= Solution
Cells store usable <free energy> in chemical disequilibria: reduced carbon compounds, <redox> carriers such as <NADH>, <ATP> relative to <ADP> and <inorganic phosphate>, <ion> <Concentration gradient>[concentration gradients] and <Cell membrane>[membrane] <voltage>. These reservoirs are maintained away from <equilibrium>; the useful quantity is the chemical or <electrochemical potential> difference, not simply a bond's isolated <energy>. Some ATP-consuming reactions can be driven because a coupled overall reaction decreases <Gibbs free energy>, even when the uncoupled synthesis would not.
Oxidation of nutrients transfers reducing equivalents to <Electron carrier>[electron carriers] and then to an <Electron transport chain>[electron-transport chain]. Coupled <proton> pumping converts part of the <redox> free-energy drop into a <Proton-motive force>[protonmotive force]. <ATP synthase> can convert that <force> into <ATP> <chemical potential>, and <ATP hydrolysis> drives <biosynthesis>, transport and mechanical motion. <Substrate-level phosphorylation> is another <ATP> source; photosynthetic cells can initially supply the disequilibrium with absorbed light. These are flows with unavoidable dissipation and <heat> release, not independent energy-creation processes. A reservoir is depleted unless metabolism replenishes it.
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