Question

Since the plasma membrane blocks the passage of all but a few molecules with specific traits it is necessary for the other molecules to ?

Answer 1

Plasma membrane is a selective as well as differential permeable membrane. The substance like fatty acid and positively charge particles can pass freely across the cell surface membrane because cell membrane have negative charge. Now if there is a substance which is required for the normal function of the cell. It select it and enters it through endcytosis or through permeases enzyme which is also known is intrinsic protein.
So we can conclude that some particles passes directly through plasma membrane while other required particles are selected by the plasma membrane

Answer 2

No- the question isn’t too clear but since the plasma membrane is the only part of the cell responsible for movement of molecules, there doesn’t need to be other molecules to replicate the same process