Final answer:
The primary structure of a protein describes the linear sequence of amino acids in a polypeptide chain.
Step-by-step explanation:
The level of structure that describes the linear arrangement of amino acids in a polypeptide is known as the primary structure. This structure is the sequence in which amino acids are arranged, and held together by peptide bonds. An example of a primary structure can be found in the protein insulin, which is composed of 51 amino acids. The primary structure is vital because it determines the final shape and function of the protein after it undergoes further folding and modifications in the secondary, tertiary, and quaternary structures.