A monomer is a small, organic molecule that acts as a fundamental molecular unit in living systems. The name comes from the Greek words mono (“one”) and meros (“part”). These simple molecules react with other similar units, forming much larger, more intricate structures. Monomers are the basic chemical subunits used to construct major biological materials necessary for structure, energy storage, and genetic information.
Defining the Building Blocks
The primary function of a monomer is to serve as the structural subunit for a polymer, which is a long chain molecule composed of many repeating monomer units. This relationship is similar to how a single brick is a monomer used to build a large wall, which is the polymer. Monomers are the individual parts, while polymers are the vast, complex structures that result from their chemical linking.
A polymer can consist of hundreds or even thousands of these individual units chemically bonded together. The properties of the resulting large molecule are determined by the specific type of monomer used and the way these units are connected.
The Major Biological Types
Life relies on four distinct classes of biological monomers to construct its essential macromolecules. Each class of monomer is responsible for building a different type of polymer with unique functions within the cell.
Monosaccharides
Monosaccharides, or simple sugars, are the monomers of carbohydrates. Glucose is the most common natural monomer, serving as the primary energy source for most organisms. Other examples include fructose and galactose, which share the same chemical formula but have a different structural arrangement. These simple sugars link together using covalent bonds called glycosidic bonds. The resulting polymers, polysaccharides, include starches for energy storage and cellulose, which forms the structural cell walls of plants.
Amino Acids
Amino acids are the monomers that build proteins, the most structurally and functionally diverse biological molecules. Organisms use 20 common types of amino acids to assemble protein chains; thus, proteins are typically copolymers made from multiple different monomers. These units link together through a covalent connection known as a peptide bond, forming a long chain called a polypeptide. Once a polypeptide folds into its specific three-dimensional shape, it becomes a functional protein.
Nucleotides
Nucleotides are the monomers of the nucleic acids, Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA). Each nucleotide is a complex molecule composed of three distinct subunits: a five-carbon sugar, a phosphate group, and a nitrogen-containing base. The specific order in which these nucleotides are arranged determines the genetic code of an organism. DNA and RNA are long polynucleotide chains that store and transmit hereditary information within cells.
Fatty Acids and Glycerol
Lipids (fats and oils) are often not classified as true polymers because they do not form long chains of repeating units in the same linear fashion as the other three types. Instead, they are assembled from smaller components, primarily fatty acids and glycerol. A typical fat molecule, such as a triglyceride, is constructed by joining three fatty acid chains to a single glycerol molecule. This structure allows lipids to function primarily in long-term energy storage and as components of cellular membranes.
The Chemistry of Assembly
The processes of building and breaking down biological polymers are governed by two opposing chemical reactions involving water. These reactions are carefully controlled by enzymes within the cell to manage energy and material needs.
Dehydration Synthesis
To link two monomers together and form a growing polymer chain, the cell uses a process called dehydration synthesis, also known as a condensation reaction. During this process, a hydroxyl group (-OH) from one monomer and a hydrogen atom (-H) from the other are removed. The removed atoms combine to form a molecule of water (\(\text{H}_2\text{O}\)), and the remaining oxygen atom forms a covalent bond between the two monomers. This process requires an input of energy and is how biological macromolecules are constructed from their smaller subunits.
Hydrolysis
The reverse reaction, which breaks a polymer down into its constituent monomers, is called hydrolysis. The term literally means “water splitting,” as this reaction consumes a water molecule added across the covalent bond linking two monomers. The water molecule is split, with a hydrogen atom (\(\text{H}\)) attaching to one monomer and the remaining hydroxyl group (\(\text{OH}\)) attaching to the other. This breaks the bond, separating the polymer into smaller components. Digestion is a prime example of hydrolysis, where enzymes break down large polymers in food into absorbable monomers like amino acids and simple sugars.