Dehydration synthesis is a fundamental chemical reaction in living organisms, representing a primary mechanism for building large, complex molecules from smaller subunits. This process is also known as a condensation reaction because it involves the joining of two molecules with the release of a water molecule. As an anabolic reaction, dehydration synthesis drives the formation of biological polymers that make up cells and tissues. This assembly constructs everything from stored starches to the proteins that perform most cellular functions.
The Chemical Steps of Dehydration Synthesis
Dehydration synthesis begins with small building blocks called monomers, which are linked into longer chains known as polymers. The reaction requires that the two monomers have specific reactive functional groups positioned to interact. One monomer contributes a hydroxyl group (-OH), while the adjacent monomer contributes a hydrogen atom (-H).
The removal of these two components results in the formation of a molecule of water (H2O), which is released as a byproduct. This loss of water is the “dehydration” part of the process. The vacancies left by the departing -OH and -H allow the two monomers to join together through a new covalent bond.
This newly formed covalent bond links the monomers, creating a larger molecule. Since the reaction involves forming bonds, it requires an input of energy to proceed. Dehydration synthesis is an endergonic process, meaning it consumes energy to drive the synthesis of the polymer chain.
Building the Major Biological Polymers
Dehydration synthesis is applied across the four major classes of biological macromolecules.
Carbohydrates
In carbohydrates, single sugar monomers like glucose are joined to form disaccharides or long-chain polysaccharides such as starch and cellulose. These connections are called glycosidic linkages, formed by the condensation reaction between two hydroxyl groups.
Proteins
For proteins, dehydration synthesis links individual amino acids together in a specific sequence to form long polypeptide chains. The resulting covalent bond between the carboxyl group of one amino acid and the amino group of the next is known as a peptide bond.
Nucleic Acids and Lipids
Nucleic acids like DNA and RNA are also assembled through this reaction. Nucleotide monomers are linked via phosphodiester bonds. Similarly, triglycerides, a type of lipid, are formed when three fatty acid molecules attach to a single glycerol molecule.
The Reverse Process Hydrolysis
The reverse process of dehydration synthesis is hydrolysis, a catabolic reaction that breaks down polymers into their constituent monomers. The term hydrolysis means “water splitting” and reflects the mechanism of the reaction. Instead of removing water to form a bond, water is consumed to break a bond.
During hydrolysis, a water molecule is added across the covalent bond linking two monomers in the polymer chain. The water molecule splits, with a hydrogen atom (-H) attaching to one monomer and the remaining hydroxyl group (-OH) attaching to the other. This addition effectively breaks the covalent bond, releasing the two individual monomers.
Hydrolysis is fundamental to digestion, allowing the body to break down large polymers consumed in food into monomers small enough to be absorbed by cells. Since breaking chemical bonds releases stored energy, hydrolysis is an exergonic process.