AP Biology Quiz: Feedback

What this quiz covers

This quiz focuses on Feedback, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Biology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. ERK activity becomes more transient because ERK reduces upstream RTK signaling capacity (correct answer)
2. ERK activity becomes more sustained because ERK increases adaptor recruitment to RTK
3. ERK activity becomes oscillatory because ERK directly dephosphorylates the ligand
4. ERK activity becomes stronger because ERK prevents RTK dimerization at the membrane
5. ERK activity becomes weaker because ERK phosphorylation activates additional RTKs

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is that ERK activity becomes more transient because ERK reduces upstream RTK signaling capacity, as the stimulus describes ERK phosphorylating the RTK to decrease adaptor binding and limit further MAPK activation. This negative feedback shortens the duration of ERK activity despite continued ligand presence. The RTK mutant lacking the phosphorylation site shows prolonged ERK activity, confirming the feedback's role in promoting transience. A tempting distractor is that ERK activity becomes more sustained because ERK increases adaptor recruitment to RTK, which reverses the feedback's actual inhibitory effect, a misconception in misidentifying negative feedback as positive. To approach such problems, trace the feedback to see if it inhibits or enhances an upstream step in the pathway.

Question 2

1. Faster signal shutoff because Ca$^{2+}$-activated kinase reduces channel opening in negative feedback (correct answer)
2. Stronger signal because Ca$^{2+}$-activated kinase increases odorant affinity in positive feedback
3. No change because phosphorylation affects only intracellular Ca$^{2+}$ buffering proteins
4. Longer signal because phosphorylation increases Na$^+$ conductance through the channel
5. Weaker signal because kinase inhibition prevents odorant from binding its receptor site

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is faster signal shutoff because Ca²⁺-activated kinase reduces channel opening in negative feedback, as the stimulus shows the kinase phosphorylates the Na⁺ channel to decrease its open probability while odorant is bound. This feedback accelerates termination of depolarization by limiting Na⁺ influx. Blocking the kinase prolongs depolarization, confirming the feedback promotes rapid shutoff. A tempting distractor is longer signal because phosphorylation increases Na⁺ conductance through the channel, which misinterprets inhibition as activation, a common misconception in phosphorylation effects. To solve these, map the feedback loop to see if it opposes or supports the initial stimulus response.

Question 3

1. M continues increasing at the same rate because K remains fully active despite M binding.
2. M oscillates randomly because M binding permanently activates K and amplifies signaling.
3. M reaches a lower steady-state level because M binding reduces K activity and limits further M production. (correct answer)
4. M returns to zero because M binding prevents hormone H from binding receptor R at the membrane.
5. M plateaus at a higher level because M binding increases E phosphorylation and accelerates M synthesis.

Explanation: This question assesses understanding of feedback regulation in signal transduction pathways. The feedback is negative, as accumulating M binds allosterically to kinase K and decreases its catalytic activity, reducing phosphorylation of E and thus limiting further M production. With constant H stimulation, M rises initially when K is fully active but then plateaus at a lower steady-state level because the inhibition curbs excessive accumulation, aligning with choice C. This creates a balanced plateau where production matches degradation. A tempting distractor is choice E, which incorrectly claims a higher plateau from increased E phosphorylation due to the misconception that M binding amplifies rather than reduces K activity. In feedback analysis, determine if the loop is positive or negative to predict behaviors like stabilization or amplification in steady states.

Question 4

1. Shorter Ca$^{2+}$ transients because Ca$^{2+}$ can no longer inhibit IP$_3$ receptors
2. Longer Ca$^{2+}$ transients because negative feedback on IP$_3$ receptors is removed (correct answer)
3. Lower Ca$^{2+}$ peaks because IP$_3$ production is reduced by Ca$^{2+}$ binding
4. No effect because Ca$^{2+}$ feedback requires new IP$_3$ receptor synthesis
5. Lower Ca$^{2+}$ release because Ca$^{2+}$ binding is needed to open IP$_3$ receptors

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is longer Ca²⁺ transients because negative feedback on IP₃ receptors is removed, as the stimulus indicates cytosolic Ca²⁺ binds the IP₃ receptor to decrease its open probability, limiting further release. This negative feedback shortens the Ca²⁺ signal despite ongoing IP₃ presence. The mutation preventing Ca²⁺ binding increases release duration, demonstrating loss of feedback prolongs the transient. A tempting distractor is shorter Ca²⁺ transients because Ca²⁺ can no longer inhibit IP₃ receptors, which inverts the feedback's inhibitory role, a misconception in misunderstanding binding's effect. When analyzing feedback, consider how it modulates the signal's temporal profile, such as duration or oscillation.

Question 5

1. Overexpressing the Ca$^{2+}$-binding regulatory protein to enhance PLC inhibition and shorten IP$_3$ production.
2. Chelating cytosolic Ca$^{2+}$ to reduce Ca$^{2+}$-dependent inhibition of PLC and prolong IP$_3$ production. (correct answer)
3. Inhibiting the GPCR to prevent PLC activation and thereby increase IP$_3$ production duration.
4. Increasing IP$_3$ receptor opening to reduce Ca$^{2+}$ release and strengthen PLC activation feedback.
5. Blocking IP$_3$ degradation to eliminate PLC feedback because IP$_3$ directly inhibits the GPCR.

Explanation: This question tests understanding of feedback regulation in signal transduction pathways. Negative feedback shortens IP₃ production as released Ca²⁺ binds a protein inhibiting PLC, despite constant neurotransmitter. Choice B alters this by chelating Ca²⁺, reducing inhibition and prolonging IP₃ production. This illustrates feedback's termination of Ca²⁺ signaling. Choice A is a tempting distractor but wrong because inhibiting PLC reduces IP₃ entirely, not prolongs it, arising from a misconception that blocking the enzyme extends its activity. When analyzing, identify inhibitors in the loop and test ways to weaken their effects on duration.

Question 6

1. Blocking Ca$^{2+}$ entry prevents the time-dependent increase in Na$^+$ current at constant ACh. (correct answer)
2. Blocking the phosphatase increases receptor desensitization, causing larger Na$^+$ currents at constant ACh.
3. Increasing ACh concentration eliminates feedback because ligand binding alone determines channel opening.
4. Inhibiting Na$^+$ influx increases Ca$^{2+}$ entry by removing negative feedback on voltage-gated channels.
5. Chelating Ca$^{2+}$ increases receptor opening by enhancing receptor phosphorylation after ACh binding.

Explanation: This question tests understanding of feedback regulation in signal transduction pathways. The pathway involves positive feedback where Na⁺ influx from ACh-bound receptors depolarizes the membrane, opening Ca²⁺ channels and increasing cytosolic Ca²⁺, which activates a phosphatase to dephosphorylate the receptor and enhance its opening probability, leading to progressively larger Na⁺ currents over time with constant ACh. Choice A illustrates this by showing that blocking Ca²⁺ entry interrupts the feedback loop, preventing the phosphatase activation and thus the time-dependent increase in Na⁺ current. This demonstrates how the feedback amplifies the signal through Ca²⁺-dependent modification of the receptor. Choice E is a tempting distractor but incorrect because chelating Ca²⁺ would actually prevent phosphatase activation and reduce receptor opening, not increase it via enhanced phosphorylation, revealing a misconception about the role of dephosphorylation in receptor sensitization. To analyze similar pathways, map out the sequence of events and identify how downstream products influence upstream components to classify feedback as positive or negative.

Question 7

1. Increasing phosphodiesterase activity to accelerate cAMP breakdown during signaling.
2. Mutating the GPCR tail to remove PKA phosphorylation sites required for reduced Gs coupling. (correct answer)
3. Decreasing epinephrine concentration to lower receptor occupancy and cAMP production.
4. Overexpressing Gi to directly inhibit adenylyl cyclase independently of receptor phosphorylation.
5. Blocking GTP binding to Gs to prevent activation of adenylyl cyclase by the receptor.

Explanation: This question tests understanding of feedback regulation in signal transduction pathways. The pathway features negative feedback where PKA, activated by cAMP, phosphorylates the GPCR tail to reduce its coupling with Gs, causing cAMP levels to decline after an initial rise despite constant epinephrine. Choice B disrupts this by mutating the GPCR to remove phosphorylation sites, preventing the reduction in Gs coupling and thus reducing the decline in cAMP. This highlights how the feedback desensitizes the receptor to limit prolonged signaling. Choice A is a tempting distractor but wrong because increasing phosphodiesterase would accelerate cAMP breakdown independently of the receptor feedback, not specifically disrupt the described mechanism, stemming from a misconception that all cAMP-lowering processes are part of the same feedback loop. To approach such questions, identify the specific feedback component and predict outcomes of interventions that target it directly.

Question 8

1. Increasing phosphatase activity that dephosphorylates MAPK to speed MAPK inactivation.
2. Mutating the scaffold to prevent MAPK-dependent phosphorylation that weakens scaffold–MAPKKK binding. (correct answer)
3. Reducing mating factor concentration to decrease GPCR activation and lower MAPK peak amplitude.
4. Inhibiting MAPKKK to block signaling initiation and prevent MAPK activation entirely.
5. Adding more scaffold protein to increase feedback strength and accelerate MAPK decline.

Explanation: This question tests understanding of feedback regulation in signal transduction pathways. Negative feedback occurs as active MAPK phosphorylates the scaffold, reducing its affinity for MAPKKK and causing MAPK activity to peak and then drop despite constant mating factor. Choice B interferes by mutating the scaffold to prevent phosphorylation, maintaining scaffold-MAPKKK binding and prolonging high MAPK activity. This shows how feedback limits cascade assembly to prevent sustained activation. Choice E is a tempting distractor but wrong because overexpressing scaffold would enhance feedback by providing more targets for phosphorylation, accelerating decline, based on a misconception that more scaffold strengthens signaling rather than feedback. A useful strategy is to diagram the feedback loop and simulate mutations to predict changes in signal dynamics.

Question 9

1. Increase phosphatase activity so phosphorylated C is dephosphorylated more rapidly during stimulation.
2. Mutate C so it cannot be phosphorylated by P while keeping N binding unchanged.
3. Decrease P activity so less Ca$^{2+}$-activated kinase is available to modify C.
4. Reduce extracellular Ca$^{2+}$ concentration to limit Ca$^{2+}$ entry through open C channels.
5. Inhibit the phosphatase that removes phosphate from C, prolonging P-dependent enhancement of C opening. (correct answer)

Explanation: This question assesses understanding of feedback regulation in signal transduction pathways. The positive feedback loop involves Ca²⁺ influx through channel C activating kinase P, which phosphorylates C to increase its open probability, enhancing further Ca²⁺ entry during sustained N stimulation. Inhibiting the phosphatase that dephosphorylates C prolongs the phosphorylated state, strengthening the feedback by maintaining higher open probability longer, as in choice E. The phosphatase normally counters the feedback by returning C to baseline, so its inhibition amplifies the effect. A tempting distractor is choice A, which suggests increasing phosphatase activity but is wrong due to the misconception that faster dephosphorylation enhances rather than weakens positive feedback. To evaluate modifications in feedback systems, consider how changes affect loop components to strengthen or weaken signal amplification.

Question 10

1. Longer cAMP signaling because negative feedback that inactivates G$\alpha$ is reduced (correct answer)
2. Shorter cAMP signaling because positive feedback that inactivates G$\alpha$ is reduced
3. No change because RGS proteins act only on adenylyl cyclase, not G proteins
4. Lower cAMP because blocking RGS phosphorylation increases PDE activity directly
5. Lower cAMP because blocking RGS phosphorylation prevents ligand binding to GPCR

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is longer cAMP signaling because negative feedback that inactivates Gα is reduced, as the stimulus describes PKA phosphorylating RGS to increase its GTPase activity on Gα, reducing adenylyl cyclase. This feedback shortens cAMP duration. Blocking RGS phosphorylation prolongs cAMP, showing reduced feedback extends signaling. A tempting distractor is shorter cAMP signaling because positive feedback that inactivates Gα is reduced, which misclassifies the feedback type, a misconception in RGS role. A strategy is to assess feedback by perturbing regulators and observing signal kinetics.

Question 11

1. Lower PLC activity because PKC phosphorylation is required for G-protein activation
2. Higher sustained PLC activity because negative feedback on receptor coupling is eliminated (correct answer)
3. No change because PKC acts only on nuclear proteins, not membrane receptors
4. Shorter signaling because receptor phosphorylation increases G-protein coupling efficiency
5. Lower DAG production because receptor mutation prevents ligand binding at the surface

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is higher sustained PLC activity because negative feedback on receptor coupling is eliminated, as the stimulus describes PKC phosphorylating the GPCR to reduce G-protein coupling while ligand is present. This negative feedback dampens ongoing PLC activation. The mutant lacking phosphorylation sites shows higher PLC activity, confirming loss of feedback sustains the signal. A tempting distractor is shorter signaling because receptor phosphorylation increases G-protein coupling efficiency, which inverts the inhibitory effect, a misconception in phosphorylation outcomes. A strategy is to classify feedback as positive or negative based on whether it reinforces or opposes the signal.

Question 12

1. Reduced signal amplification because receptor opening no longer increases Ca$^{2+}$ entry (correct answer)
2. Increased amplification because CaMKII no longer requires Ca$^{2+}$ to activate
3. Faster termination because glutamate is hydrolyzed more quickly by CaMKII
4. Longer signaling because CaMKII phosphorylation blocks Ca$^{2+}$ export pumps
5. No change because feedback occurs only through new receptor synthesis

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is reduced signal amplification because receptor opening no longer increases Ca²⁺ entry, as the stimulus shows CaMKII phosphorylates the receptor to enhance its open probability and allow more Ca²⁺ influx. This positive feedback amplifies the Ca²⁺ signal by reinforcing influx through the same receptors during glutamate binding. Blocking the phosphorylation reduces sustained Ca²⁺ rise, illustrating how the feedback loop enhances amplification. A tempting distractor is increased amplification because CaMKII no longer requires Ca²⁺ to activate, which confuses the role of feedback with the activation mechanism itself, a misconception in overlooking the target's effect on signal strength. When evaluating feedback, determine if it creates a self-reinforcing loop that boosts or a dampening one that limits the response.

Question 13

1. More sustained signaling because negative feedback that removes ROS and restores PTP activity is blocked (correct answer)
2. Less sustained signaling because positive feedback that removes ROS and restores PTP activity is blocked
3. Unchanged signaling because ROS cannot modify enzyme activity in signaling pathways
4. Reduced signaling because antioxidant inhibition prevents ligand binding to the receptor
5. Reduced signaling because receptor phosphorylation requires antioxidant enzymes to add phosphates

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is more sustained signaling because negative feedback that removes ROS and restores PTP activity is blocked, as the stimulus describes downstream kinase activating antioxidant to reduce ROS, allowing PTP recovery and signal dampening. This feedback limits phosphorylation duration. Inhibiting antioxidant prolongs phosphorylation, showing blocked feedback sustains signaling. A tempting distractor is less sustained signaling because positive feedback that removes ROS and restores PTP activity is blocked, which confuses negative with positive, a misconception in restoration. When evaluating, consider how feedback regulates signal persistence through secondary messengers like ROS.

Question 14

1. Positive feedback because PKA suppresses a phosphatase that would reverse PKA signaling (correct answer)
2. Negative feedback because PKA activates a phosphatase that removes PKA target phosphates
3. No feedback because phosphatases cannot be regulated by phosphorylation in cells
4. Negative feedback because PP inhibition reduces cAMP production by adenylyl cyclase
5. Positive feedback because PP directly increases hormone binding to the GPCR

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is positive feedback because PKA suppresses a phosphatase that would reverse PKA signaling, as the stimulus shows PKA phosphorylates PP to decrease its activity, preventing dephosphorylation of PKA targets. This feedback prolongs PKA effects by inhibiting reversal. PP inhibition keeps phosphorylation high post-hormone, confirming positive reinforcement. A tempting distractor is negative feedback because PKA activates a phosphatase that removes PKA target phosphates, which inverts the inhibitory effect on PP, a misconception in regulation. For these questions, classify feedback by its impact on sustaining or diminishing the primary signal.

Question 15

1. Enhanced response because Ca$^{2+}$ activates a positive feedback that opens more channels
2. Diminished response because Ca$^{2+}$ activates negative feedback that reduces channel activity (correct answer)
3. Unchanged response because vesicle fusion does not depend on Ca$^{2+}$ concentration
4. Enhanced response because protease inhibition decreases Ca$^{2+}$ buffering in the cytosol
5. Diminished response because protease inhibition prevents channel insertion into membranes

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is diminished response because Ca²⁺ activates negative feedback that reduces channel activity, as the stimulus shows Ca²⁺-dependent protease cleaves a protein to lower Ca²⁺ channel open probability during firing. This feedback diminishes Ca²⁺ entry over time. Inhibiting the protease increases entry, indicating the feedback normally reduces the response. A tempting distractor is enhanced response because Ca²⁺ activates a positive feedback that opens more channels, which confuses negative with positive regulation, a misconception in feedback polarity. For similar problems, examine how perturbations like inhibitors reveal the feedback's modulatory role.

Question 16

1. Lower S levels because phosphorylation is required to keep the enzyme active
2. Higher sustained S levels because negative feedback on S synthesis is removed (correct answer)
3. No change because kinase Z acts only downstream of S and cannot affect S concentration
4. Lower S levels because the mutation increases degradation of S by phosphodiesterase
5. Shorter signaling because the mutation prevents ligand binding to the receptor extracellularly

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is higher sustained S levels because negative feedback on S synthesis is removed, as the stimulus describes kinase Z phosphorylating the S-synthesizing enzyme to decrease its activity while ligand is bound. This negative feedback limits sustained S accumulation. The mutant enzyme shows higher S levels, indicating loss of feedback sustains the messenger. A tempting distractor is lower S levels because phosphorylation is required to keep the enzyme active, which misinterprets inhibition as activation, a common misconception. A strategy is to evaluate feedback by observing concentration changes in mutants or inhibitors.

Question 17

1. Ca$^{2+}$ release becomes self-limiting because Ca$^{2+}$ reduces IP$_3$ receptor opening (correct answer)
2. Ca$^{2+}$ release increases because inhibitory phosphorylation increases IP$_3$ receptor opening
3. Ca$^{2+}$ release stops because IP$_3$ receptors require phosphorylation to bind IP$_3$
4. Ca$^{2+}$ release increases because Ca$^{2+}$ directly synthesizes IP$_3$ from DAG
5. Ca$^{2+}$ remains constant because IP$_3$ receptors are not regulated by cytosolic factors

Explanation: This question assesses understanding of feedback regulation in signal transduction pathways. The feedback involves Ca²⁺ activating an enzyme that phosphorylates the IP₃ receptor at an inhibitory site, reducing its opening probability despite constant IP₃ levels. During continuous ligand stimulation, initial Ca²⁺ release occurs, but the resulting phosphorylation limits further release, making the process self-limiting as described in choice A. This negative feedback prevents excessive Ca²⁺ buildup by decreasing receptor sensitivity. A tempting distractor is choice B, which wrongly states that inhibitory phosphorylation increases receptor opening due to the misconception that all phosphorylation enhances activity rather than inhibits it. When evaluating feedback in signaling, identify whether it amplifies or dampens the signal to forecast outcomes like self-limitation or escalation.

Question 18

1. Lower signaling because kinase B can no longer activate kinase A in the cascade
2. Higher sustained signaling because negative feedback from kinase B onto kinase A is removed (correct answer)
3. No change because kinase B acts downstream and cannot influence kinase A activity
4. Shorter signaling because inhibitory-site loss increases receptor dephosphorylation by PTPs
5. Lower signaling because the mutation prevents kinase A localization to the membrane receptor

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is higher sustained signaling because negative feedback from kinase B onto kinase A is removed, as the stimulus shows kinase B phosphorylates kinase A on an inhibitory site, decreasing its activity while ligand is bound. This negative feedback limits sustained activation. The mutant lacking the site shows higher kinase B, confirming removal sustains signaling. A tempting distractor is lower signaling because kinase B can no longer activate kinase A in the cascade, which overlooks the inhibitory nature, a misconception in site function. To solve, identify if phosphorylation sites are activating or inhibitory based on context.

Question 19

1. Lower Akt activation because PIP$_3$ is degraded faster when phosphatase is inhibited
2. Higher and longer Akt activation because negative feedback limiting PIP$_3$ is removed (correct answer)
3. No change because Akt activation depends only on cytosolic Ca$^{2+}$ concentration
4. Lower PIP$_3$ because Akt phosphorylation prevents PI3K recruitment to the receptor
5. Shorter signaling because phosphatase inhibition increases receptor endocytosis rate

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is higher and longer Akt activation because negative feedback limiting PIP₃ is removed, as the stimulus describes Akt activating a phosphatase that degrades PIP₃, reducing the signal that activates Akt. This negative feedback curbs sustained Akt activity. Inhibiting the phosphatase increases PIP₃ accumulation, leading to prolonged Akt activation. A tempting distractor is lower Akt activation because PIP₃ is degraded faster when phosphatase is inhibited, which confuses inhibition with activation of the phosphatase, a misconception in regulatory logic. For transferable strategy, always verify if feedback dampens or amplifies by simulating the effect of blocking the loop.

Question 20

1. Prolonged signaling because negative feedback that inactivates the G protein is weakened (correct answer)
2. Reduced signaling because positive feedback that activates the G protein is weakened
3. No change because GAPs act only on MAPKs, not on G proteins
4. Oscillations because GAP phosphorylation causes pheromone to dissociate from the receptor
5. Reduced signaling because GAP mutation prevents receptor insertion into the membrane

Explanation: This question assesses the skill of understanding feedback regulation in signal transduction pathways. The correct answer is prolonged signaling because negative feedback that inactivates the G protein is weakened, as the stimulus describes MAPK phosphorylating GAP to increase its activity on Gα, accelerating GTP hydrolysis and reducing signaling. This negative feedback shortens response duration. The non-phosphorylatable mutant prolongs MAPK activation, showing weakened feedback extends signaling. A tempting distractor is reduced signaling because positive feedback that activates the G protein is weakened, which confuses negative with positive, a misconception in GAP function. To approach, trace the feedback's effect on upstream activators or inhibitors.