Lipids are polymers because they are formed by condensation reactions from fatty acid monomers and alcohols. While they do not share the same physical characteristics of other polymers, such as plastic, they still meet the definition of a polymer.
A polymer is a molecule that is created by combining other molecules through condensation reactions. These are called condensation reactions because they release water, and, in the case of lipids, the water is released when the fatty acid's carboxyl group (-COOH) and the alcohol's hydroxyl group (-OH) react. Despite the length of its carbon chain, a fatty acid ester is a monomer, not a polymer. It is only when it is connected to an alcohol, such as glycerol, that it becomes a polymer. When three fatty acids are connected through glycerol, they form a triglyceride, or fat. Triglycerides are one of the most common types of lipid. They give the body energy. Sterol is another common alcoholic precursor to lipids, as it forms cholesterol.
Like lipids, proteins, carbohydrates and nucleic acids are linked together through condensation reactions and broken apart through hydrolysis (water-absorbing) reactions, making them polymers as well. This class of molecules, called macromolecules, serves as the building blocks for life.