Questions: Cell Signaling and Signal Transduction

Question 1

Epinephrine (adrenaline) is a peptide hormone. How does it trigger changes inside a target cell?

• A. It diffuses through the membrane and binds a nuclear receptor
• B. It binds a surface GPCR, triggering a cascade via second messengers like cAMP
• C. It directly activates DNA transcription from outside the cell
• D. It enters via active transport and phosphorylates intracellular enzymes

Answer: (B)

Epinephrine is hydrophilic and cannot cross the lipid bilayer. It binds G protein-coupled receptors (GPCRs) on the cell surface, activating G proteins that stimulate adenylyl cyclase to produce cAMP. cAMP then activates protein kinase A (PKA), which phosphorylates downstream targets. Only lipid-soluble hormones (steroids, thyroid hormone) cross the membrane directly.

Question 2

Signal amplification in transduction means that one ligand-receptor binding event can ultimately activate thousands of downstream molecules.

• T. True
• F. False

Answer: True

Amplification is a defining feature of signal transduction cascades. Each activated enzyme can catalyze reactions on many substrate molecules, so the signal grows at each step. A single epinephrine molecule activating one GPCR can ultimately trigger the release of millions of glucose units from glycogen. This is why cells can respond to vanishingly small hormone concentrations.

Question 3

What is the role of a second messenger like cAMP, and why is one needed?

Model answer: Second messengers relay and amplify signals inside the cell. They are needed because most signaling molecules are too large or hydrophilic to cross the cell membrane, so an intracellular messenger bridges the gap between surface receptor activation and the intracellular response.

Peptide hormones and neurotransmitters bind surface receptors but stay outside the cell. cAMP, produced inside the cell in response to receptor activation, carries the signal into the cytoplasm and amplifies it — one activated receptor produces many cAMP molecules, each of which can activate a kinase, which can phosphorylate many targets. Without this relay, signals from outside could not reach intracellular machinery.