# How does the lipid bilayer form a barrier to molecules?

Jun 26, 2015

The lipid bilayer forms a barrier to molecules because it consists of closely-packed molecules with long nonpolar hydrocarbon chains.

#### Explanation:

It is perhaps more correct to say that the bilayer is a semipermeable membrane rather than a barrier.

It allows some molecules to pass through, while preventing others from doing so..

Even though the bilayer is only a few nanometres thick, it is impermeable to most water-soluble (polar) molecules.

Thus, ions, proteins, sugars, and other large molecules are kept in place on each side of the bilayer and prevented from moving to the other side.

However, nonpolar substances such as ${\text{O}}_{2}$ and ${\text{CO}}_{2}$ can easily diffuse across the membrane.

Even though water molecules are polar, they are small enough to move freely through the lipid bilayer.