MCAT Biological and Biochemical Foundations of Living Systems Quiz: 2a Cytoskeleton Cell Motility

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2a Cytoskeleton Cell MotilityQuestion 1 of 20

Migrating neurons were imaged as they advanced a leading process on a laminin substrate. Investigators quantified delivery of adhesion receptors to the front of the cell. Under control conditions, receptor-containing vesicles moved along linear tracks toward the leading edge and accumulated in the front membrane. After treatment with a microtubule motor inhibitor, vesicle movement became largely diffusive, front membrane receptor density decreased, and cells exhibited repeated protrusion attempts without sustained forward translocation.

Which conclusion about cytoskeletal dynamics is most consistent with these findings?

Actin filaments are static during migration; therefore, disrupting microtubule motors should not affect protrusion persistence.

Intermediate filaments are the primary tracks for long-range vesicle delivery to the leading edge during migration.

Inhibiting microtubule motors should enhance receptor delivery by preventing vesicles from being pulled away from the front.

Microtubule-based transport supports migration by delivering components needed for stable adhesions and coordinated forward movement.

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MCAT Biological and Biochemical Foundations of Living Systems Quiz

MCAT Biological and Biochemical Foundations of Living Systems Quiz: 2a Cytoskeleton Cell Motility

Practice 2a Cytoskeleton Cell Motility in MCAT Biological and Biochemical Foundations of Living Systems with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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This quiz focuses on 2a Cytoskeleton Cell Motility, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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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.

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Question 1

Which conclusion about cytoskeletal dynamics is most consistent with these findings?

Actin filaments are static during migration; therefore, disrupting microtubule motors should not affect protrusion persistence.
Intermediate filaments are the primary tracks for long-range vesicle delivery to the leading edge during migration.
Inhibiting microtubule motors should enhance receptor delivery by preventing vesicles from being pulled away from the front.
Microtubule-based transport supports migration by delivering components needed for stable adhesions and coordinated forward movement. (correct answer)

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, migrating neurons rely on microtubule-based transport for delivering adhesion receptors to the leading edge. Choice D is correct because it highlights how microtubules support migration by enabling vesicle delivery for stable adhesions and forward movement. Choice B is incorrect because intermediate filaments do not serve as primary tracks for vesicle transport. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Assess how motor inhibition disrupts polarized delivery and impairs motility.

Question 2

To test the mechanical role of intermediate filaments during migration, researchers compared wild-type cells to cells lacking a major intermediate filament protein. Both cell types were placed in a 3D matrix with narrow pores. Wild-type cells elongated and maintained integrity while squeezing through pores. Intermediate-filament–deficient cells initiated entry into pores but frequently developed localized membrane blebs and transient ruptures, followed by abrupt retraction; their average displacement over time decreased despite normal-looking actin-rich protrusions.

Which statement best describes the role of intermediate filaments in cell motility in this context?

Intermediate filaments primarily drive protrusion by polymerizing at the leading edge faster than actin filaments.
Intermediate filaments provide mechanical resilience that helps cells withstand deformation during confined migration. (correct answer)
Intermediate filaments inhibit migration by increasing cytoplasmic viscosity; their loss should increase displacement.
Intermediate filaments replace microtubules as the main system for polarized vesicle trafficking during migration.

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, cells in a 3D matrix with narrow pores require intermediate filaments for mechanical integrity during confined migration. Choice B is correct because it explains how intermediate filaments provide resilience to withstand deformation and prevent ruptures. Choice A is incorrect because intermediate filaments do not drive protrusion via rapid polymerization. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Note how filament deficiencies reveal roles in mechanical stress resistance.

Question 3

A team examined how microtubules contribute to directional migration in epithelial cells moving into a scratch wound. Live imaging showed microtubule plus-ends repeatedly growing toward the leading edge, while vesicles carrying membrane components accumulated near the front. When cells were treated with a microtubule depolymerizing drug, overall speed decreased and cells frequently lost a stable front–rear axis; actin-rich protrusions still formed but were short-lived and appeared at multiple edges. A separate condition used a drug that stabilizes microtubules; these cells maintained a single front but turned slowly and showed delayed repositioning of internal organelles during direction changes.

Based on the vignette, which statement best describes the role of microtubules in cell motility?

Microtubules primarily generate the protrusive force for lamellipodia extension by directly pushing the plasma membrane forward.
Microtubules help maintain polarity and support directed trafficking during migration, enabling persistent front–rear organization. (correct answer)
Microtubules are dispensable for polarity because actin polymerization alone determines where the leading edge forms over time.
Microtubule depolymerization should increase migration speed by removing internal constraints and freeing actin networks to expand.

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, epithelial cells in a scratch wound depend on microtubules for polarity and trafficking during directional migration. Choice B is correct because it describes microtubules' role in maintaining polarity and supporting directed trafficking for persistent organization. Choice A is incorrect because microtubules do not directly generate protrusive force for lamellipodia. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Examine how microtubule perturbations affect overall directionality versus local protrusions.

Question 4

In a scratch-wound assay, cells normally reorient their centrosome and microtubule network toward the wound edge before migrating. A drug that prevents microtubule polymerization was applied after the scratch. Cells still formed actin-rich protrusions but showed poor directional persistence and frequently changed direction. Based on the vignette, which statement best describes microtubule function in directional migration?

Microtubules contribute to front–rear organization and directional persistence, likely by coordinating polarized trafficking during migration. (correct answer)
Microtubules directly polymerize to push the plasma membrane forward; actin-rich protrusions are incidental and not linked to movement.
Microtubules are static structural rods; preventing their polymerization should not affect directional persistence once protrusions form.
Microtubules primarily generate contractile force at the rear; inhibiting polymerization should increase persistence by reducing rear contraction.

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, preventing microtubule polymerization after scratching leads to poor directional persistence despite protrusions. Choice A is correct because it accurately describes microtubules' role in front-rear organization and persistence. Choice B is incorrect because it wrongly attributes direct membrane pushing to microtubules. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Examine polarity markers like centrosome orientation in wound assays.

Question 5

In migrating neurons, mitochondria accumulate near the leading process where ATP demand is high. A lab disrupted microtubules and observed that mitochondria became dispersed and leading-edge advance slowed, even though actin polymerization events were still detectable at the cortex. Which outcome would be expected if microtubule-based transport is inhibited during migration?

Migration would be unaffected because mitochondria move primarily by diffusion in the cytosol during motility.
Migration speed would increase because disrupting microtubules frees actin monomers to polymerize more efficiently at the leading edge.
Reduced delivery of organelles and cargo to the front would impair sustained migration, even if actin polymerization can still initiate protrusions. (correct answer)
Cells would lose all protrusive activity because microtubules, not actin, are required for lamellipodial formation at the membrane.

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, disrupting microtubules disperses mitochondria and slows leading-edge advance despite actin events. Choice C is correct because it accurately describes how impaired transport reduces sustained migration. Choice B is incorrect because it wrongly suggests speed increases by freeing actin monomers. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Monitor organelle positioning to assess transport's impact on motility.

Question 6

In a study of collective migration, epithelial sheets were subjected to cyclic stretching while moving across a substrate. Cells at the leading edge maintained forward movement under moderate stretch, but when an intermediate filament–disrupting compound was added, the sheet began to fragment at cell–cell junctions during stretch cycles. Individual cells still formed actin-based protrusions, yet coordination across the sheet deteriorated and net advancement slowed.

What outcome would be expected if intermediate filament function is inhibited during mechanically stressed collective migration?

Improved coordination because actin networks become more rigid and can transmit forces between cells more effectively.
Reduced tissue-level integrity and slower net advance because cells are less able to tolerate and distribute mechanical strain. (correct answer)
No change in sheet cohesion because microtubules alone determine the strength of cell–cell junctions under stretch.
Faster net advance because intermediate filaments normally block protrusion formation at the leading edge.

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, epithelial sheets under cyclic stretching depend on intermediate filaments for maintaining integrity during collective migration. Choice B is correct because it predicts reduced integrity and slower advance due to impaired strain distribution without intermediate filaments. Choice A is incorrect because rigid actin would not improve coordination under stress. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Evaluate how mechanical perturbations highlight roles in tissue-level cohesion.

Question 7

In a study of amoeboid motility, researchers tracked single-cell migration on a soft collagen-coated surface while imaging F-actin at the leading edge. Cells were briefly exposed to a low dose of an actin polymerization inhibitor that preferentially reduces new filament growth at barbed ends. During exposure, cells showed fewer membrane protrusions and a marked decrease in forward displacement, but remained viable and continued slow shape fluctuations. After washout, protrusions and net migration recovered within minutes. Based on these observations, which conclusion about actin dynamics is most consistent with the role of actin in amoeboid movement?

Simplified schematic: Front (leading edge): G-actin → F-actin (polymerization) → protrusion Rear: F-actin disassembly → monomers recycled

Intermediate filaments generate the protrusive force at the front, while actin is mainly static scaffolding that does not require turnover.
Microtubule polymerization at the leading edge is the primary driver of protrusion, so inhibiting actin should minimally affect net migration.
Inhibiting actin polymerization should increase migration speed by reducing cytoplasmic viscosity and allowing faster forward flow.
Actin polymerization at the leading edge provides a pushing force for protrusion, and reversible turnover enables rapid recovery after inhibitor washout. (correct answer)

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, a low-dose actin polymerization inhibitor reduces new filament growth, leading to fewer protrusions and decreased migration, with recovery after washout. Choice D is correct because it accurately describes actin polymerization providing protrusive force and reversible turnover enabling rapid recovery. Choice B is incorrect because it incorrectly attributes protrusion primarily to microtubules rather than actin. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Always verify how perturbations like inhibitors affect specific filament dynamics in motility assays.

Question 8

Migrating epithelial cells were imaged while a fluorescently labeled vesicle marker tracked delivery of membrane components to the leading edge. When microtubules were selectively destabilized, vesicles accumulated near the cell center and leading-edge expansion slowed, even though cortical actin still exhibited transient polymerization. Which conclusion about microtubules in cell migration is most consistent with the findings?

Intermediate filaments are responsible for vesicle transport during migration; microtubule destabilization should not affect vesicle localization.
Microtubules are the primary source of protrusive force at the membrane, so vesicle accumulation indicates reduced microtubule pushing against the cortex.
Microtubules inhibit migration by sequestering vesicles; destabilizing them should increase vesicle delivery and speed up expansion.
Microtubules support migration by facilitating intracellular transport to the leading edge, enabling sustained protrusion and membrane remodeling. (correct answer)

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, destabilizing microtubules causes vesicle accumulation centrally and slows leading-edge expansion despite actin polymerization. Choice D is correct because it accurately describes microtubules' role in transport for sustained protrusion. Choice B is incorrect because it wrongly claims microtubules provide primary protrusive force. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Track vesicle delivery to understand microtubule contributions to migration.

Question 9

A research group compared two cell lines migrating on the same extracellular matrix. Cell line 1 displayed rapid, rounded amoeboid movement with transient protrusions; cell line 2 displayed slower, elongated movement with more persistent front–rear polarity. When both lines were treated with a mild actin polymerization inhibitor, line 1 showed a large drop in speed, whereas line 2 showed a modest drop but a pronounced loss of protrusion stability. The investigators proposed that differences in reliance on actin-driven protrusion dynamics contributed to the distinct motility modes. What outcome would be expected if actin polymerization is inhibited, consistent with the vignette?

Amoeboid-like movement increases because inhibiting actin polymerization reduces membrane tension, allowing faster protrusions.
Elongated movement is enhanced because reduced actin polymerization shifts force generation to microtubules, increasing polarity and speed.
Both motility modes are unaffected because actin polymerization is redundant with intermediate filament assembly at the leading edge.
Amoeboid-like movement is disproportionately impaired because rapid protrusion formation depends strongly on actin polymerization dynamics. (correct answer)

Explanation: This question assesses understanding of how different cell migration modes depend on actin polymerization dynamics. Amoeboid movement relies heavily on rapid, transient actin-based protrusions, while elongated mesenchymal movement involves more stable structures. In the vignette, the amoeboid cell line showed a larger speed reduction than the elongated line when actin polymerization was inhibited. Choice D is correct because it accurately describes how amoeboid-like movement is disproportionately impaired due to its strong dependence on rapid actin polymerization dynamics. Choice B is incorrect because inhibiting actin polymerization does not enhance movement or shift force generation to microtubules. When comparing migration modes, recognize that amoeboid movement's reliance on dynamic actin cycling makes it particularly sensitive to polymerization inhibitors.

Question 10

In a study of amoeboid motility, researchers tracked single cells moving through a 3D collagen matrix. Live imaging showed repeated cycles of leading-edge protrusion followed by rear retraction. When cells were treated with a low dose of an actin polymerization inhibitor, protrusions became shorter-lived and net displacement over 10 minutes decreased, despite continued membrane ruffling. In a separate condition, mild stabilization of existing actin filaments reduced the frequency of protrusion–retraction cycles and also reduced net displacement. The authors concluded that efficient movement required both assembly and turnover of actin at the front. Based on these findings, which conclusion about cytoskeletal dynamics is most consistent with the observed changes in cell motility?

Microtubule polymerization at the leading edge is the primary driver of protrusive force, so inhibiting actin polymerization should have minimal effect on net displacement.
Dynamic actin remodeling, including polymerization and depolymerization, is required to sustain productive protrusions that translate into forward movement. (correct answer)
Intermediate filaments generate the pushing force for membrane protrusion, so stabilizing actin should increase net displacement by reducing cytoskeletal noise.
Actin filaments function as static struts; therefore, stabilizing them should preserve protrusions and increase migration speed in a collagen matrix.

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility, specifically actin dynamics in amoeboid movement. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, both actin polymerization inhibition and stabilization reduced net displacement, indicating that dynamic actin remodeling is essential for productive cell movement. Choice B is correct because it accurately describes how both polymerization and depolymerization of actin are required to sustain productive protrusions that translate into forward movement. Choice A is incorrect because microtubules do not drive protrusive force at the leading edge - this is primarily an actin-based process. When evaluating cytoskeletal function in motility, consider that efficient movement requires not just assembly but also turnover of cytoskeletal components to enable continuous remodeling and adaptation.

Question 11

To test how leading-edge actin contributes to amoeboid motility, investigators briefly applied a localized actin polymerization inhibitor to only one side of a migrating cell (left edge) while leaving the opposite side untreated. Within minutes, the cell's protrusions became biased toward the untreated side and the cell's trajectory curved away from the inhibited region, despite no change in overall cell viability.

Which statement best describes the role of actin polymerization in this cell's motility?

Actin filaments act as a static scaffold, so localized inhibition should have no effect unless the entire cortex is depolymerized.
Actin polymerization mainly functions to anchor microtubules at the centrosome, so local inhibition should not alter directional protrusions.
Actin polymerization is required only for rear retraction, so local inhibition at the leading edge should increase forward speed.
Actin polymerization provides a spatially controllable protrusive force, so locally reducing polymerization shifts protrusion formation and redirects migration. (correct answer)

Explanation: This question assesses understanding of how localized actin polymerization controls directional cell movement. The cytoskeleton generates spatially regulated protrusive forces through localized actin polymerization at the cell periphery. In the vignette, inhibiting polymerization on one side causes the cell to redirect movement toward the untreated side, demonstrating spatial control of protrusion formation. Choice D is correct because it accurately describes how actin polymerization provides controllable protrusive force that determines migration direction. Choice C is incorrect because it misattributes actin's role to rear retraction only, contradicting the observed effect on leading-edge protrusions. When analyzing directional cell motility, recognize that localized regulation of actin polymerization at the leading edge is a primary mechanism for steering cell movement.

Question 12

In a migration assay, amoeboid cells were subjected to alternating periods of high and low mechanical confinement. Under high confinement, cells displayed short-lived, actin-rich protrusions and maintained movement by repeatedly forming new protrusions. Under low confinement, protrusions persisted longer and cells migrated with fewer protrusion cycles. When actin polymerization was partially inhibited, the largest drop in net displacement occurred specifically under high confinement.

Based on the vignette, which conclusion about actin dynamics is most consistent?

Actin polymerization becomes less important under confinement because microtubules dominate force generation when space is limited.
Actin polymerization is especially important under confinement because frequent formation of new protrusions is needed to sustain forward movement. (correct answer)
Partial inhibition of actin polymerization should increase displacement under confinement by reducing protrusion turnover and conserving energy.
Intermediate filaments are expected to compensate for reduced actin polymerization by generating protrusive force at the leading edge.

Explanation: This question assesses understanding of how mechanical confinement affects the importance of actin dynamics in cell motility. The cytoskeleton's role in generating protrusions becomes particularly critical when cells navigate confined spaces that require frequent directional adjustments. In the vignette, high confinement correlates with shorter-lived protrusions and greater sensitivity to polymerization inhibition, indicating increased reliance on dynamic actin remodeling. Choice B is correct because it recognizes that confinement necessitates frequent formation of new protrusions to maintain forward movement when individual protrusions cannot persist. Choice A is incorrect because it suggests microtubules replace actin's role under confinement, contradicting the observed importance of actin-rich protrusions. When evaluating cell motility under different mechanical conditions, consider that physical constraints often increase, rather than decrease, the cell's dependence on dynamic cytoskeletal remodeling.

Question 13

Amoeboid cells were exposed to a reversible inhibitor of actin polymerization for 5 minutes and then washed out. During inhibition, cells rounded and showed minimal forward movement. Within minutes after washout, cells rapidly re-extended leading-edge protrusions and resumed directional migration. The rapid recovery occurred without new protein synthesis.

Which statement is most consistent with the observed recovery of motility after inhibitor washout?

Motility resumes because intermediate filaments polymerize quickly and replace actin as the primary driver of protrusion.
Recovery requires transcriptional upregulation of tubulin to rebuild microtubule tracks that directly generate protrusive force.
Actin-based motility depends on dynamic filament assembly from existing monomers, enabling rapid restoration of protrusions when polymerization resumes. (correct answer)
The inhibitor likely increased actin polymerization during treatment, and washout removed this enhancement, restoring normal speed.

Explanation: This question assesses understanding of the reversible nature of actin-based motility and its dependence on existing cellular components. The cytoskeleton can rapidly reassemble from available monomers once polymerization inhibition is removed, enabling quick restoration of motility. In the vignette, cells resume migration within minutes after washout without requiring new protein synthesis, indicating reliance on existing actin monomers. Choice C is correct because it accurately describes how dynamic filament assembly from existing monomers enables rapid recovery of protrusions and motility. Choice B is incorrect because it invokes transcriptional upregulation and microtubule-based force generation, contradicting both the rapid timescale and the actin-based mechanism. When evaluating recovery from cytoskeletal perturbations, consider that the dynamic equilibrium between monomeric and filamentous actin allows rapid structural reorganization without new synthesis.

Question 14

In a 3D collagen matrix, two cell populations migrate with similar speeds in wide channels. In narrow channels that require substantial deformation, Population X maintains integrity, while Population Y shows increased blebbing and occasional tearing. Population Y has reduced intermediate filament expression but normal actin polymerization at the front. Based on the vignette, which conclusion is most consistent?

Reduced intermediate filaments should increase migration success in narrow channels by making the cytoskeleton more fluid and less prone to damage.
Intermediate filaments determine the rate of actin polymerization; reduced intermediate filaments should primarily decrease leading-edge actin assembly.
Microtubules provide the dominant mechanical resilience in confinement; intermediate filament levels should not affect tearing.
Intermediate filaments help cells withstand mechanical stress during deformation; reduced intermediate filaments can impair successful migration in confinement. (correct answer)

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, reduced intermediate filaments in Population Y lead to blebbing and tearing in narrow channels despite normal actin. Choice D is correct because it accurately describes their support for mechanical stability in confinement. Choice B is incorrect because it wrongly links them to actin polymerization rate. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Vary channel dimensions to test resilience in deformation.

Question 15

In an assay of amoeboid motility, cells were treated with a drug that caps actin filament barbed ends, reducing the rate of new filament elongation at the leading edge. Imaging showed diminished lamellipodial extension and reduced chemotactic accuracy, while overall ATP levels were unchanged. Which outcome would be expected if barbed-end elongation is inhibited during chemotaxis?

Cells show reduced protrusion and impaired directional movement because forward extension relies on rapid actin elongation at the leading edge. (correct answer)
Cells show increased protrusion because capped filaments recruit more monomers to uncapped ends, amplifying polymerization.
Cells maintain chemotaxis because microtubules substitute for actin to generate lamellipodia under all conditions.
Cells lose motility because intermediate filaments cannot be transported without actin barbed-end elongation.

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, capping actin barbed ends reduces elongation, leading to diminished lamellipodia and reduced chemotactic accuracy. Choice A is correct because it accurately describes how inhibited elongation impairs protrusion and directional movement. Choice B is incorrect because it wrongly suggests capping increases protrusion by recruiting monomers. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Evaluate how end-specific inhibitors affect filament growth in chemotaxis assays.

Question 16

Amoeboid cells were placed in a microchannel with a chemoattractant gradient. Investigators selectively inhibited actin filament disassembly (reducing monomer recycling) without directly blocking polymerization. Cells initially formed a leading-edge protrusion but then progressively lost the ability to generate new protrusions and slowed over several minutes. Which outcome would be expected if actin disassembly is inhibited during sustained migration?

Conceptual flow: F-actin disassembly → G-actin pool → new F-actin at front → continued protrusion

Cells switch to microtubule-based protrusion, so inhibiting actin disassembly primarily increases microtubule-driven lamellipodia formation.
Sustained migration is enhanced because preventing disassembly preserves filaments that can be reused without additional polymerization.
Migration is unaffected because actin monomers are synthesized rapidly enough to replace recycled monomers during movement.
Sustained migration is impaired because reduced disassembly limits the G-actin pool needed for continued polymerization at the leading edge. (correct answer)

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, inhibiting actin disassembly reduces monomer recycling, leading to initial protrusions but progressive loss of new ones and slowed migration in a chemoattractant gradient. Choice D is correct because it accurately describes how reduced disassembly limits the G-actin pool for sustained polymerization and migration. Choice B is incorrect because it wrongly claims preventing disassembly enhances migration by preserving filaments. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Examine how recycling mechanisms sustain long-term motility in gradient-based assays.

Question 17

In a mixed-population assay, two groups of migrating cells were exposed to repeated compressive pulses. Group 1 maintained migration speed and shape; Group 2 slowed and showed persistent elongation after each pulse. Group 2 had experimentally reduced intermediate filament crosslinking, while actin polymerization at the leading edge remained responsive. Based on the vignette, which conclusion is most consistent?

The phenotype indicates actin is static; persistent elongation implies actin filaments cannot depolymerize after protrusion.
Intermediate filament crosslinking is required for microtubule-based vesicle transport; reduced crosslinking slows migration by trapping vesicles at the rear.
Reduced intermediate filament crosslinking should increase speed by decreasing cytoskeletal resistance to actin polymerization.
Intermediate filament network integrity supports recovery from mechanical perturbations, enabling sustained migration despite repeated deformation. (correct answer)

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, reduced intermediate filament crosslinking in Group 2 leads to slowed migration and persistent elongation after compression. Choice D is correct because it accurately describes their role in recovery from perturbations. Choice B is incorrect because it wrongly links crosslinking to microtubule transport. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Apply pulses to test recovery in mixed populations.

Question 18

A migrating cell line was engineered so that vesicles carrying membrane proteins could not bind to microtubule-associated transport machinery. Imaging showed normal actin-rich ruffling, but the leading edge failed to expand persistently and the cell frequently retracted. Which conclusion about microtubules in cell motility is most consistent?

Microtubules generate traction forces against the substrate; vesicle binding defects should not affect edge persistence.
Microtubules primarily resist compression; loss of vesicle binding should stiffen the cell and increase persistent protrusion.
Actin ruffling alone is sufficient for persistent leading-edge expansion; the retractions suggest intermediate filaments are missing.
Microtubules support persistent protrusion by enabling targeted delivery of membrane and associated components to the leading edge. (correct answer)

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, preventing vesicle binding to microtubules leads to failed persistent expansion and frequent retractions despite ruffling. Choice D is correct because it accurately describes microtubules' support for targeted delivery. Choice B is incorrect because it wrongly claims microtubules resist compression via vesicle binding. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Engineer specific defects to isolate transport roles in protrusion persistence.

Question 19

Researchers mechanically stretched migrating cells and monitored cytoskeletal rearrangements. Cells with intact intermediate filaments recovered shape after stretch and continued moving, whereas cells with disrupted intermediate filaments showed persistent deformation and reduced migration persistence, despite normal actin-rich protrusions. Which statement best describes the role of intermediate filaments in motility under mechanical stress?

Intermediate filaments act as a load-bearing network that preserves cellular integrity and supports persistent migration when external forces deform the cell. (correct answer)
Intermediate filaments generate ATP needed for actin polymerization; disruption reduces persistence by lowering available energy.
Intermediate filaments are the main tracks for directional vesicle transport; disruption should prevent leading-edge membrane delivery but not affect deformation recovery.
Intermediate filaments are static and not involved in migration; persistent deformation indicates microtubules have depolymerized.

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, disrupting intermediate filaments leads to persistent deformation and reduced persistence after stretching, despite protrusions. Choice A is correct because it accurately describes their load-bearing role under stress. Choice B is incorrect because it wrongly attributes ATP generation to them. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Apply mechanical perturbations to reveal stress-handling mechanisms.

Question 20

Amoeboid cells in culture were treated with a compound that increases actin filament nucleation, producing many short filaments. Immediately after treatment, cells formed numerous small protrusions in multiple directions but showed reduced net displacement over time. Which conclusion about actin organization and cell motility is most consistent with these results?

Excessive, uncoordinated actin nucleation can fragment protrusive activity and reduce persistent polarity, decreasing net migration despite more protrusions. (correct answer)
Increasing actin nucleation should always increase migration speed because more filaments necessarily produce greater forward force.
The reduced displacement indicates actin is not involved in protrusion; microtubules must be the primary determinant of membrane extension.
The phenotype is best explained by intermediate filament overgrowth, which directly creates multiple membrane protrusions.

Explanation: This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, increasing actin nucleation produces many small protrusions but reduces net displacement due to uncoordinated activity. Choice A is correct because it accurately describes how excessive nucleation fragments protrusions and decreases persistent polarity. Choice B is incorrect because it wrongly assumes more filaments always increase speed. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Balance nucleation rates to maintain coordinated motility in treatments.