What are the main steps in the electron transport chain?

The electron transport chain (ETC) consists of a series of four protein complexes located in the inner mitochondrial membrane in eukaryotes or the plasma membrane in eukaryotes. It plays a crucial role in generating ATP (adenosine triphosphate) during the oxidative phosphorylation stage of cellular respiration. 

There are four main steps in the electron transport chain, which involve the transfer of electrons from NADH to molecular oxygen: 

1. Electrons are removed from molecules such as NADH (nicotinamide adenine dinucleotide) and FADH2 (flavin adenine dinucleotide), which are products of earlier stages of cellular respiration. Electrons from NADH are transferred to Complex I (NADH dehydrogenase) and electrons from FADH2 are transferred to Complex II (Succinate dehydrogenase).
2. As electrons move through the electron transport chain protons (H+) are actively pumped across the inner mitochondrial membrane, creating an electrochemical gradient. 
3. Molecular oxygen (O2) acts as the final electron acceptor, combining with H+ ions and electrons to form water.  
4. The proton gradient drives protons back into the mitochondrial matrix through ATP synthase, an enzyme complex embedded in the mitochondrial matrix. As protons move through ATP synthase, the enzyme uses the energy released by the proton flow to synthesize ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi).