Cellular Transport

  • Simple diffusion: movement of particles along their concentration gradient directly through the cell membrane.
    • Transport of gases (O2 and CO2) occurs via simple diffusion, as the membrane is very permeable to small, nonpolar molecules.
  • Facilitated diffusion: Facilitated by transmembrane proteins without the expenditure of energy.
    • Carrier-mediated transport: Transport across the cell membrane by carrier proteins (which undergo conformational changes as the substrate is transported, unlike channel proteins).
    • Saturation of carrier proteins significantly affect transport.
  • Active transport refers to the movement of a substance against its concentration gradient.  This process requires energy as well as transport proteins.
    • In primary active transport, the energy required for transport against the concentration gradient is provided by the hydrolysis of ATP.
    • In secondary active transport, this energy is generated by co-transport of a separate but wsubstance down its concentration gradient.  Transport of glucose against the concentration gradient occurs via the Na/glucose symporter, which is found in the intestinal and renal tubular epithelium and is used to transfer glucose intracellularly from the lumen.
  • Endocytosis allows for cellular uptake through the formation of membrane-bound, typically clathrin-coated, vesicles.  Uptake of cholesterol by cells occurs by means of receptor-mediated endocytosis (mediated by the LDL receptor).
  • The difference in the concentration of the substrate across the membrane is called concentration gradient (ΔP).  The higher the concentration gradient the higher the rate of transport of the molecule or ion across the membrane.
  • The larger the surface area (SA) the greater the rate of diffusion.
  • If the thickness of the membrane (T) is higher (eg, lung fibrosis) then the rate of diffusion would be slower.
  • High molecular weight (MW) compounds diffuse slowly.
  • A high oil-water partition coefficient means that a solute is much more soluble in oil than in water.

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