4.4 Cytoskeleton

Objectives

  • Compare and contrast cilia and flagella.

  • Compare the roles of microfilaments, intermediate filaments, and microtubules.

  • Describe the cytoskeleton.

  • Summarize the differences among the components of prokaryotic cells, animal cells, and plant cells.

The Cytoskeleton

  • The cytoskeleton is located within the cytoplasm.

  • It consists of three main types of filaments:

    • Microfilaments

    • Intermediate filaments

    • Microtubules

  • Functions of the cytoskeleton:

    • Maintains cell shape

    • Anchors organelles

    • Allows the movement of cytoplasm and vesicles

    • Enables cell motility

Microfilaments

  • Microfilaments are the narrowest fibers of the cytoskeleton.

  • Characteristics:

    • Diameter: Approximately 7 nm

    • Composition: Comprised of two intertwined strands of the globular protein actin, also known as actin filaments.

  • Functions of microfilaments:

    • Facilitate cellular movement and motility

    • Provide rigidity and shape for the cell

    • Allow movement necessary for:

    • Cell division

    • Cytoplasmic streaming

    • Muscle contraction

    • Enable cells to change shape rapidly through assembly or disassembly.

    • Example: White blood cells utilize microfilaments to find and phagocytize pathogens.

Intermediate Filaments

  • Intermediate filaments are composed of wound fibrous protein strands.

  • Characteristics:

    • Diameter: Ranges from 8 to 10 nm

    • Size: Between that of microfilaments and microtubules.

  • Composition:

    • Comprised of various types of fibrous proteins, notably keratin (found in hair, nails, and skin).

  • Functions of intermediate filaments:

    • Serve a purely structural role, bearing tension to maintain the cell's shape.

    • Anchor organelles, such as the nucleus, in place within the cell.

Microtubules

  • Microtubules are small, hollow tubes.

  • Functions of microtubules:

    • Help the cell resist compression.

    • Provide a track along which vesicles move through the cell.

    • Pull replicated chromosomes to opposite ends of a dividing cell.

    • Form the structural elements of:

    • Centrioles

    • Flagella

    • Cilia

  • Composition:

    • Composed of 13 polymerized dimers of α-tubulin and β-tubulin.

Flagella and Cilia

  • Flagella

    • Long, whip-like structures that extend from the plasma membrane, allowing cell movement.

    • Examples include: Sperm, Euglena, and some prokaryotes.

  • Cilia

    • Short, hairlike structures that also extend from the plasma membrane and move entire cells or substances along the outside surface.

    • Found in: Fallopian tubes and the respiratory tract.

  • Common Structural Feature:

    • Both flagella and cilia share a "9+2" microtubule arrangement.

    • This consists of 9 microtubule doublets surrounding 2 central microtubules.

Differences Among Cell Types

Table 1: Components of Prokaryotic and Eukaryotic Cells

  • Overview:

    • Compares the presence and function of various cell structures across prokaryotic cells, animal cells, and plant cells.

    • Cell structures evaluated include:

    • Plasma membrane

      • Function: Separates cell from external environment, controls the passage of organic molecules, ions, water, oxygen, and wastes into and out of the cell.

      • Present in Prokaryotes: Yes, Animal Cells: Yes, Plant Cells: Yes.

    • Cytoplasm

      • Function: Provides turgor pressure to plant cells as fluid inside the central vacuole; site of many metabolic reactions; medium in which organelles are found.

      • Present in all cell types.

    • Nucleus

      • Function: Houses DNA and directs synthesis of ribosomes and proteins.

      • Present in Prokaryotes: No, Animal Cells: Yes, Plant Cells: Yes.

    • Ribosomes: Protein synthesis

      • Present in all cell types.

    • Mitochondria: ATP production and cellular respiration.

      • Present in Animal Cells and Plant Cells but not in prokaryotes.

    • Other structures (e.g., peroxisomes, lysosomes, etc.) are similarly evaluated.

Summary of Cytoskeletal Elements

  • The cytoskeleton consists of three protein elements:

    • Microfilaments provide rigidity, shape, and enable movement through interaction with myosin.

    • Intermediate filaments bear tension and anchor organelles.

    • Microtubules provide resistance to compression, transport vesicles, and separate chromosomes during cell division.

  • Microtubules form structural components for centrioles, flagella, and cilia.