Membrane Transport

Overview of Cell Membrane

• Plasma Membrane: Acts as a fence, regulating substance movement.
• Composition: Mainly phospholipid molecules with scattered protein molecules.
  • Phospholipid Structure:
    • Polar hydrophilic (water-loving) head
    • Non-polar hydrophobic (water-fearing) tails
  • Forms a bilayer with heads facing out and tails facing in.
  • Cytosol: Inner part of the cell.
  • Interstitial Fluid: Surrounds the cell.

Types of Membrane Transport

• Passive Transport: Does not require energy.
• Active Transport: Requires cellular energy.

Passive Transport Processes

1. Simple Diffusion

   • Movement from high concentration to low concentration.
   • Example: Sugar cube dissolving in water.
   • Occurs down a concentration gradient.
   • Solutes: Small, non-polar molecules (e.g., O2, CO2, fatty acids).

2. Facilitated Diffusion

   • For small, charged or polar solutes.
   • Requires assistance from plasma membrane proteins.
   • Types:
     • Channel Mediated Diffusion: Ions move through water-filled protein channels.
       • Leak Channel: Continuously open.
       • Gated Channel: Opens due to a stimulus.
     • Carrier Mediated Diffusion: Involves carrier proteins that change shape to transport molecules.
       • Example: Glucose binding to a carrier protein.

3. Osmosis

   • Passive movement of water through a selectively permeable membrane.
   • Occurs when there is a concentration difference of water.
   • Water can move through phospholipid molecules or aquaporins (water channels).
   • Example: Water moves from areas of higher to lower concentration of solutes.

Active Transport Processes

1. Active Transport

   • Movement against concentration gradient (low to high concentration).
   • Uses cellular energy.
   • Types:
     • Primary Active Transport: Ion pumps (e.g., sodium/potassium pump).
       • Example of sodium and potassium movement involving ATP.
     • Secondary Active Transport: Uses energy from movement of a second substance.
       • Types:
         • Symport: Two substances move in the same direction.
         • Antiport: Two substances move in opposite directions.

2. Vesicular Transport

   • Transport of large substances using vesicles.
   • Types:
     • Exocytosis: Materials secreted from the cell.
       • Vesicles from the Golgi apparatus fuse with the plasma membrane.
     • Endocytosis: Plasma membrane engulfs substances to form a vesicle.
       • Forms of Endocytosis:
         • Phagocytosis: Engulfing large particles (cell eating).
         • Pinocytosis: Engulfing droplets of fluid (cell drinking).
         • Receptor Mediated Endocytosis: Uses receptors to bind molecules for transport.

Key Points

• Both vesicular transport processes (endocytosis and exocytosis) require energy.
• Understanding these processes is crucial for grasping how substances move across cellular membranes.