Cytoskeleton and cell Wall

Cytoskeleton & Cell Wall Dynamics

Complete Masterclass for CSIR-NET, GATE & DBT-BET

"Cells are not just squishy water balloons. They crawl, they divide, they haul cargo, and they defend themselves with rigid armor. Let's explore the structural cables and protective walls that keep cells alive and moving."

1. Functions of Actin (Microfilaments)

Actin filaments are the thinnest cytoskeletal fibers (~7 nm). They are incredibly dynamic, formed by the polymerization of globular G-actin into filamentous F-actin, a process heavily dependent on ATP.

• Cell Shape & Cortex: Actin forms a dense web just beneath the plasma membrane (the cell cortex), giving the cell structural rigidity.
• Cell Motility: Rapid polymerization at the leading edge pushes the membrane forward, forming structures like Lamellipodia (sheet-like) and Filopodia (spike-like).
• Muscle Contraction: Actin slides past Myosin motor proteins to contract muscle fibers.
• Cytokinesis: Forms the Contractile Ring that pinches dividing animal cells into two during the final stage of mitosis.

🧠 THE TREADMILLING PHENOMENON: Actin filaments are polar. ATP-bound G-actin adds rapidly to the (+) barbed end. As ATP hydrolyzes to ADP, the filament becomes unstable, and ADP-actin falls off the (-) pointed end. The filament stays the same length but acts like a moving treadmill!

Live Animation: Actin Treadmilling

ATP-Actin adds to the (+) end. ADP-Actin falls off the (-) end.

2. Microtubules & Molecular Motors

Microtubules are massive (~25 nm) hollow tubes made of α- and β-tubulin dimers. They rely entirely on GTP hydrolysis for their dynamic instability. They act as the primary transport highways of the cell and form the mitotic spindle to separate chromosomes.

The Cellular Delivery Trucks: Motor Proteins

These proteins literally "walk" along microtubules, burning one ATP for every step they take.

Motor Protein	Direction	Primary Function
Kinesin	Moves toward (+) End (Cell Periphery)	Anterograde Transport (Moves vesicles outwards).
Dynein	Moves toward (-) End (MTOC / Nucleus)	Retrograde Transport. Also powers the beating of Cilia and Flagella.

3. Intermediate Filaments (The Cell's Rebar)

Unlike Actin and Microtubules, Intermediate Filaments (~10 nm) do NOT use ATP/GTP, and they lack polarity (no + or - ends). They are incredibly stable and provide pure mechanical tensile strength to prevent the cell from tearing under stress.

📌 CSIR EXAM TIP: You must memorize the specific tissue locations of Intermediate Filaments:
• Keratin: Epithelial cells (Skin, hair, nails).
• Vimentin: Mesenchymal cells (Connective tissue, fibroblasts).
• Neurofilaments: Neurons (Axon structural support).
• Nuclear Lamins (A, B, C): Inside ALL animal nuclei (supports the nuclear envelope).

4. The Cell Wall (Plants, Fungi & Bacteria)

Animal cells only have a plasma membrane, but plants, fungi, and bacteria have a rigid Cell Wall to prevent osmotic lysis (bursting) and provide structural support.

• Plant Cell Wall: Primarily made of Cellulose (a β-1,4-linked glucose polymer), Hemicellulose, and Pectin. It has a flexible Primary wall and a rigid Secondary wall (often containing lignin).
• Fungal Cell Wall: Primarily made of Chitin (a polymer of N-acetylglucosamine).
• Bacterial Cell Wall: Made of Peptidoglycan (alternating chains of NAM and NAG cross-linked by short peptides).

Visual Comparison: Gram-Positive vs. Gram-Negative

Notice the massive Peptidoglycan difference and the Outer Membrane of Gram-Negatives.

🔥 Master Comparison & Final Revision

Cytoskeleton Type	Protein Base	Energy	Key Function
Microfilaments	Actin	ATP	Cell shape, crawling, muscle contraction.
Microtubules	Tubulin	GTP	Intracellular transport, mitotic spindle.
Intermediate Filaments	Keratin, Vimentin, etc.	None	Pure mechanical and tensile strength.

🧠 Rapid Fire Cell Wall Recall:
• Plants: Cellulose.
• Fungi: Chitin.
• Gram-Positive Bacteria: Thick Peptidoglycan, Teichoic acids.
• Gram-Negative Bacteria: Thin Peptidoglycan, Outer Membrane, LPS (Endotoxin), Porins.