Lecture Notes on Membrane Transport

Introduction

• Discussing membrane transport mechanisms.
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Simple Diffusion

• Definition: A passive process allowing molecules to move from high to low concentration.
• Key Points:
  • No energy (ATP) required.
  • Molecules that can pass through:
    • Respiratory gases: Oxygen (O2) and carbon dioxide (CO2).
    • Lipid-soluble molecules:
      • Steroid hormones (e.g., testosterone, estrogen).
      • Lipid soluble drugs.
  • Cell Membrane Structure:
    • Composed of phospholipids (polar heads & nonpolar fatty acid tails).
    • Polar molecules are repelled, nonpolar molecules can pass freely.
    • "Like dissolves like" principle applies to diffusion.
• Factors Affecting Rate of Diffusion:
  • Surface Area: Increased surface area increases diffusion.
  • Concentration Gradient: A higher gradient increases diffusion.
  • Thickness of Membrane: Thicker membranes decrease diffusion.
  • Molecule Weight: Heavier molecules diffuse slower.

Facilitated Diffusion

• Definition: Similar to simple diffusion but requires transport proteins.
• Key Points:
  • Still a passive process (no ATP required).
  • Utilizes proteins (channels/carriers) for transport.
  • Osmosis: Movement of water through aquaporins from high to low concentration.
• Transport Channels:
  • Leaky Channels: Always open, crucial for potassium in neurons.
  • Voltage-gated Channels: Open at specific voltage thresholds; important for action potentials.
  • Ligand-gated Channels: Open when a specific chemical binds (e.g., acetylcholine at the neuromuscular junction).
  • Mechanically gated Channels: Open in response to physical pressure (e.g., in pain receptors).

Active Transport

Primary Active Transport

• Definition: Direct use of ATP to move molecules against their concentration gradient.
• Key Points:
  • Sodium-Potassium ATPase: Pumps 3 Na+ out and 2 K+ in, crucial for maintaining gradients.
  • Calcium ATPases: Pumps calcium into sarcoplasmic reticulum during muscle relaxation.
  • Proton Pumps: Pumps protons into stomach lumen to create hydrochloric acid.

Secondary Active Transport

• Definition: Indirect use of ATP; relies on primary active transport to create gradients.
• Key Points:
  • Uses the gradient created by primary active transport.
  • Systems can be symport (same direction) or antiport (opposite direction).
  • Example: Sodium-glucose co-transporter in kidneys.

Vesicular Transport

• Endocytosis: Taking substances into the cell.

  • Types:
    • Pinocytosis: Cellular drinking (absorption of fluids and solutes).
    • Phagocytosis: Cellular eating (usually by white blood cells to engulf pathogens).
    • Receptor-mediated endocytosis: Specific uptake of molecules (e.g., LDL uptake in liver).

• Exocytosis: Release of substances from the cell (e.g., neurotransmitters, hormones).

  • Involves vesicles fusing with the cell membrane to expel contents.

Summary

• Covered types of membrane transport mechanisms: simple diffusion, facilitated diffusion, active transport, and vesicular transport.
• Importance of understanding these mechanisms for both cellular function and medical relevance.