Cell Membrane Lecture Notes

Introduction

• Today's topic: Cell Membrane
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• Visual aids and supplementary notes available on the website

Overview of Cell Membrane

• Purpose: Barrier between intracellular and extracellular fluids.
• Structure Components:
  • Membrane lipids
  • Membrane proteins
  • Glycocalyx (Glycoprotein and glycolipid network)

Structure of Cell Membrane

1. Membrane Lipids

• Phospholipids: Key component of the membrane:

  • Outer and inner membranes made of phosphates and sphingocides (sphingosines).
  • Charged: Negatively charged phosphates make them hydrophilic (water-attracting).
  • Outer membrane: Phosphatidylcholine and sphingomyelin.
  • Inner membrane: Phosphatidylserine and phosphatidylethanolamine.

• Fatty Acids:

  • Hydrophobic (water-repelling), preventing interaction with extracellular fluid.
  • Types: Saturated (straight) and unsaturated (with double bonds, causing kinks).

• Cholesterol:

  • Stabilizes membrane structure, influencing fluidity.

2. Membrane Proteins

• Integral Proteins:

  • Span the entire membrane (transmembrane proteins), allowing for transport across the membrane.
  • Types include ion channels and carrier proteins.

• Peripheral Proteins:

  • Weakly attached to the membrane, involved in signaling and structural support.

3. Glycocalyx

• Composed of glycoproteins and glycolipids.
• Functions:
  • Regulates water movement, reducing cell dehydration.
  • Provides antigenic functions, helping the immune system distinguish between host and foreign cells.

Functions of Cell Membrane

1. Glycocalyx Functions

• Water Regulation:

  • Controls movement of water in and out of the cell.

• Antigenic Function:

  • Helps immune system recognize normal host cells vs. foreign cells.
  • Example: Blood typing based on glycoproteins present on red blood cells.

2. Membrane Lipid Functions

• Fluidity:

  • Influenced by:
    • Temperature: Higher temperature increases fluidity.
    • Cholesterol: Stabilizes and influences spacing of phospholipids.
    • Fatty Acid Types: Unsaturated fatty acids increase fluidity due to kinks; saturated fatty acids decrease fluidity.

• Transport:

  • Simple Diffusion: Small nonpolar, lipid-soluble molecules can diffuse through the membrane (e.g., O2, CO2).
  • Lateral and Transverse Diffusion: Phospholipids can move laterally or flip between inner and outer membrane via specific enzymes (flippases and flopases).

3. Membrane Protein Functions

• Transport Proteins:

  • Facilitate movement of large polar and water-soluble molecules across the membrane.

• Exocytosis and Endocytosis:

  • Membrane proteins assist in transporting materials into and out of the cell through vesicle fusion.

• Receptor Functions:

  • Receive signals from hormones, triggering responses within the cell.

• Cell Communication:

  • Gap junctions allow ions to flow between adjacent cells.

• Enzymatic Activity:

  • Membrane proteins can act as enzymes, catalyzing reactions on the cell surface or within cells.

• Cell Attachment:

  • Membrane proteins link cells to the extracellular matrix (e.g., hemidesmosomes provide structural support).

Conclusion

• Reviewed structure and functions of cell membrane components: lipids, proteins, glycocalyx.
• Encouragement to further explore transport mechanisms and cell signaling.