Fatty acids are defined by the presence of a carboxyl group (–COOH), the functional group that makes them an “acid.” A fatty acid is a fundamental type of lipid molecule, characterized by a long hydrocarbon chain attached to this terminal carboxyl group. This structure is consistently found in all fatty acids, whether saturated or unsaturated, classifying them as carboxylic acids. The carbon chain typically ranges from 4 to 28 carbons, though most common fatty acids in biology contain an even number of carbons between 12 and 20.
Defining the Fatty Acid Structure
Every fatty acid molecule consists of two distinct regions that grant it a dual nature, known chemically as amphipathic. At one end is the long hydrocarbon chain, often referred to as the “tail,” which is nonpolar and hydrophobic, meaning it repels water. This tail is composed solely of carbon and hydrogen atoms, making it similar to a small oil or wax molecule.
The other end features the carboxyl group, the “head” of the molecule, which is highly polar and hydrophilic, meaning it readily interacts with water. Because the hydrophobic tail is usually much longer than the hydrophilic head, the entire molecule is generally water-insoluble.
The Chemical Significance of the Carboxyl Group
The carboxyl group’s structure, consisting of a carbon atom double-bonded to one oxygen and single-bonded to a hydroxyl (–OH) group, gives the molecule its acidic properties. Under physiological conditions, such as the neutral pH found in the human body, the hydrogen atom readily dissociates as a proton (H+). This ability to donate a proton is the chemical definition of an acid, which is why the molecule is named a fatty acid.
Once the proton is released, the carboxyl group becomes negatively charged (–COO-), forming a carboxylate anion. This charged, reactive end is the primary site for esterification, the molecule’s most important biological function. Through this reaction, the carboxyl group forms an ester bond with the alcohol group of a glycerol molecule. This process creates major storage and structural lipids, such as triglycerides and phospholipids.
Structural Variations: Saturated and Unsaturated Chains
While the carboxyl group is constant, the structure of the hydrocarbon chain differentiates one fatty acid from another and dictates its physical properties. The chain is categorized based on the types of bonds connecting the carbon atoms within the tail. Saturated fatty acids contain only single bonds, allowing the molecule to remain straight and pack tightly together, which results in a solid state at room temperature.
Unsaturated fatty acids, in contrast, contain one or more double bonds between carbon atoms. A fatty acid with a single double bond is monounsaturated, while those with multiple double bonds are polyunsaturated. These double bonds introduce a rigid bend or “kink” into the chain. This structural irregularity prevents the molecules from packing closely, which lowers the melting point and causes unsaturated fatty acids to be liquid at room temperature, such as in most plant oils.