electron transport chain

An electron transport chain (ETC) couples electron transfer between an electron donor (such as NADH) and an electron acceptor (such as O₂) to the transfer of H⁺ ions (protons) across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate (ATP). Electron transport chains are the cellular mechanisms used for extracting energy from sunlight in photosynthesis and also from redox reactions, such as the oxidation of sugars (respiration).

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⁺ and succinate to fumarate that generates a proton gradient. Although some bacteria have electron transport chains with components similar to those in chloroplasts or mitochondria, others use different electron donors and acceptors. Both the respiratory and photosynthetic electron transport chains are major sites of premature electron leakage to oxygen, generating superoxide and potentially resulting in increased oxidative stress.