= Electron transport chain
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An <electron transport chain> transfers <electrons> from an <electron donor> through <electron carriers> to a terminal <electron acceptor>. In the <mitochondrial respiratory chain>, some transfers drive <protons> across the <inner mitochondrial membrane>; their return through <ATP synthase> can drive <ATP> formation.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 14, Electron-Transport Chains and Their Proton Pumps, Summary: carrier sequence, outward proton transport and ATP synthase]{ref}
The useful separation is between an <electron> route and a <proton> route. The former ends in a <redox> <chemical reaction>; the latter changes an <electrochemical gradient> across a membrane. <Energy coupling> connects them without making <electrons> and <protons> interchangeable. An <oxygen> readout consequently need not be an <ATP> readout.
This article focuses on the <mitochondrial respiratory chain>. <Bacteria> can use other terminal <electron acceptors> in <anaerobic respiration>. In <chloroplast> <light-dependent reactions>, light supplies <energy> and linear <electron> transport connects <water> <oxidation> to <NADP+> <reduction>, releasing <oxygen>. Thus neither <oxygen> consumption nor the mitochondrial arrangement defines every <electron transport chain>.<Biology 2e by Mary Ann Clark and others>[§7.5, Anaerobic Cellular Respiration; §8.2, Generating an Energy Carrier: linear transport from water to NADP+]{ref}
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