n electron transport chain (ETC) is a series of compounds that transfer electrons from electron donors toelectron acceptors via redox reactions, and couples this electron transfer with the transfer of protons (H+ions) across a membrane. This creates an electrochemical proton gradient that drives ATP synthesis, or the generation of chemical energy in the form of adenosine triphosphate (ATP). Final acceptor of electrons in electron transport chain is molecular oxygen.

Electron transport chains are used for extracting energy via redox reactions from sunlight in photosynthesisor, such as in the case of the oxidation of sugars, cellular respiration. In eukaryotes, an important electron transport chain is found in the inner mitochondrial membrane where it serves as the site of oxidative phosphorylation through the use of ATP synthase. It is also found in the thylakoid membrane of thechloroplast in photosynthetic eukaryotes. In bacteria, the electron transport chain is located in their cell membrane.

In chloroplasts, light drives the conversion of water to oxygen and NADP+ to NADPH with transfer of H+ ions across chloroplast membranes. In mitochondria, it is the conversion of oxygen to water, NADH to NAD+ andsuccinate to fumarate that are required to generate the proton gradient.