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.”