Question

Aspirin is a weak acid with a pka of 3.5. it is absorbed into the blood through the cells lining the stomach and the small intestine. absorption requires passage through the plasma membrane, the rate of which is determined by the polarity of the molecule: charged and highly polar molecules pass slowly, whereas neutral hydrophobic ones pass rapidly. the ph of the stomach contents is about 1.5, and the ph of the contents of the small intestine is about 6. is more aspirin absorbed into the bloodstream from the stomach or from the small intestine? clearly justify your choice. can you identify and describe other molecules whose absorption in the stomach is ph dependent?

Answer 1

Aspirin is a weak acid with a Pka of 3.5. With this Pka of 3.5, it is in a neutral or what is known as protonated  form at a pH below 2.5. At higher levels of pH, it becomes increasingly protonated or anionic, therefore, it is better absorbed in more acidic environment of the stomach. This is because absorption requires passage through the plasma membrane, the rate which  is determined by the polarity of the molecule. Charged and highly polar molecules pass, slowly, while the neutral hydrophobic ones pass rapidly.