Through ATP coupling, the hydrolysis of ATP can allow the second, thermodynamically unfavorable reaction to proceed. . As the resulting delta is negative, this will be thermodynamically favorable.

Exergonic reactions are reactions that release energy. Endergonic reactions are reactions that require energy to proceed. Exergonic reactions can occur spontaneously, or are thermodynamically favorable. Endergonic reactions cannot occur spontaneously, or are thermodynamically unfavorable.

An endergonic reaction requires input of energy; delta G will be positive. Hydrolysis of ATP, cellular respiration, and catabolism (breakdown of a large molecule) are exergonic processes. The Na+/K+ pump is endergonic, as it requires energy. This process is coupled with ATP hydrolysis to allow it to proceed.

ATP hydrolysis is exergonic. ATP phosphorylation is the reverse of ATP hydrolysis, and thus is endergonic.

ATP coupling is the process where hydrolysis of ATP (a thermodynamically favorable, negative delta G, or spontaneous reaction) is coupled with a thermodynamically unfavorable reaction (a reaction that cannot proceed without energy input).

Given that the hydrolysis of 1 mole of ATP has delta G of , if 2 moles of ATP are hydrolyzed, delta . Reaction D has a delta G of ; when coupled with the hydrolization of 2 moles of ATP, delta G will be . As this is negative, it is indicated that this will be a spontaneous reaction, or can proceed without input of energy.