Membrane Transport

Overview

• Cells have a plasma membrane that regulates substance movement.
• The membrane is mainly composed of phospholipid molecules with protein molecules scattered.
• Phospholipid molecules have:
  • Polar, hydrophilic heads.
  • Non-polar, hydrophobic tails.
• Membrane forms a bilayer; heads face outward and tails inward.

Types of Membrane Transport

• Passive Transport Processes (no energy required):
  • Simple diffusion
  • Facilitated diffusion
  • Osmosis
• Active Transport Processes (requires energy):
  • Active transport
  • Vesicular transport

Passive Transport

Diffusion

• Movement from high to low concentration.
• Simple Diffusion
  • Occurs with small, non-polar solutes.
  • Examples: Oxygen (O2), Carbon Dioxide (CO2), small fatty acids.
• Facilitated Diffusion
  • For small, charged or polar solutes.
  • Utilizes plasma membrane proteins.
  • Channel Mediated Diffusion: Uses protein channels for ions.
    • Leak channels (always open)
    • Gated channels (open due to stimulus)
  • Carrier Mediated Diffusion: Uses carrier proteins, changes shape for solute transport.

Osmosis

• Passive movement of water through selectively permeable membrane.
• Water moves via:
  • Between phospholipid molecules.
  • Through aquaporins (water channels).

Active Transport

Active Transport

• Movement against concentration gradient (low to high).
• Primary Active Transport
  • Uses ion pumps and ATP.
  • Example: Sodium-potassium pump.
• Secondary Active Transport
  • Uses energy from another substance moving down its gradient.
  • Symport: Two substances move in same direction.
  • Antiport: Two substances move in opposite directions.

Vesicular Transport

• Transport of large substances via vesicles.
• Exocytosis: Secreting materials outside the cell.
• Endocytosis: Intake of materials into the cell.
  • Phagocytosis: "Cell eating"; large particles engulfed.
  • Pinocytosis: "Cell drinking"; engulfs droplets of fluid.
  • Receptor-Mediated Endocytosis: Uses receptors to bind molecules.

Key Takeaways

• Passive transport doesn’t require energy; active transport does.
• Diffusion, osmosis, and facilitated diffusion are passive.
• Active transport includes primary and secondary transport, as well as vesicular transport.
• Vesicular transport requires energy, includes exocytosis and different forms of endocytosis.