Phospholipids are fat derivatives in which one fatty acid has been replaced by a phosphate group and one of several nitrogen-containing molecules.
Example 2.4.1: Phosphatidyl ethanolamine (also known as cephalin)
The hydrocarbon chains are hydrophobic (as in all fats). However, the charges on the phosphate and amino groups (in red) make that portion of the molecule hydrophilic. The result is an amphiphilic molecule.
Phospholipids like phosphatidyl ethanolamine are major constituents of cell membranes. These molecules form a phospholipid bilayer with their hydrophilic (polar) heads facing their aqueous surroundings (e.g., the cytosol) and their hydrophobic tails facing each other.