Enzymes are biological catalysts made of proteins that allow reactions to occur more easily in cells. They speed up reactions without being used up in the process. Enzymes are found in the cytoplasm and organelles and are involved in breaking down molecules and synthesizing new ones. They work by orienting reactants in a specific way so that reactions can occur at the active site of the enzyme. The rate of enzymatic reactions depends on factors like temperature, amounts of reactants and products, pH, and other substances present that can inhibit the enzyme or alter its shape.