The role of oxygen is critical with respect to Electron Transport System (ETS) because:

i) Oxygen plays a crucial role at the end of ETC because it removes the hydrogen from the electron transport system.

ii) Oxygen acts as the final hydrogen acceptor in the electron transport chain.

iii) If oxygen is not present, then the electrons could not pass through the co-enzymes.

iv) Oxygen is responsible for the formation of the proton pump in the ETS process as it gives energy by the oxidation-reduction process.

v) If oxygen is absent, then there will be no formation of proton pump and ATP will not be produced by oxidative phosphorylation.