biology : class11 Signal Termination : Receptors, Second Messengers, Gene Regulation : Hormone Action MCQ Questions and Answers
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Signal Termination
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MCQ Questions
1. A hormone activates a G protein-coupled receptor and causes a rapid rise in cytosolic cAMP. Which change would most directly terminate the second-messenger signal without necessarily removing the hormone from the extracellular fluid?
A.
Activation of adenylyl cyclase
B.
Inhibition of phosphodiesterase
C.
Hydrolysis of cAMP to 5′-AMP by phosphodiesterase
D.
Conversion of ATP into cAMP
Correct Answer:
Hydrolysis of cAMP to 5′-AMP by phosphodiesterase
Explanation:
Phosphodiesterase terminates a cAMP signal by hydrolyzing cAMP into 5′-AMP, thereby lowering the concentration of the active second messenger even if the receptor remains occupied.
2. A receptor tyrosine kinase remains bound to its hormone, but its downstream signaling rapidly declines. Which mechanism most directly explains this reduction in signaling?
A.
Dephosphorylation of activated signaling proteins by protein phosphatases
B.
Continued autophosphorylation of the receptor
C.
Increased recruitment of adaptor proteins
D.
Increased synthesis of the hormone
Correct Answer:
Dephosphorylation of activated signaling proteins by protein phosphatases
Explanation:
Protein phosphatases remove phosphate groups from activated receptors or downstream proteins, switching off phosphorylation-dependent signaling even while the hormone remains bound.
3. The alpha subunit of a heterotrimeric G protein contributes to signal termination primarily by which event?
A.
Binding permanently to the beta-gamma subunits
B.
Hydrolyzing bound GTP to GDP
C.
Converting GDP into ATP
D.
Increasing the affinity of the receptor for its hormone
Correct Answer:
Hydrolyzing bound GTP to GDP
Explanation:
The G alpha subunit is active when GTP-bound. Its intrinsic GTPase activity converts GTP to GDP, returning the subunit to its inactive state and terminating its direct signaling.
4. A hormone-stimulated cell has a normal receptor and normal adenylyl cyclase activity, but cAMP remains elevated for an unusually long time. Which defect is most likely?
A.
Overactive phosphodiesterase
B.
Defective cAMP phosphodiesterase
C.
Excessive protein phosphatase activity
D.
Accelerated receptor internalization
Correct Answer:
Defective cAMP phosphodiesterase
Explanation:
A defective phosphodiesterase would slow cAMP degradation, causing prolonged elevation of cAMP and delayed termination of signaling.
5. Which sequence best represents receptor-mediated termination of signaling after prolonged exposure to a high concentration of a hormone?
A.
Receptor phosphorylation, arrestin binding, internalization, and degradation or recycling
B.
Receptor activation, increased ligand binding, and permanent membrane insertion
C.
Second-messenger synthesis, receptor gene amplification, and increased surface expression
D.
Ligand synthesis, receptor dephosphorylation, and irreversible activation
Correct Answer:
Receptor phosphorylation, arrestin binding, internalization, and degradation or recycling
Explanation:
Many activated membrane receptors are phosphorylated and bind arrestin, which prevents further coupling and promotes internalization. The receptor may then be recycled or degraded, reducing signaling.
6. If intracellular Ca2+ acts as a second messenger, which process most directly terminates a Ca2+-dependent signal?
A.
Opening additional Ca2+ channels in the plasma membrane
B.
Sequestration of Ca2+ into the endoplasmic reticulum or extrusion from the cytosol
C.
Activation of phospholipase C
D.
Release of more Ca2+ from intracellular stores
Correct Answer:
Sequestration of Ca2+ into the endoplasmic reticulum or extrusion from the cytosol
Explanation:
Ca2+-dependent signaling declines when cytosolic Ca2+ is removed by pumps and transporters that move it into intracellular stores or out of the cell.
7. A hormone receptor is removed from the cell surface and placed in an endocytic vesicle, but the hormone remains inside the vesicle. What is the immediate signaling consequence?
A.
The receptor is necessarily activated more strongly
B.
The receptor is physically separated from cytosolic signaling partners at the plasma membrane
C.
The hormone is converted directly into a second messenger
D.
The receptor begins synthesizing cAMP without a ligand
Correct Answer:
The receptor is physically separated from cytosolic signaling partners at the plasma membrane
Explanation:
Internalization can terminate or reduce signaling by removing the receptor from the plasma membrane, where it normally interacts with G proteins or other signaling components.
8. Which intervention would prolong a signaling pathway that uses IP3 and Ca2+ as second messengers?
A.
Accelerating IP3 dephosphorylation and Ca2+ uptake into the endoplasmic reticulum
B.
Inhibiting IP3 degradation and slowing cytosolic Ca2+ clearance
C.
Increasing protein phosphatase activity
D.
Promoting receptor internalization
Correct Answer:
Inhibiting IP3 degradation and slowing cytosolic Ca2+ clearance
Explanation:
Signal termination requires removal of IP3 and restoration of low cytosolic Ca2+. Inhibiting IP3 degradation and Ca2+ clearance prolongs both second-messenger signals.
9. A cell exposed continuously to a hormone shows progressively lower responses despite an unchanged extracellular hormone concentration. Which combination most plausibly produces this desensitization?
A.
Increased receptor number and reduced phosphatase activity
B.
Receptor phosphorylation, recruitment of inhibitory proteins, and receptor internalization
C.
Increased second-messenger degradation and increased receptor recycling to the membrane
D.
Reduced receptor phosphorylation and increased coupling to effector enzymes
Correct Answer:
Receptor phosphorylation, recruitment of inhibitory proteins, and receptor internalization
Explanation:
Persistent stimulation commonly promotes receptor phosphorylation, inhibitory-protein binding, and internalization. These mechanisms reduce receptor coupling and terminate or weaken the signal.
10. A hormone is rapidly removed from the bloodstream by enzymatic degradation. Which statement best describes the effect on its target-cell signaling?
A.
Signal termination is promoted because receptor occupancy falls
B.
Signal termination is prevented because degraded hormone activates receptors longer
C.
Second messengers remain elevated regardless of receptor occupancy
D.
Receptor internalization becomes impossible
Correct Answer:
Signal termination is promoted because receptor occupancy falls
Explanation:
Hormone degradation lowers the extracellular ligand concentration, reducing receptor occupancy and preventing continued receptor activation, thereby contributing to signal termination.
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