All life on Earth depends on the construction and dismantling of organic molecules, known as macromolecules. These compounds perform nearly every function within a cell, from storing genetic information to providing structural support and catalyzing chemical reactions. The complexity and diversity of these large molecules are achieved by linking together much smaller, simpler building blocks in specific sequences. Understanding the chemical reaction that connects these units is fundamental to cellular biology.
Monomers and Polymers: The Foundational Concepts
Biological macromolecules are constructed from basic chemical units called monomers. The name monomer is derived from the Greek words mono- meaning “one” and -mer meaning “part.” Monomers are the foundational, repeating components that are chemically bonded together to create much larger structures.
When hundreds or thousands of these monomers join in a continuous chain, they form a polymer, from the Greek words poly- meaning “many” and -mer meaning “part.” The chemical structure of the polymer is determined by the specific type and arrangement of the monomers it contains.
The relationship between these two molecules is the basis for the four main classes of biological molecules found in all living organisms. Monomers combine using covalent bonds to create the complex structures necessary for cellular function.
Dehydration Synthesis: The Polymerization Process
The chemical reaction responsible for linking monomers together to form polymers is called dehydration synthesis, which is also known as a condensation reaction. The name itself describes the process: “dehydration” refers to the loss of a water molecule, and “synthesis” means to create or build something. This anabolic reaction successfully joins two smaller molecules into one larger one.
During the reaction, a hydrogen atom (\(\text{H}\)) is removed from one monomer, and a hydroxyl group (\(\text{OH}\)) is removed from the other. These combine to form a molecule of water (\(\text{H}_2\text{O}\)), which is released as a byproduct of the synthesis. This removal of the water molecule creates a vacant bonding site on each of the two joining monomers.
The empty sites on the two monomers then allow them to share electrons, forming a new, strong covalent bond between them. This new bond permanently links the two units, extending the length of the growing polymer chain. This process repeats multiple times as additional monomers are added, releasing a molecule of water for every attachment. The reaction requires an input of energy to proceed. Furthermore, the entire process is highly regulated and sped up by specialized proteins known as enzymes.
Hydrolysis: The Reverse Reaction
While dehydration synthesis builds polymers, hydrolysis is the reverse process used to break them down back into their individual monomer units. Hydrolysis means “water breaking,” and it cleaves the bond between two monomers through the addition of a water molecule.
A water molecule (\(\text{H}_2\text{O}\)) is inserted across the covalent bond connecting two monomer units. The water molecule splits, with a hydrogen atom (\(\text{H}\)) attaching to one monomer and the remaining hydroxyl group (\(\text{OH}\)) attaching to the other. This addition of water successfully breaks the covalent bond, separating the polymer into two smaller components.
Hydrolysis is an indispensable biological process, particularly in the digestive system. Large polymers consumed in food, such as starches and proteins, must be broken down into their smaller, absorbable monomer components, such as simple sugars and amino acids. Digestive enzymes catalyze these reactions.
The constant, balanced interplay between dehydration synthesis and hydrolysis allows organisms to maintain and adjust their supply of biological macromolecules as needed. Unlike dehydration synthesis, hydrolysis reactions usually release energy upon the breaking of chemical bonds.
Major Biological Macromolecules Formed by These Reactions
Carbohydrates, for instance, are synthesized when monosaccharide monomers, like glucose, link together to form long polysaccharide polymers such as starch or cellulose. These large sugar molecules are broken down via hydrolysis when the body needs to access the stored energy.
Proteins are another class of polymers built this way, using amino acid monomers. The dehydration synthesis reaction creates a peptide bond between two amino acids, and multiple repetitions form a polypeptide chain, which then folds into a functional protein. Similarly, nucleic acids, which include DNA and RNA, are polymers constructed from nucleotide monomers.
The diversity found in life is a direct result of these simple reactions, as different sequences and combinations of monomers create a vast array of polymers with unique shapes and functions. Whether it is the storage of genetic code in DNA or the structural integrity provided by cellulose in a plant cell wall, these large molecules all owe their existence to the mechanism of dehydration synthesis.