Biomembranes and Cell Architecture

Introduction to Biomembranes

  • Study of biomembranes and cell architecture essential for understanding cell structure and function.

General Structure of Biomembranes

  • Barrier Function: Membranes act as impermeable barriers to water-soluble molecules.
  • Membrane Proteins: Comprise 25-35% of the genome; crucial for many cell functions.
  • Phospholipid Bilayer: Approximately 5 nm thick; 10 nm thick when including proteins.

Fluid Mosaic Model

  • Describes the arrangement of lipids and proteins in biomembranes.

Lipid Bilayer Properties

  • Flexibility: The lipid bilayer is a flexible 2D fluid, restricted by the surrounding water. Critical for functionality.
  • Lateral Diffusion: Very fast (~2 µm/s).
  • Flip-Flop: Rare; takes place once a month in synthetic membranes.
  • Flippases: Enzymes facilitating the flip-flop of lipids across membranes.

Classes of Lipids in Biomembranes

  • Types of Lipids:
    • a. Phosphoglycerides
    • b. Sphingolipids
    • c. Sterols
    • d. All of the above
  • Principal Classes of Lipids: Phosphatidylserine (phospholipid), cholesterol (sterol), galactocerebroside (glycolipid).

Fluidity of the Lipid Bilayer

  • Fluidity Factors:
    1. Lipid Composition
    2. Structure of Hydrophobic Tails
    3. Temperature
  • Importance of Fluidity:
    • Allows diffusion of molecules (e.g., signaling).
    • Affects membrane fusion and inheritance.

Effect of Hydrocarbon Tails

  • Tail Length: Shorter tails increase fluidity. Common lengths: 14-24 carbon atoms (18-20 are predominant).
  • Degree of Saturation:
    • Unsaturated fats (with double bonds) lead to increased fluidity due to less stable packing.

Role of Cholesterol

  • Amphipathic Nature: Cholesterol makes up about 20% of the plasma membrane.
  • Functions:
    • Fills gaps between phospholipids, making membranes more rigid and less permeable.

Temperature Effects on Membrane Fluidity

  • Phase Transition: Temperature changes can cause a transition from gel-like to fluid-like states within membranes.

Membrane Assembly and Lipid Distribution

  • Membrane assembly begins at the Endoplasmic Reticulum (ER):
    • Smooth ER creates lipids using free fatty acids.
    • Newly synthesized lipids are added to the cytosolic layer of the membrane.
  • Scramblases: Randomly transfer phospholipids between layers.
  • Asymmetrical Distribution: Most membranes have different lipid compositions in each layer, maintained by flippases.

Membrane Leaflets

  • Definitions:
    • Cytosolic Leaflet: Faces cytosol.
    • Exoplasmic Leaflet: Faces exterior (not in contact with cytosol).
  • Conserved Asymmetry: Maintained during membrane budding and fusion.

Membrane Protein Interactions

  • Types of Membrane Proteins:
    • Integral Proteins: Span the membrane; often amphipathic.
    • Peripheral Proteins: Loosely attached to the membrane surface.

Membrane Protein Structure and Function

  • Transmembrane Proteins: Typically cross the membrane as an α-helix, enriched in hydrophobic amino acids.
  • Channels: Create aqueous pores for water-soluble molecules to pass through.

Detergents and Membrane Proteins

  • Detergents solubilize membranes by disrupting lipid bilayers and forming micelles for ease of study.

Cell Cortex and Membrane Support

  • Cell Cortex: A protein network supporting and reinforcing the plasma membrane.
  • Example: Red blood cell cortex composed of spectrin, critical for maintaining cell shape.

Movement of Membrane Proteins

  • Diffusion Across Membranes: Can be restricted by the cytoskeleton or by anchoring to other proteins.

Lipid Rafts

  • Microdomains in Membranes: Regions with higher concentrations of cholesterol, sphingolipids, and specific proteins, less fluid than surrounding areas.

Measuring Membrane Flow: FRAP Method

  • Fluorescence Recovery After Photobleaching (FRAP):
    1. Express fluorescently tagged proteins.
    2. Bleach a small area with a laser.
    3. Measure the rate of fluorescence recovery to analyze mobility.

Glycocalyx: The Cell Surface Coating

  • Cells have carbohydrates on the outer surface, mainly in the form of glycoproteins and proteoglycans.
  • Functions include protection, cell recognition, and adhesion, emphasized by the term “sugar coating.”