The substrate of an enzyme is the specific molecule or compound that an enzyme acts upon. Enzymes are biological catalysts that speed up chemical reactions in living organisms, and each enzyme is typically specific to a particular substrate due to the unique shape of its active site. When the substrate binds to the enzyme's active site, it forms an enzyme-substrate complex, facilitating the conversion of the substrate into products as a result of the enzyme's catalytic action. The specificity of enzymes ensures that metabolic processes occur efficiently within cells.