Cell Biology
Cytoskeleton
Cell's internal scaffolding — provides shape, enables movement, organizes interior
The cytoskeleton is a network of protein filaments giving cells shape, supporting organelles, and enabling movement. Three main types: (1) Microfilaments (actin) — thinnest (~7 nm); cell shape, migration, division. (2) Microtubules (tubulin) — thickest (~25 nm); transport, mitosis, cilia/flagella. (3) Intermediate filaments — midrange; mechanical support (keratin, desmin). Dynamic: continuously assembled/disassembled. Motor proteins (myosin, kinesin, dynein) walk along filaments — transport organelles, drive contraction. Critical for cell biology, development, disease (muscular dystrophy, neurodegenerative).
- Three filament typesMicrofilaments, microtubules, intermediate filaments
- MicrofilamentsActin polymers; ~7 nm diameter
- MicrotubulesTubulin polymers; ~25 nm diameter; hollow tubes
- Intermediate filamentsVarious proteins; ~10 nm; mechanical strength
- Motor proteinsMyosin (actin), kinesin/dynein (microtubules)
- DynamicConstantly assembling/disassembling
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Why cytoskeleton matters
- Cell biology. Foundational structure.
- Cell division. Mitosis depends on it.
- Cell movement. Migration, crawling.
- Transport. Vesicle movement in cell.
- Cancer. Metastasis involves cytoskeletal changes.
- Drug targets. Many cancer drugs target cytoskeleton.
- Development. Cell shape changes drive morphogenesis.
Common misconceptions
- Cytoskeleton is static. Dynamic; rapid turnover.
- Cytoskeleton just for shape. Many functions.
- Only animal cells have it. Plants, bacteria, archaea also have homologs.
- Filaments are simple ropes. Complex polymers with polarity.
- One filament type does all. Three distinct types, different functions.
- Movement = energy-free. ATP-driven motors.
Frequently asked questions
What are microfilaments?
Actin polymers. Thinnest filaments (~7 nm). Highly dynamic — assembled/disassembled continuously. Functions: cell shape, motility (lamellipodia, filopodia), endocytosis, cytokinesis (cleavage furrow), muscle contraction. Toxin: phalloidin (binds actin tightly; used as marker). Drug: cytochalasin (disrupts assembly; used in research). Most abundant cellular protein in many cells.
What are microtubules?
Tubulin polymers. Hollow tubes (~25 nm). Made of α/β tubulin dimers; arranged in 13 protofilaments. Highly dynamic — constant addition at + end. Functions: vesicle transport, mitotic spindle, cilia/flagella beating, cell shape. Drugs: taxol (stabilizes; chemotherapy), colchicine, vinca alkaloids (depolymerize; chemotherapy). Microtubule poisons widely used in cancer.
What are intermediate filaments?
Larger family; proteins vary by cell type. Keratin (epithelial), desmin (muscle), neurofilaments (neurons), vimentin (mesenchymal), lamin (nuclear). Most stable cytoskeleton element — slow turnover. Provides mechanical strength. Mutations: skin diseases (epidermolysis bullosa from keratin defect), heart problems (desmin), muscular dystrophy.
What are motor proteins?
Walk along filaments using ATP. Myosin: walks on actin; muscle contraction, intracellular movement. Kinesin: microtubules, anterograde (toward + end; outward from cell body). Dynein: microtubules, retrograde (toward - end; inward). Each motor: head domain (binds filament + hydrolyzes ATP), tail domain (binds cargo). Steps in 8-nm increments. Critical for transport.
How does cytoskeleton enable mitosis?
Microtubules form mitotic spindle. Centrosomes (microtubule organizing centers) move to opposite poles. Spindle microtubules extend and capture chromosomes at kinetochores. Microtubules then shorten — pulling chromosomes apart. Disrupted by drugs: vinca alkaloids, taxol. Used in cancer (rapidly dividing cells more sensitive).
How does cytoskeleton drive cell movement?
Actin polymerization at leading edge pushes membrane forward (lamellipodia, filopodia). Cell adheres via integrins. Myosin contracts back of cell. Cycle repeats. Crawling movement. Examples: macrophages tracking pathogens, fibroblasts in wound healing, cancer cells metastasizing. Speed: 1-100 µm/min.
How is cytoskeleton involved in disease?
Many diseases. (1) Cancer metastasis: cytoskeletal changes enable migration. (2) Muscular dystrophy: dystrophin defect; cytoskeleton-membrane connection. (3) Charcot-Marie-Tooth: peripheral neuropathy from cytoskeleton mutations. (4) Lissencephaly: smooth brain; tubulin mutations affect neuronal migration. (5) ALS: neurofilament aggregation. Critical for cell function across diseases.