Plasma membranes of human cells are made up from phospholipids. The components of phospholipids are two fatty acids, a glycerol molecule and a phosphate group, which can be modified by an alcohol. Phospholipids have two non-polar hydrophobic tails and one polar hydrophilic head.
Since the phosphate group is polar, negatively charged, and hydrophilic, it is attracted to fluid found in intracellular and extracellular areas.
The phosphate group makes the difference between phospholipids, since it contains additional groups of molecules, such as serine, choline or ethanolamine.
Phosphatidylserine contains the group serine attached to a phosphate head and it is present in all biological membranes. In humans, the majority (around 15%) of shosphatidylserine is found in the brain, a smaller percent being encountered in the lungs, kidneys, heart, liver, and blood plasma.