Describe the characteristics of the phospholipid bilayer that permit small hydrophobic lipid molecules to pass directly across the membrane.

The phospholipid bilayer forms the outermost layer of cells called the plasma membrane of the cell.

The phospholipids are arranged in the bilayer with the globular head composed of glycerol and the phosphate on the outer side (hydrophilic) and the hydrophobic carbon chain in the inner side.

The hydrophobic molecule present in the layer provides a characteristic feature to the membrane that is known as selective permeability. The selective permeability allows the non-polar or uncharged molecule to pass through the membrane and not the large polar molecules.

This is because the non-polar molecules can interact with the hydrophobic portion (tails) which could allow the molecules to pass through the membrane but the polar molecules are repelled by the hydrophobic protein so require special channels.