Chapter 5: Active Transport and Bulk Transport

Active Transport

• Definition: Movement of molecules from low to high concentration, against the concentration gradient, requiring energy.
• Characteristics:
  • Opposite of passive transport.
  • Always involves a protein transporter (cannot pass directly through cell membrane).
  • Can move molecules against their electrochemical gradient (e.g., moving protons toward a more positive solution).
  • Energy Requirement: Always needed, unlike passive transport (diffusion, facilitated diffusion, osmosis).

Types of Active Transport

1. Primary Active Transport:

   • Uses ATP directly to move substances against their concentration or electrochemical gradient.
   • Example: Sodium-potassium pump (or sodium potassium ATPase)
     • Moves 3 Na+ out and 2 K+ into the cell using 1 ATP.
     • Creates a net negative charge inside the cell due to more positive ions being moved out than in.

2. Secondary Active Transport:

   • Does not use ATP directly, relies on a gradient created by primary active transport.
   • Uses an electrochemical gradient (e.g., sodium gradient) to move molecules against their gradient.
   • Example: Sodium-glucose transporter
     • Sodium moves down its gradient, releasing energy to move glucose against its gradient.

Electrochemical Gradients

• Arise from different concentrations of ions inside and outside the cell, contributing to a membrane potential.
• Example: Sodium-potassium pump contributes to a net negative charge inside the cell.

Types of Transport Proteins

• Uniporters: Move one molecule or ion.
• Symporters: Move two different molecules in the same direction.
• Antiporters: Move two different molecules in opposite directions (e.g., sodium-potassium pump).

Bulk Transport

• Used for molecules too large to pass through transport proteins or for moving large quantities at once.
• Types:
  • Endocytosis: Importing materials into the cell.
    • Phagocytosis (cell eating): Engulfing large particles or cells.
    • Pinocytosis (cell drinking): Engulfing solutes in fluid.
    • Receptor-Mediated Endocytosis: Specific molecules bind to receptors before being engulfed.
  • Exocytosis: Exporting materials out of the cell.
    • Vesicles fuse with the cell membrane, releasing contents outside the cell.

Examples and Applications

• Sodium-Potassium Pump:

  • Antiporter and electrogenic pump.
  • Important for maintaining membrane potential and cellular functions.

• Familial Hypercholesterolemia:

  • Genetic disorder affecting LDL receptors.
  • Causes high LDL levels in blood, increasing risk of heart disease.

Conclusion

• Active transport and bulk transport are essential for maintaining cellular homeostasis and performing vital functions, such as nutrient uptake and waste removal.

This concludes Chapter 5. Chapter 6 will cover metabolism, energy, and enzymes.