When the products store more energy than the reactants, outside energy is required for the reaction to proceed, this is an endergonic reaction. In the body, endergonic reactions must be coupled with exergonic reactions. If activation energy did not exist, our components would spontaneously break down into simpler molecules and we would all burst into flames. Proteins called enzymes lower the amount of energy needed to overcome the energy barrier preventing the exergonic reactions from occurring. Reactions in the body usually occur in a long series.
Metabolism is the sum of all the reactions that occur in the cell.
Specific enzymes are required at each of these steps. A reaction which breaks down a complex molecule into simpler molecules is called a degradation reaction and usually releases energy. A reaction which combines simple molecules to form a more complex molecule is called a synthetic reaction and usually requires the input of energy. When an enzyme binds to a reactant it can now be called a substrate. The site on an enzyme that binds to the substrate is called its active site.