Unit 7: The Cytoskeleton and Cell Motility
Three filament systems and the motor proteins that walk, pull, and crawl along them to move cargo, divide a cell, and drive a whole cell across a surface.
8 topics · 34 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Actin Filament Structure and Treadmilling
- How it worksHow does ATP-bound actin monomer addition at the plus end and loss at the minus end create treadmilling?
- How it worksWalk me through how ATP hydrolysis inside an actin filament destabilizes the older subunits near the minus end.
- Common mistakeWhy isn't an actin filament a permanent structure once it's assembled?
- Worked exampleFind how fast an actin filament's plus end grows if monomers add at 10 subunits per second and each adds 2.7 nanometers of length.
Your turn1 of 2
Find how fast an actin filament's plus end grows if monomers add at 19 subunits per second and each adds 2.724 nanometers of length.Actin-Binding Proteins and the Cell Cortex
- How it worksHow does a crosslinking protein like alpha-actinin bundle parallel actin filaments together?
- How it worksHow does the Arp2/3 complex nucleate a branched actin network at a cell's leading edge?
- Common mistakeWhy doesn't actin polymerize into one fixed shape and just stay that way for the rest of the cell's life?
- Going furtherWhat happens to the actin cortex's stiffness when a capping protein is added to stop filament growth?
Your turn1 of 2
A crawling cell's actin cortex is a loose branched mesh at its leading edge but becomes a tight parallel bundle in a different region a few minutes later. A student assumes that once actin polymerizes into a given shape, it stays that way for the rest of the cell's life. Why is this wrong?Microtubule Structure and Dynamic Instability
- How it worksHow does GTP hydrolysis inside a growing microtubule set up a cap that decides whether it grows or shrinks?
- How it worksWalk me through a catastrophe event where a microtubule flips from steady growth to rapid shrinkage.
- Common mistakeWhy isn't microtubule dynamic instability a sign that something has gone wrong inside the cell?
- Ties togetherHow does this same dynamic instability let microtubules search the cytoplasm to find and capture kinetochores during mitosis?
Your turn1 of 2
A microtubule is observed growing steadily for a minute and then suddenly shrinking rapidly back almost to nothing, only to begin growing again shortly after. A student assumes this rapid switching means something has gone wrong inside the cell. Why is this incorrect?The Centrosome and Microtubule Organization
- How it worksHow does the centrosome nucleate new microtubules using gamma-tubulin ring complexes?
- Common mistakeWhy isn't a centrosome required for a cell to build microtubules at all?
- Going furtherWhat happens to spindle formation when a cell's centrosomes are destroyed with a laser before mitosis?
Your turn1 of 2
Plant cells and many differentiated animal cells (like neurons) lack a centrosome, yet they still build extensive microtubule networks. A student assumes a centrosome is absolutely required for a cell to build microtubules at all. Why is this wrong?Intermediate Filaments and Mechanical Resilience
- How it worksHow do intermediate filaments form a rope-like structure that resists stretching rather than compression?
- How it worksWalk me through how keratin filaments anchor at desmosomes to spread mechanical stress across a tissue.
- Common mistakeWhy doesn't an intermediate filament disassemble and reassemble as constantly as actin or microtubules do?
- Going furtherWhat happens to skin cells when a keratin gene mutation weakens the intermediate filament network?
Your turn1 of 2
Actin filaments and microtubules constantly polymerize and depolymerize within a cell, but intermediate filaments are comparatively much more stable over time. A student assumes intermediate filaments must be completely static once assembled. Why doesn't an intermediate filament disassemble and reassemble as constantly as actin or microtubules do?Kinesin and Dynein: Walking Along a Microtubule
- How it worksWalk me through how kinesin's two heads take alternating steps toward a microtubule's plus end.
- How it worksHow does dynein use ATP hydrolysis to walk toward a microtubule's minus end instead?
- Common mistakeWhy isn't a motor protein simply dragged along by cytoplasmic streaming instead of actively walking?
- Worked exampleFind how long it takes kinesin to carry cargo 10 micrometers down an axon at a speed of 1 micrometer per second.
- Going furtherWhat happens to organelle distribution in a cell when the gene for a kinesin motor is knocked out?
Your turn1 of 2
Find how long it takes kinesin to carry cargo 29 micrometers down an axon at a speed of 1.76 micrometers per second.Myosin Motors and Actin-Based Movement
- How it worksHow does myosin's power stroke pull an actin filament forward once ATP hydrolysis releases stored energy?
- How it worksWalk me through how myosin II filaments sliding past actin filaments shortens an entire sarcomere.
- Worked exampleFind how far a muscle fiber shortens if each of its 500 sarcomeres contracts by 1 micrometer.
- Common mistakeWhy doesn't a muscle contract by the actin and myosin filaments themselves getting shorter?
- Going furtherWhat happens to a muscle cell's contraction when ATP runs out entirely, as in rigor mortis?
Your turn1 of 2
Find how far a muscle fiber shortens if each of its 1423 sarcomeres contracts by 1.47 micrometers.Cilia, Flagella, and Axonemal Structure
- How it worksHow does the 9+2 arrangement of microtubules inside an axoneme convert sliding into a bending wave?
- How it worksHow does dynein sliding one microtubule doublet against its neighbor bend the whole cilium?
- Worked exampleFind the beat frequency of a cilium that completes 20 full beating cycles in 2 seconds.
- Common mistakeWhy isn't a cilium's beating powered by the cell body pushing it back and forth?
- Going furtherWhat happens to airway clearance when a genetic defect leaves a cilium's dynein arms missing?
Your turn1 of 2
Find the beat frequency of a cilium that completes 58 full beating cycles in 4 seconds.