inorganic ADP phosphate CHAPTER 8 Energy and Enzymes: An Introduction to Metabolic Pathways 143 A Coupled reaction Activated 7.3 substrate Energy released P ADP to synthesize BP AG Exer Energy released AG to synthesize AB Pi ADP Uncoupled reaction onic reaction Energy inputy Energy required to synthesize AB Products Progress of reaction Reactants FIGURE 8.9 Exergonic Phosphorylation Reactions Are Coupled to Endergonic Reactions. In cells, many reactions only occur if one reactant is activated by phosphorylation. The phosphorylated reactant molecule has high enough free energy that the subsequent reaction is exergonic. In this graph, the free energy being tracked on the represents and the 7,3 that released when ATP For simplicity, the free energy in ADP and Pi is not