Study Notes on Membrane Structure

Chapter 11: Membrane Structure

Key Terms

  • Extracellular Matrix: The network of proteins and carbohydrates found outside cells that provides structural and biochemical support.
  • Oligosaccharide: A carbohydrate polymer made up of a small number of monosaccharides (typically 3-10).
  • Integral Protein: A type of protein that is permanently attached to the biological membrane.
  • Intracellular Matrix: The complex network of structural proteins, fibers, and other substances found within cells.
  • Peripheral Protein: A protein that is loosely attached to the membrane's surface, often involved in signaling pathways.

Structure of Membranes

  • Membranes form the plasma membrane and surround organelles such as:
    • Endoplasmic Reticulum
    • Nucleus
    • Peroxisome
    • Lysosome
    • Golgi Apparatus
    • Vesicle
    • Mitochondrion

Functions of Membranes

  • Physical Barrier: Defines the boundaries of the cell and its compartments.
  • Selective Permeability: Allows certain molecules to enter and exit the cell, maintaining homeostasis.
  • Communication: Facilitates signal reception and response through receptors.
  • Cell Adhesion: Mediates adhesion between cells for tissue formation.
  • Flexibility: Membranes are flimsy yet flexible, allowing movement and expansion in response to environmental changes.
    • Reference: Lazer Tweezers demonstration (link provided).

Composition of Cell Membranes

  • Lipids
  • Proteins
    • Types of proteins include:
    • Integral Proteins
    • Peripheral Proteins
  • Examples of lipids:
    • Glycolipid
    • Phospholipid
    • Cholesterol

Fluid Mosaic Model

  • Describes the structure of the plasma membrane as a fluid lipid bilayer with a mosaic of various proteins embedded within.
  • Comparison of early models:
    • Davson-Danielli (1935): Proposed a simple bilayer model.
    • Singer-Nicolson Model (1972): Updated the view to represent the fluidity and dynamic nature of lipid and protein interactions.

Major Classes of Membrane Lipids

  1. Cholesterol
  2. Glycolipids
  3. Phospholipids

Types of Phospholipids

  • Key examples include:
    • Phosphatidylserine: Contains phosphoserine as the polar head group.
    • Phosphatidylcholine: Contains phosphocholine as the polar head group.
    • Phosphatidylinositol: Contains phosphoinositol as the polar head group.

Formation of Lipid Bilayers

  • Phospholipids spontaneously form bilayers in aqueous environments due to their amphipathic nature, with:
    • Polar heads facing the aqueous environment.
    • Hydrophobic tails facing inward, away from water.

Asymmetry of Cell Membranes

  • Membrane bilayers are asymmetric in terms of lipid composition:
    • Variation in types and amounts of
      • Phospholipids
      • Cholesterol
      • Proteins
      • Glycolipids (often found more on the extracellular side).

Fluidity of Membranes

  • Factors influencing fluidity:

    1. Number of double bonds in hydrocarbon tails (cis isomer increases fluidity).
    2. Length of hydrocarbon tails (shorter chains increase fluidity).
    3. Amount of cholesterol (impacts rigidity and fluidity).

  • Lateral Diffusion: Phospholipids can rotate and move laterally but flipping between layers (flip-flop) is rare.

    • FRAP (Fluorescence Recovery After Photobleaching) used to observe movement.
    • Exceptions: Certain proteins (Flippase, Floppase, Scramblase) facilitate movement across bilayers.

Membrane Protein Functions

  • Four Major Functions:

    1. Transporters: Facilitate the passage of molecules through the membrane.
    2. Anchors: Link plasma membrane to the extracellular matrix and cytoskeleton.
    3. Receptors: Receive and propagate signals for communication.
    4. Enzymes: Provide catalytic activity within the membrane.

  • Mechanisms: Involved in endocytosis (uptake) and exocytosis (secretion) of substances.

  • Structural Roles: Stabilize and shape the membranes.

Classes of Membrane Proteins

  1. Integral Membrane Proteins

    • Types:
      • Integral Monotopic
      • Single pass
      • Multipass
      • Multi-subunit

  2. Peripheral Membrane Proteins

    • Loosely associated with the membrane, usually hydrophilic.

  3. Lipid-Anchored Proteins

    • Covalently attached to lipid molecules embedded in the bilayer.

Membrane Protein Types

  • Integral Monotopic Proteins: Embedded in one leaflet with no transmembrane domain.
  • Singlepass Proteins: Contain a single transmembrane domain typically as an alpha helix.
  • Multipass Proteins: Contain multiple transmembrane domains, can form channels or pores for transport.
  • Multi-subunit Proteins: Composed of separate polypeptide units aggregating in the membrane.

Isolation of Membrane Proteins

  • Detergents: E.g., SDS or Triton X-100 required for isolating membrane proteins by disrupting membranes.

Structure of the Plasma Membrane

  • Supported by Cell Cortex: An underlying actin protein network that determines cell shape and plays a role in division and movement.
  • Spectrin: Major protein in the cell cortex of red blood cells, providing support and maintaining shape.
    • Mutation can lead to conditions such as anemia due to abnormal shapes of RBCs.

Glycocalyx

  • Located on the extracellular side of the plasma membrane, composed of:

    • Glycoproteins
    • Glycolipids
    • Proteoglycans

  • Functions of Glycocalyx:

    • Cell-cell recognition.
    • Immune recognition and response.
    • Adhesion in tissues.
    • Provides protection from chemical injury.