3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.3. Pharmacology
                  3.1.3.1. Pharmacology principles
                      3.1.3.1.1. Pharmacokinetics
 3.1.3.1.1.1. Drug passage across cell membrane 

Drug passage across cell membrane

[PHW2:p4-6]

Methods of crossing the cell membrane

  • Passive diffusion
    * Non-ionised drugs
  • Facilitated diffusion
    * e.g. steroid and amino acid absorption from the gut
  • Active transport
    * e.g. Na/K ATPase
    * Can be primary active transport, or secondary active transport (co-transport or antiport)
  • Pinocytosis

Factors influencing diffusion rate

  • Molecular size
    * Graham's law = diffusion rate is inversely proportional to the square root of the MW
  • Concentration gradient
    * Fick's law = diffusion rate is proportional to the concentration gradient
    * Bowman's principle = Use of less potent drugs requires larger doses, which actually increases concentration gradient and hasten onset of action
  • Ionisation
    * Low ionisation increases diffusion rate
    * Weak bases at high pH and weak acids at low pH are non-ionised, thus faster diffusion rate
  • Lipid solubility
    * High lipid solubility increases diffusion rate
  • Protein binding
    * Only unbound fraction of the drug is available for diffusion