Coupled reaction pathways connect favourable chemistry with cellular work.
Key knowledge
Cells couple ATP hydrolysis to otherwise energetically unfavourable processes through shared reaction steps or conformational changes. The Gibbs-energy change belongs to the overall reaction under cellular conditions; breaking an ATP bond alone is not the source of released free energy.
Conditions and scope
Coupling requires a physical reaction mechanism; merely placing two reactions together does not make them coupled.
The hydrolysis free-energy change depends on activities and conditions, rather than one fixed value for every cell.
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References
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Cells couple ATP hydrolysis to otherwise energetically unfavourable processes through shared reaction steps or conformational changes. The Gibbs-energy change belongs to the overall reaction under cellular conditions; breaking an ATP bond alone is not the source of released free energy.
The source's ATP-coupled ion pump is a membrane transport example.
Myosin interaction with actin converts ATP-dependent molecular cycling into force. ATP binding permits detachment, while hydrolysis prepares subsequent cycling. In the skeletal-muscle model, calcium binding to troponin changes tropomyosin positioning and exposes actin binding sites; neural excitation couples membrane activity to this regulation.
Actin filaments, microtubules and intermediate filaments organize different structural and dynamic functions in eukaryotic cells. Myosin acts on actin; microtubule motors move cargo along microtubules. Polymer assembly and motor activity contribute to shape, intracellular movement, division and cell motility.
The canonical glycolytic pathway converts one six-carbon glucose into two three-carbon pyruvates. Investment and payoff stages yield net two ATP and two NADH per glucose under the usual accounting. Substrate-level phosphorylation directly transfers phosphate to ADP; continued flux requires regeneration of oxidized NAD⁺.
A respiratory electron-transport chain couples favourable electron transfers to an electrochemical proton gradient across a membrane. ATP synthase can use proton return to synthesize ATP. In the mitochondrial aerobic example oxygen is the terminal acceptor; electron transfer, proton pumping and ATP production have distinct steps.
Aminoacyl-tRNA synthetases couple amino acids to cognate tRNAs using ATP-dependent activation. Recognition and, in relevant synthetases, hydrolytic editing reduce mischarging. The ribosome reads the tRNA codon adaptor, so charging fidelity and codon recognition are separate accuracy stages.
Rab GTPases and effectors help recruit and tether transport carriers at appropriate membranes. Compatible trans-SNARE assembly draws membranes together and contributes to fusion. After fusion, ATP-dependent NSF machinery disassembles SNARE complexes for reuse. Docking and fusion are separable stages.