What is facilitated diffusion? Describe the mechanism of active transport across the plasma membrane with suitable example. (IAS 2022/15 Marks Marks)

Facilitated Diffusion

Facilitated diffusion is a type of passive transport that moves molecules across the cell membrane without using energy, relying on specialized transport proteins.

• Facilitated diffusion is the passive movement of molecules along a concentration gradient.
• It requires the assistance of membrane proteins to help molecules pass through the plasma membrane.
• Mechanism:
  
  • Transport Proteins: Specific carrier or channel proteins embedded in the cell membrane assist in the transport of molecules.
    
    • Carrier Proteins: Bind with molecules (like glucose) and change shape to move them across the membrane.
    • Channel Proteins: Create a water-filled pathway for ions and polar molecules, enabling their passage.
  • No Energy Requirement: Facilitated diffusion does not use ATP; it relies on the existing concentration gradient.
  • Examples:
    
    • Glucose Transport: Glucose molecules enter cells via GLUT transporters.
    • Ion Transport: Potassium and chloride ions pass through channel proteins in neurons.

Active Transport Across the Plasma Membrane

Active transport is the movement of molecules across the plasma membrane against their concentration gradient, requiring cellular energy.

• Active transport is the process by which cells move molecules from an area of lower concentration to an area of higher concentration.
• This process requires energy in the form of ATP.
• Mechanism:
  
  • Energy Requirement: Active transport requires ATP because molecules are moved against the concentration gradient.
  • Transport Proteins Involvement:
    
    • Pumps: Specific proteins known as pumps move molecules actively across the membrane.
    • Example: Sodium-potassium pump in animal cells.
    • Endocytosis and Exocytosis: Larger molecules or particles are engulfed or expelled by membrane invaginations.
    • Endocytosis: Cells take in substances by enclosing them in vesicles (e.g., uptake of nutrients).
    • Exocytosis: Cells expel waste or secrete substances using vesicles (e.g., secretion of hormones).
  • Example - Sodium-Potassium Pump:
    
    • This pump maintains the electrochemical gradient in nerve and muscle cells.
    • It pumps three sodium ions out of the cell and two potassium ions into the cell, consuming one molecule of ATP per cycle.

Conclusion

Facilitated diffusion and active transport are two important mechanisms by which molecules are transported across the plasma membrane. While facilitated diffusion relies on transport proteins to facilitate the movement of molecules down their concentration gradient, active transport requires energy input to move molecules against their concentration gradient.