Final answer:
Eicosanoids are not found in plasma membranes; instead, plasma membranes consist of phospholipids, cholesterol, proteins, and carbohydrates.
Step-by-step explanation:
The component that is NOT found in plasma membranes is eicosanoids. The plasma membrane is primarily composed of phospholipids, cholesterol, proteins, and carbohydrates. Phospholipids form the bilayer structure of the membrane, proteins serve various functions including transport and cell recognition, cholesterol provides flexibility, and carbohydrates are attached to proteins and lipids on the membrane's exterior surface, mostly for cell recognition and adhesion.
Eicosanoids, while being signaling molecules derived from fatty acids, are not a structural component of the plasma membrane. They are typically involved in activities such as inflammation and immune function. Therefore, eicosanoids would be the correct answer to the question.