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Cytoskeleton Types and Comparison Table
| Category | Microtubules (Tubulin Polymers) | Microfilaments (Actin Filaments) | Intermediate Filaments (Keratin, Lamin) |
|---|---|---|---|
| Structure | Hollow tubes (13 columns of tubulin dimers) | Two intertwined strands of actin | Fibrous proteins supercoiled into thicker cables |
| Diameter | 25 nm (15 nm lumen) | 7 nm | 8–12 nm |
| Protein Subunits | Tubulin (α-tubulin + β-tubulin) | Actin | Keratin, Lamin, others |
| Main Functions | Maintain cell shape (compression-resisting) Cell motility (cilia, flagella) Chromosome movement Organelle movement |
Maintain cell shape (tension-bearing) Interact with myosin ➔ sliding mechanism Muscle contraction (via sliding, not shortening of filaments) Cytoplasmic streaming Cell motility (pseudopodia) Cell division (cleavage furrow) |
Maintain cell shape (tension-bearing) Anchorage of nucleus & organelles Formation of nuclear lamina (Lamin) |
| Contractile? | ❌ Not contractile | ✅ Yes (actin works with myosin to cause contraction by sliding) | ❌ Not contractile |
| Examples | Spindle fibers, cilia, flagella | Muscle fibers, cleavage furrow | Keratin (skin, hair), Lamin (nucleus) |
| Balloon Analogy | Like pillars inside balloon → resist compression (pushing forces) | Like ropes pulling balloon surface → resist tension and allow sliding action | Like safety net inside balloon → resist tension (stretching) |
Note for Students: Actin is called a contractile protein because it works with myosin to cause contraction by sliding, not by shrinking itself. This sliding action shortens the muscle or cell ➔ leading to contraction!
Centrioles: Proximal vs Distal Region Detailed Table
| Feature | Proximal Part | Distal Part |
|---|---|---|
| Location | Near nucleus / center of centrosome | Away from nucleus / towards cell periphery |
| Central Hub | Present ➔ Proteinaceous hub | Absent |
| Radial Spokes | Present ➔ Connect hub to triplet microtubules | Disappear ➔ Linker proteins hold triplets |
| Microtubule Organization | Active ➔ Works as Microtubule Organizing Center (MTOC) | Stable ➔ Supports distal structure (non-MTOC) |
| Function Identity | Initiates spindle formation in cell division | Connects to cilia/flagella ➔ cell motility |
| Example Connection | Forms centrosome ➔ MTOC | Forms basal body ➔ cilia/flagella |
💡 Memory Tip: Proximal part has hub & spokes ➔ works as MTOC (organizer).
Distal part has no hub ➔ acts as structural base for cilia/flagella.
Distal part has no hub ➔ acts as structural base for cilia/flagella.
Centrosome vs Centriole vs Basal Body vs Cilia/Flagella Table
| Feature | Centrosome | Centriole | Basal Body | Cilia/Flagella |
|---|---|---|---|---|
| Definition | Organelle with 2 centrioles + amorphous pericentriolar material (PCM) | Cylindrical structure with 9 triplet microtubules | Modified centriole anchoring cilia/flagella | Hair-like projections for movement |
| Structure | 2 centrioles at right angles + PCM | Proximal: hub + spokes Distal: stable tubular triplets |
Like centriole ➔ anchors axoneme | Axoneme: 9+2 microtubule array Covered by plasma membrane |
| Function | MTOC ➔ spindle formation | Forms centrosome or basal body | Initiates growth of cilia/flagella | Causes cell/ fluid movement |
| Amorphous Material | PCM ➔ unstructured protein cloud for microtubule nucleation | ❌ No amorphous material | ❌ No amorphous material | ❌ No amorphous material |
| Example Role | Spindle formation in mitosis/meiosis | Forms basal bodies in animal cells | Anchor point for 9+2 axoneme | Movement ➔ sperm tail, respiratory tract |
| Special Notes | MTOC in animal cells | Present in animals, absent in most plants | Derived from centriole | Prokaryotic flagella ≠ eukaryotic (structurally different) |
💡 Explanation Tip:
Amorphous pericentriolar material means the cloudy, unstructured proteins that surround centrioles in centrosomes. This material helps in nucleating and organizing microtubules. It is called amorphous because it has no fixed shape like organelles.
9+2 array of cilia/flagella: 9 doublet microtubules outside + 2 singlets in center ➔ allows bending and movement.
Amorphous pericentriolar material means the cloudy, unstructured proteins that surround centrioles in centrosomes. This material helps in nucleating and organizing microtubules. It is called amorphous because it has no fixed shape like organelles.
9+2 array of cilia/flagella: 9 doublet microtubules outside + 2 singlets in center ➔ allows bending and movement.
Note: “Nucleating” means starting the formation of microtubules. The amorphous pericentriolar material (PCM) in the centrosome helps the first tubulin units come together and begin building microtubules — like laying the foundation bricks before constructing the full structure.
