Final answer:
The element is not part of the plasma membrane, which comprises a lipid bilayer, proteins, and cholesterol.
Step-by-step explanation:
The component that is not part of the plasma membrane is an element. The plasma membrane is primarily composed of a phospholipid bilayer, with embedded proteins, and contains cholesterol which provides flexibility to the membrane. Phospholipids indeed form the bilayer, and proteins can be integral or peripheral, serving as transporters or recognition sites respectively. Cholesterol is found within the tail layer of the bilayer, not visible from either the extracellular or intracellular surface.