Polarized membranes are formed out of two layers of lipids having hydrophilic heads on the outside of the membrane with hydrophobic tails on the inside of the membrane. The dual nature of the lipid layers allows the membrane to selectively transport various specific compounds into the cell.
The most common type of lipid that forms a polar membrane is a phospholipid which has a polar head containing a phosphate group. This is known as an ionic head. Besides the lipids that form the lipid bi-layer, there is polar membrane proteins that act as enzymes (functional compounds) and carrier molecules for actively transporting other molecules across the membrane.