Energy coupling in biological processes. The reaction coordinate diagram on the left shows an energetically unfavorable biosynthetic reaction. The middle diagram shows a favorable reaction such as the hydrolysis of ATP. By coupling biosynthetic reactions having a positive (unfavorable) ΔG value with the breakage of high-energy bonds having a negative ΔG of greater absolute value, the overall reaction has a negative ΔG (shown in the diagram on the right). At equilibrium, the coupled reaction favors synthesis of the biomolecule over its breakdown.