Chemistry

The key insight is free energy coupling: when you link an unfavorable reaction (positive $\Delta G$) to a highly favorable one (large negative $\Delta G$), their $\Delta G$ values add together. If the exergonic reaction releases enough energy, the total $\Delta G$ becomes negative, making the whole process spontaneous even though the endergonic half alone would never happen.

The key insight is free energy coupling: when you link an unfavorable reaction (positive $\Delta G$) to a highly favorable one (large negative $\Delta G$), their $\Delta G$ values add together. If the exergonic reaction releases enough energy, the total $\Delta G$ becomes negative, making the whole process spontaneous even though the endergonic half alone would never happen.