4: Membrane transport mechanisms

Section 1: Membrane Transport Mechanisms

The plasma membrane is a selectively permeable barrier, essential for maintaining cellular homeostasis by regulating the movement of ions, nutrients, and waste products. Transport mechanisms are broadly categorized as passive or active, depending on their energy requirement.

Passive Transport: Movement occurs down concentration or electrochemical gradients without cellular energy (ATP).

• Simple Diffusion: Small, nonpolar molecules (e.g., O$_2$, CO$_2$, lipids) dissolve in the lipid bilayer and move directly through it. Rate depends on the concentration gradient, lipid solubility, molecular size, temperature, and membrane surface area.
• Facilitated Diffusion: Hydrophilic molecules (e.g., glucose, amino acids, ions) diffuse through specific transmembrane proteins down their gradient.
  • Channel Proteins: Form aqueous pores. Ion channels (e.g., Na$^+$, K$^+$) are often gated (voltage, ligand, or mechanically regulated).
  • Carrier Proteins (Transporters): Bind specific solutes and undergo conformational change to shuttle them across (e.g., GLUT transporters for glucose).

Active Transport: Movement occurs against a gradient, requiring energy.

• Primary Active Transport: Uses ATP directly to pump solutes. The classic example is the Na$^+$/K$^+$-ATPase pump (sodium-potassium pump), which hydrolyzes ATP to transport 3 Na$^+$ out and 2 K$^+$ into the cell, maintaining crucial electrochemical gradients vital for excitability and secondary transport.
• Secondary Active Transport (Co-transport): Uses the energy stored in an ion gradient (usually Na$^+$) established by primary transport to move another solute.
  • Symport (Co-transport): Both solutes move in the same direction (e.g., Na$^+$/glucose symporter in intestinal epithelia).
  • Antiport (Exchange): Solutes move in opposite directions (e.g., Na$^+$/Ca$^{2+}$ exchanger, Na$^+$/H$^+$ exchanger).

Vesicular Transport: For large molecules, complexes, or bulk fluid.

• Endocytosis: Membrane invagination forms vesicles bringing material into the cell.
  • Phagocytosis: "Cell eating" of large particles (e.g., bacteria by macrophages).
  • Pinocytosis: "Cell drinking" of extracellular fluid and dissolved solutes.
  • Receptor-Mediated Endocytosis: Highly specific uptake of ligands (e.g., LDL, hormones) bound to surface receptors (e.g., clathrin-coated pits).
• Exocytosis: Vesicles from the Golgi fuse with the plasma membrane, releasing contents (e.g., neurotransmitters, hormones, mucus) out of the cell.