complex that forms the channel through which protons cross the inner membrane.

The passage of protons through the channel is coupled to the catalytic site of the F₁ component for the production of ATP. For each ATP produced, 4H⁺ pass through F₀ from the intermembrane space to the matrix down the electrochemical proton gradient.