Signal transduction – Which statement accurately describes a hydrophilic (water-soluble) signaling molecule and its mode of action?

Introduction:
Hydrophilic signaling molecules (e.g., many peptide hormones, catecholamines) cannot readily cross the hydrophobic core of the plasma membrane. Their biology illustrates the principle that information can be transmitted across membranes without the ligand entering the cell, using receptors and second-messenger systems.

Given Data / Assumptions:

• Hydrophilic ligands are water-soluble and generally membrane-impermeant.
• Receptors for hydrophilic ligands are usually transmembrane proteins at the cell surface.
• Signal amplification often occurs via second messengers such as cAMP, IP3, DAG, and Ca2+.

Concept / Approach:
Hydrophilic ligands bind extracellular domains of cell-surface receptors (GPCRs, receptor tyrosine kinases) to initiate intracellular signaling cascades. The downstream effectors and second messengers propagate and amplify the signal to change gene expression, metabolism, secretion, or contractility. Steroid hormones are typically hydrophobic and use intracellular receptors—opposite of the hydrophilic paradigm.

Step-by-Step Solution:

Verification / Alternative check:
Epinephrine stimulates hepatic glycogen breakdown via a GPCR → cAMP pathway without epinephrine entering hepatocytes, proving the principle of membrane-initiated signaling.

Why Other Options Are Wrong:

• Cytosolic/nuclear receptors: typical of hydrophobic ligands (steroids, thyroid hormone).
• Direct intracellular action without receptors: contradicts membrane impermeability and receptor necessity.
• “Necessarily a steroid”: steroids are hydrophobic, not hydrophilic.
• Covalent entry via transporters: not a standard requirement for signaling.

Common Pitfalls:
Assuming all hormones enter cells; many act entirely through surface receptors and second messengers.

Final Answer:
It often triggers a membrane-initiated signaling cascade that produces cellular effects