Transport Mechanisms in Biology: Active vs. Passive

Canoeing Metaphor

• Current of the River: Represents different types of transport in biology.
  • Downstream (with the current): Represents passive transport.
  • Upstream (against the current): Represents active transport.

Passive Transport

• Definition: Movement of substances along a gradient without the use of energy.
  • Concentration Gradient:
  • Movement from high concentration to low concentration.
  • Example: Molecules in a tube of water move from areas of high to low concentration, achieving equilibrium over time.
  • Electrical Gradient:
  • Movement along an electrical difference.
  • Example: Positive particles are drawn towards negative particles while avoiding other positive ones, and vice versa.
  • Energy Requirement: No energy is needed for passive transport; it happens naturally.

Active Transport

• Definition: Movement of substances against a gradient, requiring energy.
  • Direction: Moves from low concentration to high concentration or against an electrical gradient.
  • Example: Sodium-Potassium Pump
  • Cell Membrane Structure:
    • More potassium (K^+) ions inside the cell than outside.
    • More sodium (Na^+) ions outside the cell than inside.
  • Passive Transport Example: Potassium ions can passively move inside the cell through a channel because the concentration is higher inside.
  • Active Transport Example: Sodium ions are actively transported out of the cell using the sodium-potassium pump.
    • Process uses ATP (energy) to change the shape of the protein in the pump, allowing sodium to be pumped out against both concentration and electrical gradients.

Summary of Energy Usage

• Passive Transport: No energy required; occurs naturally based on gradients.
• Active Transport: Requires energy (ATP) to move substances against gradients. It is essential for maintaining the necessary concentrations of ions in cells, such as sodium and potassium, for proper function.