Small molecules, called monomers, are assembled into long chains called polymers by a dehydration reaction. Let's look at this reaction more closely. An enzyme covalently bonds two monomers to each other with the loss of a water molecule. One monomer provides a hydrogen (H), while the other provides a hydroxyl group (-OH), and water is formed as a by-product. This reaction is repeated as monomers are added to the chain one by one, making a polymer. Polymers are disassembled to monomers by hydrolysis. A different enzyme breaks the bond between monomers by adding a water molecule, with a hydrogen from water attaching to one monomer and the hydroxyl group, -OH, attaching to the other.