1)passive diffusion through the membrane phospholipids &

2) passive filtration trough aqueous pores

small molecules up to MW 600 (ethanol, ureum)

• down a conc. gradient

• influenced by

• ionisation

• lipophilicity

• blood flow

Role of blood flow and ionisation

both create a gradient across the membrane

Role of ionisation

• Only a neutral form can be absorbed via lipid layer, so pKa&ph have an influence.

  benzoic acid pKa=4. Ph below 4 = absorbtion, because the molecule is in its uncharged state. In the stomach.

  aniline pKa=7. PH abow 7 = absorbtion, molecule is then neutral. In the intestine.