Reaction coupling is the thermodynamic principle that an energetically unfavorable reaction (positive ΔG) can be driven forward by summing it with a sufficiently favorable one (large negative ΔG), because Gibbs free energy is a state function whose changes are additive across the steps of an overall process. In biochemistry the canonical exergonic partner is ATP hydrolysis — ATP plus water yielding ADP and inorganic phosphate — whose strongly negative ΔG exceeds in magnitude the positive ΔG of typical biosynthetic condensations, so the coupled sum is negative and the composite process becomes spontaneous. This concept sits at the junction of bioenergetics and general-chemistry thermodynamics, and it supplies the quantitative justification for calling ATP the cell's energy currency across biosynthesis, muscle contraction, and active transport.
Reaction coupling is the thermodynamic principle that an energetically unfavorable reaction (positive ΔG) can be driven forward by summing it with a sufficiently favorable one (large negative ΔG), be…