Where do water-soluble hormones bind receptors?

• A. Inside the nucleus
• B. Bind to receptors on the surface of the cell
• C. Inside the cytoplasm
• D. Located in mitochondria

Answer: (B)

Water-soluble hormones cannot cross the lipid bilayer because they are polar and hydrophilic. To convey their signal, they bind to receptors on the surface of the cell. This binding activates signal transduction pathways inside the cell, often through second messengers like cAMP, IP3, DAG, or calcium, which then alter enzyme activity or gene expression without the hormone entering the cell. For example, many peptide hormones and catecholamines trigger these surface receptors to start cascades that change metabolism or cell behavior.

In contrast, lipid-soluble hormones (like steroids) can diffuse through the membrane and bind receptors inside the cell—in the cytoplasm or nucleus—where they directly influence gene transcription. The options placing receptors Inside the nucleus, Inside the cytoplasm, or in mitochondria would require the hormone to cross the membrane, which water-soluble hormones do not do, so those locations aren’t correct for this type of hormone.