Final answer:
Tyrosine derivatives are categorized separately from polypeptide hormones because they are water insoluble and bind to cell surface receptors, initiating signaling pathways, unlike water-soluble peptide hormones which can also bind to surface receptors but have different structures.
Step-by-step explanation:
Tyrosine derivatives are treated as a separate class of hormones from the polypeptide class due to their solubility and receptor interaction characteristics. Amino acid-derived hormones such as epinephrine, which is derived from tyrosine, are generally water insoluble and thus cannot diffuse through the lipid bilayer of plasma membranes. This class of hormones binds to receptors on the outer surface of cells and triggers a chain of events within the cell through second messenger systems like cAMP. In contrast, peptide hormones are typically water-soluble and consist of short or long chains of amino acids. These hormones also bind to receptors on the cell surface and initiate a signaling pathway leading to the intended cellular response.