An enzyme complex is a structure made up of a weak protein, molecule, or atom that is weakly connected to a protein. There are 4 main enzyme complexes involved in the electron transport chain, all of which lie embedded in the inner mitochondrial membrane.
Complex I: NADH dehydrogenase
Complex II: Succinate dehydrogenase
Complex III: Cytochrome b and c1
Complex IV: Cytochrome c oxidase
The electron transport chain (ETC) is a key component of oxidative phosphorylation, the last stage of cellular respiration. The chain begins when two electron carriers, NADH (nicotinamide adenine dinucleotide) and FADH2 (flavin adenine dinucleotide), donate their electrons to Complex I and Complex II respectively. The donated electrons are then transferred to the next two complexes in the chain, Complex III and Complex IV. The last complex in the ETC, Complex IV, transfers the electrons to oxygen, the final electron acceptor in cellular respiration. The proton gradient that is generated as protons move through the electron transport chain drives the production of ATP molecules in oxidative phosphorylation.