Final answer:
Octylglucoside is not naturally found in eukaryotic membranes, whereas sphingomyelin is a natural component involved in membrane stability and signaling.
Step-by-step explanation:
The molecule not naturally found in a eukaryotic membrane is Octylglucoside. Eukaryotic plasma membranes are primarily comprised of a bilayer of phospholipids including phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and sphingomyelin, often enriched with cholesterol.
These components are crucial for the membrane's structure and function, influencing fluidity and playing roles in signal transduction and cell communication. Octylglucoside, on the other hand, is not a natural component of these membranes, but rather a nonionic detergent used experimentally to solubilize membrane proteins without denaturing them. In contrast, sphingomyelin is a type of sphingolipid found naturally within the eukaryotic plasma membrane, contributing to membrane stability and cell signaling.