The basic idea of a catalyst(enzymes). Enzymes are proteins that lower the energy of activation. Substrate molecules approach to the active site of enzyme, and are chemically bonded, and released. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates sufficient for life. Since enzymes are selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell.