Core idea
An exergonic reaction can drive an endergonic reaction only when a mechanism couples them. The combined free-energy change is the sum of both ΔG values.
Equation: ΔG(total) = ΔG(donor) + ΔG(receiver).
If the total is negative and the transfer mechanism supplies enough usable energy, the linked cellular task can proceed.
Common misconception
Two reactions occurring nearby are not automatically coupled. Their mechanisms must share an intermediate, enzyme, conformational change or other direct transfer route.