Biological macromolecules include proteins, carbohydrates, nucleic acids, and lipids. Lipids, such as fats, oils, and waxes, are defined by being hydrophobic, meaning they do not dissolve in water. They serve many functions, including long-term energy storage and insulation. Unlike the other three macromolecules, lipids do not follow the standard model of molecular construction based on a single, repeating building block.
The Monomer-Polymer Framework
Biological macromolecules are typically created through polymerization, where small molecular units called monomers link together repeatedly to form a long chain known as a polymer. This framework applies to proteins, which are built from amino acid monomers, and carbohydrates, which are formed by linking simple sugar monomers. The expectation that all macromolecules follow this repeating-unit pattern leads to the question of the lipid monomer. However, lipids are structurally distinct and do not form indefinite chains of identical subunits, meaning they are not considered true polymers.
Glycerol and Fatty Acids: The Lipid Building Blocks
Lipids are constructed from two distinct types of smaller molecules: glycerol and fatty acids. Because these components are not repeating and are chemically different, they are better described as building blocks rather than a single monomer. Glycerol acts as a backbone, a small three-carbon alcohol molecule with three hydroxyl (-OH) groups that provide sites for chemical attachment.
Fatty acids consist of a long hydrocarbon chain attached to a carboxyl (-COOH) group at one end. The structure of this chain determines the fat’s properties. Saturated fatty acids have only single bonds, allowing the chain to remain straight, which often makes the fat solid at room temperature. In contrast, unsaturated fatty acids contain one or more double bonds, which introduce a physical bend or “kink” into the chain. This structural feature prevents tight packing and typically results in a fat that is liquid at room temperature.
How Lipids Assemble
The most common type of lipid, the triacylglycerol (triglyceride), is formed by chemically joining the glycerol backbone to three fatty acid chains. This assembly occurs through dehydration synthesis, where a molecule of water is removed for each fatty acid attached. The hydroxyl group from the glycerol reacts with the carboxyl group of the fatty acid, forming a chemical bond known as an ester linkage. Three ester linkages are formed to create the final triglyceride molecule, releasing a total of three water molecules.