Phospholipids are the triglyceride lipids where one fatty acid is replaced by phosphoric acid, attached to additional nitrogenous group like choline in lecithin phospholipid.
The two fatty acids hydrocarbon chains and glycerol functions as hydrophobic (water-repellant) and forms the non-polar tail of the phospholipid molecule; whereas phosphate and nitrogenous compound, choline functions as hydrophilic (water-loving) polar head of the molecule. The presence of these hydrophilic (polar) and hydrophobic (non-polar) groups the same molecule gives the amphipathic nature to it. In the aqueous medium, phospholipids form the bilayer due to the presence of polar and non-polar groups. The non-polar ends of the molecules always face towards each other withion the membrane and the polar ends are oriented towards aqueous phase because of their attraction towards the water.
Non-polar ends cannot dissolve in water because they cannot form hydrogen bonding with the lattice structure of water. So, bipolarity of phospholipids forms a thin layer on the surface of an aqueous medium.
Arrangement in Cell Membrane:
Phospholipids (lipid) assume single and double layered (bilayer) configuration in the cell membrane. When two single layers of polar lipids come together to form a double layer, the outer hydrophilic face of each single layer orients itself towards the solution and the hydrophobic portion becomes immersed in the core of the bilayer. This structure of lipid bilayer hels the membrane to function as selective permeable.