The phospholipids are composed of an alcohol group, generally glycerol or sphingosine, to which two long chain fatty acids and a phosphate group are attached. The phosphate group is at one end and forms a structure known as a "polar head," and is the hydrophilic part of the molecule, while long chain fatty acids form what is known as "apolar tail" or hydrophobic tail. Phospholipids in an aqueous medium form miscelas, structures where the phosphate group's polar heads are in contact with the aqueous medium, while hydrophobic tails are oriented inwards (something similar to oil drops in water). In the cells, both the internal medium (cytoplasm) and the external medium are aqueous media, therefore phospholipids, which are the major component of cell membranes, are oriented forming lipid bilayers. The polar heads are oriented towards the inner cell in the inner layer, and to the cellular bacteria in the outer layer. The hydrophobic tails are oriented towards the inside of the bilayer.
