Dehydration synthesis is the fundamental chemical process by which living organisms construct the large, complex molecules necessary for life. This major anabolic reaction involves building larger structures from smaller components, known as monomers, into long chains or vast networks called polymers. The name of the reaction reveals its function: dehydration refers to the removal of water, and synthesis indicates the creation of a new molecule.
The Chemical Mechanism and Hydrolysis
The formation of a new, larger molecule through dehydration synthesis relies on the interaction of specific functional groups on the reacting monomers. Typically, one monomer contributes a hydroxyl group (\(\text{-OH}\)), and the other contributes a hydrogen atom (\(\text{-H}\)) to the reaction. These components are removed from the monomers and combine to form a molecule of water (\(\text{H}_2\text{O}\)), which is released as a byproduct.
The two remaining molecular fragments form a new, stable covalent bond. This bond links the two monomers, creating a dimer, or allowing a monomer to extend a growing polymer chain. Because a water molecule is condensed out of the reactants, this reaction is also referred to as a condensation reaction.
The inverse of this construction process is called hydrolysis, which breaks down large polymers into individual monomers. Hydrolysis literally means “water splitting,” and it involves the addition of a water molecule across the covalent bond connecting two monomers. The water molecule splits, with a hydrogen atom (\(\text{-H}\)) attaching to one monomer and the remaining hydroxyl group (\(\text{-OH}\)) attaching to the other. This process breaks the covalent bond, releasing a monomer from the polymer chain.
Constructing Carbohydrates and Proteins
The principles of dehydration synthesis are applied across different classes of biological molecules, starting with carbohydrates and proteins. Carbohydrates are constructed by linking simple sugar monomers, called monosaccharides, such as glucose, into larger structures. When two monosaccharides join, the dehydration reaction forms a covalent bond known as a glycosidic linkage.
This linkage forms disaccharides, like sucrose, and much larger polysaccharides, such as starch and cellulose. Starch and cellulose are long chains of glucose monomers linked by repeated dehydration synthesis reactions. The specific location and orientation of the glycosidic linkage determine the final shape and function of the resulting polysaccharide.
Proteins are built via dehydration synthesis from smaller units called amino acids. The amino acids link together in a linear sequence, with the carboxyl group (\(\text{-COOH}\)) of one amino acid reacting with the amino group (\(\text{-NH}_2\)) of the next. This reaction removes a water molecule and forms a specialized covalent bond known as a peptide bond.
Multiple peptide bonds link dozens to thousands of amino acids together into a long, unbranched chain called a polypeptide. The precise sequence of amino acids in this polypeptide chain dictates how the protein folds into its complex three-dimensional structure.
Constructing Nucleic Acids and Lipids
Dehydration synthesis is used for assembling the nucleic acids, DNA and RNA, which carry the cell’s genetic information. These molecules are polymers built from nucleotide monomers, each consisting of a sugar, a phosphate group, and a nitrogenous base. Nucleotides are joined by connecting the phosphate group of one to the sugar component of the next in the growing chain.
Specifically, the hydroxyl group on the \(3′\) carbon of the existing sugar molecule reacts with the phosphate group of the incoming nucleotide. This dehydration reaction forms a phosphodiester bond, a strong covalent link that forms the backbone of the entire DNA or RNA strand. The continuous formation of these bonds creates the long, directional polymer chains that store and transmit genetic instructions.
The synthesis of lipids, such as fats (triglycerides), also utilizes dehydration synthesis, though the final structure is not a polymer chain like the other three biomolecule classes. A fat molecule is formed when three long fatty acid chains are each joined to a single glycerol molecule. The carboxyl end (\(\text{-COOH}\)) of each fatty acid reacts with a hydroxyl group (\(\text{-OH}\)) on the glycerol backbone.
This process releases three water molecules, one for each fatty acid chain attached, and forms three ester linkages. These ester bonds hold the fatty acids to the glycerol, creating the characteristic non-polar, energy-rich structure of a fat molecule.
The Cellular Context of Synthesis
Within the living cell, dehydration synthesis is an anabolic process that requires a significant input of energy to proceed. Building complex molecules is thermodynamically unfavorable, so the cell couples these reactions with the breakdown of adenosine triphosphate (ATP), which releases the necessary energy. Specific protein catalysts, known as enzymes, are required to accelerate the rate of these reactions.
These enzymes ensure that dehydration synthesis occurs quickly and accurately at the relatively low temperatures and neutral \(\text{pH}\) conditions found within the body. The location of synthesis varies depending on the molecule being built; for instance, protein synthesis occurs on ribosomes, while many components of lipids are synthesized in the smooth endoplasmic reticulum.