An electron transport chain (ETC) couples electron transfer between an electron donor, such as NADH, and an electron acceptor, such as oxygen, to the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of ATP, adenosine triphosphate. Electron transport chains are the cellular mechanisms used for extacting energy from sunlight in photosynthesis, and also redox equations, such as the oxidation of sugars, in cellular respiration.
For chloroplasts, light drives the conversion of water to oxygen and NADP+ to NADPH with transfer of H+ ions across the chloroplast membranes. In mitochondria, it is the conversion of oxygen to water, NADH to NAD+ and succinate to fumerate that generates a proton gradient.