Cell Transport Mechanisms

Cell Membrane Structure

• Double layer of lipids
• Embedded proteins and carbohydrates

Types of Transport

1. Simple Diffusion

   • Small nonpolar molecules (e.g., O2, CO2) diffuse rapidly
   • Small polar molecules (e.g., H2O) diffuse slowly
   • Large nonpolar molecules (e.g., Vitamin A) diffuse very slowly
   • Large polar molecules (e.g., Glucose) and charged ions (e.g., Na+, K+, Cl-) unlikely to cross without assistance

2. Facilitated Transport

   • Uses membrane-bound transport proteins
   • Examples:
     • Aquaporins (water channels)
     • Chloride channels
     • Glucose transporters

3. Bulk Transport

   • Requires energy (ATP)
   • Types:
     • Endocytosis: Engulfing material into the cell
       • Phagocytosis: "Cell eating" (e.g., macrophages engulf bacteria)
       • Pinocytosis: "Cell drinking" (ingestion of extracellular fluid)
       • Receptor-Mediated Endocytosis: Specific molecules like LDL are ingested
     • Exocytosis: Expelling material from the cell
       • Involves the Golgi apparatus packaging proteins, lipids, and hormones into vesicles
       • Vesicles transported via the cytoskeleton and motor proteins

Endocytosis Details

• Phagocytosis:

  • Example: Macrophages engulfing streptococcus
  • Formation of phagosome, fusion with lysosome creates phagolysosome
  • Digestion via lysosomal enzymes and acidic pH
  • Waste expelled by fusion with cell membrane

• Pinocytosis:

  • Involves invagination of cell membrane to form vesicles
  • Non-specific solute intake
  • Vesicles carried by motor proteins into the cytosol

• Receptor-Mediated Endocytosis:

  • Involves clathrin-coated pits on cell membrane
  • Example: LDL cholesterol
  • Separation and recycling of receptors
  • Enzyme digestion in lysosomes

Exocytosis Details

• Originates from Golgi apparatus
• Vesicles transported by cytoskeleton
• Fusion with cell membrane to release contents

Key Concepts

• Both endocytosis and exocytosis require ATP
• Dynamic cytoskeleton enables cellular movement and transport

These notes summarize the mechanisms by which cells transport materials across their membranes, focusing on diffusion, facilitated transport, and bulk transport processes such as endocytosis and exocytosis.