Membrane Proteins and Transport Across Membranes

Permeability of the Lipid Bilayer

  • Lipid bilayer basics: Impermeable to most water-soluble molecules.
  • Facilitated transport: Specialized membrane transport proteins aid the passage of specific small, water-soluble molecules.

Classes of Membrane Transport Proteins

  1. Channel Proteins:

    • Provide a hydrophilic pore for specific ions and polar molecules.
    • Can be gated (open by signals) or non-gated (always open).
    • Example: K+ Leak Channels (always open).

  2. Transporter Proteins:

    • Bind specific solutes and undergo conformational changes to transport them across the membrane.
    • Types include:
      • Uniporters: Transport one solute along its gradient (e.g., GLUT Uniporter for glucose).
      • Symporters: Transport two solutes in the same direction (e.g., Na+-glucose symporter).
      • Antiporters: Transport two solutes in opposite directions (e.g., Na+-H+ exchanger).
      • ATP-driven pumps: Move solutes against their gradient using ATP energy (e.g., Na+-K+ pump).

Passive and Active Transport

  • Passive transport: Movement of solutes without energy input, down concentration gradients through channels and uniporters.
  • Active transport: Movement against concentration gradients requiring energy.
    • Types:
      • Gradient-driven (symport/antiport)
      • ATP-driven (P-type pumps, V-type pumps, ABC transporters)

Electrochemical Gradients

  • Created by membrane potential and concentration gradients. Critical for maintaining cellular functions and driving active transport.

Membrane Potential

  • Difference in electrical charge across the membrane, created primarily by K+ leak channels and Na+-K+ pumps.
  • Functions include:
    • Driving gradient-driven transport.
    • Electrical signaling (communication in neurons).
    • Regulating intracellular pH by transporting H+ ions.

Transcellular Transport of Glucose

  • Mechanism: Uses asymmetrically located transporters on epithelial cells to absorb glucose from gut and transport to bloodstream.
  • Key Transporters:
    • Apical side: Na+-glucose symporter.
    • Basolateral side: GLUT Uniporter and Na+-K+ pump.

Summary of Key Concepts

  • Active vs. Passive transport: Active requires energy and moves against gradients; passive does not.
  • Types of transport proteins: Channel proteins (passive) and transporter proteins (can be passive or active).
  • Membrane potential: Key role in cellular processes, maintained by specific pumps and channels.