Cell Transport Mechanisms

Cell Membrane Structure

• Barrier made of a double layer of lipids
• Embedded protein and carbohydrate components

Types of Molecule Movement

1. Simple Diffusion

   • Small nonpolar molecules (e.g., Oxygen, Carbon Dioxide)
   • Small polar molecules (e.g., Water)
     • Cross slowly
   • Large nonpolar molecules (e.g., Vitamin A)
     • Very slow
   • Large polar molecules and charged ions (e.g., Glucose, Sodium, Potassium, Chloride)
     • Unlikely to cross on their own

2. Transport Proteins

   • Channels (e.g., Aquaporins for water, Chloride channels)
   • Carriers (e.g., Glucose transporter)

3. Bulk Transport

   • Endocytosis: Engulfing extracellular material
   • Exocytosis: Expelling material into extracellular space
   • Both require energy (ATP)

Types of Endocytosis

1. Phagocytosis

   • "Cell eating"
   • Used by macrophages, neutrophils
   • Process:
     • Macrophage engulfs bacteria
     • Forms a phagosome
     • Fuses with lysosome to form phagolysosome
     • Acidic environment and enzymes degrade bacteria
     • Expels debris

2. Pinocytosis

   • "Cell drinking"
   • Invaginates extracellular fluid and solutes
   • Forms a vesicle
   • Non-specific uptake

3. Receptor-mediated Endocytosis

   • Specific molecules bind receptors (e.g., LDL)
   • Involves clathrin-coated pits
   • Forms vesicle, separates LDL from receptor
   • Receptor recycling

Exocytosis

• Begins in the Golgi apparatus
• Packages proteins, lipids, hormones
• Transported using cytoskeleton and motor proteins
• Vesicle fuses with cell membrane, releases contents

Recap

• Endocytosis: Cells engulf materials (types: phagocytosis, pinocytosis, receptor-mediated)
• Exocytosis: Cells expel materials
• Both require ATP

Important Concepts

• Use of ATP in transport processes
• Role of cytoskeleton in cell transport and structure
• Integration of vesicles and cellular components in transport systems