Carbon's unique chemical bonding with other atoms contributes to its ability to form biomolecules, such as macromolecules, which serve as building blocks for many larger kinds of molecules or polymers.
Because of the carbon atom's ability to form four covalent bonds with other atoms, it serves as the backbone in the formation of numerous biological molecules. Many biological molecules are polymers. A polymer is a large molecule made of repeating smaller units of similar structures that are bonded together. A monomer is a single molecule that when combined with other monomers forms a larger molecule called a polymer. Think of a monomer as an individual boxcar. The boxcars are joined together to form the train, which represents a polymer. Polymers are used to create a macromolecule, which is a complex molecule that contains a large number of atoms. A condensation reaction is a multistep reaction that combines two molecules together with the elimination of a water molecule. The bond occurs where a hydroxyl () group is present. Enzymes remove the from one molecule and the H from another, forming a covalent bond between the two molecules. Since several reactions occur during the condensation reaction, the intermediaries (H and ) created after each step eventually chemically bond to create water. That is, hydrogen will chemically bond to the hydroxyl group () to form water.
Hydrolysis is a process by which a covalent bond between two molecules is broken by the addition of water. An group and an H atom derived from a water molecule are attached to the sites that are selected by the activity of specific enzymes.
In living things, hydrolysis reactions are important for biochemical reactions that rely on enzymes called hydrolases. These types of reactions drive the breakdown of polymers such as proteins, fats, starches, and complex sugars. For example, hydrolases will break down proteins into their smaller subunits called amino acids. In the process of the breakdown, peptide bonds that are used to hold the amino acids together to make a protein molecule are cleaved, or broken.