Why is a lipid bilayer in cell membrane considered as a barrier to the diffusion of polar molecules?

Because some materials can come in and some materials must stay out

Explanation:

Cell membrane is a structure that covers or bounds the cytoplasmic layer of every living cell. It is made up of mostly lipids. According to this question, it is regarded as selectively permeable. This means that it selects what goes into the cell and what comes out.

The cell membrane is capable of this because of its amphipathic nature i.e contains both hydrophilic and hydrophobic regions. Because the cell membrane controls what goes into the cell and what comes out of the cell, it is referred to as selectively permeable as some materials can come in and some materials must stay out.