Cytoskeleton: Microtubules, Microfilaments and Functions

The cytoskeleton is a dynamic network of protein filaments inside the cytoplasm that gives the cell shape, strength and internal organisation. Its three main components are microtubules, microfilaments and intermediate filaments.

It matters because cells need to move, divide, transport materials and maintain form. The cytoskeleton acts like both scaffolding and railway tracks, constantly rebuilding to meet changing demands.

Key Concepts

• Microtubules: hollow tubes of tubulin protein, about 25 nanometres in diameter; form spindle fibres and tracks for motor proteins.
• Microfilaments: thin actin filaments, about 7 nanometres in diameter; support cell shape and movement.
• Intermediate filaments: rope-like fibres about 10 nanometres in diameter; provide tensile strength.
• Motor proteins: kinesin, dynein and myosin that walk along filaments carrying cargo using ATP.
• Centrosome: main microtubule-organising centre in animal cells.

Unlike static bones, cytoskeletal filaments assemble and disassemble rapidly by adding or removing subunits.

How It Works

1. Tubulin dimers add to the plus end of microtubules causing growth, or detach causing shrinkage in dynamic instability.
2. Actin monomers polymerise into filaments that push the membrane during crawling and cytokinesis.
3. Motor proteins bind vesicles and slide along filaments to deliver them to specific locations.
4. During mitosis, microtubules form the spindle that separates chromosomes equally.

Regulatory proteins control filament length and organisation according to cell signals.

Worked Example

Imagine a nerve cell axon 1 metre long in the human body. Vesicles made in the cell body must reach the terminal without diffusion, which would be far too slow. Kinesin motors carry vesicles along microtubules at about 1 micrometre per second. At this speed, travel over 1 metre or 1,000,000 micrometres would take about 1,000,000 seconds, roughly 11 to 12 days, which matches known slow axonal transport rates for some cargo, while faster vesicles using multiple motors can reach up to 400 millimetres per day. This calculated transport explains why cytoskeletal tracks are essential for long cells.

Common Misconceptions

• Misconception: Cytoskeleton is a rigid skeleton like bones. Correction: it is highly dynamic and remodels within seconds.
• Misconception: Only microtubules matter for movement. Correction: actin microfilaments power crawling, muscle contraction and cell division ring formation.
• Misconception: Plant cells lack cytoskeleton because they have walls. Correction: plant cells use microtubules and actin for division, growth and cytoplasmic streaming.

Real-World Applications

The cytoskeleton enables white blood cells to chase bacteria, muscles to contract via actin-myosin interaction, and plant cells to orient cellulose deposition. Drugs that stabilise or disrupt microtubules, such as paclitaxel and colchicine, are used in cancer therapy and gout treatment by interfering with cell division.

To sum up, microtubules, microfilaments and intermediate filaments together form a flexible internal framework. It supports shape, enables movement and organises transport, making it indispensable for cell life.