Protein, carbohydrate, and fat macromolecules are broken down into their simplest monomeric units by hydrolytic enzymes produced at different locations in the digestive tract. Many are secreted into the lumen of the gut, and others remain associated with the membranes of the microvilli. All of these enzymes cleave the chemical bonds of macromolecules through hydrolysis, a reaction that adds a water molecule (see Figure 3.4B). Digestive enzymes are classified according to the substances they hydrolyze: proteases break the bonds between adjacent amino acids in proteins; carbohydrases hydrolyze carbohydrates; peptidases break down peptides; lipases, fats; and nucleases, nucleic acids. We will discuss the digestive enzymes of humans in detail in Key Concept 50.3.