General and Systematic Veterinary Pharmacology

FACILITATED DIFFUSION

• Facilitated diffusion is a transport mechanism across biological membranes that involve a special “carrier molecule” in the membrane which facilitates the movement of certain molecules across the membrane.
• As in passive diffusion, facilitated diffusion involves no energy to move the drug molecules and the direction of the drug movement is determined by the concentration gradient.
• In addition, once the equilibrium is attained, the number of drug molecules crossing the membrane in either direction remains the same.
• This process proceeds more rapidly than simple diffusion and even translocates non-diffusible substrates, but along their concentration gradient.
  
• However, the transporter may become saturated and other compounds may compete or inhibit the transport.
• Examples of facilitated diffusion are:
  • Vitamin B12 absorbed from the gut .
  • Transport of sugars and aminoacids across membranes.