The Cytoskeleton: Support System for Eukaryotic Cells
The cytoskeleton is an intricate network of protein "tracks" and tubules found within the cytosol of eukaryotic cells.
It serves as a fundamental structural framework with diverse functions that are crucial for cell viability and activity.
Key Functions:
Transportation System: Facilitates the movement of substances within the cell.
Physical Support: Maintains the cell's characteristic three-dimensional shape.
Cell Division Aid: Plays a vital role in processes related to cell division.
Cell Connection: Helps link cells to one another, forming tissues.
Cellular and Subcellular Movement: Enables entire cells or specific parts of a cell to move.
Major Components of the Cytoskeleton
The cytoskeleton comprises three primary components: microfilaments, intermediate filaments, and microtubules.
These components are differentiated by their protein composition, diameter, and how they assemble into larger structures.
An intricate meshwork is formed by other proteins that connect these main components.
Microfilaments
Description: The thinnest component of the cytoskeleton.
Structure: Long rod-like structures.
Composition: Primarily composed of the protein actin.
Diameter: Approximately 7 nanometers.
Prevalence: Actin microfilament networks are present in nearly all eukaryotic cells.
Functions and Examples:
Muscle Contraction: Essential for muscle contraction, working in conjunction with another protein called myosin (further detailed in section 26.4A).
Mechanical Strength: Provides strength to cells, allowing them to withstand stretching and compression forces.
Cell Anchoring: Aids in anchoring one cell to another (as discussed in section 3.6).
Intermediate Filaments
Description: Named for their diameter, which is intermediate between microfilaments and microtubules.
Diameter: Around 10-nanometer.
Composition: The specific proteins forming intermediate filaments vary depending on the cell type.
Functions:
Shape Maintenance: Forms an internal scaffold within the cytosol to maintain the cell's shape.
Stress Resistance: Crucial for resisting mechanical stress placed upon the cell.
Cell Binding: Assists in binding some cells together (also described in section 3.6).
Microtubules
Structure: Hollow tubes.
Composition: Assembled from a protein called tubulin.
Diameter: Measures 23 nanometers.
Dynamic Nature: The cell possesses the ability to rapidly alter the length of microtubules by either adding or removing tubulin molecules.
Multiple Functions in Eukaryotic Cells:
Intracellular Transport: Acts as "trackways" along which substances are transported within the cell. Specialized motor proteins effectively "walk" along these tracks, carrying organelles, vesicles, or other cargo.
Chromosome Segregation: Plays a critical role in splitting a cell's duplicated chromosomes apart during cell division (further elaborated in chapter 8).
Organization:
Animal Cells: Microtubules are organized by structures known as centrosomes.
Plant Cells: Typically lack centrosomes and instead assemble microtubules at various scattered sites throughout the cell.
Centrosomes and Centrioles:
A centrosome contains two centrioles, which are visible in figure 3.6.
Centrioles are indirectly responsible for producing the cellular extensions that enable movement: cilia and flagella (illustrated in figure 3.23).
Cilia and Flagella: Cellular Extensions for Movement
These extensions are indirectly produced by centrioles and are vital for the locomotion of certain cells.
Cilia
Appearance: Short, numerous extensions that resemble a fringe.
Functions and Examples:
Swimming in Protists: Some protists, such as the Paramecium (figure 15.32), use thousands of cilia for "swimming" in aquatic environments (see section 15.4D).
Mucus Clearance in Respiratory Tract: In the human respiratory tract, the coordinated movement of cilia generates a wave that propels mucus and trapped dust particles up and out of the airways.
Egg Cell Movement: Cilia are also responsible for moving an egg cell along the female reproductive tract.
Flagella
Occurrence: Typically, flagella appear singly or in pairs.
Size: A flagellum is substantially longer than a cilium.
Appearance: They are often described as tail-like.
Movement: Exhibit a distinctive whiplike motion to propel cells.
Functions and Implications:
Sperm Motility: Sperm cells in many species, including humans, possess prominent flagella that enable them to swim.
Infertility: Individuals whose sperm cells have defective flagella are infertile because the sperm cannot swim to reach the egg cell.
Assisted Reproductive Technologies: Certain assisted reproductive technologies can help restore fertility in affected males. For instance, in intracytoplasmic sperm injection (Table 30.4), a sperm cell is directly inserted into an egg cell, circumventing the need for sperm motility.