Most biological molecules are very large and are built by assembling small molecules, or monomers, into long chains. The resulting molecules are called macromolecules, or polymers.
A process of linking monomers, called dehydration condensation, involves the removal of two hydrogen atoms and one oxygen atom to form water. One way this might happen is diagrammed above, where several generic monomers are shown with -OH groups that could be used for linking.
The animation illustrates the joining together of monomers by condensation (catalyzed by a polymerase enzyme) and the reverse process, in which added water results in hydrolysis (catalyzed by a hydrolase enzyme).