Polar substances have little ability to diffuse across cell membrane. This is because the main component in the cell membrane is the lipid and polar substances are not lipid soluble. But non polar substances cross easily. Substances that are polar have several other mechanisms to compensate for their deficiency. They include facilitated diffusion across channel, active transport which is energy consuming, receptor mechanism which allows the substance no access to the interior of the cell but could influence cellular behaviour by proxy through attachment to its receptor. Finally enzymatic activation of the cell membrane could cause chemical reactions within the cell or the release of metabolites that can cross the cell membrane barrier easily.
Substance may be polar or non-polar
Other substance of low molecular weight of 100 – 200 are transported through channels found in the integral protein. Ions are transported through cell membrane by an electrochemical gradient which depends on transmembrane potential of the cell membrane.
Aspirin (Acetyl salicylic acid)
Aspirin – acetylsalicylic acid is an analgesic-antipyretic drug which behaves as acid
(S H) ↔ (H+) (S)
This drug when given by mouth passes from gastric mucosa to the plasma. The high acidity in the stomach makes the weak acidic drug dissociate less and hence absorb faster into plasma, by acting as nonpolar substance. But in the intestine (duodenum, jejunum and ileum) absorption is less since it meets an alkaline medium and dissociates more, acting as a polar substance and diffusion through the intestinal mucosa is less. Hence aspirin is rapidly absorbed in the stomach but less so in the duodenum.
Given a gastric pH of 1.4, and aspirin has a pKa of 4.4
pH = pKa + log A-/HA
1.4 =4.4 + log A-/HA
-3 = log A-/HA
therefore ratio of A to HA = 1:0.001
In the plasma however given a plasma pH of 7.4 and a pKa of aspirin of 4.4
applying Henderson-Hasselbalch equation we have
A-/HA = 103
Therefore ratio is 1:1000.
Hence dissociation increases and absorption decreases.
School of Medicine
Creator: Oluwole Ogunranti