A phospholipid consists of a hydrophilic polar head group and a hydrophobic tail. The polar head group contains one or more phosphate groups (PO43-). The hydrophobic tail is made up of two fatty acyl chains. When many phospholipid molecules are placed in water, their hydrophilic heads tend to face water and the hydrophobic tails are forced to stick together, forming a bilayer.
Polar Head Groups
Most phospholipid head groups belong to phosphoglycerides, which contain glycerol joining the head and the tail. Examples of phosphoglycerides include phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol, etc.
Fatty Acyl Chains
The fatty acyl chain in biomembranes usually contains even number of carbon atoms. They may be saturated (neighboring C atoms are all connected by single bonds) or unsaturated (some neighboring C atoms are connected by double bonds).
Note: In the bond-line representation,