The electron transport system occurs in the inner membrane of the mitochondria. The electron transport system contains mainly six components arranged in the sequence of NAD, FMN, c ytochrome b, c ytochrome c, c ytochrome a and c ytochrome a$${_3}$$.
Electrons from NADH produced during citric acid are oxidized by an NADH dehydrogenase (complex I) and electrons are then transferred to ubiquinone located within the inner member. Ubiquinone also receives reducing equivalents via FADH $${_2}$$ , through the activity of the enzyme, succinate dehydrogenase (complex II). The reduced ubquinone called ubquinol is then oxidised with the transfer of electrons to cytochrome c, (complex III) cytochrome c is a small protein attached to outer surface of the inner membrane and is the mobile carrier for transfer of electrons between complex between III and IV. Complex IV is cytochrome c oxidase complex containing cytochromes a and a $${_3}$$ and 2 copper centers.
When electrons pass from one carrier to another via complex I to IV, in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP.
At each step of electron transport, the electron acceptor has a higher electron affinity than the electron donor. The energy from such electron transport is utilised in transporting protons (H+) from the matrix, across the inner membrane to its outer chamber.
This creates a higher proton concentration outside the inner membrane than in the matrix. This difference in proton concentration on the outer and inner sides of the inner mitochondrial membrane is known as proton gradient.
At the end of the chain the electrons and the accompanying protons are combined with oxygen to form water. Oxygen is thus, the terminal electron acceptor of the mitochondrial respiratory chain.