Proteins are considered polymers because they are made up of long chains of amino acids linked together by peptide bonds. These amino acids join in a specific sequence to form a polypeptide chain, and a protein can be made of one or more of these chains. The structure and function of proteins are determined by the sequence of amino acids, allowing for a great variety of proteins with different functions.
In contrast, lipids do not fit the definition of polymers. While lipids are large molecules made up of smaller subunits, they do not form long chains through a repetitive process like polymers do. Instead, lipids are typically made of glycerol and fatty acids, which do not repeat in a chainlike fashion. For example, triglycerides consist of one glycerol molecule connected to three fatty acid chains, but this is not repeated to form a long chain. Therefore, even though lipids are complex molecules, they lack the characteristic repetitive structure of polymers.