Cytoskeleton, Extracellular Structures, and Cell Membrane Dynamics

Types and Properties of Cytoskeletal Elements

• Overview of Cytoskeletal Fiber Types:
  • Microfilaments: The thinnest cytoskeletal elements within the cell.
  • Microtubules: The thickest cytoskeletal elements within the cell.
  • Intermediate filaments: Cytoskeletal elements that possess a diameter intermediate between microfilaments and microtubules (thinner than microtubules, thicker than microfilaments).
• Monomer Subunit Composition:
  • Microtubules: Structural polymers composed of monomer units of the protein tubulin.
  • Microfilaments: Structural polymers composed of monomer units of the protein actin.
  • Intermediate filaments: Structural polymers composed of varying protein monomer subunits depending on cell type.

Microfilament Functions and Structural Cellular Adaptations

• Physical Architecture of Microfilaments:
  • Microfilaments exist as solid rods composed completely of solid protein throughout their structure, unlike microtubules which are hollow tubes.
  • Each individual microfilament is structured as a twisted double chain consisting of two intertwined actin polymers or string-like protein chains.
• The Cellular Cortex:
  • A specialized network or dense group of microfilaments situated immediately beneath the plasma membrane, just inside the cell boundary, forms the cortex.
  • Function of the cortex: It provides structural support to give the cell its specific three-dimensional shape.
  • Biological significance of cell shape: Cellular architecture directly dictates and regulates cell function; alterations to the cortex change cell shape, thereby altering cell function.
• Intestinal Epithelial Cells and Microvilli:
  • Anatomical context: Human anatomy features a single small intestine lined with specialized epithelial cells.
  • Role of epithelial cells: These cells extract and absorb vital nutrients from digested food passing through the lumen of the small intestine.
  • Structure of Microvilli:
  • Microvilli (singular: microvillus) are finger-like plasma membrane and cytoplasm extensions that project outward into the interior space of the small intestine.
  • Function: Microvilli significantly increase the surface area of the intestinal epithelial cells.
  • Biological importance: By expanding the functional surface area, intestinal cells absorb a much higher volume of nutrients compared to cells lacking microvilli.
  • Clinical implication: Proper microvilli function prevents malnourishment, a state where an individual fails to absorb necessary dietary nutrients required for health.
  • Cytoskeletal support: Microvilli are densely filled with microfilaments, providing the structural framework necessary to maintain microvillar extension and enable optimal nutrient extraction.

Microfilament Motility Mechanisms Across Organisms

• Mechanism of Cell Motility via Motor Proteins:
  • Cell motility is defined as the physical movement of a cell from one location to another.
  • Myosin Motor Proteins: Myosin is the specific motor protein that binds to and physically walks along microfilaments (actin tracks).
  • Dynamic action: As the myosin motor protein walks along actin microfilaments, it physically bends the microfilaments.
  • Contrast with Microtubule Motors: While dynein motor proteins walk along microtubule tracks (such as during a duck walk demonstration), myosin operates exclusively on actin microfilaments.
  • Result: Microfilament bending caused by myosin action physically propels or moves the cell.
• Amoeboid Movement and Pseudopodia:
  • Organisms: Single-celled organisms such as amoebas utilize crawling motility to move across surfaces.
  • Pseudopodia Structure: Extensions of the cytosol (cytoplasm) extending outward from the amoeba cell body.
  • Role of microfilaments: Pseudopodia are filled with microfilaments that continuously alter their structural orientation inside the cytoplasm.
  • Locomotion mechanism: Structural realignments of microfilaments drive movement of the pseudopodia, which consequently pulls and advances the entire single-celled amoeba forward.
• Cytoplasmic Streaming in Plant Cells:
  • Definition: Cytoplasmic streaming is the continuous, circular movement (circulation) of cytosol within plant cells.
  • Driving mechanism: Microfilament movement inside the plant cell actively drives the circular flow of the surrounding cytosol.
  • Function: Circulating cytoplasm distributes dissolved nutrients, solutes, and cellular substances directly to their designated destination sites throughout the plant cell.
• Skeletal Muscle Contraction in Humans:
  • Muscle classification: Human movement is powered by skeletal muscle contraction, distinct from cardiac muscle and smooth muscle types.
  • Microscopic structure: Skeletal muscles consist of bundles of individual muscle fibers (skeletal muscle cells).
  • Molecular mechanism:
  • Skeletal muscle cells contain microfilaments (intertwined actin chains) paired with myosin motor proteins.
  • Myosin motor proteins physically walk along actin microfilaments within the muscle cell.
  • Walking action causes the actin microfilaments to bend and slide.
  • Bending of microfilaments causes individual skeletal muscle cells to shorten and contract.
  • Synchronized contraction across a massive number of skeletal muscle cells results in full skeletal muscle contraction, generating body movement (such as leg muscle contraction enabling running).

Intermediate Filaments Structure and Organelle Localisation

• Physical Size and Permanence:
  • Intermediate filament diameter: Ranges from $8\text{ to }12\,\text{nm}$.
  • Structural role: Intermediate filaments constitute the more permanent structural framework and scaffolding of the cytoskeleton.
  • Dynamic stability: Unlike microfilaments (which rapidly reorganize during processes like pseudopodia formation) and microtubules, intermediate filaments are durable and less prone to frequent structural changes.
• Organelle Anchoring and Cell Shape:
  • Overall shape support: Maintains the permanent three-dimensional architectural integrity of the cell.
  • Organelle fixation: Locks specific organelles into fixed cellular positions (anchoring), ensuring organelles remain localized where their biochemical functions are most required.

Plant Cell Walls and Extracellular Structures

• Extracellular vs. Intracellular Materials:
  • Intracellular: Refers to molecules, structures, or substances located inside the cell boundary.
  • Extracellular: Refers to molecules, structures, or substances located outside the plasma membrane.
  • Secretion process: Cells synthesize substances internally and export (secrete) them to the cell exterior, where extracellular materials carry out specialized supportive and protective roles.
• Phylogenetic Distribution of Cell Walls:
  • Animal cells: Completely lack cell walls.
  • Plant cells: Possess rigid cell walls surrounding their plasma membranes.
  • Prokaryotic cells: Bacteria and other prokaryotes possess cell walls surrounding their plasma membranes.
  • Fungi: Possess cell walls composed of chitin rather than cellulose.
  • Protists: Certain protists possess cell walls surrounding their plasma membranes.
• Functions and Chemical Composition of Plant Cell Walls:
  • Functions:
  • Provides structural protection for plant tissues.
  • Maintains the three-dimensional shape of individual plant cells.
  • Prevents cell lysis (bursting) resulting from excessive osmotic water absorption.
  • Molecular composition:
  • Composed of the polysaccharide cellulose embedded within a matrix of non-cellulose polysaccharides and structural proteins.
  • Monomer unit of cellulose: Cellulose is a linear polymer composed of repeating units of the beta form of glucose ($\beta\text{-glucose}$).
• Structural Layers of the Plant Cell Wall:
  • Primary Cell Wall: A thin, flexible layer synthesized first during cell growth. It separates the middle lamella from the secondary cell wall (if present).
  • Middle Lamella: A thin layer positioned between the primary cell walls of adjacent neighboring plant cells within the same plant organ. Rich in sticky polysaccharides called pectins that adhere neighboring cells together.
  • Secondary Cell Wall: A thick layer located between the plasma membrane and the primary cell wall of the same cell. Present only in specific plant species (not all plant cells produce a secondary cell wall).
  • Layering sequence between two adjacent plant cells (analogous to a multi-layered Trace Leches or three milks cake):
  • Plasma Membrane of Cell 1
  • Secondary Cell Wall of Cell 1
  • Primary Cell Wall of Cell 1
  • Middle Lamella (central boundary)
  • Primary Cell Wall of Cell 2
  • Secondary Cell Wall of Cell 2
  • Plasma Membrane of Cell 2

Animal Cell Extracellular Matrix (ECM) and Integrin Signaling

• Structural Composition of the ECM:
  • Animal cells lack cell walls and are instead embedded in an Extracellular Matrix (ECM).
  • ECM consists primarily of glycoproteins: proteins covalently bonded to one or more carbohydrate (sugar) chains.
  • Key ECM Glycoproteins: Fibronectins, collagen, and proteoglycans.
• Integrin Receptors and Transmembrane Signaling:
  • Integrin Structure: Integrins are specialized transmembrane receptor proteins that extend completely across the plasma membrane, bridging the extracellular and intracellular environments.
  • Binding mechanism: Extracellular matrix glycoproteins (such as fibronectin) physically bind to the external domains of integrin proteins.
  • Signal Transduction:
  • Binding triggers mechanical or chemical signal transmission through the integrin protein across the plasma membrane into the cytoplasm.
  • Mechanical signaling occurs via connections between integrins and the internal cytoskeleton framework.
  • Signals travel to the nucleus, where cellular genetic material (DNA arranged in units of inheritance called genes) resides.
  • Signal impact on gene expression: ECM signals alter nuclear gene activity, increasing the transcription and expression of specific genes to synthesize corresponding proteins.

Higher-Level Biological Organization and Cell Junctions

• Hierarchy of Biological Structures:
  • Tissue: An organized group of similar cells performing a common specific function.
  • Organ: An organized group of different tissues working together to carry out a complex function (e.g., the heart pumping blood).
  • Organ System: A collection of organs operating in coordination to fulfill major physiological needs (e.g., the circulatory system transporting blood throughout the body).
• Cell Junctions in Plant Tissues — Plasmodesmata:
  • Plasmodesmata (singular: plasmodesma) are open microscopic channels that penetrate plant cell walls to connect adjacent plant cells.
  • Function: Enable direct intercellular transport of water molecules and dissolved solute molecules between neighboring plant cells.
  • Transported cargo: Water, RNA molecules dissolved in water, and proteins dissolved in water.
• Specialized Cell Junctions in Human/Animal Epithelial Tissues:
  • Tight Junctions:
  • Physical nature: Membranes of neighboring epithelial cells are tightly pressed and fused together by specialized proteins.
  • Function: Forms an impenetrable barrier or continuous sheet preventing extracellular fluid (such as blood) from leaking across epithelial tissue layers (e.g., in human skin).
  • Desmosomes:
  • Physical nature: Anchoring junctions that rivet neighboring epithelial cells together into strong sheets.
  • Function: Provide structural elasticity and mechanical strength to tissues subject to stretching (e.g., skin tissue).
  • Gap Junctions:
  • Physical nature: Communicating channels formed by membrane proteins connecting adjacent cells.
  • Function: Allow direct passage of ions and small molecules from the cytoplasm of one cell into the cytoplasm of a neighboring cell, facilitating rapid intercellular communication.

System Integration in Immune Response

• Functional Integration of Cellular Components:
  • Organelles and structures inside a cell operate as an integrated team (analogous to a coordinated soccer team) rather than in isolation.
• Example — Macrophage Phagocytosis:
  • Macrophage role: A large human white blood cell responsible for engulfing and destroying invading bacterial pathogens to protect the organism.
  • Coordinated Structural Mechanics during Phagocytosis:
  • Plasma Membrane: Surrounds, invaginates, and engulfs the target bacterial cell via phagocytosis.
  • Cytoskeleton: Directly guides, directs, and maneuvers the internalized bacterial cell within the cell interior.
  • Lysosomes: Act as the cell's garbage disposal units, fusing with the phagocytic vesicle to digest, break down, and recycle foreign bacterial components.

Cell Membrane Structure, Transmembrane Transport, and Fluid Mosaic Model

• Neural Plasticity and Memory:
  • Intercellular communication between neighboring brain cells (neurons) occurs across plasma membranes and forms the physiological basis for learning and memory creation.
• Mechanisms of Membrane Transport:
  • Passive Transport:
  • Movement of small molecules across the plasma membrane without cellular energy input ($\text{ATP}$).
  • May occur spontaneously or via facilitated transport proteins.
  • Active Transport:
  • Movement of substances across the plasma membrane requiring expenditure of cellular energy derived from the hydrolysis of $\text{ATP}$.
  • Absolutely requires specific transmembrane transport proteins to pump substances against concentration gradients.
  • Vesicular Transport:
  • Transport of large macromolecules or bulky substances into or out of the cell wrapped in lipid-lined membrane sacs called vesicles.
  • Endocytosis: Importation of substances into the cell via vesicles.
  • Exocytosis: Exportation of substances out of the cell via vesicles.
• Biochemical Structure of the Plasma Membrane:
  • Main constituents: Lipids (principally phospholipids), proteins, and carbohydrates (sugars).
  • Phospholipid Bilayer Structure:
  • Composed of two aligned layers of phospholipids.
  • Phospholipids are amphipathic: possess a hydrophilic (water-loving) head and a hydrophobic (water-fearing) fatty acid tail.
  • Bilayer arrangement: Hydrophobic tails face inward toward each other, forming a nonpolar interior core, while hydrophilic heads face outward toward the aqueous cytosol and extracellular fluid.
  • Transmembrane Protein Orientation:
  • Membrane proteins are typically amphipathic.
  • Hydrophilic protein regions extend into the aqueous environments on either side of the membrane in contact with hydrophilic phospholipid heads.
  • Hydrophobic protein regions reside within the nonpolar core in contact with hydrophobic phospholipid tails.
• Fluid Mosaic Model:
  • Model description: The membrane is a structural mosaic of distinct protein molecules floating or bobbing laterally within a dynamic, fluid phospholipid bilayer.
  • Non-random protein distribution: Proteins are not randomly dispersed throughout the lipid bilayer; they are precisely localized to specific membrane regions where their specialized functions are required.
  • Clinical Context Note: MGUS (Monoclonal Gammopathy of Undetermined Significance) represents a clinical disease topic associated with plasma cell protein secretion.