Final answer:
Protein hormones begin their action by binding to receptors on the plasma membrane, initiating a signaling pathway inside the cell that leads to the desired cellular response.
Step-by-step explanation:
Protein hormones act initially on target cells by attaching to receptors on the plasma membrane. These hormones are water-soluble and cannot diffuse through the lipid bilayer of cell membranes. Instead, they bind to receptors on the cell surface, which triggers a series of events inside the cell. This binding activates a signaling pathway involving various intracellular molecules, also known as second messengers, which then carry out the effects of the hormone. This contrasts with steroid hormones, which are lipid-soluble and can pass through the plasma membrane to bind to intracellular receptors and directly influence gene transcription.