Lecture Notes: Cell Membrane Structure and Function

Introduction

• Overview of the cell membrane as a barrier between intracellular and extracellular fluid.
• Key components include membrane lipids, proteins, and the glycocalyx.

Structure of the Cell Membrane

Membrane Lipids

• Phospholipids:
  • Outer Membrane: Contains phosphatidylcholine and sphingomyelin.
  • Inner Membrane: Contains phosphatidylserine and phosphatidylethanolamine.
  • Negatively charged, hydrophilic, allowing interactions with water.
• Fatty Acids:
  • Two types: Saturated (no double bonds) and Unsaturated (with double bonds causing kinks).
  • Hydrophobic, located in the membrane's interior.
• Cholesterol:
  • Provides stability and affects membrane fluidity.

Membrane Proteins

• Integral/Transmembrane Proteins: Span the entire membrane, allowing molecule transport.
• Peripheral Proteins: Attach loosely to the membrane, involved in signaling.

Glycocalyx

• Composed of glycoproteins and glycolipids.
• Functions in water retention and antigen recognition.

Functions of the Cell Membrane

Glycocalyx

• Water Regulation: Prevents dehydration by regulating water movement.
• Antigenic Function: Allows immune system to distinguish between host and foreign cells.
  • Examples: Immune system recognition, blood typing.

Membrane Lipids

• Fluidity:
  • Influenced by temperature, cholesterol, and types of fatty acids.
  • High temperature increases fluidity; cholesterol can stabilize or separate phospholipids.
  • Saturated fatty acids decrease fluidity; unsaturated increase it.
• Transport:
  • Simple diffusion of small, non-polar, lipid-soluble molecules.
  • Lateral and transverse diffusion within the membrane.

Membrane Proteins

• Transport: Channels and carriers for large, polar, water-soluble molecules.
• Signal Reception: Hormones activate receptors, triggering intracellular responses.
• Cell Adhesion: Proteins link cells together (e.g., tight junctions, desmosomes).
• Enzymatic Activity: Catalyze reactions on cell surfaces.
• Communication: Gap junctions allow ionic flow between cells.
• Extracellular Matrix Attachment: Connects cells to external structures, e.g., hemidesmosomes.

Conclusion

• Cell membrane described as a "fluid mosaic" due to its dynamic and flexible nature composed of moving phospholipids and embedded proteins.
• Different methods of substance transport include simple diffusion through phospholipids or passage through protein tunnels in the membrane.