Membrane Transport

Overview

• Membrane transport refers to the movement of substances into and out of a cell.
• Cells have a plasma membrane that regulates this movement.
• The membrane consists mainly of phospholipid molecules with scattered protein molecules.
• Phospholipid Structure:
  • Polar hydrophilic heads (water-loving)
  • Non-polar hydrophobic tails (water-fearing)
  • Forms a bilayer with heads facing outwards and tails inwards.

Types of Membrane Transport

1. Passive Transport Processes

   • Do not require energy expenditure.
   • Types: Simple diffusion, facilitated diffusion, osmosis.

2. Active Transport Processes

   • Require cellular energy.
   • Types: Active transport, vesicular transport.

Passive Transport

Simple Diffusion

• Movement from high to low concentration.
• Occurs with small, non-polar solutes.
• Examples: Oxygen (O2), Carbon Dioxide (CO2), small fatty acids.

Facilitated Diffusion

• Applies to small, charged or polar solutes blocked by the phospholipid bilayer.
• Requires assistance from membrane proteins.
• Types:
  • Channel-mediated diffusion:
    • Ions move through water-filled protein channels.
    • Types of Channels:
      • Leak channels (continuously open)
      • Gated channels (open due to stimulus)
  • Carrier-mediated diffusion:
    • Movement of polar molecules like glucose.
    • Carrier proteins change shape during transfer.

Osmosis

• Movement of water through a selectively permeable membrane.
• Water can move:
  • Between phospholipid molecules.
  • Through aquaporins (protein channels).

Active Transport

Active Transport

• Movement against concentration gradient (low to high concentration).
• Types:
  • Primary Active Transport:
    • Ion pumps move ions against gradient using ATP.
    • Example: Sodium-potassium pump.
  • Secondary Active Transport:
    • Uses energy from a second substance moving down its gradient.
    • Types:
      • Symport: Both substances move in the same direction.
      • Antiport: Substances move in opposite directions.

Vesicular Transport

• Involves the movement of large substances.
• Types:
  • Exocytosis:
    • Secreting materials from the cell.
    • Vesicles fuse with plasma membrane to release contents.
  • Endocytosis:
    • Plasma membrane engulfs substances to form vesicles.
    • Forms:
      • Phagocytosis (cell eating): Engulfment of large particles.
      • Pinocytosis (cell drinking): Engulfment of interstitial fluid droplets.
      • Receptor-mediated endocytosis: Involves binding of molecules to receptors before vesicle formation.

Key Points

• Both endocytosis and exocytosis require energy.
• Membrane transport is crucial for maintaining cellular homeostasis.