Plasma Membranes 2

Cellular Biology & Homeostasis

Cell Membrane Part 2

  • Presentation details by Clara Camargo, DVM, Cert AqV

Learning Objectives

  • Membrane flexibility and fluidity: Understand the concepts of flexibility and fluidity in membranes.

  • Movement of Phospholipids: Describe how phospholipids move within the membrane.

  • Role of Cholesterol: Explain how cholesterol serves as a fluidity buffer.

  • Extracellular Matrix (ECM): Define the ECM, its main components, and functions in cell physiology.

Cell Membrane Fluid Mosaic Model for Membrane Structure

  • Key Features:

    • Compartmentalization: Separation of cellular compartments.

    • Properties: Membrane characteristics.

    • Fusion and Fission: Processes involving membrane interactions.

    • Components: Various types of biomolecules that make up the membrane.

    • Self-healing properties: Ability of membranes to seal themselves.

    • Visualization Techniques: Techniques such as electron microscopy to study membranes.

    • Phospholipids: Major building blocks of membranes.

Membrane Dynamics

Features of Biological Membranes

  • Flexibility: Ability to change shape without compromising integrity.

  • Fluidity: Capability of the membrane to flow.

Structure and Flexibility of Lipid Bilayer

  • Influenced by:

    • Lipid Composition: Types of lipids present.

    • Temperature Changes: Membrane properties vary with temperature.

    • Noncovalent Interactions: Lipid interactions affect fluidity.

History of Membrane Research

  • 1970 Recognition: Discovery of individual lipid molecules' capability to diffuse freely within bilayers.

  • Study Preparations:

    • Liposomes: Spherical vesicles used as model membranes.

    • Black Membranes: Planar bilayers to measure permeability properties.

Movement of Phospholipids

Types of Movement

  • Lateral Diffusion: Movement within the same leaflet, occurring readily.

  • Transversal Diffusion (Flip-Flop): Rarely occurs without catalysis.

  • Factors Influencing Movement:

    • Hydrocarbon tail size

    • Temperature

Catalysis of Phospholipid Movement

  • Flippases: Move specific phospholipids from outer to inner leaflet.

  • Floppases: Transport specific lipids from inner to outer leaflet.

  • Scramblases: Facilitate bidirectional lipids movement and disrupt membrane asymmetry.

Membrane Fluidity and Lateral Diffusion

  • Techniques to Measure:

    • Fluorescence Microscopy: Observes membrane fusion.

    • FRAP: (Fluorescence Recovery After Photobleaching) demonstrates lateral diffusion.

Regulation of Fluidity

  • Membrane Phase Transition: Changes between liquid-crystalline and paracrystalline states.

  • Adaptation of Organisms: Ability to adjust fatty acids to maintain fluidity.

Factors Affecting Fluidity

  1. Phospholipid Content: Length and saturation of fatty acids influence fluidity.

  2. Cholesterol Content: Affects the membrane’s permeability and stability.

  3. Temperature: Varies lipid movement and state of the bilayer.

Implications of Temperature Changes

  • Low Temperatures: Reduced lipid movement leads to a rigid, gel-like state.

  • Moderate to High Temperatures: Increased fluidity, transitions to liquid-crystalline state.

Influence of Fatty Acids on Membrane Fluidity

  • Unsaturated Fatty Acids: Create kinks that space tails apart, lowering melting point and preventing paracrystalline structure.

  • Saturated Fatty Acids: Pack closely, increasing the phase transition temperature and making the membrane more rigid.

Cholesterol's Role in Membrane Fluidity

  • Cholesterol Content: Can be present up to one molecule per phospholipid, enhancing stability.

  • Temperature Effects:

    • At high temperatures: Cholesterol stabilizes the bilayer.

    • At low temperatures: Acts as antifreeze, preventing fatty acids from clumping.

Extracellular Matrix (ECM)

  • Definition: A network of macromolecules present within tissues.

  • Components:

    • Basement Membrane: Provides support and structure.

    • Interstitial Matrix: Contains gels of polysaccharides and proteins that buffer compressive forces.

Functions of ECM

  • Support: Structural and biochemical support to cells.

  • Cell Adhesion and Communication: Facilitates interaction between cells.

Types of ECM in Connective Tissue

  • Bone: Rigid structure consisting of collagen and minerals.

  • Loose Connective Tissue: Contains reticular fibers and ground substance.

  • Blood: Specialized fluid connective tissue.

Fibroblasts in ECM Production

  • Role: Produce and secrete ECM macromolecules, primarily collagen fibers.

  • Differentiation: Can become chondroblasts, osteoblasts, or myofibroblasts depending on tissue type.

Major Classes of ECM Macromolecules

  1. Glycosaminoglycans (GAGs): Charged polysaccharides that form ground substance with proteins (proteoglycans).

  2. Fibrous Proteins: Collagen for structure and elastin for elasticity.

  3. Non-collagen Fibrous Proteins: Include fibronectin and laminin for stability and function.

Collagen Structure and Functions

  • Characteristics: Long, stiff, triple-stranded proteins that provide structural integrity.

  • Distribution: Found in various tissues with specific functions depending on type.

Elastin and Its Role in ECM

  • Function: Provides elasticity to tissues.

  • Structure: Cross-linked network allowing extension and recoil similar to rubber bands.

Fibronectin in ECM

  • Role: Adhesive glycoprotein involved in tissue repair, cellular attachment, and motility.

Laminin and Basal Lamina

  • Function: Organizes the structure of basal lamina; critical for epithelial integrity.

Integrins and Their Functions

  • Role: Transmembrane proteins facilitating cell adhesion and signaling, connecting cytoskeleton to ECM.