What defines the primary structure of a protein?

The primary structure of a protein is defined by the specific sequence of amino acids that are linked together through peptide bonds. This linear arrangement is crucial because the order of the amino acids determines how the protein will fold into its higher-level structures, such as secondary, tertiary, and quaternary structures. Each amino acid in the sequence plays a unique role in the overall function of the protein, and even a single change in this sequence can significantly affect the protein's properties and functionality.

The other options relate to different aspects of protein structure. The three-dimensional shape of a protein is described by its tertiary structure and is influenced by the primary structure but is not the primary characteristic itself. The number of functional proteins is not a defining feature of primary structure; instead, it pertains to the protein as a whole in a biological context. Lastly, while chemical bonds between amino acids (like peptide bonds) are essential for forming the primary structure, they do not define it; rather, they facilitate its formation by connecting the amino acids in sequence.