The cytoskeleton and cell movement

MCAT content library / Biology & biochemistry / The cytoskeleton and cell movement

Match a transport, movement, division, or mechanical phenotype to the cytoskeletal system that can produce it.

About 16 minutes, including the perturbation dataset and checks.

Give each filament system a job

The eukaryotic cytoskeleton is built from actin filaments, microtubules, intermediate filaments, and associated proteins. They cooperate, but they are not interchangeable. Actin is central to cell-cortex shape, cell crawling, contractile structures, and the cleavage furrow during cytokinesis. Microtubules form polarized tracks for long-range intracellular transport, organize the mitotic spindle, and support eukaryotic cilia and flagella. Intermediate filaments contribute tensile strength and help tissues resist mechanical stress.²

A useful experimental question is therefore not “Which filament is in the cell?” but “Which measured function failed while the others were preserved?”

Concept: motors convert ATP into directed movement

Molecular motors bind polarized cytoskeletal tracks and use ATP hydrolysis to move cargo or slide filaments relative to one another. Myosin motors operate on actin filaments; kinesin and dynein families operate on microtubules. Different motors can move in opposite directions on the same polarized track, so a passage can localize a defect by showing which direction of cargo movement is lost.³

Motor activity is distinct from filament assembly. A microtubule can remain physically present while a motor defect prevents vesicles from moving along it. Conversely, a motor can be intact but unable to transport cargo if the track is depolymerized.

Worked application: three selective perturbations

An original epithelial-cell experiment uses three selective perturbations. Investigators measure cortical shape, long-range vesicle transport, chromosome segregation, and survival after repeated mechanical stretch.

Perturbation A best matches actin disruption because the cortical phenotype is dominant. Perturbation B best matches microtubule disruption because both long-range vesicle movement and chromosome segregation collapse. Perturbation C best matches intermediate-filament disruption because mechanical resistance fails while transport and mitosis remain comparatively preserved.

The table intentionally avoids one-to-one perfection. Real cellular systems are coupled, so disrupting one component can produce secondary effects. The inference comes from the pattern of strongest defects, not from assuming every phenotype has only one structural cause.

Pattern: structure and transport fail differently

Suppose vesicles accumulate near the cell center while microtubules remain intact. That finding suggests a transport-protein problem more strongly than complete microtubule loss. If ATP depletion stops directed vesicle motion but the tracks remain visible, the experiment further supports an energy-dependent motor step.

By contrast, a spindle defect with normal ATP and intact motor proteins can point toward microtubule assembly or attachment. Keep the level of the defect explicit: filament, motor, cargo attachment, or energy supply.

Rule: eukaryotic cilia are not bacterial flagella

Eukaryotic cilia and flagella are membrane-covered structures built around microtubules and powered by dynein-driven sliding. Bacterial flagella are structurally different and rotate using a membrane-associated motor rather than a microtubule axoneme. Similar names for cell movement do not imply homologous machinery.

Check your reasoning

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Sources

1. AAMC. Content Category 2A.
2. NCBI Bookshelf. The Cytoskeleton and Cell Behavior.
3. NCBI Bookshelf. Molecular Motors.