What is the electron transport system?

Electron transport system is a series of four protein complexes located in the inner membrane of the mitochondria in eukaryotes and the plasma membrane in prokaryotes. A protein complex refers to a structure that is made up of a weak protein, molecule or atom that is weakly connected to a protein. 

The four protein complexes couple redox reactions, creating an electrochemical gradient that results in the generation of adenosine triphosphate (ATP) molecules. This process is called oxidative phosphorylation and occurs in the mitochondria.