Phospholipids - the biological membranes consist of phospholipids. In phospholipids there are only two non-polar fatty acid molecules bound to glycerol.
The third component that binds glycerol is a phosphate group (hence the name phospholipid) which, in turn, may be linked to other organic molecules. Thus each phospholipid contains a hydrophobic portion - represented by fatty acids - and a hydrophilic portion - corresponds to the phosphate group and its associated molecules. A remarkable fact is that when placed in water, phospholipid molecules can take the form of a sphere known as the micelle: the hydrophilic polar portions are distributed on the periphery, while the hydrophobic tails are within the distant micelles from water.
In cells, phospholipids from biological membranes (plasma membrane and many organelles) are arranged forming bilayers. The hydrophilic portions are in contact with water from the internal and external cellular media, while the hydrophobic portions are internally located on the membrane, away from the water, reminiscent of a loaf sandwich.