Yes, the isolated drugs posses higher biological activity than extracts. But some times, mixed phytochemicals (extracts) may have better biological activity than the isolated drugs.

Yes, pure compounds which have been isolated possess stronger biological activity than extracts in most cases. However, extract may also possess relatively good activity. So, probably you should check extract first before proceeding to isolation which may yield better outcome and shorten your time for isolation

We can't say that. some compounds may do not show effect alone and some time it ,may show effect when combine with other molecules energetically. Some times, one compound may mask the activity of another compound. So we can't say all natural compounds increases it activity in its pure form.

A compound, by definition, already has a defined identity and its purity is supposed to have been established before it is termed a compound. A natural substance in an impure mixture is different from a natural compound in its free state. When drug solutions of the two are made, their concentrations must differ when the same mass of materials is dissolved in the same volume of solvent. The pure compound must, by virtue of its purity, have a higher concentration than the impure substance which distributes itself in the solvent alongside its contaminants. The observed increased concentration of the pure compound is higher than that which will be observed for the impure substance provided the drug action is directly dependent on its concentration.