Cytoskeleton and ECM

Cytoplasm of Eukaryotic Cells

  • Definition: Portion of a eukaryotic cell interior not occupied by nucleus; includes organelles like mitochondria and cytosol.

  • Cytosol:

    • Semifluid substance where organelles are suspended, providing a medium for biochemical reactions and facilitating the transport of materials within the cell.

    • In animal cells, cytosol occupies >50% of internal volume, contributing to the cell's shape and influencing various metabolic processes.

  • Major cellular activities include protein synthesis, fat synthesis, and energy release from sugars, showcasing the cytoplasm's role as a hub for cellular metabolism and function.

  • The Cytoskeleton

    • Initially thought to be an amorphous fluid but is a structured network of microfilaments, microtubules, and intermediate filaments, providing essential structural support to the cell.

    • Function: Provides internal framework for cell shape and organization, involved in cell movement and division, playing a crucial role during processes like cytokinesis.

      • Major roles include contraction, enabling muscle movement, cell motility via flagella and cilia, and chromosome movement during cell division, ensuring genetic material is properly segregated.

    • Accounts for interactions with organelles and macromolecules, with approximately 80% of proteins in cytosol associated with cytoskeleton, indicating its importance in maintaining cellular organization and facilitating biochemical pathways.

Structural Elements of the Cytoskeleton

  • Microtubules

    • Largest structural components of the cytoskeleton, serving roles in motility and chromosomal movement, acting as tracks for motor proteins that transport cellular components.

    • Diameter: 25 nm (outer), 15 nm (inner), reflecting their role in providing rigidity and support to the cell.

    • Composed of tubulin dimers (α-tubulin and β-tubulin) arranged into hollow cylinders, enabling dynamic instability which allows rapid growth and shrinkage in response to cellular needs.

    • Functions:

      • Organizes cytoplasm, assists intracellular movement, shapes cells, positions organelles, and regulates macromolecule distribution, thereby influencing cell behavior and function.

      • Plays critical roles in ciliary and flagellar movement and mitotic spindle formation, essential for processes such as locomotion and cell division.

  • Microfilaments

    • Diameter: ~7 nm; smallest cytoskeletal components, essential for various cellular functions.

    • Primarily composed of actin, arranged in helical filaments (F-actin), allowing for high flexibility and dynamic restructuring.

    • Functions:

      • Muscle contraction, locomotion, cytoplasmic movement, and cleavage furrow formation during cytokinesis, demonstrating its role in cell shape and movement.

      • Polarity influences growth direction; differs in assembly and disassembly at opposite ends, enabling cells to adapt rapidly to changes in their environment.

  • Intermediate Filaments

    • Diameter: 8–12 nm; more stable and least soluble, contributing to the mechanical strength of cells.

    • Serve as scaffolding support and tension-bearing structures in cells, providing resilience against mechanical stresses.

    • Consists of diverse proteins based on tissue type; can function as diagnostic markers in tissue pathology.

    • Structure: Typically form dimers that align to create protofilaments, forming a larger composite structure that contributes to overall cellular architecture.

Extracellular Structures

  • Cells have extracellular matrices (ECM) or cell walls, which provide support and structure, playing a vital role in cellular communication and tissue formation.

    • Animal Cells: ECM composed mainly of collagen fibers and proteoglycans, which help to regulate cell processes like motility, division, recognition, adhesion, and differentiation, influencing overall tissue integrity.

    • Plant & Fungal Cells: Rigid cell walls made of cellulose fibers and polysaccharides, providing protection and support while facilitating growth through turgor pressure.

    • Bacterial & Archaeal Cells: Cell walls made of peptidoglycans, providing rigidity and shape to the cells, which is crucial for survival in different environments.

  • ECM varies across cell types and tissues, significantly influencing mechanical properties and cellular interactions, affecting not only individual cellular behavior but also tissue functionality.

  • Plant cells connect through plasmodesmata; animal cells utilize gap junctions for intercellular communication, facilitating coordination and signaling between cells.

  • Other junctions in animal cells include tight and adhesive junctions, each serving unique cellular functions essential for maintaining tissue integrity and regulating permeability.