Describe the cytoskeleton (microfilaments, microtubules, intermediate filaments)

2.3.11 · Biology › Organelles & Their Functions

YEH HAI KYA

Naming diameter ke hisaab se hoti hai: microfilaments sabse choti hain, microtubules sabse badi, "intermediate" beech mein aati hai. Yeh yaad rakhlo aur phir kabhi order confuse nahi hoga.

Figure — Describe the cytoskeleton (microfilaments, microtubules, intermediate filaments)

1. Microfilaments (Actin filaments)

POLAR kyun? Har G-actin monomer asymmetric hota hai aur ek hi orientation mein add hota hai, isliye poore filament ka ek direction hota hai. Yeh polarity motor proteins (myosin) ko batati hai ki kaunsi taraf chalna hai.

YEH KAAM KAISE KARTE HAIN / functions

Cell shape & cortex — membrane ke theek neeche ek dense actin mesh deformation ko resist karta hai.
Muscle contraction — actin + myosin sliding.
Cell crawling — actin leading edge pe polymerise hota hai taaki membrane ko aage push kar sake (lamellipodia).
Cytokinesis — contractile actin ring ek animal cell ko pinch karke do mein karta hai.

2. Microtubules

YEH KAAM KAISE KARTE HAIN / functions

Intracellular highways — motor proteins kinesin (+ end ki taraf, baahir) aur dynein (− end ki taraf, andar) vesicles aur organelles ko inke saath kheenchte hain.
Spindle fibres — mitosis mein chromosomes ko separate karte hain.
Cilia & flagella — 9+2 pattern mein arrange hote hain (2 central tubules ke around 9 doublets).
Centrioles — 9 triplets jo spindle ko organise karte hain.

3. Intermediate Filaments

ALAG KYUN? Inका kaam sirf mechanical strength dena hai — tension bear karna taaki cells/tissues phat na jayein. Twisted rope ki tarah bani hain: bahut saari strands ek saath wound, har direction mein mazboot (isliye polarity irrelevant hai).

KAISE / functions

Nucleus aur organelles ko jagah par anchor karte hain.
Tissues ko tensile strength dete hain (skin, hair, nails = keratin).
Nuclear lamina nuclear envelope ko support karta hai.

Common mistake Common errors ko steel-man karna

Mistake A: "Microtubules sabse choti hain kyunki 'micro' chota lagta hai." Sahi kyun lagta hai: dono words "micro" se start hote hain, aur tubules delicate lagte hain. Fix: Strands ki number se order karo: microfilament = 2 strands (7 nm) < intermediate = rope (10 nm) < microtubule = 13-protofilament hollow tube (25 nm). Hollow tube sabse moti hai.

Mistake B: "Intermediate filaments bhi vesicles carry karte hain." Sahi kyun lagta hai: yeh bhi doosron jaisi lambi fibres hain, isliye transport natural lagta hai. Fix: IFs non-polar hain aur inke paas koi motor proteins nahi — kinesin/dynein ko polar microtubule tracks chahiye; myosin ko polar actin chahiye. Polarity nahi ⇒ directional walking nahi ⇒ transport nahi.

Mistake C: "Cilia actin se move karte hain." Sahi kyun lagta hai: actin cells ko move karta hai (crawling), isliye hum assume karte hain ki cilia bhi yahi karta hai. Fix: Cilia & flagella microtubule structures hain (9+2), dynein se powered. Actin crawling karta hai; microtubules beating/whipping karte hain.

Recall Feynman: 12-saal ke bachche ko samjhao

Ek tent imagine karo. Mote poles jo usse khada rakhte hain aur jinpe tum ropes slide kar sakte ho cheezein move karne ke liye = microtubules (jinpe chote delivery-men jaise kinesin/dynein chalte hain). Patli springy threads tent ki skin ke paas jo usse flex aur crawl karne deti hain = microfilaments (actin). Mazboot criss-cross ropes jo fabric ke andar sili hain taaki yeh phat na jaaye = intermediate filaments. Teen kaam: cheezein move karo, shape change karo, phat mat jaao.

Flashcards

Teen cytoskeletal filament types badhte diameter ke order mein kya hain?

Microfilaments (actin, ~7 nm) → intermediate filaments (~10 nm) → microtubules (~25 nm)

Microfilaments kaunse protein se bante hain?

G-actin (globular actin helical strands mein polymerise hoti hai)

Microtubules kaunse proteins se bante hain?

α-tubulin aur β-tubulin dimers

Microtubules aur microfilaments directional transport support kyun kar sakte hain lekin intermediate filaments nahi?

Yeh polar hain (inke paas + aur − ends hote hain), jo motor proteins ko direction dete hain; intermediate filaments non-polar hain jinke paas koi motors nahi

Do microtubule motor proteins aur unki direction batao.

Kinesin (+ end ki taraf, baahir); dynein (− end ki taraf, andar)

9+2 arrangement kya hai?

Cilia/flagella mein microtubule pattern: 2 central microtubules ke around 9 outer doublets

Cytokinesis mein contractile ring kaunsa filament banata hai?

Microfilaments (actin) plus myosin

Skin ko tensile strength kaunsa filament deta hai, aur kis protein ke zariye?

Intermediate filaments, keratin ke zariye

Har tubulin dimer se kitna axial length add hota hai?

~8 nm

Muscle-contraction failure mein most likely kaunse do proteins involved hain?

Actin (microfilament) aur myosin

Connections

Centrosome and Centrioles — MTOC jo microtubules nucleate karta hai
Mitosis — spindle fibres microtubules hain jo chromosomes kheenchte hain
Muscle Contraction — actin–myosin sliding filament model
Cilia and Flagella — 9+2 microtubule machines
Nuclear Envelope — lamins (intermediate filaments) lamina banate hain
Motor Proteins (Kinesin, Dynein, Myosin) — cytoskeletal tracks pe engines
Cell Membrane — actin cortex iske neeche hota hai aur ise shape deta hai