bio 2.9

Mechanisms of Transport

Passive Transport

• Diffusion

  • Movement of molecules from a high concentration to a low concentration.
  • Example molecules:
  • Small non-polar molecules: Nitrogen, Oxygen, Carbon Dioxide.
  • Small amounts of very small polar molecules like water can diffuse across the cell membrane in minor quantities.

• Facilitated Diffusion

  • Movement through transport proteins.
  • Involves both large and small polar molecules, as well as charged ions.
  • Examples: Sodium (Na⁺), Potassium (K⁺) require channel proteins.
  • Water moves via aquaporins.
  • Osmosis is the diffusion of water based on differences in solute concentration.

Active Transport

• Overview
  • Active transport moves molecules or ions against their concentration gradient (from low to high concentration).
  • Utilizes protein pumps which are carrier proteins requiring metabolic energy (e.g., ATP).
  • Essential for establishing and maintaining concentration gradients.

Transport of Large Molecules

• Endocytosis

  • The process where the cell uses energy to intake macromolecules by forming new vesicles from the plasma membrane.
  • Types of endocytosis:
  • Phagocytosis: Ingestion of large particles or cells.
  • Pinocytosis: Ingestion of liquids or small particles.
  • Receptor-Mediated Endocytosis: Targeted uptake by binding specific ligands.

• Exocytosis

  • Internal vesicles fuse with the plasma membrane to secrete large macromolecules out of the cell.

Key Takeaways

• Passive Transport does not require energy and involves the net movement from high to low concentration.
  • Water diffuses in small amounts via simple diffusion but in large quantities via facilitated diffusion through aquaporins.
• Active Transport requires energy to move against the gradient, from low to high concentration.
• Large molecules are transported into cells via endocytosis and out via exocytosis.