= Energy coupling
{title2=$\Delta G_{\mathrm{net}}=\Delta G_1+\Delta G_2$}
<Energy coupling> links an <endergonic reaction> to an <exergonic reaction> through a <reaction mechanism>, so that the combined transformation has a favourable change in <Gibbs free energy>. <ATP> <hydrolysis> is a common driving <chemical reaction>. A shared <reaction intermediate>, produced in one step and consumed in another, can physically connect the transformations; merely placing two independent <chemical reactions> in the same <chemical solution> does not establish <energy coupling>. See https://www.ncbi.nlm.nih.gov/books/NBK26838/[Alberts et al., Molecular Biology of the Cell: coupled reactions and activated intermediates].
\Image[/atp-energy-coupling.png]
{title=<Energy coupling>: $+20-30.5=-10.5\,\mathrm{kJ\,mol^{-1}}$ under the same stipulated conditions}
{description=Original diagram of cumulative changes in <Gibbs free energy> as two contributions are added. The levels are bookkeeping steps, not a <reaction mechanism>, actual <reaction intermediates>, elapsed time, or <activation energy> barriers. A <reaction mechanism> must enforce the joint transformation for the net favourable value to drive the desired conversion.}
For a computed example, stipulate an <endergonic reaction> $\mathrm A\longrightarrow\mathrm B$ with $\Delta G_1^{\circ\prime}=+20\,\mathrm{kJ\,mol^{-1}}$ under precisely the same biochemical <standard state> conditions as the quoted <ATP> <hydrolysis> value. Assume a <reaction mechanism> couples one <ATP> <hydrolysis> to each A-to-B conversion. The formal sum is
$$
\begin{aligned}
\mathrm A&\longrightarrow\mathrm B,
&\Delta G_1^{\circ\prime}&=+20\,\mathrm{kJ\,mol^{-1}},\\
\mathrm{ATP}+\mathrm{H_2O}&\longrightarrow\mathrm{ADP}+\mathrm P_i,
&\Delta G_2^{\circ\prime}&=-30.5\,\mathrm{kJ\,mol^{-1}},\\
\mathrm A+\mathrm{ATP}+\mathrm{H_2O}&\longrightarrow\mathrm B+\mathrm{ADP}+\mathrm P_i,
&\Delta G_{\mathrm{net}}^{\circ\prime}&=-10.5\,\mathrm{kJ\,mol^{-1}}.
\end{aligned}
$$
The sum $+20+(-30.5)=-10.5$ gives an <exergonic reaction>. The $+20$ is a chosen teaching value, not a measured property of an identified substance. Outside these reference conditions, add the actual $\Delta G_1$ and $\Delta G_2$, each evaluated using consistent <thermodynamic activities>; mixing a standard value for one <chemical reaction> with an unrelated cellular value for the other gives no justified prediction.
Why do the changes in <Gibbs free energy> add? <Gibbs free energy> is a <state function>. Adding consistent reaction changes cancels any shared <reaction intermediate> and leaves the difference between the combined <reactants> and combined <reaction products>. This explains the thermodynamic accounting; it does not construct the <reaction mechanism>. In many <ATP>-dependent routes, <phosphorylation> transfers a <phosphate> group to a <substrate> or an <enzyme>, and subsequent transformation consumes that modified material. The shared material is what connects the steps. Other <reaction mechanisms> use <ATP> differently. See https://www.ncbi.nlm.nih.gov/books/NBK26838/[Alberts et al., Molecular Biology of the Cell: ATP-driven biosynthesis].
The useful achievement is that a common regenerated <nucleotide> can connect many different processes in <metabolism>. The net negative change in <Gibbs free energy> permits the coupled conversion, but supplies no <reaction rate>: <enzymes> can lower <activation energy> barriers without changing the endpoint difference. <Energy coupling> changes which overall <chemical reaction> is being considered; <catalysis> accelerates an available route. See https://openstax.org/books/biology-2e/pages/6-2-potential-kinetic-free-and-activation-energy[OpenStax Biology 2e, 6.2: free-energy changes and reaction barriers].
A historical landmark is Fritz Lipmann's https://doi.org/10.1002/9780470122464.ch4[1941 essay, Metabolic Generation and Utilization of Phosphate Bond Energy]{ref}. Its sections on the <phosphate> cycle, generation and utilization of phosphate-bond <energy>, and group transfer organized these processes around a reusable chemical carrier. The historical terminology should be read with the modern distinction between net <hydrolysis> and the cost of breaking a <chemical bond>.
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