Question

When is facilitated diffusion necessary?

Answer 1

When you need to move molecules from an area of high concentration to an area of low concentration, but energy is not required.

Answer 2

did that help

Answer 3

Cells do not have to undergo a state or phase where they "Need" or it is "neccessary" facilitated diffusion to take place. Its a natural process occurring across cell surface membranes from high concentration to low concentration. Facilitated diffusion is quite different than normal diffusion. As the molecules are polar they cannot pass though the phospholipid bilayer directly. These molecules are transported by carrier proteins through channel proteins specific to these molecule e.g Glucose.
There you go (: