Why is facilitated diffusion necessary?

to cross the plasma membrane.

Explanation:

Two classes of proteins that mediate facilitated diffusion are generally distinguished: carrier proteins and channel proteins.

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.