Energy metabolism comprises reactions that capture, transfer, and use energy to support cellular processes. Oxidation of nutrients can reduce electron carriers and support synthesis of ATP, which then couples favorable chemical change to otherwise unfavorable work.
ATP is a transferable chemical intermediate, not a substance that creates energy. In oxidative phosphorylation, electron transfer helps establish an electrochemical gradient across a membrane, and ATP synthase uses that gradient. Conservation of energy and the direction of free-energy changes constrain the whole process.
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