Cell Biology: Cytoskeleton Dynamics, Extracellular Matrix, and Cellular Junctions

Course Administration and Topic 3 Learning Objectives

• Assessment Details:

  • Quiz 3 is scheduled for October 2nd.
  • Availability window: Access opens at 12:01 AM on Friday, October 2nd, and closes at 11:59 PM on Sunday, October 4th.
  • Time constraint: Once initiated, there is a strict $20\,\text{minute}$ time limit to complete the evaluation.
  • Session restriction: Exiting and resuming the quiz at a later point is not permitted.
  • Curriculum coverage: Topic 3 (Cell Structure), aligning with Chapter 6 (Concepts 6.2–6.7).

• Institutional Schedule and Office Hours:

  • September 30th: University closed in observation of the National Day for Truth and Reconciliation.
  • Student help sessions:
    • Monday: 11:00 AM – 1:00 PM.
    • Thursday: 2:30 PM – 4:00 PM.

• Core Competencies and Learning Outcomes:

  • Analyze constraints on cellular dimensions governed by the surface area-to-volume ratio.
  • Contrast the structural organizations of prokaryotic (bacterial) and eukaryotic cells.
  • Elaborate on the endomembrane theory and its relevance to eukaryotic cell evolution.
  • Characterize the structural anatomy and physiological roles of endomembrane organelles.
  • Evaluate the endosymbiont theory regarding the evolutionary origins of mitochondria and chloroplasts.
  • Compare and contrast the architecture, dimensions, and primary roles of the three cytoskeleton filaments.
  • Differentiate the chemical compositions and architectures of cell walls in bacteria, fungi, and plants.
  • Delineate the morphological configurations and functional properties of the four principal cellular junctions.
  • Diagram the molecular linkages bridging the extracellular matrix ($\text{ECM}$) with the intracellular cytoskeleton.
  • Accurately diagram and annotate structural components of a typical bacterium, plant cell, and animal cell.

Overview of the Cytoskeleton

• Definition and Distribution:
  • The cytoskeleton constitutes a complex network of protein fibers extending throughout the cytoplasm.
  • Cytoskeletal frameworks occur across both bacteria and eukaryotes.
  • In eukaryotic cells, the cytoskeleton consists of three distinct fibrous components:
    • Microtubules (represented in green fluorescence microscopy).
    • Microfilaments (represented in red fluorescence microscopy).
    • Intermediate filaments (intermediate in diameter; not highlighted in standard two-color tubulin/actin stainings).

• Dynamic Remodeling and Mechanical Integrity:

  • The cytoskeleton is dynamic, continuously polymerizing, depolymerizing, and rearranging intracellular contents.
  • Provides essential structural scaffolding to sustain cellular morphology, which is particularly vital for animal cells that lack rigid cell walls.
  • Confers mechanical resistance against both tensile (pulling) and compressive (pushing/crushing) mechanical forces.

• Motility and Transport Functions:

  • Coordinates intracellular movements, relocating organelles and trafficking transport vesicles.
  • Microtubules function as intracellular "highways" along which specialized motor proteins travel.
  • Drives whole-cell locomotion and surface fluid propulsion via specialized external projections (cilia and flagella).
  • Facilitates amoeboid locomotion via pseudopodial cellular extensions through coordinated cytoskeletal reorganization.

Cytoskeletal Structural Elements Comparison

• Systematic Comparison of Cytoskeletal Filaments:

  • Microtubules (Tubulin Polymers):

    • Structure: Hollow, unbranched cylindrical tubes.
    • Outer Diameter: Approximately $25\,\text{nm}$.
    • Protein Subunits: Tubulin heterodimers consisting of one $\alpha\text{-tubulin}$ and one $\beta\text{-tubulin}$ polypeptide.
    • Primary Functions: Resistance to compressive forces (compression-bearing "girders"); motility via cilia and flagella; chromosome segregation during cell division (mitotic spindle apparatus); organelle and vesicle translocation tracks.

  • Microfilaments (Actin Polymers):

    • Structure: Solid rods composed of two intertwined helical strands of actin monomers.
    • Outer Diameter: Approximately $7\,\text{nm}$.
    • Protein Subunits: Actin (specifically globular actin monomers).
    • Primary Functions: Maintenance of cellular morphology by resisting tensile (pulling) forces; muscle contraction via interaction with myosin motors; cell crawling via amoeboid pseudopodia; plant cytoplasmic streaming; division of animal cells (cleavage furrow formation).

  • Intermediate Filaments:

    • Structure: Tough, fibrous proteins supercoiled into thick, rope-like structural cords.
    • Outer Diameter: Ranges from $8\,\text{nm}$ to $12\,\text{nm}$.
    • Protein Subunits: Diverse family of fibrous polypeptides depending on tissue type, notably keratins, vimentins, and nuclear lamins.
    • Primary Functions: Bearing severe tension across tissue layers; reinforcement of cellular shape; nuclear lamina formation to support the nuclear envelope; mechanical anchoring of the nucleus and other stable organelles.

Microtubules: Structure, Centrosomes, and Molecular Motors

• Molecular Architecture and Assembly Dynamics:
  • Microtubules are hollow protein cylinders measuring $25\,\text{nm}$ in diameter.
  • Composed of tubulin heterodimers, each containing one $\alpha\text{-tubulin}$ and one $\beta\text{-tubulin}$ subunit non-covalently bound together.
  • Grow longitudinally through the reversible accretion of tubulin dimers.
  • Undergo dynamic disassembly, releasing free tubulin dimers into the cytosolic pool for recycling.
  • Structurally polar with distinct biochemical termini:
    • Polymerization (addition) and depolymerization (removal) occur at different rates at the two ends.
    • The "plus end" ($\text{+\,end}$) exhibits significantly higher rates of assembly and disassembly compared to the "minus end" ($\text{-\,end}$).
  • Function mechanically as compression-resisting structural struts, comparable to steel girders in architectural frameworks.

• Microtubule-Organizing Centers (MTOCs) and Centrosomes:
  • In animal cells, microtubule nucleation and spatial organization are coordinated by the centrosome, positioned adjacent to the cell nucleus.
  • Each animal centrosome contains a pair of centrioles arranged perpendicularly to one another.
  • A centriole is configured into a ring composed of nine sets of triplet microtubules ($9 + 0$ arrangement).

• Motor Proteins and Cytoplasmic Vesicle Transport:
  • Act as transport routes for targeted cytoplasmic translocation of vesicles and membrane-bound organelles.
  • Translocation requires specialized motor proteins (such as kinesins and dyneins) that attach to cargo vesicles via specific receptor proteins.
  • Motor proteins utilize energy derived from adenosine triphosphate ($\text{ATP}$) hydrolysis to drive conformational shifts in their mechanical "feet," sequentially stepping along the microtubule lattice.

Microtubule-Based Motility: Cilia and Flagella

• Comparison of Cilia and Flagella:
  • Cilia:
    • Typically present in large numbers on exposed cell surfaces.
    • Relatively short in overall length.
    • Move with an oar-like rowing stroke consisting of an alternating power stroke and recovery stroke.
    • Propel fluids over stationary tissue surfaces (e.g., ciliated epithelial cells of the human respiratory tract propelling mucus and trapped particulates away from the lungs).
  • Flagella:
    • Usually limited to one or a small number per cell.
    • Considerably longer than cilia.
    • Generate an undulating, snake-like waveform that exerts force along the axis of movement.
    • Propel single cells through aqueous environments (e.g., flagellated swimming of eukaryotic sperm cells).

• Ultrastructural Organization ($9 + 2$ and $9 + 0$ Patterns):
  • Both cilia and flagella are encased by an extension of the cell's outer plasma membrane.
  • The Axoneme ($9 + 2$ Pattern):
    • The core shaft features a circular ring of nine doublets of microtubules surrounding two central single microtubules.
    • Flexible cross-linking proteins interconnect adjacent outer doublets.
    • Radial spokes extend inward from each outer doublet toward the central pair.
  • The Basal Body ($9 + 0$ Pattern):
    • Anchors the ciliary or flagellar axoneme into the cell cortex.
    • Structurally homologous to a centriole, consisting of nine triplets of microtubules arranged in a ring without central microtubules.
    • The two central singlet microtubules of the axoneme terminate above the transition zone and do not enter the basal body.

• Mechanics of Axonemal Bending:
  • Mechanical bending is driven by large motor protein complexes termed axonemal dyneins.
  • Dynein arms are attached to the A tubule of each doublet and reach toward the B tubule of the neighboring doublet.
  • Utilizing $\text{ATP}$ hydrolysis, dynein "feet" walk along the adjacent B tubule, pulling it longitudinally.
  • Because the doublets are physically anchored by flexible protein cross-links and radial spokes, the sliding motion is restricted and converted into localized lateral bending.
  • Dynein activation occurs alternately on opposing sides of the axoneme, causing coordinated oscillation.

Microfilaments (Actin Filaments)

• Structural Composition:
  • The thinnest elements of the eukaryotic cytoskeleton, possessing a uniform diameter of $7\,\text{nm}$.
  • Composed of globular actin (subunits) polymerized into solid, twisted double-helical chains.
  • Capable of forming branched, interconnected networks when cross-linked by accessory proteins.
  • Universally present in all eukaryotic cells.
  • Exhibit dynamic assembly and disassembly, allowing rapid structural reorganizations.

• Mechanical Support and the Cell Cortex:
  • Function primarily to bear tensile (pulling) stresses within the cytoplasm.
  • A three-dimensional network of microfilaments beneath the plasma membrane forms the cell cortex.
  • Gives the outer cytoplasmic zone a semi-solid gel consistency that helps support cell shape, contrasting with the fluid inner sol state.
  • Bundles of microfilaments form the structural core of absorptive cellular projections, such as intestinal microvilli, anchoring them into underlying intermediate filaments.

• Motility Functions Mediated by Actin:
  • Muscle Contraction:
    • Parallel arrays of actin filaments interdigitate with thicker filaments of the motor protein myosin.
    • Myosin head projections walk along actin filaments toward their plus ends, pulling the actin filaments toward the center of the sarcomere.
    • This sliding-filament mechanism shortens muscle cells during contraction.
  • Amoeboid Movement:
    • Locomotion in organisms such as Amoeba and mammalian white blood cells relies on pseudopodia formation.
    • Driven by localized actin polymerization near the leading edge, coupled with gel-to-sol cytoplasmic transitions and myosin-mediated cortical contraction at the trailing end.
  • Cytoplasmic Streaming:
    • A circular flow of cytosol around the large central vacuole in plant cells.
    • Facilitated by actin-myosin interactions, accelerating the distribution of organelles, nutrients, and metabolites through large plant cells.

Intermediate Filaments

• Properties and Composition:
  • Possess an intermediate diameter ranging between $8\,\text{nm}$ and $12\,\text{nm}$.
  • Exclusive to animal cells; absent in plant cells and prokaryotes.
  • Display high tensile strength, making them specialized tension-bearing elements.
  • Heterogeneous in molecular composition, assembled from diverse fibrous protein subunits depending on cell lineage:
    • Keratins: Present in epithelial cells, epidermal structures, and hair.
    • Vimentins and Desmins: Present in mesenchymal and muscle tissues.
    • Neurofilaments: Reinforce nerve cell axons.
    • Lamins: Form the internal nuclear lamina lining the nuclear envelope.
  • More permanent fixtures than microtubules or microfilaments; do not regularly undergo rapid depolymerization and repolymerization.

• Physiological Roles:
  • Maintain cellular morphology and withstand heavy mechanical shear forces (remain intact even after microtubules and microfilaments are chemically extracted).
  • Preserve organelle structural integrity under mechanical stress.
  • Form the nuclear lamina, a lattice network reinforcing the inner surface of the nuclear envelope.
  • Anchor the nucleus and other stable cytoplasmic organelles in fixed spatial positions.
  • Provide external structural durability in epidermal appendages (e.g., hair, claws, nails, and outer stratum corneum layers composed of keratin).

Plant Cell Walls and Extracellular Structures

• Biological Roles of Plant Cell Walls:

  • Extracellular protective structures secreted outside the plasma membrane of plant cells.
  • Maintain cell morphology, provide skeletal support to the whole plant, and prevent excessive water uptake under hypotonic conditions.
  • Considerably thicker than the lipid bilayer plasma membrane.

• Comparative Composition Across Biological Kingdoms:

  • Plants: Composed predominantly of cellulose microfibrils embedded in a matrix of other polysaccharides and proteins.
  • Fungi: Composed of chitin polymers.
  • Bacteria: Composed of cross-linked peptidoglycan.

• Architectural Structure of Plant Cell Wall Layers:

  • Primary Cell Wall:
    • Relatively thin and flexible wall layer secreted initially by young, growing plant cells.
  • Middle Lamella:
    • Thin extracellular layer positioned between the primary walls of adjacent cells.
    • Enriched in sticky polysaccharides called pectins, which glue neighboring cell walls together.
  • Secondary Cell Wall:
    • Deposited between the plasma membrane and the primary cell wall in mature, non-elongating cells (e.g., xylem/wood cells).
    • Deposited in multiple laminated layers containing lignin, providing compressive strength and structural support.
  • Plasmodesmata:
    • Cytoplasmic channels perforating the cell wall to allow continuous symplastic transport between adjacent plant cells.

The Extracellular Matrix (ECM) of Animal Cells

• Organization and Synthesis:

  • Animal cells lack rigid cell walls and instead synthesize an intricate extracellular matrix ($\text{ECM}$).
  • Connects individual cells into functional tissues and facilitates cell-cell communication.
  • Composed of glycoproteins and carbohydrate-containing macromolecules synthesized intracellularly and secreted via the endomembrane system.

• Core Molecular Components:

  • Collagen:
    • Glycoprotein that forms strong extracellular fibers resisting mechanical pulling forces.
    • Accounts for approximately $40\%$ of total protein mass in the human body.
  • Proteoglycan Complexes:
    • Consist of a central core protein non-covalently linked to numerous carbohydrate-rich glycosaminoglycan chains.
    • Hundreds of proteoglycans attach non-covalently along a single long polysaccharide backbone.
    • Polycellular carbohydrate groups trap water molecules, creating a hydrated gel that resists compressive forces.
  • Fibronectin:
    • Extracellular glycoprotein that cross-links collagen fibers and proteoglycans to integrin receptor proteins on the cell surface.
  • Integrins:
    • Transmembrane heterodimeric receptor proteins embedded within the plasma membrane.
    • Extracellularly bind to fibronectins; intracellularly attach to microfilaments of the actin cytoskeleton via adaptors.
    • Transmit mechanical and chemical signals bidirectionally across the membrane, coordinating changes in internal cellular activity in response to external environmental cues.

Intercellular Junctions in Animal and Plant Tissues

• Cell Adhesion and Sealing Junctions:

  • Tight Junctions:
    • Plasma membranes of neighboring cells are pressed together and stitched by specialized trans-membrane sealing proteins.
    • Establish an impermeable continuous barrier preventing interstitial fluid, ions, and solutes from leaking across an epithelial sheet.
    • Maintain distinct apical and basolateral fluid compartments (e.g., preventing water loss across the epidermis and fluid migration across intestinal linings).
  • Desmosomes (Anchoring Junctions):
    • Rivet-like multiprotein complexes that mechanically fasten neighboring cells together into strong sheets.
    • Anchored into the cytoplasm by intermediate filaments (composed of keratin proteins).
    • Distribute mechanical tensile strain across tissues (e.g., torn muscles involve the mechanical rupture of desmosomes between cells).

• Communicating Junctions:

  • Gap Junctions (Animal Communicating Junctions):
    • Composed of transmembrane protein rings (connexons) creating aqueous pores that directly connect the cytoplasms of neighboring cells.
    • Allow the direct passage of ions, sugars, amino acids, and second messengers without exposing them to the extracellular space.
    • Enable rapid electrical and chemical synchronization across cellular populations (e.g., coordinating uniform contraction throughout cardiac muscle tissue).
  • Plasmodesmata (Plant Communicating Junctions):
    • Microscopic membrane-lined channels traversing plant cell walls to connect the cytoplasm and endoplasmic reticulum of adjacent cells.
    • Unify the plant body into a functional continuous cytoplasm (the symplast).
    • Permit free diffusion of water and small solutes.
    • Dynamically adjust their channel diameter to permit regulated transit of larger macromolecules (proteins and RNAs) along internal cytoskeletal microfilaments.