Lecture: The Cell Membrane

Introduction

• Purpose of the Cell Membrane: Acts as a barrier between intracellular and extracellular fluid.
• Main Components:
  • Membrane lipids
  • Membrane proteins
  • Glycocalyx

Structure of the Cell Membrane

Membrane Lipids

• Main Functions: Form the basic structure of the cell membrane.
• Components:
  • Phospholipids: Form the lipid bilayer, hydrophilic heads face water, hydrophobic tails face each other.
    • Outer: Phosphatidylcholine, Sphingomyelin
    • Inner: Phosphatidylserine, Phosphatidylethanolamine
  • Fatty Acids:
    • Saturated: Straight chains
    • Unsaturated: Double bonds create kinks
  • Cholesterol: Stabilizes membrane, affects fluidity.

Membrane Proteins

• Functions: Span the membrane, serve multiple roles.
• Types:
  • Integral Proteins: Span the entire membrane (e.g., ion channels, carrier proteins).
  • Peripheral Proteins: Weakly attached, interact with membrane surfaces.

Glycocalyx

• Structure: Network of glycoproteins and glycolipids on the cell's exterior.
• Functions:
  • Prevents dehydration
  • Antigenic function (immune system, blood typing)

Functions of the Cell Membrane

Glycocalyx Functions

• Water Regulation: Controls movement of water in/out of cell.
• Antigenic Recognition:
  • Immune cells recognize host vs. foreign cells.
  • Blood typing and compatibility.

Membrane Lipid Functions

• Fluidity:
  • Influenced by temperature, cholesterol, and types of fatty acids.
    • High temperature: Increases fluidity
    • Presence of cholesterol: Can either stabilize (reduce fluidity) or prevent packing (increase fluidity)
    • Fatty Acids: Saturated (decrease fluidity), Unsaturated (increase fluidity)
• Transport:
  • Simple diffusion of small, nonpolar, lipid-soluble molecules (e.g., O2, CO2).
  • Lateral diffusion and transverse diffusion (movement within the membrane).

Membrane Protein Functions

• Transport: Facilitate movement of large, polar, water-soluble molecules through channels or carriers.
• Signal Transduction: Receptors trigger intracellular responses.
• Cell Adhesion: Link cells together (e.g., tight junctions, desmosomes).
• Enzymatic Activity: Catalyze reactions on inner or outer cell surfaces.
• Communication: Gap junctions allow ions/molecules to pass directly between cells.
• Attachment: Anchor cells to extracellular matrix (e.g., hemidesmosomes).

Conclusion

• The cell membrane's structure and components are crucial for its function as a protective barrier, facilitator of cellular communication, and contributor to cellular stability and function.
• Understanding the cell membrane's components, such as lipids, proteins, and glycocalyx, is essential for comprehending how cells interact with their environment and maintain homeostasis.