Polar and nonpolar?

Question

When biomedical researchers design drugs that must enter cells to be effective, they sometimes add methyl (CH3) groups, which are nonpolar, all over the outer surface to make the drug molecules more likely to pass through cell membranes. Conversely, when researches design drugs that act on the exterior of cell membranes, they sometimes add a charged group, which are polar, to decrease the likelihood that the drugs will pass through membranes and enter cells. Explain why these strategies make sense given what you know about cells and cell membranes...

Answer 1

The plasma membrane is made of a lipid bilayer. Nonpolar compounds can pass through the membrane easily. So they are absorbed more easily, and they are retained in the body longer. On the other hands, polar compounds are excreted by the kidneys faster, and thus do not last long. One good example is penicillin. It is excreted so fast that in the first clinical trial, they had to re-isolate penicillin from the urine of the patient. That is why scientists add methyl groups to drugs to increase their potency.

Answer 2

Cell membranes are non polar being made up principally of long hydrocarbon chains derived from fatty acids and attached to glycerol.

Since like dissolves like, non polar substances will find it easy to penetrate into a cell membrane while polar molecules will find it difficult especially large ones.