Cell Transport Mechanisms

Cell Membrane Structure

• Composition: Double layer of lipids with embedded proteins and carbohydrates.
• Function: Acts as a barrier, controlling material transport in and out of the cell.

Molecule Transport Across Cell Membrane

1. Diffusion

   • Small Non-polar Molecules: Rapid diffusion (e.g., oxygen, carbon dioxide).
   • Small Polar Molecules: Slow diffusion (e.g., water).
   • Large Non-polar Molecules: Very slow diffusion (e.g., vitamin A).
   • Large Polar and Charged Molecules: Unlikely to cross on their own (e.g., glucose, ions like sodium, potassium, and chloride).

2. Transport Proteins

   • Channels: e.g., Aquaporins for water, chloride channels for chloride ions.
   • Carriers: e.g., Glucose transporter.

Bulk Transport Methods

Endocytosis

• Purpose: Engulfs extracellular material into the cell.
• Types:
  1. Phagocytosis: "Cell eating," used by white blood cells to consume bacteria, debris, etc.
     • Process: Pseudopods surround target, engulf it, form phagosome, which fuses with a lysosome to digest the contents.
  2. Pinocytosis: "Cell drinking," non-specific uptake of extracellular fluid and solutes.
     • Process: Plasma membrane forms a vesicle around fluid, smaller than a phagosome.
  3. Receptor-Mediated Endocytosis: Specific uptake using membrane receptors (e.g., LDL uptake).
     • Process: LDL binds receptor in clathrin-coated pits, vesicle forms, LDL separates from the receptor in endosome, receptors recycled.

Exocytosis

• Purpose: Expels material from the cell into the extracellular space.
• Process: Golgi apparatus packages proteins, lipids, and hormones, vesicles travel to cell membrane, fuse, and release contents.

Energy Requirement

• Both endocytosis and exocytosis require energy in the form of ATP for transporting substances in and out of the cell.

Recap

• Endocytosis: Cell engulfs materials (phagocytosis, pinocytosis, receptor-mediated).
• Exocytosis: Cell expels materials.
• ATP: Necessary for both processes.