Living organisms are constructed from complex, giant molecules known as macromolecules. These large biological molecules form the structural components of cells and tissues and carry out the complex functions necessary for life. Macromolecules are built from smaller, simpler subunits called monomers. The process of assembling these monomers into long chains, or polymers, is a fundamental chemical reaction that underpins all biological growth and maintenance.
The Chemical Mechanism of Assembly
The primary method by which biological monomers are joined together to form macromolecules is known as dehydration synthesis, or a condensation reaction. This anabolic process is defined by the removal of a water molecule to facilitate the formation of a new covalent bond between two subunits. One monomer contributes a hydroxyl (-OH) group, and the other contributes a hydrogen (-H) atom.
When these groups are removed, they combine to form a molecule of water (H₂O), which is released. The resulting bonding sites on the two monomers then join together, creating a stable, larger molecule. This reaction requires an input of energy and is catalyzed by specific enzymes, which accelerate the reaction rate.
The process of linking these units is known as polymerization. The dehydration step is repeated continuously as each monomer is added to the growing chain, allowing for the rapid construction of very long polymer chains.
Diversity in Structure and Linkages
While the core chemical mechanism remains the same, the specific monomers and resulting bonds vary significantly among the major classes of macromolecules.
Carbohydrates
Carbohydrates function primarily for energy storage and structure. They are built from simple sugar monomers called monosaccharides, such as glucose and fructose. These sugars are linked together through the removal of water to form a covalent connection called a glycosidic linkage. The resulting polymers, known as polysaccharides, can be straight chains like cellulose, or highly branched chains like glycogen.
Proteins
Proteins utilize twenty different types of amino acid monomers and demonstrate great structural complexity. The dehydration reaction joins the carboxyl group of one amino acid to the amino group of a second, forming a specialized covalent bond known as a peptide bond. A chain of amino acids linked by these bonds forms a polypeptide, which represents the protein’s primary structure. The precise sequence of these amino acids dictates the final three-dimensional folding of the protein, determining its specific function.
Nucleic Acids
Nucleic acids are responsible for storing and transmitting genetic information. They are constructed from nucleotide monomers, each containing a sugar, a phosphate group, and a nitrogenous base. When two nucleotides are joined, the phosphate group of one unit links to the sugar of the next through a phosphodiester linkage. This connection creates the backbone of DNA and RNA strands, forming the long informational polymers necessary for heredity.
Lipids
Lipids represent a unique case because many forms do not technically qualify as true polymers. Triglycerides, used for long-term energy storage, are assembled via dehydration synthesis, but the resulting structure is not a repeating chain. This assembly occurs when three fatty acid molecules are joined to a single glycerol molecule. The resulting covalent bonds formed are referred to as ester linkages.
The Necessary Role of Degradation
The construction of macromolecules is constantly balanced by the need for their breakdown and recycling, a process called hydrolysis. This catabolic reaction is the reverse of dehydration synthesis, requiring the addition of a water molecule. Hydrolysis literally means “to break with water.”
When a water molecule is introduced across a covalent bond, the molecule splits, releasing the individual monomers. The hydrogen atom attaches to one monomer, and the hydroxyl group attaches to the other, reversing the bond formation. This process is necessary for digestion and allows cells to break down old or damaged polymers, making the constituent monomers available for building new structures.