Questions: Cell Signaling: External Signals to Internal Response

Question 1

A single epinephrine molecule binding to a GPCR on a liver cell can trigger the release of millions of glucose molecules from glycogen. What mechanism produces this enormous amplification?

• A. Each epinephrine molecule binds multiple receptors simultaneously, activating them in parallel
• B. Each step in the cascade activates many downstream molecules: one receptor activates many G proteins, each G protein activates an enzyme producing many cAMP molecules, each cAMP activates many kinases, and so on
• C. Epinephrine crosses the membrane and directly activates the enzymes responsible for glycogen breakdown
• D. The liver stores pre-activated signaling proteins that only require a small trigger to release all at once

Answer: (B)

Signal amplification is a cascade effect: at each step, one activated molecule activates many downstream molecules. One GPCR activates ~10-100 G proteins; each activated adenylyl cyclase produces many cAMP molecules per second; each cAMP molecule activates a protein kinase A that phosphorylates many target proteins. The amplification multiplies at every node. This explains why hormones circulating at nanomolar or picomolar concentrations can produce dramatic physiological effects — the signal is not just transmitted but vastly amplified. Option C is wrong: most hydrophilic hormones (including epinephrine) cannot cross the plasma membrane and work entirely through surface receptors.

Question 2

A researcher blocks all G protein-coupled receptors in a cell but leaves receptor tyrosine kinases intact. Which cellular process would most likely be most severely impaired?

• A. Long-term transcriptional responses to growth factors, since RTKs primarily mediate fast responses
• B. Rapid ion flux changes in neurons, since GPCRs control ion channel gating in all neurons
• C. Hormonal responses requiring cAMP as a second messenger, such as epinephrine-stimulated glycogen breakdown
• D. All cell division, since GPCRs are the primary pathway for growth factor signaling

Answer: (C)

cAMP is produced by adenylyl cyclase, which is activated by the G protein Gαs — a downstream effector of GPCRs. Blocking all GPCRs prevents cAMP production from this route, disrupting all cAMP-dependent processes including epinephrine-stimulated glycogen breakdown and many hormonal responses. Option A has it backwards — RTKs are primarily responsible for growth and differentiation (longer-term effects), while GPCRs often mediate faster responses. Option B is partially true for some neurons but overstated — many neuronal GPCRs modulate synaptic transmission, but ligand-gated ion channels (a separate receptor class) operate independently of GPCRs.

Question 3

Cell signaling pathways are linear chains: one receptor activates one pathway, which produces one response.

• T. True
• F. False

Answer: False

This is the most common misconception about signal transduction. Real signaling is a network, not a chain. Cross-talk is pervasive: the same second messenger (e.g., Ca²⁺) can be elevated by multiple different receptors; the same kinase can be activated by different upstream inputs; a single pathway can branch to activate multiple downstream targets. Furthermore, pathways have feedback loops — positive feedback amplifies and accelerates responses; negative feedback terminates and modulates them. A growth factor that promotes proliferation in one cell type may trigger differentiation in another, depending on what other signals are present and which downstream targets are expressed. The context-dependence of signaling responses is a direct consequence of this network architecture.

Question 4

Some signaling receptors are located inside the cell rather than on the plasma membrane, but these primarily respond to signals synthesized within the cell.

• T. True
• F. False

Answer: False

Intracellular receptors respond to extracellular signals — they just respond to signals that are hydrophobic enough to cross the plasma membrane. Steroid hormones (cortisol, estrogen, testosterone), thyroid hormones, and vitamin D are all synthesized outside the target cell, circulate in the bloodstream, diffuse across the plasma membrane, and bind to intracellular receptors (often in the cytoplasm or nucleus). These receptor-ligand complexes then directly regulate gene transcription. The distinction is not 'inside vs. outside origin' but 'lipid-soluble vs. water-soluble': hydrophilic signals cannot cross membranes and require surface receptors; hydrophobic signals can cross and use intracellular receptors.

Question 5

Why does signal amplification through a second-messenger cascade allow a small number of hormone molecules in the bloodstream to produce a large physiological response, and what is the structural feature of the cascade that enables this?

Model answer: Signal amplification works because each activated molecule in the cascade activates many copies of the next molecule. This is possible because the activated proteins are enzymes (or activate enzymes): a single activated kinase can phosphorylate hundreds of substrate proteins per minute before it is inactivated. A single GPCR can activate ~10-100 G proteins during its active lifetime; each activated adenylyl cyclase produces thousands of cAMP molecules; each cAMP-activated protein kinase phosphorylates many targets. The multiplication at every step is why the cascade is called a 'cascade' — the amplification is not additive but multiplicative. A single hormone molecule binding one receptor can ultimately alter the activity of millions of intracellular proteins within seconds.

This amplification is also why signaling needs to be tightly regulated. Runaway amplification would be catastrophic — uncontrolled cell growth, inappropriate metabolic shifts, etc. The cascade is therefore paired with multiple off-switches: phosphodiesterases degrade cAMP, phosphatases remove phosphate groups added by kinases, receptor internalization removes receptors from the surface, and inhibitory signals activate brakes at multiple nodes. The gain is high, but the control is also high.