Chapter 5: Composition and Function of Cell Membranes

Functions of the Cell Membrane

• Outer Border: Acts as an outer border for cells and membranous organelles.
• Control Entry/Exit: Regulates what enters and exits the cell.
• Signal Reception: Receives environmental signals initiating cellular responses.
• Adhesion: Allows cells to adhere to neighboring cells (e.g., tight junctions, desmosomes, GAP junctions).

Structure of the Cell Membrane

• Phospholipid Bilayer: Composed mainly of phospholipids with nonpolar tails facing inward and polar heads facing outward.
• Fluid Mosaic Model: Includes cholesterol, proteins, carbohydrates allowing lateral movement.
• Discovery: Structure identified in the 1950s via electron microscopy showing a bilayer.
• Thickness: Typical membrane is 5-10 nm thick.
• Composition:
  • 50% proteins
  • 40% lipids
  • 10% carbohydrates

Phospholipid Composition

• Amphipathic Nature: Contains both hydrophilic (water-loving) and hydrophobic (water-fearing) parts.
• Fatty Acids:
  • Saturated: Single bonds, solid structure.
  • Unsaturated: Includes CIS double bonds, kinks, more fluid.
• Self-arrangement: Form bilayer, liposomes, or micelles.

Proteins in the Cell Membrane

• Functions:
  • Transporters
  • Receptors
  • Enzymes
  • Adhesion
• Types:
  • Peripheral Proteins: Surface-bound.
  • Integral Proteins: Embedded with hydrophilic and hydrophobic regions (e.g., lysine, leucine).

Carbohydrates

• Location: Only on the exterior, forming glycoproteins or glycolipids.
• Functions:
  • Cell-cell recognition
  • Attachment
• Blood Type: Defined by glycoproteins on RBC's surface (A, B, AB, O).
• Immune Response: CD4 glycoproteins involved in HIV recognition.

Membrane Fluidity

• Importance: Prevents membrane from being too rigid or too loose.
• Influencing Factors:
  • Phospholipid Type: Saturated vs. unsaturated fatty acids.
  • Temperature: Warmer temps increase fluidity, colder temps decrease.
  • Cholesterol: Buffers fluidity, prevents freezing/rigidity extremes.

Membrane Asymmetry

• Interior vs. Exterior: Different compositions and functions.
  • Interior Proteins: Bind to cytoskeleton.
  • Exterior Proteins: Bind to extracellular matrix.
• Glycoproteins: Bind to hormones or cell signals.

Selective Permeability

• Allows: Selective entry/exit of molecules.
• Cytosol vs. Extracellular Environment: Different compositions, e.g., sodium and potassium ion concentrations.

Transport Across Membrane

• Passive Transport: No energy required.
• Active Transport: Energy required.

Next video covers passive transport.