The Cytoskeleton

For many years, the cytoplasm was assumed to be a featureless fluid in which organelles floated freely. We now know that eukaryotic cells contain a highly organised and dynamic protein scaffold called the cytoskeleton. OCR 2.1.1 (f) requires you to understand its three major components, their ultrastructure, and their roles in maintaining cell shape, providing mechanical support, enabling intracellular transport, and generating cell movement.

Key Definition: The cytoskeleton is a dynamic, three-dimensional network of protein filaments extending throughout the cytoplasm of a eukaryotic cell. It determines cell shape, supports organelles, drives intracellular movement, and enables cell motility.

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Why Cells Need a Cytoskeleton

A cell without a cytoskeleton would be:

• Shapeless, unable to maintain form against internal and external pressures.
• Incapable of organising organelles at specific locations.
• Unable to move vesicles, chromosomes, or mRNA from one place to another.
• Unable to change shape for processes such as phagocytosis, cytokinesis, or cell migration.
• Incapable of developing specialised projections such as microvilli, cilia, and flagella.

The cytoskeleton solves all of these problems through three distinct but interacting systems of protein filaments: microfilaments, intermediate filaments, and microtubules.

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Microfilaments

Microfilaments are the thinnest of the three cytoskeletal elements, typically 7 nm in diameter. They are made of the protein actin.

Structure

• Composed of globular G-actin monomers that polymerise to form a long, double-helical chain known as F-actin (filamentous actin).
• Each filament is polar, with a "plus" (+) end that grows faster than the "minus" (−) end. This polarity is crucial for directional movement.
• Microfilaments can assemble and disassemble quickly as actin monomers are added or removed, making them a highly dynamic structure.

Roles of Microfilaments

1. Cell shape and mechanical support — a dense network of actin filaments just inside the plasma membrane forms the cell cortex, giving the cell surface shape and stiffness. Changes in the cortex drive amoeboid movement and the formation of microvilli.
2. Cell movement — polymerising actin pushes the leading edge of a migrating cell forward, creating lamellipodia and filopodia. This is essential in phagocytes moving to infection sites and in developing tissues.
3. Cytokinesis — after mitosis, a contractile ring of actin and myosin forms around the equator of the cell and tightens like a purse-string, pinching the cell into two. This physical division of the cytoplasm is driven entirely by the cytoskeleton.
4. Muscle contraction — in muscle cells, actin interacts with myosin motor proteins to slide filaments past one another and shorten the sarcomere. This is the basis of the sliding filament model.
5. Cytoplasmic streaming in plant cells — actin–myosin interactions drive bulk movement of cytoplasm around the cell, distributing organelles and nutrients.
6. Microvilli — the actin-based core of each microvillus gives it rigidity, maintaining the brush border of intestinal epithelial cells.

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Intermediate Filaments

Intermediate filaments are so-named because their diameter (10 nm) is intermediate between that of microfilaments (7 nm) and microtubules (25 nm). Unlike the other two, intermediate filaments are not polar and are considerably more stable and less dynamic.

Structure

• Built from a diverse family of proteins; the specific protein depends on the tissue.
• Each monomer is a long, rod-like protein that dimerises and then assembles into tetramers. Tetramers stack end-to-end and laterally to form a cable-like filament.
• The cable-like arrangement gives intermediate filaments exceptional tensile strength — much like the strands of a rope.

Examples of Intermediate Filament Proteins

Protein	Tissue/location	Function
Keratins	Epithelial cells, hair, nails, skin	Mechanical strength of epithelial sheets and skin appendages
Vimentin	Connective tissue, leukocytes	Maintains cell integrity in tissues under mechanical stress
Desmin	Muscle cells	Connects adjacent sarcomeres and holds myofibrils in register
Neurofilaments	Neurons	Structural support of axons, particularly large-diameter axons
Lamins	Nuclear lamina (inside the nuclear envelope)	Maintains shape of the nucleus and anchors chromatin

Roles of Intermediate Filaments

1. Mechanical strength — the primary role. Intermediate filaments resist tensile forces (stretching), preventing cells from being torn apart under mechanical stress.
2. Anchoring organelles — in epithelial cells, keratin filaments anchor the nucleus and other organelles in position. Lamins anchor chromatin within the nucleus.
3. Structural continuity across tissues — intermediate filaments in adjacent cells connect through desmosomes, forming a continuous mechanical network that distributes stress throughout a tissue (essential in skin and heart muscle).
4. Maintaining nuclear shape — the nuclear lamina is a mesh of intermediate filament proteins (lamins) lining the inner nuclear membrane.

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Microtubules

Microtubules are the thickest of the three cytoskeletal components, with an outer diameter of about 25 nm. They are hollow tubes built from the protein tubulin.

Structure

• Formed from dimers of α-tubulin and β-tubulin. Dimers polymerise end-to-end to form protofilaments.
• Thirteen protofilaments arrange in parallel to form a hollow cylindrical tube 25 nm across.
• Microtubules are polar, with a slow-growing (−) end and a fast-growing (+) end. The (−) ends are typically anchored in microtubule organising centres (MTOCs), especially the centrosome in animal cells.
• Microtubules are highly dynamic, exhibiting "dynamic instability" — rapid growth alternating with sudden catastrophic shrinking. This allows the cytoskeleton to rebuild itself quickly.
• Some microtubules, such as those of cilia and flagella, are stabilised in the famous "9 + 2" arrangement: nine pairs of microtubules arranged in a ring around a central pair.