What defines the structure of a polypeptide in biology?

The structure of a polypeptide is defined by a chain of amino acids. Amino acids are the building blocks of proteins, and when they are linked together through peptide bonds, they form a polypeptide chain. The sequence of amino acids in the chain determines the specific characteristics and function of the protein, as the unique arrangement influences how the polypeptide folds into its three-dimensional structure. This structure is crucial for the protein's activity.

In the context of biological macromolecules, the other options do not represent the structure of polypeptides. Nucleotides are the building blocks of nucleic acids, such as DNA and RNA. Fatty acids are components of lipids, not proteins. Sugars are monomers of carbohydrates and do not contribute to the formation of polypeptides. Each of these classes of macromolecules plays distinct roles in biological systems, but only amino acids are relevant to polypeptide formation.