Biology Week 2 Lecture 6: Cytoskeleton, Intracellular Transport, and Cell-Cell Interactions

Overview of the Cytoskeleton

  • Primary Functions:

    • Structural Framing: Provides shape and structural integrity to the cell. Analogous to two-fourths (24\frac{2}{4}) of a house's structural framing; while the plasma membrane defines the outer boundary, the cytoskeleton dictates the overall physical architecture.
    • Intracellular Transport: Functions as an internal railroad system, providing molecular tracks for moving materials, vesicles, and organelles.
    • Cellular Motility: Mediates whole-cell movement, including muscle contraction and amoeboid migration toward chemoattractants.
    • Organelle Positioning and Cell Division: Controls the spatial orientation of organelles within the cytoplasm and manages the structural segregation of cellular components during cell division.

  • Three Main Cytoskeletal Components:

    • Microtubules
    • Actin Filaments (also termed microfilaments)
    • Intermediate Filaments

Microtubules

  • Structural Characteristics:

    • Analogous to bicycle frame tubing—strong, rigid, hollow cylinders.
    • Possess inherent structural polarity, characterized by a positive end (+end+\text{end}) and a negative end (−end-\text{end}), similar to the positive and negative terminals of a battery.
    • Polarity provides directional cues essential for intracellular transport mediated by motor proteins.
    • Present in every eukaryotic cell type.

  • Molecular Composition and Assembly:

    • Composed of two distinct protein monomers: α-tubulin\alpha\text{-tubulin} and β-tubulin\beta\text{-tubulin}.
    • α-tubulin\alpha\text{-tubulin} and β-tubulin\beta\text{-tubulin} bind together to form a heterodimer (a protein dimer composed of two non-identical subunits).
    • Heterodimers stack end-to-end into protofilaments, which arrange side-by-side to form a hollow tube.

  • Dynamic Instability and Bonding:

    • Subunits within the hollow tube are joined by non-covalent bonds.
    • Because non-covalent bonds are relatively weak, tubulin units can be rapidly added or removed from either end.
    • Allows microtubules to dynamically change length through growth (expansion) and shrinkage.

Actin Filaments (Microfilaments)

  • General Properties:

    • Also referred to as microfilaments.
    • Universal component of all eukaryotic cells.
    • Exhibit distinct structural polarity (+end+\text{end} and −end-\text{end}), crucial for orienting motor protein movement.
    • Flexible, bendable, non-hollow filamentous structures.
    • Drive cell shape determination, cell motility, and mechanical contraction.

  • Structure and Rope Analogy:

    • Composed of a single monomeric protein type: actin.
    • Analogous to a two-strand rope made of two intertwined actin strands.
    • A two-strand arrangement yields high flexibility; adding more strands increases structural rigidity.
    • Monomers are linked by non-covalent bonds, permitting dynamic extension and shortening via monomer addition or subtraction.

  • Assembly Dynamics (The Ikea Model):

    • When reconfiguring within the cell, actin filaments rarely depolymerize entirely into individual, isolated actin monomers.
    • Follows an "Ikea model": similar to moving furniture by disassembling it into its largest movable sub-assemblies rather than individual screws and hardware, actin filaments break down into smaller filament fragments, transport to new cell locations, and reassemble into new structural arrays.

  • Cell Motility via Membrane Extension:

    • Pushing Event: At the leading edge of a migrating cell (e.g., an amoeba forming pseudopodial extensions), actin polymerization attached to the plasma membrane pushes the membrane outward.
    • Pulling Event: At the trailing edge, actin filament contraction shrinks the network, pulling the rear plasma membrane forward and advancing the cell.

Intermediate Filaments

  • Distinctive Properties:

    • Unlike microtubules and actin filaments, intermediate filaments are not present in all cell types; they are restricted primarily to specialized cells in multicellular organisms.
    • Absent in organisms with exoskeletons (e.g., insects/bugs), which rely on external structures for mechanical support.
    • Lack structural polarity (there is no positive or negative end).
    • Composed of non-uniform, highly variable protein subunits.
    • Assembly involves random, non-directional subunit association, making their exact assembly mechanisms far less understood than those of microtubules or actin.

  • Function and Mechanical Strength:

    • Function exclusively to provide high mechanical strength and structural resistance to physical stress.
    • Analogous to twisting hundreds of individual ropes together into an extremely dense, rigid cable.
    • Do not undergo rapid growth/shrinkage dynamic cycles for cellular transport.

  • Keratin:

    • The most prevalent and thoroughly studied class of intermediate filaments.
    • Assembles into dense protein networks forming tough, non-motile structures such as hair, fingernails, claws, and scales.
    • While individual keratin monomers are weak on their own, their aggregated, multi-stranded coiled assembly generates extreme physical toughness.

Intracellular Transport and Motor Proteins

  • Motor Protein Mechanics:

    • Act as molecular "trains" running along polarized cytoskeletal "tracks" (microtubules and actin filaments).
    • Require filament polarity (+end+\text{end} and −end-\text{end}) to determine directionality.
    • Utilize cellular energy (ATP hydrolysis) to step along filament tracks.
    • Consist of a walking domain that binds the filament track and a tail domain that attaches to cellular cargo.

  • Transport Cargo Types:

    • Individual functional proteins.
    • Membrane-bound vesicles (e.g., moving secretory vesicles from the Endoplasmic Reticulum to the Golgi apparatus along microtubule tracks).
    • RNA molecules.
    • Other cytoskeletal filaments (driving sliding motions).

  • Ciliary and Flagellar Movement via Microtubules:

    • Respiratory Cilia: Ciliated epithelial cells lining the human respiratory tract continuously beat to move mucus and trapped foreign pathogens upward and out of the airways.
    • Flagella: Provide whip-like propulsion for cell locomotion (e.g., sperm cells).
    • Bending Mechanism: A microtubule-associated motor protein binds one microtubule while its cargo tail is anchored to a neighboring parallel microtubule. As the motor protein walks along the adjacent microtubule, it forces the two parallel tubes to slide past one another. Because the tubes are fixed at their base, this sliding force causes the hollow tubes to bend. Repeated, coordinated cycles produce a whip-like beating motion.
    • Note: Bacterial flagella operate via an entirely different rotary motor mechanism rather than this eukaryotic sliding/bending microtubule mechanism.

  • Muscle Contraction via Actin Filaments:

    • Driven by the motor protein myosin walking along actin filaments.
    • Myosin tails link adjacent actin filaments.
    • As myosin heads walk along the polarized actin tracks, they pull opposing actin filaments toward each other, shortening the sarcomere and driving muscle contraction.

Cytoskeleton Concept Verification and Exam Strategy

  • Concept Analysis Question:

    • Question: Which of the following statements regarding microtubules is true?
      • A. Microtubules are important in the movement of cilia. (Correct)
      • B. Microtubules are involved in muscle contraction. (Incorrect: Muscle contraction depends on actin filaments and myosin)
      • C. Microtubules are made up of one type of tubulin monomer. (Incorrect: Microtubules are composed of heterodimers of α-tubulin\alpha\text{-tubulin} and β-tubulin\beta\text{-tubulin})
      • D. Microtubules are homodimers. (Incorrect: Microtubules are composed of heterodimers)

  • Key Term Definitions:

    • Heterodimer: A macromolecular complex formed by two structurally distinct protein subunits (e.g., α-tubulin\alpha\text{-tubulin} bound to β-tubulin\beta\text{-tubulin}).
    • Homodimer: A macromolecular complex formed by two identical protein subunits.
    • Nexin: An inter-microtubule protein link that maintains structural arrangement during ciliary beating.

  • Test-Taking Strategy:

    • When evaluating multiple-choice options, locate key functional terms and pair them directly with their corresponding cytoskeletal component (e.g., connect cilia to hollow microtubule structures, and muscle contraction directly to flexible actin filaments).

Tissue Organization and Extracellular Interactions

  • Higher Organizational Scheme:

    • Multicellular organisms organize cells into higher functional tiers: Cells →\rightarrow Tissues →\rightarrow Organs →\rightarrow Organ Systems →\rightarrow Organism.
    • While individual free-floating cells exist (e.g., blood cells), the vast majority of cells operate as integrated, anchored collectives.

  • Gastrointestinal (GI) Tract Luminal Structure:

    • The intestinal lumen features finger-like mucosal extensions called villi.
    • Surface Area to Volume Ratio: Villi drastically increase the exposed plasma membrane surface area relative to luminal volume, maximizing nutrient absorption capacity.
    • A flat intestinal lining drastically reduces surface area, restricting nutrient uptake.
    • Pathology: Certain gastrointestinal infections cause villi blunting or collapse, impairing nutrient absorption and altering fluid retention.

  • Extracellular Matrix (ECM) and Secretions:

    • Cells secrete functional molecules into their surrounding external environment; some diffuse away, while others remain tightly bound to the cell exterior.
    • Plant cells and prokaryotes possess rigid cell walls to maintain structural form.
    • Animal cells lack cell walls and rely on secreted ECM components to anchor to neighboring cells, lipid structures, or basement membranes.

Cell Junctions and Cellular Communication

  • Desmosomes and Tight Junctions:

    • Desmosomes: Complex multi-protein structures that physically link adjacent cell membranes together tightly.
    • Tight Junctions: Formed via desmosomal connections, tight junctions seal the intercellular spaces between adjacent epithelial cells in tissues like the intestinal lumen.
    • Function: Prevents paracellular fluid movement between cells, forcing nutrients and water to pass transcellularly through the cell under active regulation.
    • Clinical Application: Disrupting tight junctions permits massive, unregulated water loss between intestinal cells into the gut lumen, causing severe diarrhea. Antidiarrheal agents like Pepto-Bismol help counteract fluid loss associated with junctional disruption.

  • Gap Junctions (Animal Cells):

    • Protein channels/tunnels bridging the plasma membranes of neighboring animal cells.
    • Allow direct transport of signaling molecules, ions, and small solutes between cell cytoplasm, enabling rapid intercellular communication.

  • Plasmodesmata (Plant Cells):

    • Membrane-lined channels penetrating the rigid cell walls of adjacent plant cells.
    • Provide cytoplasmic continuity and material transport between plant cells, acting as the functional equivalent of animal gap junctions within a walled tissue matrix.

Exam Preparation and Administrative Notes

  • Upcoming Examination: Scheduled for Wednesday.
  • Required Testing Materials: Bring #2 pencils to the examination session.