Glycolysis

2nd Law of Thermodynamics

Because the overall free-energy change for coupled reactions equals the sum of the individual free energy changes, a thermodynamically unfavorable reaction, such as a typical biochemical processes, can be driven by a thermodynamically favorable reaction coupled to it, such as the cleavage of ATP. The activities of many protein enzymes are coupled to ATP hydrolysis. However, a few other compounds in biological systems have to capacity to carry out phosphate hydrolysis with even greater negative free energy than ATP, such as phosphoenolpyruvate, acetyl phosphate, and phosphocreatine, which means that these substances have the ability to transfer a phosphate group to ADP. ATP releases its free energy coupled to biochemical processes and recharges coupled to the hydrolysis of such molecules as phosphoenolpyruvate.