Whats the definition of selective permeability?

Answer 1

Selective Permeability is the characteristic of cell membranes that allows some chemicals to pass through them more easily than others. The chemicals that a membrane allows to pass through it depends on several of the chemicals' characteristics including size and electrical charge.

for example:
Small, uncharged molecules like water or carbon dioxide, tend to pass through membranes easily, but big chemicals or chemicals with an electrical charge tend to be stopped. Of course, there are other ways specific chemicals get through. Hormones are very large moleclues, but they pass through cell membranes easily. This is because they are very similar to the phospholipids that make up membranes. Hormones actually become part of the membrane for a moment and then break away from it on the other side!