Monomers are small molecules that serve as building blocks for larger, more complex structures called polymers. Polymers are long chains, with monomers as their individual links. These larger molecules are ubiquitous in biological systems, performing functions from storing genetic information to providing structural support within cells.
Building Blocks Unite: Dehydration Synthesis
The primary chemical reaction responsible for joining monomers to form polymers is called dehydration synthesis, also known as a condensation reaction. This process involves forming a new chemical bond between two monomers, accompanied by the removal of a water molecule. One monomer contributes a hydroxyl group (-OH) and the other a hydrogen atom (-H), which combine to form H2O. The remaining atoms then form a covalent bond, linking the two monomers.
Each time a new monomer is added to a growing polymer chain, another molecule of water is released. This mechanism allows for the repetitive addition of monomer units, gradually extending the polymer chain into a much larger molecule. The process requires an input of energy and is facilitated by specific enzymes.
The Chemical Bonds of Life’s Macromolecules
The way monomers bond is specific to the type of macromolecule formed, resulting in distinct chemical linkages. In carbohydrates, such as starches and cellulose, monosaccharide monomers like glucose are linked by glycosidic bonds. These bonds form between the hydroxyl groups of two monosaccharides, creating a disaccharide or polysaccharide chain.
Proteins, polymers of amino acids, are formed through the creation of peptide bonds. A peptide bond forms between the carboxyl group of one amino acid and the amino group of another, releasing a water molecule. These bonds create the polypeptide chains that fold into functional proteins.
Nucleic acids, including DNA and RNA, consist of nucleotide monomers joined by phosphodiester bonds. This bond forms between the phosphate group of one nucleotide and the sugar of another, forming the sugar-phosphate backbone of the nucleic acid strand.
For lipids, fatty acid monomers attach to a glycerol molecule via ester linkages. Each ester bond forms when a hydroxyl group from glycerol reacts with the carboxyl group of a fatty acid, releasing a water molecule. These specific bond types give each class of macromolecule its unique structural and functional properties.
Breaking Apart: The Hydrolysis Reaction
Just as monomers are joined by removing water, polymers can be broken down into their constituent monomers through a reverse process called hydrolysis. The term “hydrolysis” literally means “water splitting.” In this reaction, a water molecule is added across the bond linking two monomers, splitting the bond and restoring the original functional groups to each monomer. For example, a water molecule would be added to break a peptide bond, yielding two separate amino acids.
This process is fundamental to digestion, where large food polymers like proteins, carbohydrates, and fats are broken down into smaller, absorbable monomers in the digestive tract. The body uses these freed monomers as building blocks for new polymers or as a source of energy. Hydrolysis reactions are also catalyzed by specific enzymes, ensuring breakdown occurs efficiently and precisely where needed.