Rucete ✏ AP Biology In a Nutshell
8. Cell Communication and Signaling — Practice Questions 2
This chapter explores how cells send, receive, and respond to signals through receptors, secondary messengers, and feedback systems to maintain homeostasis or amplify changes.
Need a quick review?
📘 Go to the Concept Summary
(Multiple Choice — Click to Reveal Answer)
1. Which feature allows hydrophobic ligands to interact with intracellular receptors?
(A) They require membrane transport proteins.
(B) They dissolve in water easily.
(C) They diffuse through the phospholipid bilayer.
(D) They bind ion channels.
Answer
(C) — Hydrophobic ligands can pass through the membrane without assistance and bind intracellular receptors.
2. A mutation causes a receptor to remain constantly active even in the absence of a ligand. What is the most likely result?
(A) No cellular response will occur.
(B) The signal will be delayed.
(C) The pathway will be overactivated.
(D) The receptor will degrade immediately.
Answer
(C) — A constitutively active receptor continuously signals even without ligand binding, potentially leading to overstimulation.
3. Which part of a receptor protein determines ligand specificity?
(A) Secondary messenger site
(B) Transmembrane domain
(C) Ligand-binding domain
(D) GTP-binding pocket
Answer
(C) — The ligand-binding domain has a specific shape that recognizes and binds particular signaling molecules.
4. Which of the following is most likely to cause an interruption in a signal transduction pathway?
(A) Increased membrane permeability
(B) Mutation in a secondary messenger
(C) ATP synthesis in mitochondria
(D) Passive diffusion of glucose
Answer
(B) — Mutations in secondary messengers like cAMP can disrupt entire signaling cascades.
5. What is the main function of a second messenger like IP₃ or DAG in cell signaling?
(A) Bind directly to DNA
(B) Act as receptors on the plasma membrane
(C) Amplify and relay signals from receptors to internal targets
(D) Transport ligands through the bloodstream
Answer
(C) — Second messengers help relay the signal from the membrane to internal components, amplifying the response.
6. Which event would most likely occur first in a typical signal transduction pathway?
(A) Kinase phosphorylation
(B) Ligand binding to a receptor
(C) DNA transcription
(D) Protein synthesis
Answer
(B) — Signal transduction begins when a ligand binds to its receptor on the target cell.
7. Which of the following best describes the function of a scaffold protein in signaling?
(A) It degrades secondary messengers.
(B) It transports ligands between cells.
(C) It holds multiple kinases in place to streamline signaling.
(D) It binds DNA to initiate transcription.
Answer
(C) — Scaffold proteins enhance signaling efficiency by holding components of a pathway close together.
8. What is a key characteristic of synaptic signaling?
(A) It requires endocrine hormones.
(B) It uses physical cell-cell junctions.
(C) It occurs over long distances in the body.
(D) It relies on neurotransmitter release between adjacent neurons.
Answer
(D) — Synaptic signaling involves neurotransmitters crossing synaptic gaps between neurons or neurons and other cells.
9. Which of the following responses would most likely result from a signal transduction pathway activated by epinephrine?
(A) Increased glucose uptake by liver cells
(B) Muscle cell contraction due to Ca²⁺ increase
(C) Breakdown of glycogen to glucose
(D) Cell death by apoptosis
Answer
(C) — Epinephrine binding to liver cell receptors activates pathways that break down glycogen into glucose for quick energy.
10. Which statement correctly distinguishes paracrine signaling from endocrine signaling?
(A) Paracrine signals act locally; endocrine signals travel systemically via the bloodstream.
(B) Endocrine signaling only occurs in the nervous system.
(C) Paracrine signaling requires gap junctions.
(D) Endocrine signals affect only neighboring cells.
Answer
(A) — Paracrine signaling affects nearby cells, while endocrine signals travel longer distances through the circulatory system.
11. What is the main role of a kinase enzyme in a signal cascade?
(A) Transfer DNA into the nucleus
(B) Catalyze phosphorylation of target proteins
(C) Destroy excess ligands
(D) Bind to GTP and activate GPCRs
Answer
(B) — Kinases activate or inactivate proteins by transferring phosphate groups, driving signal progression.
12. Which condition would most likely disrupt signal transduction in a liver cell responding to glucagon?
(A) Defective G-protein activation
(B) Increased oxygen levels
(C) Normal ATP synthesis
(D) Presence of insulin
Answer
(A) — A defective G-protein would prevent the glucagon signal from being transduced properly, halting downstream effects.
13. What kind of ligand-receptor interaction involves a ligand binding that opens a channel for ions to pass through?
(A) GPCR activation
(B) Tyrosine kinase signaling
(C) Ligand-gated ion channel
(D) Intracellular receptor activation
Answer
(C) — Ligand-gated ion channels open to allow specific ions to enter or exit in response to ligand binding.
14. What is the effect of epinephrine binding to β-adrenergic receptors on heart muscle cells?
(A) Slower heart rate
(B) Increase in cyclic AMP production
(C) Immediate transcription of stress genes
(D) Closure of calcium channels
Answer
(B) — Epinephrine binding to β-adrenergic GPCRs activates adenylate cyclase, increasing cAMP production.
15. In feedback regulation of signaling, which of the following is an example of negative feedback?
(A) Signal causes more ligand release.
(B) Response inhibits the signaling pathway.
(C) Signal activates transcription of more receptors.
(D) Cell amplifies its response with each cycle.
Answer
(B) — Negative feedback reduces the activity of a signaling pathway once the desired response is achieved.
16. What type of signaling molecule is most likely to pass directly through the cell membrane without a receptor on the surface?
(A) Protein hormone
(B) Peptide neurotransmitter
(C) Steroid hormone
(D) Charged ion
Answer
(C) — Steroid hormones are lipid-soluble and can diffuse directly through the lipid bilayer to bind intracellular receptors.
17. Which of the following would likely occur if a ligand fails to bind to its receptor?
(A) The receptor activates its target protein anyway.
(B) Signal transduction is halted and no cellular response occurs.
(C) The receptor is phosphorylated automatically.
(D) A second messenger is still produced by default.
Answer
(B) — Without ligand binding, the receptor cannot activate, so the signal pathway does not proceed.
18. Which molecule is commonly associated with activating protein kinase A (PKA)?
(A) Calcium ion
(B) cAMP
(C) GTP
(D) IP₃
Answer
(B) — Cyclic AMP (cAMP) binds and activates protein kinase A in many signal transduction pathways.
19. Which is a characteristic of endocrine signaling that distinguishes it from paracrine signaling?
(A) It is limited to a single cell.
(B) It uses direct contact between cells.
(C) It acts on distant target cells via the circulatory system.
(D) It produces immediate local effects.
Answer
(C) — Endocrine signaling involves hormones that travel through the blood to reach distant target tissues.
20. Which statement best describes a key role of the plasma membrane in signal transduction?
(A) It blocks all signal molecules from entering.
(B) It contains receptors that initiate signaling when a ligand binds.
(C) It directly produces second messengers.
(D) It digests signal proteins through enzymes.
Answer
(B) — Receptors embedded in the membrane detect ligands and start the signal transduction process.
21. What happens immediately after a ligand binds to a receptor tyrosine kinase (RTK)?
(A) The receptor releases calcium ions.
(B) The receptor dimerizes and autophosphorylates.
(C) G-proteins are inactivated.
(D) Transcription begins in the nucleus.
Answer
(B) — Ligand binding causes RTKs to dimerize and phosphorylate their own tyrosine residues to start downstream signaling.
22. What would be the effect of blocking adenylate cyclase in a signaling pathway that relies on cAMP?
(A) cAMP levels would rise rapidly.
(B) No effect would be seen.
(C) The pathway would stop due to lack of cAMP.
(D) Calcium ion levels would spike.
Answer
(C) — Adenylate cyclase produces cAMP; blocking it would stop cAMP-dependent signaling.
23. Which of the following cell responses would be classified as a nuclear response to signaling?
(A) Activation of ion channels
(B) Phosphorylation of an enzyme
(C) Activation of a transcription factor to regulate gene expression
(D) Temporary increase in cytosolic calcium
Answer
(C) — A nuclear response affects gene expression, often by activating or repressing transcription factors.
24. What feature distinguishes a G-protein from a kinase in signal transduction?
(A) Kinases bind DNA, G-proteins do not.
(B) G-proteins hydrolyze GTP, while kinases transfer phosphate groups to proteins.
(C) Kinases only exist in the nucleus.
(D) G-proteins act as hormones.
Answer
(B) — G-proteins use GTP to turn "on" or "off," while kinases add phosphate groups to proteins to alter activity.
25. Which of the following is most likely to cause desensitization of a receptor?
(A) Mutation in a secondary messenger
(B) Prolonged exposure to a high concentration of ligand
(C) Low ligand availability
(D) Temporary absence of the receptor gene
Answer
(B) — Continuous high ligand exposure can cause a receptor to become less responsive, leading to desensitization or internalization.
26. A cell is exposed to a ligand that activates a GPCR, but downstream signaling does not occur. Which is the most likely cause?
(A) Ligand failed to bind to the receptor.
(B) The receptor was phosphorylated instead of dimerized.
(C) The associated G-protein cannot exchange GDP for GTP.
(D) The signal entered the nucleus too quickly.
Answer
(C) — If the G-protein cannot activate by exchanging GDP for GTP, the downstream signaling cascade cannot begin.
27. Which of the following best explains why a single ligand can trigger multiple cellular outcomes in different tissues?
(A) Each tissue uses the same pathway.
(B) The ligand breaks into different components.
(C) Target cells have different receptors and signal transduction machinery.
(D) Ligands can enter the cell directly and modify DNA.
Answer
(C) — Cellular context, such as specific receptor types and intracellular proteins, determines the outcome of ligand binding.
28. A mutation causes constant activation of a receptor tyrosine kinase. What is the likely result in the cell?
(A) The pathway is shut off by phosphatases.
(B) The cell remains unresponsive to the ligand.
(C) The signal is continuously propagated, possibly leading to uncontrolled growth.
(D) DNA replication is permanently blocked.
Answer
(C) — Constant receptor activation without ligand control can result in overactive signaling, often seen in cancer cells.
29. Which scenario best describes cross-talk between signaling pathways?
(A) One receptor blocks the function of another.
(B) A signal is stopped before entering the nucleus.
(C) Two different pathways share components or influence each other's activity.
(D) One pathway completely replaces another pathway’s function.
Answer
(C) — Cross-talk occurs when pathways intersect or share molecules, modifying cellular outcomes.
30. What would be the most direct consequence of a mutation that prevents a transcription factor from entering the nucleus?
(A) Phosphorylation cascades would fail.
(B) Second messengers would be degraded.
(C) Gene expression would not be altered in response to the signal.
(D) Calcium ions would accumulate in the cytoplasm.
Answer
(C) — Transcription factors must enter the nucleus to regulate gene expression; if blocked, the final response cannot occur.
31. Why do second messengers like cAMP and IP₃ contribute to signal amplification?
(A) They create new receptors on the cell surface.
(B) One molecule can activate many downstream targets rapidly.
(C) They inhibit feedback inhibition loops.
(D) They directly block phosphorylation cascades.
Answer
(B) — Second messengers increase the efficiency and magnitude of signaling by activating multiple targets.
32. What is the consequence of reduced phosphodiesterase activity in a cell using cAMP signaling?
(A) Increased ATP consumption
(B) Prolonged activation of protein kinase A
(C) Inactivation of G-protein
(D) Decrease in calcium ion concentration
Answer
(B) — Phosphodiesterase degrades cAMP. Without it, cAMP levels stay elevated, prolonging kinase activity.
33. A researcher disables all GPCRs in a set of cells. Which of the following responses would most likely be affected?
(A) Passive glucose transport
(B) Activation of protein kinase A by cAMP
(C) Synthesis of DNA polymerase
(D) Formation of lysosomes
Answer
(B) — GPCRs often activate adenylate cyclase, which produces cAMP. Without GPCRs, this signaling would be blocked.
34. What distinguishes signal amplification from signal diversification?
(A) Amplification increases the number of molecules activated; diversification results in different responses from the same signal.
(B) Amplification only occurs in the nucleus.
(C) Diversification depends solely on ATP concentration.
(D) Amplification and diversification are always the same process.
Answer
(A) — Amplification boosts signal strength, while diversification allows different outcomes in different contexts.
35. Which of the following is most directly responsible for deactivating a G-protein after it has transmitted a signal?
(A) Binding of calcium
(B) Hydrolysis of GTP to GDP
(C) Rebinding to the ligand
(D) Phosphorylation by a kinase
Answer
(B) — G-proteins deactivate themselves by hydrolyzing bound GTP to GDP.
36. Describe how a G-protein becomes inactivated after signal transmission.
Answer
After transmitting the signal, the G-protein hydrolyzes bound GTP to GDP, returning to its inactive form and dissociating from the effector enzyme.
37. How does signal amplification enhance the efficiency of a cellular response?
Answer
Amplification allows one ligand-receptor interaction to activate many downstream molecules, leading to a strong cellular response from a small initial signal.
38. Explain the role of scaffold proteins in organizing signaling pathways.
Answer
Scaffold proteins hold multiple components of a signaling pathway close together, increasing specificity and efficiency while preventing cross-talk.
39. What could be the cellular consequence of a defective phosphatase in a phosphorylation cascade?
Answer
Without phosphatase activity, proteins remain phosphorylated longer than necessary, potentially leading to prolonged or inappropriate cellular responses.
40. How can the same signaling molecule result in different responses in different cell types?
Answer
Different cells may express different receptors or signal transduction proteins, allowing the same ligand to trigger unique responses based on cell context.
41. What is the function of second messengers like IP₃ in signal transduction?
Answer
IP₃ binds to receptors on the endoplasmic reticulum, triggering calcium release into the cytoplasm and initiating specific cellular responses.
42. Why does endocrine signaling typically take longer than synaptic signaling?
Answer
Endocrine signals travel long distances through the bloodstream, requiring more time to reach target cells compared to rapid synaptic transmission across small gaps.
43. How do intracellular receptors activate gene expression?
Answer
Lipid-soluble ligands cross the membrane and bind intracellular receptors, which act as transcription factors by directly influencing DNA transcription in the nucleus.
44. Describe how negative feedback regulates signal transduction pathways.
Answer
Negative feedback occurs when the output of a pathway inhibits earlier steps, reducing receptor activity or enzyme function to prevent overstimulation.
45. Why is it important for cells to tightly regulate calcium ion concentration?
Answer
Calcium is a potent secondary messenger; unregulated levels can cause inappropriate activation of enzymes and cell death.
46. What happens if a cell’s GPCR remains permanently activated?
Answer
Prolonged GPCR activation leads to continuous signal transduction, which may result in excessive cell growth or metabolic imbalance.
47. How do paracrine and autocrine signaling differ in their target cells?
Answer
Paracrine signals affect nearby cells, while autocrine signals act on the same cell that produced the signal.
48. Explain the advantage of using phosphorylation cascades in cell signaling.
Answer
Phosphorylation cascades allow signal amplification, rapid response, and fine control through multiple regulatory steps.
49. What is the role of the ligand-binding domain in a membrane receptor?
Answer
The ligand-binding domain recognizes and binds specific signaling molecules, triggering receptor activation and downstream signaling.
50. Summarize the general steps of a cell signal transduction pathway.
Answer
Signal transduction involves reception (ligand binding), transduction (amplification through intermediates), and response (activation of gene expression or other cellular processes).