Biological systems rely on organic molecules categorized by size. The term “macromolecule” suggests a very large structure, distinct from the smaller components that build it. Understanding the relationship between these small building blocks and the large structures they form is necessary to accurately classify fundamental units like amino acids.
Defining the Building Blocks: Amino Acids
Amino acids are small organic compounds that serve as the fundamental subunits for constructing proteins. The structure is built around a central carbon atom, called the alpha (\(\alpha\)) carbon. Attached to this central point are four distinct chemical groups: a hydrogen atom, an amino group, and a carboxyl group.
The fourth attachment is the side chain, or R-group, which is unique to each of the twenty standard amino acids. This R-group determines the specific chemical properties of the amino acid. Amino acids function as foundational units used in biological assembly, chemically joined together to form much larger, functional chains.
Characteristics of Macromolecules
Macromolecules are defined as extremely large, complex molecules necessary for life, built from smaller organic components. The high molecular weight distinguishes them from simpler compounds. To be classified as a biological macromolecule, a molecule must exhibit structural complexity and be integral to cellular function, providing structure, storing energy, or carrying out chemical reactions.
Many biological macromolecules are characterized by their repetitive structure, formed by linking many similar subunits into a long chain. This repetitive organization is a defining feature of these immense molecules.
The Difference Between Monomers and Polymers
The classification of amino acids is resolved by understanding the relationship between monomers and polymers. Amino acids are correctly identified as monomers, which are single, unattached units. The corresponding large molecule formed from these units is a polymer, which falls into the category of a biological macromolecule.
The polymer formed by amino acids is a polypeptide chain, which folds into a functional protein. This polymerization involves repeatedly joining the carboxyl group of one amino acid to the amino group of the next. The resulting covalent bond between the two units is specifically called a peptide bond.
The reaction that forms this peptide bond is known as dehydration synthesis, where a molecule of water is removed to create the link. Proteins are composed of chains containing anywhere from a few dozen to many thousands of amino acid monomers. Therefore, the amino acid is the small building block, while the entire protein, the polymer, is the true macromolecule.
The Other Biological Macromolecules
The monomer/polymer relationship established by amino acids and proteins is a consistent theme across most biological macromolecules. Besides proteins, there are three other major classes of large biological molecules. Carbohydrates, such as starches and cellulose, are macromolecules built from smaller, repeating units.
The monomers for carbohydrates are monosaccharides, such as glucose, which link together to form long polysaccharide polymers. Similarly, nucleic acids, which include DNA and RNA, are macromolecules that store and transmit genetic information. Their building blocks are nucleotides, which join into long polynucleotide chains.
Lipids, the fourth class, represent a slight exception to the strict monomer-polymer rule, as they are not always built from repeating units. However, they are still considered macromolecules due to their large size and functions, such as forming cell membranes and storing energy.