Describe the cytoskeleton (microfilaments, microtubules, intermediate filaments)

WHAT it is

The naming follows diameter: microfilaments are the smallest, microtubules the largest, "intermediate" sits in between. Remember that and you can never confuse the order.

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

1. Microfilaments (Actin filaments)

WHY polar? Each G-actin monomer is asymmetric and adds in one orientation, so the whole filament has direction. This polarity lets motor proteins (myosin) know which way to walk.

HOW they work / functions

Cell shape & cortex — a dense actin mesh just under the membrane resists deformation.
Muscle contraction — actin + myosin sliding.
Cell crawling — actin polymerises at the leading edge to push the membrane forward (lamellipodia).
Cytokinesis — the contractile actin ring pinches an animal cell into two.

2. Microtubules

HOW they work / functions

Intracellular highways — motor proteins kinesin (walks toward + end, outward) and dynein (walks toward − end, inward) drag vesicles & organelles along them.
Spindle fibres — separate chromosomes in mitosis.
Cilia & flagella — arranged in the 9+2 pattern (9 doublets ringing 2 central tubules).
Centrioles — the 9 triplets that organise the spindle.

WHY different? Their job is pure mechanical strength — bearing tension so cells/tissues don't rip apart. Built like twisted rope: many strands wound together, strong in any direction (so polarity is irrelevant).

HOW / functions

Anchor the nucleus and organelles in place.
Give tissues tensile strength (skin, hair, nails = keratin).
Nuclear lamina supports the nuclear envelope.

Formula Why microtubules are the "tracks" — geometry of length

A microtubule is built from rings of 13 protofilaments, each a chain of 8 nm tubulin dimers. If a filament has $N$ dimers added end-to-end along one protofilament, its length is: $L = N \times d, \qquad d = 8\ \text{nm}$ Derivation from scratch: length is added one dimer at a time; each dimer contributes a fixed axial rise $d$ . After $N$ additions the total rise is just the sum $\underbrace{d+d+\dots+d}_{N} = Nd$ . To grow a $10\ \mu\text{m} = 10000\ \text{nm}$ microtubule: $N = \frac{L}{d} = \frac{10000}{8} = 1250 \text{ dimers per protofilament.}$ With 13 protofilaments, total dimers $= 13 \times 1250 = 16250$ . This shows polymer length is discrete — it changes in fixed 8 nm jumps, the molecular basis of dynamic instability.

Common mistake Steel-manning the common errors

Mistake A: "Microtubules are smallest because 'micro' sounds tiny." Why it feels right: both words start with "micro", and tubules sound delicate. Fix: Order by number of strands: microfilament = 2 strands (7 nm) < intermediate = rope (10 nm) < microtubule = 13-protofilament hollow tube (25 nm). The hollow tube is the fattest.

Mistake B: "Intermediate filaments carry vesicles too." Why it feels right: they're long fibres like the others, so transport seems natural. Fix: IFs are non-polar and have no motor proteins — kinesin/dynein need polar microtubule tracks; myosin needs polar actin. No polarity ⇒ no directional walking ⇒ no transport.

Mistake C: "Cilia move using actin." Why it feels right: actin moves cells (crawling), so we assume it moves cilia. Fix: Cilia & flagella are microtubule structures (9+2), powered by dynein. Actin does crawling; microtubules do beating/whipping.

Recall Feynman: explain to a 12-year-old

Imagine a tent. The thick poles that hold it up and let you slide ropes along to move things = microtubules (with little delivery-men called kinesin/dynein walking along them). The thin springy threads near the tent's skin that let it flex and crawl = microfilaments (actin). The tough criss-cross ropes stitched through the fabric so it doesn't rip = intermediate filaments. Three jobs: move stuff, change shape, don't tear.

Flashcards

What are the three cytoskeletal filament types in order of increasing diameter?

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

Which protein makes microfilaments?

G-actin (globular actin polymerises into helical strands)

Which proteins make microtubules?

α-tubulin and β-tubulin dimers

Why can microtubules and microfilaments support directional transport but intermediate filaments cannot?

They are polar (have + and − ends), giving motor proteins a direction; intermediate filaments are non-polar with no motors

Name the two microtubule motor proteins and their direction.

Kinesin (toward + end, outward); dynein (toward − end, inward)

What is the 9+2 arrangement?

The microtubule pattern in cilia/flagella: 9 outer doublets surrounding 2 central microtubules

Which filament forms the contractile ring in cytokinesis?

Microfilaments (actin) plus myosin

Which filament gives skin tensile strength, and via which protein?

Intermediate filaments, via keratin

What is the axial length added per tubulin dimer?

~8 nm

A muscle-contraction failure most likely involves which two proteins?

Actin (microfilament) and myosin

Connections

Centrosome and Centrioles — the MTOC that nucleates microtubules
Mitosis — spindle fibres are microtubules pulling chromosomes
Muscle Contraction — actin–myosin sliding filament model
Cilia and Flagella — 9+2 microtubule machines
Nuclear Envelope — lamins (intermediate filaments) form the lamina
Motor Proteins (Kinesin, Dynein, Myosin) — engines on the cytoskeletal tracks
Cell Membrane — actin cortex underlies and shapes it

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, cell ek paani se bhari thaili jaisi hai. Agar andar koi dhaancha (scaffold) na ho to cell bina shape ke blob ban jaayegi, organelles idhar-udhar tairti rahengi, aur cell na chal paayegi na divide kar paayegi. Isi liye nature ne cytoskeleton banaya — teen tarah ke protein filaments ka network jo shape deta hai, strength deta hai, aur cargo transport ke liye "patri" (tracks) banata hai.

Teen types yaad rakho diameter ke hisaab se (chhote se bade): Microfilament (actin, ~7 nm, sabse patla) — yeh movement, cell crawling aur muscle contraction karta hai. Intermediate filament (~10 nm, rope jaisa) — yeh sirf strength deta hai taaki tissue phate na (jaise skin mein keratin). Microtubule (tubulin, ~25 nm, hollow tube, sabse mota) — yeh transport ki highway hai jispar kinesin aur dynein motor proteins chalte hain, aur mitosis mein spindle bhi yahi banata hai.

Ek important point: microfilament aur microtubule polar hote hain (un ke + aur − end hote hain), isliye motor proteins direction samajh ke chal sakte hain. Lekin intermediate filament non-polar hai, isliye usme transport nahi hota — sirf taakat. Yeh distinction exam mein bahut puchha jaata hai, isliye "polarity = transport possible" yaad rakho.

Trick: FIT — Filament, Intermediate, Tubule (patle se mote). Aur function tag: Actin = Action (movement), Tubule = Transport, Intermediate = Integrity. Bas itna pakka karlo to poora topic clear.

Describe the cytoskeleton (microfilaments, microtubules, intermediate filaments)

WHAT it is

1. Microfilaments (Actin filaments)

2. Microtubules

3. Intermediate Filaments

Flashcards

Connections

Concept Map

Hinglish (regional understanding)

Test yourself — Organelles & Their Functions

Connections