Structure & Function of Membranes

Introduction to the Fluid-Mosaic Model

• Proposed in 1972 to describe cell membranes.
• Fluid: Phospholipid bilayer where molecules move constantly.
• Mosaic: Various proteins of different sizes embedded in the bilayer.

Key Components of Cell Membranes

• Phospholipid Bilayer

  • Composed of phospholipids with hydrophilic heads and hydrophobic tails.
  • Arrangement forms a bilayer, with heads facing outward and tails inward.

• Cholesterol

  • Provides stability by reducing fluidity.
  • Has hydrophilic and hydrophobic regions that bind with fatty acid tails of phospholipids.

• Proteins

  • Intrinsic Proteins:
    • Embedded through both sides of the bilayer.
    • Include channel and carrier proteins for transport.
  • Extrinsic Proteins:
    • Located on one side of the bilayer.
    • Involved in support and cell signaling.

• Glycoproteins and Glycolipids

  • Glycoproteins: Proteins with attached carbohydrates.
  • Glycolipids: Lipids with attached carbohydrates.
  • Functions involve cell adhesion, recognition, and signaling.

Phospholipid Bilayer

• Hydrophilic heads face the water, hydrophobic tails face inward.
• Creates a hydrophobic center, preventing water-soluble substances from passing.
• Lipid-soluble substances can dissolve and pass through.

Cholesterol in Membranes

• Stabilizes membrane by decreasing fluidity.
• Cholesterol molecules have hydrophilic and hydrophobic parts that interact with phospholipids.

Proteins in Membranes

• Intrinsic Proteins:
  • Span entire bilayer.
  • Facilitate transport of large molecules and ions.
• Extrinsic Proteins:
  • Located on surface.
  • Provide structural support and facilitate cell communication.

Roles of Glycoproteins and Glycolipids

1. Cell Adhesion: Cells attach to one another.
2. Cell Recognition: Cells identify each other.
3. Cell Signaling: Cells communicate with each other.

Types of Cell Membranes

1. Cell-Surface Membranes
   • Surround cells, control substance entry and exit.
2. Membranes Around Organelles
   • Encase organelles, separate them from cytoplasm.
   • Allow compartmentalization within the cell.

• Permeability: Partially permeable, allowing selective molecular passage.