Lecture Notes: Membrane Transport Mechanisms

Passive Movement

• Definition: Movement of molecules across the membrane without external energy.
• Diffusion: Passive process where molecules spread from high to low concentration.
  • Simple Diffusion:
    • Molecules pass directly through the membrane.
    • Factors affecting rate:
      • Concentration gradient
      • Partition coefficient
      • Diffusion coefficient
      • Membrane thickness
      • Surface area
    • Common for lipophilic molecules.
  • Facilitated Diffusion:
    • Utilizes channel or carrier proteins.
    • Channel Proteins: For small molecules like ions.
    • Carrier Proteins:
      • Transport larger molecules.
      • Characteristics:
        • Specificity
        • Affinity
        • Saturation

Active Transport

• Definition: Movement of molecules against their concentration gradient (uphill), requires energy.
• Types:
  • Primary Active Transport:
    • Directly uses ATP hydrolysis for energy.
    • Example: Sodium-Potassium Pump
      • Sodium (Na+) is transported out of the cell.
      • Potassium (K+) is transported into the cell.
      • Driven by ATP hydrolysis (ATP to ADP).
      • Also known as a pump.
      • Maintains high Na+ outside and high K+ inside.
  • Secondary Active Transport:
    • Uses sodium gradient created by Na+/K+ pump.
    • Sodium moves passively into the cell, releasing potential energy.
    • Co-transport:
      • Molecule X moves into the cell with sodium.
    • Counter-transport:
      • Molecule Y is moved out as sodium moves in.
    • Sodium moving into the cell powers the transport of other molecules.

Key Concepts

• Electrochemical Gradient: Combination of chemical and electrical gradient.
• Energy Utilization:
  • Primary transport directly uses ATP.
  • Secondary transport uses energy from sodium gradient.

Important Points

• Passive processes do not require energy, active processes do.
• Sodium-potassium pump is crucial for maintaining cellular ion balance.
• Secondary active transport leverages sodium gradients to power movement of other molecules.