Cell Membrane Lecture Notes

Introduction

• Overview of cell membrane structure and function
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Structure of the Cell Membrane

Purpose

• Acts as a barrier between intracellular and extracellular fluids.

Components of the Cell Membrane

1. Membrane Lipids

   • Phospholipids: Major component forming the bilayer.
   • Fatty Acids: Extend from phosphate heads, hydrophobic in nature.
   • Cholesterol: Provides stability to the membrane.

2. Membrane Proteins

   • Integral Proteins: Span the entire membrane (transmembrane proteins).
   • Peripheral Proteins: Weakly attached to the membrane, do not span completely.

3. Glycocalyx

   • Sugar residues attached to proteins and lipids, forming a protective layer on the outer surface of the cell.
   • Involves glycoproteins and glycolipids.

Detailed Components

Membrane Lipids

• Phospholipid Bilayer: Formed by hydrophilic phosphate heads facing water and hydrophobic fatty acid tails facing inward.
• Types of Phospholipids:
  • Outer Membrane: Phosphatidylcholine, sphingomyelin.
  • Inner Membrane: Phosphatidylserine, phosphatidylethanolamine.
• Hydrophilic vs Hydrophobic:
  • Phosphate heads are hydrophilic (interact with water).
  • Fatty acid tails are hydrophobic (avoid water).
• Cholesterol:
  • Stabilizes membrane and influences fluidity.

Membrane Proteins

• Integral Proteins: Include transmembrane proteins that allow passage of substances across the membrane.
• Peripheral Proteins: Weakly attached, can move within the membrane.
• Functions:
  • Transport of large, polar, or water-soluble molecules.
  • Enzymatic activity, signal transduction (receptor function).
  • Cell recognition and communication (e.g., antigenic functions).
  • Cell adhesion (tight junctions, desmosomes).

Glycocalyx

• Composed of glycoproteins and glycolipids.
• Functions:
  • Reduces dehydration by regulating water movement.
  • Plays a role in immune recognition (distinguishing between self and non-self).

Functions of the Cell Membrane

Glycocalyx Functions

• Holds onto water, reducing cell dehydration.
• Provides antigenic functions for immune system recognition (MHC1 complex example).
• Blood typing based on glycocalyx on red blood cells.

Membrane Lipids Functions

1. Fluidity:

   • Influenced by temperature, presence of cholesterol, and types of fatty acids.
   • High temperatures increase fluidity; cold temperatures decrease fluidity.
   • Saturated fatty acids decrease fluidity; unsaturated fatty acids increase fluidity.

2. Transport:

   • Simple diffusion: Small, nonpolar, and lipid-soluble substances can easily diffuse through the membrane.
   • Lateral diffusion: Phospholipids and proteins can move within the membrane.
   • Transverse diffusion: Enzymes (floppases and flippases) assist in moving phospholipids between layers.

Membrane Proteins Functions

• Transport Functions: Allow for passage of larger, polar molecules through protein channels.
• Receptor Functions: Trigger cellular responses upon binding with hormones or signaling molecules.
• Cell-to-Cell Communication: Facilitates signaling between adjacent cells (e.g., gap junctions).
• Enzymatic Reactions: Catalyze reactions occurring at the membrane surface.
• Attachment: Connect cells to the extracellular matrix.

Conclusion

• Summary of the cell membrane components and their functions.
• Emphasis on the complexity and importance of the cell membrane in cellular function.