Many important biological molecules are made of repeating subunits, called monomers. When many monomers join, the result is a polymer. For example, amino acid monomers join to form a protein polymer, and glucose monomers combine to form a complex carbohydrate polymer. Biological polymers form by dehydration synthesis reactions. As you can see here, each of the monomers in this reaction has a hydrogen (H) and a hydroxyl (OH) group. In the course of the reaction, the hydrogen is removed from one monomer, and the hydroxyl group from the other. The hydrogen and hydroxyl combine to form water, and a bond links the two monomers. Hydrolysis is the opposite of a dehydration synthesis reaction. During a hydrolysis reaction, a polymer is reduced to its monomer subunits by the addition of water. In fact, the word Hydrolysis literally means to “break water”. The hydroxyl group from a water molecule attaches to one monomer, and the remaining hydrogen attaches to the other monomer. In other words, water is used to break the bond holding monomers together. Let’s do a quick recap. During dehydration synthesis, monomers join to form polymers, and water is released. The opposite happens during hydrolysis, where water is added to the reaction to break a polymer into monomers.