coupled reactions
Coupled reactions are how nature gets an 'uphill' change to happen by hitching it to a steeper 'downhill' one — much like using a heavy descending weight on a pulley to haul a lighter load up. A reaction that would never run on its own (its Gibbs energy wants to rise) can be forced forward if you link it to a second reaction that releases so much free energy that the pair, taken together, still goes downhill overall.
The bookkeeping is simple addition. When two reactions share a common intermediate and proceed as one, their Gibbs energy changes add up. As long as the combined ΔG is negative, the whole coupled process is spontaneous — even though one half of it, viewed alone, was unfavorable. The favorable partner effectively 'pays the bill' for the unfavorable one out of its own free-energy budget.
Why it matters: this is the trick that makes life possible. Living cells stitch demanding, energy-storing reactions — building proteins, pumping ions, contracting muscle — onto the splitting of the molecule ATP, whose large free-energy release drives the unfavorable steps. The honest caveat: coupling needs a genuine mechanical link (a shared intermediate, an enzyme, a catalyst), not merely two reactions happening side by side in the same beaker.
In a cell, joining two amino acids has a positive ΔG and won't go on its own. The cell couples it to splitting ATP, which has a large negative ΔG; added together the total is negative, so the bond forms.
A favorable reaction pays the free-energy bill for an unfavorable one.
Coupling never breaks the second law — it doesn't make an unfavorable reaction favorable in isolation; it just bundles it into a larger system whose total free energy still falls. The 'energy currency' ATP is the cell's universal coupling agent.