How do peptide hormones differ from steroid hormones in their mechanism of action?

• A. Peptide hormones bind membrane receptors and use second messengers
• B. Peptide hormones diffuse into cells and affect gene transcription
• C. Steroid hormones bind membrane receptors
• D. Peptide hormones require enzymatic activation within the nucleus

Answer: (A)

Peptide hormones act by binding to receptors on the cell surface and triggering second messenger systems inside the cell. Because they are water-soluble, they cannot cross the lipid cell membrane, so their first step is to engage membrane receptors, which then activate pathways like cAMP or IP3/DAG to produce rapid cellular responses. Steroid hormones, on the other hand, are lipid-soluble and diffuse across the membrane to bind receptors inside the cell—often in the cytoplasm or nucleus—where the hormone–receptor complex acts as a transcription factor to regulate gene expression, leading to slower, longer-lasting effects. So the description that peptide hormones bind membrane receptors and use second messengers best captures their mechanism.