Rucete ✏ AP Biology In a Nutshell
8. Cell Communication and Signaling — Practice Questions 3
This chapter reviews how cells communicate and regulate their internal activities through different signaling mechanisms, secondary messengers, and feedback responses.
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(Multiple Choice — Click to Reveal Answer)
1. Which structure allows adjacent animal cells to exchange ions and small molecules directly?
(A) Plasmodesmata
(B) Gap junctions
(C) Tight junctions
(D) Receptor tyrosine kinases
Answer
(B) — Gap junctions create direct channels between neighboring animal cells for communication.
2. A mutation prevents the activation of a G-protein after ligand binding. Which step in signaling would be blocked first?
(A) Ligand-receptor binding
(B) G-protein activation
(C) Production of secondary messenger
(D) Cellular response
Answer
(B) — Without G-protein activation, the cascade of signal transduction cannot proceed.
3. Which feature is characteristic of paracrine signaling?
(A) Signals travel through the bloodstream.
(B) Signals diffuse locally to nearby cells.
(C) Signals involve direct cell-cell contact.
(D) Signals affect only the secreting cell itself.
Answer
(B) — Paracrine signaling affects nearby cells within a short distance from the source.
4. Which outcome is most likely if a hydrophobic ligand binds an intracellular receptor?
(A) Activation of transcription inside the nucleus
(B) Opening of ion channels
(C) Activation of G-proteins
(D) Phosphorylation of membrane proteins
Answer
(A) — Hydrophobic ligands bind to receptors inside the cell, often influencing gene transcription.
5. What is the first step in a typical cell signaling pathway?
(A) Phosphorylation of target proteins
(B) Ligand binding to a receptor
(C) Synthesis of secondary messengers
(D) Release of hormones into the bloodstream
Answer
(B) — Signal reception always begins with the ligand binding to its specific receptor.
6. Which receptor type undergoes dimerization and autophosphorylation after ligand binding?
(A) G-protein-coupled receptor
(B) Receptor tyrosine kinase
(C) Ligand-gated ion channel
(D) Intracellular steroid receptor
Answer
(B) — Receptor tyrosine kinases dimerize and autophosphorylate to initiate signal transduction.
7. In synaptic signaling, what is the primary molecule released by neurons to propagate the signal?
(A) Growth factor
(B) Neurotransmitter
(C) Hormone
(D) Steroid
Answer
(B) — Neurotransmitters are released across synapses to transmit signals between neurons or between neurons and other cells.
8. Which event would happen first in a phosphorylation cascade?
(A) Dephosphorylation of a kinase
(B) Activation of a receptor
(C) Entry of calcium ions into the cytoplasm
(D) Transcription of target genes
Answer
(B) — The initial activation of a receptor triggers the start of the phosphorylation cascade.
9. What type of feedback mechanism reduces the intensity of a signal after a cellular response is achieved?
(A) Positive feedback
(B) Negative feedback
(C) Cross-talk inhibition
(D) Homeostatic amplification
Answer
(B) — Negative feedback dampens the pathway activity after the necessary response has occurred.
10. In signal transduction, what is the role of secondary messengers like cAMP or IP₃?
(A) Binding ligands on the plasma membrane
(B) Amplifying and distributing the signal inside the cell
(C) Degrading ligands before they reach the receptor
(D) Synthesizing DNA in the nucleus
Answer
(B) — Secondary messengers relay and amplify signals after primary ligand binding to receptors.
11. What happens when a ligand-gated ion channel is activated?
(A) ATP is hydrolyzed.
(B) Ions move rapidly across the membrane.
(C) G-proteins are deactivated.
(D) DNA replication begins immediately.
Answer
(B) — Activation of a ligand-gated ion channel allows ions to flow across the membrane, changing cell voltage or ion concentration.
12. Which of the following can act as a first messenger in a signaling pathway?
(A) cAMP
(B) GTP
(C) Calcium ion
(D) Insulin hormone
Answer
(D) — Hormones like insulin are examples of first messengers that bind to receptors to initiate signaling.
13. What enables specificity in cellular responses to signaling molecules?
(A) Speed of ligand diffusion
(B) Number of mitochondria present
(C) Type of receptor and intracellular machinery
(D) Amount of ATP available
Answer
(C) — Specific responses arise from different receptors and signal transduction proteins expressed by the cell.
14. Which process would be impaired if a cell had defective adenylate cyclase?
(A) ATP hydrolysis
(B) Production of cyclic AMP
(C) Activation of receptor tyrosine kinases
(D) Opening of voltage-gated ion channels
Answer
(B) — Adenylate cyclase catalyzes the formation of cAMP, a critical secondary messenger.
15. Which of the following best defines "signal transduction" in cellular communication?
(A) Production of ATP from glucose
(B) Movement of ions through gap junctions
(C) Conversion of a ligand binding event into a specific cellular response
(D) Direct secretion of hormones into the bloodstream
Answer
(C) — Signal transduction refers to converting an extracellular signal into a specific functional change within the cell.
16. What would most likely happen if a cell’s ligand-gated ion channels remained open too long?
(A) The cell would continuously release hormones.
(B) Ion imbalances could disrupt membrane potential and cellular functions.
(C) Secondary messengers would stop being produced.
(D) G-proteins would remain inactive.
Answer
(B) — Prolonged ion flow would disturb the cell's internal balance, possibly impairing or damaging the cell.
17. Which of the following best defines the role of a receptor in a signaling pathway?
(A) Degrading ligands before signal transmission
(B) Generating new ligands
(C) Recognizing the ligand and initiating a response
(D) Transporting ions across the membrane
Answer
(C) — Receptors detect external signals (ligands) and begin intracellular signaling cascades.
18. Which condition would most likely prevent calcium ions from functioning as secondary messengers?
(A) Defective ligand-receptor binding
(B) Malfunction of GTP-binding proteins
(C) Blockage of ion channels in the endoplasmic reticulum
(D) Increased production of insulin
Answer
(C) — If calcium cannot be released from the ER due to blocked channels, it cannot act as a secondary messenger.
19. Which statement about intracellular receptors is correct?
(A) They always activate ion channels.
(B) They typically bind hydrophilic ligands.
(C) They often act as transcription factors when activated.
(D) They are located on the plasma membrane.
Answer
(C) — Intracellular receptors often directly regulate gene expression by acting as transcription factors after ligand binding.
20. What allows G-proteins to switch between active and inactive states?
(A) Phosphorylation by kinases
(B) Binding and hydrolysis of GTP
(C) Secretion of neurotransmitters
(D) Direct ion flow across membranes
Answer
(B) — G-proteins become active when bound to GTP and inactive when GTP is hydrolyzed to GDP.
21. Which secondary messenger is directly responsible for activating protein kinase A (PKA)?
(A) Calcium ions
(B) Cyclic AMP (cAMP)
(C) IP₃
(D) GTP
Answer
(B) — cAMP activates protein kinase A, triggering further steps in the signaling cascade.
22. Which process in signal transduction pathways most directly leads to amplification of the original signal?
(A) Activation of multiple secondary messengers
(B) Ligand degradation
(C) Receptor internalization
(D) Termination of the kinase cascade
Answer
(A) — Secondary messengers like cAMP or IP₃ amplify the signal by activating many downstream targets.
23. Which best describes juxtacrine signaling?
(A) Hormones travel through the bloodstream.
(B) Ligands are secreted into extracellular fluid.
(C) Direct cell-to-cell contact transmits the signal.
(D) Ligands diffuse over long distances.
Answer
(C) — Juxtacrine signaling occurs when two adjacent cells communicate through direct contact.
24. What is the purpose of ligand-induced receptor dimerization in tyrosine kinase signaling?
(A) To inhibit signal transduction
(B) To cause rapid breakdown of ATP
(C) To bring two receptor units together for cross-phosphorylation
(D) To open ion channels directly
Answer
(C) — Dimerization brings receptor units together, allowing them to phosphorylate each other and activate downstream signaling.
25. Which of the following outcomes is most likely if secondary messenger production is blocked inside a cell?
(A) Signal reception would still occur, but no internal response would happen.
(B) The ligand would be degraded faster.
(C) The nucleus would immediately initiate DNA replication.
(D) G-proteins would continue to function normally without interruption.
Answer
(A) — Signal binding could still occur at the membrane, but the lack of secondary messengers would prevent internal signal propagation and cellular response.
26. A mutation causes a GPCR to remain active even after the ligand is removed. What would most likely happen inside the cell?
(A) No response would occur.
(B) The signaling pathway would stay activated continuously.
(C) The receptor would immediately degrade.
(D) The secondary messengers would deactivate automatically.
Answer
(B) — A constitutively active GPCR would keep stimulating the pathway even without ligand presence.
27. What distinguishes receptor tyrosine kinases from G-protein-coupled receptors in their method of signal transduction?
(A) RTKs phosphorylate themselves, GPCRs activate G-proteins.
(B) GPCRs are only found inside the nucleus.
(C) RTKs require secondary messengers, GPCRs do not.
(D) GPCRs directly synthesize ATP after ligand binding.
Answer
(A) — RTKs self-phosphorylate to propagate signals, while GPCRs activate intermediary G-proteins.
28. If phosphatases were inhibited during a signaling event, what would be the most likely cellular effect?
(A) Premature termination of the signal
(B) Persistent activation of kinases and their targets
(C) Rapid degradation of ligands
(D) Immediate deactivation of all second messengers
Answer
(B) — Without phosphatases, proteins remain phosphorylated longer, extending the signal.
29. What is the most direct effect of inhibiting adenylate cyclase in a signal transduction pathway relying on cAMP?
(A) Excessive calcium release
(B) Blocking production of cAMP and preventing downstream responses
(C) Immediate receptor degradation
(D) Overactivation of G-proteins
Answer
(B) — Adenylate cyclase produces cAMP; its inhibition stops cAMP-dependent signaling.
30. In the context of signaling, what best describes the concept of cross-talk between pathways?
(A) Different pathways activating each other
(B) Pathways competing for ATP
(C) Signaling molecules degrading each other
(D) Direct receptor-to-receptor binding at the membrane
Answer
(A) — Cross-talk refers to interaction where one signaling pathway influences another, modifying the overall response.
31. What would happen if a cell lost the ability to hydrolyze GTP on its G-proteins?
(A) G-proteins would remain inactive.
(B) G-proteins would stay active longer, potentially overactivating downstream pathways.
(C) Signal transduction would not initiate.
(D) Secondary messengers would be degraded faster.
Answer
(B) — Without GTP hydrolysis, G-proteins would stay active indefinitely, causing persistent signaling.
32. Which event initiates calcium release from intracellular stores during signal transduction?
(A) GTP hydrolysis
(B) Opening of ligand-gated ion channels in the plasma membrane
(C) IP₃ binding to receptors on the endoplasmic reticulum
(D) Activation of transcription factors
Answer
(C) — IP₃ binds to ER receptors, triggering calcium release into the cytoplasm.
33. A mutation causes adenylate cyclase to become overactive. What effect would this have on the cell?
(A) Decrease in cAMP concentration
(B) Prolonged activation of protein kinase A and amplified responses
(C) Immediate receptor deactivation
(D) Suppression of calcium release
Answer
(B) — Overactive adenylate cyclase produces excess cAMP, prolonging PKA activation and the cellular response.
34. Which situation is an example of a nuclear response to a signal transduction pathway?
(A) Activation of ATP synthase
(B) Opening of voltage-gated ion channels
(C) Transcription of a gene regulated by a transcription factor
(D) Phosphorylation of cytoplasmic enzymes
Answer
(C) — A nuclear response involves changes to gene expression, such as transcription initiation.
35. How does the structure of receptor tyrosine kinases enable them to trigger multiple cellular responses from a single ligand-binding event?
(A) They open ion channels in the membrane.
(B) They phosphorylate many different intracellular proteins.
(C) They directly activate DNA replication.
(D) They hydrolyze ATP to GTP after ligand binding.
Answer
(B) — RTKs, after dimerization and autophosphorylation, can activate multiple downstream proteins simultaneously.
36. Explain why ligand binding to a receptor is considered the first step in signal transduction.
Answer
Ligand binding initiates the signaling process by causing a structural change in the receptor, which triggers a series of intracellular events leading to a cellular response.
37. How does the structure of G-proteins contribute to their role as molecular switches?
Answer
G-proteins can bind either GDP (inactive) or GTP (active), allowing them to alternate between off and on states depending on ligand-receptor interactions.
38. What would happen to signal transduction if a mutation blocked phosphorylation of receptor tyrosine kinases?
Answer
Without phosphorylation, downstream signaling proteins could not bind, preventing activation of signaling cascades and cellular responses.
39. Describe the importance of secondary messengers like cAMP in cell communication.
Answer
Secondary messengers like cAMP amplify the initial signal and distribute it inside the cell, enabling rapid and widespread activation of multiple pathways.
40. Why must cells regulate calcium ion concentration tightly in the cytosol?
Answer
Calcium acts as a potent secondary messenger, and uncontrolled increases can activate inappropriate responses or cause cell damage.
41. How does feedback inhibition contribute to the control of signal transduction pathways?
Answer
Feedback inhibition stops or slows signaling once the desired cellular response is achieved, preventing overstimulation and conserving energy.
42. Predict what would happen if a phosphodiesterase enzyme became hyperactive in a cAMP signaling pathway.
Answer
Excessive phosphodiesterase activity would break down cAMP too quickly, reducing activation of downstream targets like protein kinase A and weakening the signal response.
43. What distinguishes endocrine signaling from paracrine signaling at the molecular level?
Answer
Endocrine signaling involves hormone molecules traveling through the bloodstream to distant targets, whereas paracrine signals diffuse locally to nearby cells.
44. How do cells achieve specificity in their response to the same ligand?
Answer
Cells achieve specificity by expressing different receptor types, secondary messengers, or intracellular proteins that interpret and process the signal differently.
45. Describe the role of IP₃ in releasing calcium ions from internal stores.
Answer
IP₃ binds to receptor channels on the endoplasmic reticulum, causing them to open and release stored calcium ions into the cytoplasm.
46. Explain why phosphorylation cascades are important for amplifying cellular signals.
Answer
Each activated kinase can phosphorylate many target molecules, rapidly amplifying the signal and enabling a strong cellular response from a small initial stimulus.
47. What could be a potential outcome if a ligand fails to dissociate from its receptor?
Answer
If the ligand remains bound, the receptor could stay continuously active, leading to prolonged or inappropriate cellular responses such as uncontrolled growth.
48. Why are intracellular receptors typically associated with slower but longer-lasting responses?
Answer
Intracellular receptors often regulate gene expression directly, which takes longer to manifest but leads to sustained changes in protein production and cell behavior.
49. How does the activation of transcription factors represent a nuclear response to signaling?
Answer
Activated transcription factors enter the nucleus, bind to DNA, and promote or repress the transcription of specific genes, altering cellular function.
50. Summarize the three key stages of a typical signal transduction pathway.
Answer
The three stages are: 1) reception of the signal at the receptor, 2) transduction and amplification of the signal through intermediates, and 3) generation of a specific cellular response.