Transport Mechanisms Across Membranes

Facilitated Transport

  • Specific and saturable transport mechanism for ions and small molecules.
  • Facilitated transporters include channels, gates, and transporters.
  • Driven by concentration gradients (no ATP required).
  • Movement of hydrophilic substances occurs through protein-lined pathways, avoiding the hydrophobic membrane interior.
  • Faster than predicted passive diffusion and exhibits specificity for molecules.

Mechanisms for Transport

Types of Membrane Proteins

  1. Channels
    • Ion and small hydrophilic molecules flow down their concentration gradient.
    • Non-gated (always open) or gated (open/close in response to signals).
  2. Transporters
    • Include uniporters, symporters, and antiporters, moving molecules with varying affinities.
    • Require concentration gradients and undergo conformational changes.
    • Significant for facilitated transport.
  3. ATP-powered Pumps
    • Use energy from ATP hydrolysis to transport ions and small molecules against concentration gradients.
    • Pump types include V-Class, F-Class, ABC-type, and P-Class pumps.

K+ Resting Channel

  • Selectivity
    • Only K+ ions properly interact with polar amino acids, shedding their hydration shell for passage through the channel.
    • K+ channels are integral membrane proteins with size-selective permeability, leading to significant membrane potential due to concentration gradients.

Active Transport

Primary Active Transport

  • Requires ATP to function, actively moving ions against their gradients.
  • Key Pumps:
    • P-class Pumps (e.g., Muscle Ca2+-ATPase and Na+/K+-ATPase)
      • Muscle Ca2+-ATPase: pumps 2 Ca2+ out of cytosol using ATP. Conformational change upon phosphorylation.
      • Na+/K+-ATPase: pumps 3 Na+ out and 2 K+ in per ATP, essential for maintaining low cytoplasmic Na+ and high K+ levels, creating a negative membrane charge.
    • ABC-class Pumps: Can transport a variety of molecules, not just ions, across membranes by utilizing ATP hydrolysis.

Secondary Active Transport (Co-Transport)

  • Involves coupled transport for different molecules, using ion gradients generated by ATP-powered pumps.
  • Types include antiporters and symporters (e.g., Na+-Glucose Symporter), where Na+ moves down its gradient while glucose is transported against its.

Integrating Transport and Cytoskeletal Structures

  • Importance of cytoskeletal and cell adhesion structures (like hemidesmosomes, desmosomes, and adherens junctions) in membrane transport and function of epithelial cells.
  • Co-transport mechanisms (e.g., Na+/glucose in epithelial cells) effectively utilize established gradients to facilitate import and export processes between lumen and blood.