In a chemical reaction, new molecules are created when existing chemical bonds are broken and/or new chemical bonds are formed. For example, in the first step in the cellular respiration pathway, an oxygen-hydrogen bond in a glucose molecule is broken, allowing the oxygen atom to form a new bond with the phosphorus atom in a PO32- molecule. In this case, the original molecules (glucose and PO32-) are the reactants, and the new molecule (glucose-6-phosphate) is the product.

Most chemical reactions are very simple and occur through collisions. Two molecules bang into each other with enough force to break bonds and form new ones. However, chemical reactions in a metabolic pathway need to be extremely precise and chemical reactions through random collisions are not. If a glucose molecule and a PO32- molecule were to collide, it is hard to know what would happen. Depending on the geometry of the collision, the product could be glucose-2-phosphate, and glucose-2-phosphate might not continue down the cellular respiration pathway. Some products of a random collision might even be toxic to the cell.

To control chemical reactions, most chemical reactions inside of a cell are catalyzed by proteins known as enzymes. Enzymes work by providing sites where specific chemical reactions can occur.